Insect trace fossils elucidate depositional 
environments and sedimentation at a 
dinosaur nesting site from the Cretaceous 
(Campanian) Two Medicine Formation of 
Montana
Authors: William J. Freimuth and David J. Varricchio 
©2019 This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://
creativecommons.org/licenses/by-nc-nd/4.0/
Freimuth, William J., and David J. Varricchio. “Insect Trace Fossils Elucidate Depositional 
Environments and Sedimentation at a Dinosaur Nesting Site from the Cretaceous (Campanian) 
Two Medicine Formation of Montana.” Palaeogeography, Palaeoclimatology, Palaeoecology 534 
(November 2019): 109262. doi:10.1016/j.palaeo.2019.109262.
Made available through Montana State University’s ScholarWorks
scholarworks.montana.edu
Insect trace fossils elucidate depositional environments and sedimentation at a dinosaur nesting Formatted
site from the Cretaceous (Campanian) Two Medicine Formation of Montana 
William J. Freimuth*a and David J. Varricchioa
aDepartment of Earth Sciences, Montana State University, 226 Traphagen Hall, PO Box 173480, 
Bozeman, MT 59717, U.S.A. 
*Corresponding author: willie.j.freimuth@gmail.com
Abstract 
We describe the diversity and abundance of insect (specifically hymenopterans and coleopterans) 
pupation structures in the Upper Cretaceous (Campanian) Two Medicine Formation at the Egg 
Mountain locality, western Montana, U.S.A., an important dinosaur nesting site. The study interval 
comprises a massive calcareous siltstone and indurated silty limestone horizons interpreted as the 
product of cumulative paleosols. A 7 m by 11 m area was quarried with a jackhammer at intervals of 
12.5 cm thickness for a 1.5 m thick stratigraphic section. The ichnoassemblage comprises four 
morphotypes (small, medium, large, and wide) assigned to Fictovichnus sciuttoi, of which three 
represent wasp (hymenopteran) cocoons while the fourth (wide) type potentially was produced by a 
coleopteran. Medium and small F. sciuttoi are dominant while large and wide Fictovichnus are less 
common and absent in some sample intervals. Other probable insect traces include partial perforations in 
cocoons (Tombownichnus), isolated burrows, and an enigmatic hemispherical trace. Material is 
representative of a depauperate Celliforma ichnofacies. Pervasive cocoons and other traces throughout 
the sequence suggest persistent soil conditions suitable for insect nesting and pupation, and suggest an 
absence of sediment pulses of sufficient thickness to prohibit thorough colonization. Peaks in pupation 
chamber abundance may reflect episodes of reduced sedimentation rates otherwise unseen in the 
absence of primary bedding structures. Well-drained and friable soil conditions favorable for insect 
nesting also may help explain the abundance of dinosaur nests and other vertebrate nesting events in 
associated strata as well as the presence of small terrestrial forms. 
 
Keywords: ichnology; hymenopterans; coleopterans; Egg Mountain; Pupichnia 
 
1. Introduction 
 Historically, ichnological studies have primarily focused on marine settings (e.g., Howard, 
1975). More recently, studies have shifted to the terrestrial realm, integrating the ichnofacies paradigm 
and exploring ichnological implications in pedogenesis and depositional processes, paleoecology, and 
evolutionary trends (e.g., Hasiotis and Bown, 1992; Genise et al., 2000; Hasiotis, 2003; Melchor et al., 
2012). Specifically, insect trace fossils have been recognized as valuable paleoenvironmental indicators 
(Genise, 2016). While insect trace fossils commonly occur in leaves and wood (e.g., Labaneira, 2006; 
Falcon-Lang et al. 2015), their associations in paleosols are useful indicators of paleoenvironments, as 
nesting insects are typically constrained by different environmental parameters (e.g., Genise et al., 
2000).  
The oldest records of insect-dominated ichnofacies occur in the Cretaceous with the Celliforma 
ichnofacies. Alonso-Zarza et al. (2011) describe an example of the Celliforma ichnofacies comprised of 
bee cells (Celliforma ispp.) and beetle pupation structures (Fictovichnus gobiensis) from the 
Campanian-Maastrichtian Mercedes Formation of Uruguay. In a second Cretaceous example of the 
Celliforma ichnofacies, Martin and Varricchio (2010) describe wasp cocoons (Fictovichnus sciuttoi) and 
 
potentially bee (Rebuffoichnus) and moth (Teisseirei) pupation chambers from the Cretaceous 
(Campanian) Two Medicine Formation. In both instances, the presence of the Celliforma ichnofacies 
implies semi-arid environments with bare and well-drained soil conditions favored by soil-dwelling 
insects (Genise et al, 2010). 
Here, we describe an assemblage of insect trace fossils at Egg Mountain, a dinosaur nesting 
locality from the Upper Cretaceous (Campanian) Two Medicine Formation of Montana (Horner, 1984; 
Horner, 1987). Along with abundant dinosaur eggshell and small vertebrates, this locality preserves a 
rich assemblage of insect traces first recognized and interpreted as carrion beetle cocoons (Horner, 1984; 
Horner, 1987). The locality is marked by a distinct absence of primary sedimentary structures and 
bedding markers, making interpretation of its depositional history difficult. Despite the paucity of 
stratigraphic contacts, Horner (1987) describes three distinct nesting horizons based on subtle lithologic 
differences and concentrations of eggshell and skeletal remains. Insect cocoons occur throughout 
surrounding strata and have been useful in paleoenvironmental interpretations of the area (Martin and 
Varricchio, 2011). Specifically, we examine the stratigraphic abundance and diversity of insect traces 
through a section at the Egg Mountain, and apply an approach that others have used for marine strata 
(Gingras et al., 1999; Dashtgard et al., 2011; Gingras et al., 2011), to assess environmental conditions 
and general patterns of deposition and erosion (e.g., Sánchez et al., 2010).  
 
2. Geological Setting 
 The geology of the Two Medicine Formation (Upper Cretaceous) is well known, largely due to 
extensive studies related to abundant dinosaur eggs, trace fossils, and skeletal remains (e.g., Horner, 
1979; Horner, 1982; Varricchio and Horner, 1993; Chin, 2007). 40Ar/39Ar dates near the base and top of 
the Two Medicine Formation indicate a Campanian age of between 74.076 ± 0.095 and 80.044 ± 0.190 
 
Ma (Rogers et al., 1993). Two Medicine Formation deposition occurred at a paleolatitude of 56°N 
(Fricke et al., 2010), and a semi-arid to subhumid climate and seasonality are inferred throughout the 
deposition of the formation, as indicated by the abundance of caliche nodules, taphonomic data 
supporting episodic drought, growth interruptions in conifer fossils, and herbivorous dinosaur coprolites 
(Lorenz and Gavin, 1984; Rogers, 1990; Falcon-Lang, 2003; Chin, 2007). Facies are interpreted as 
intergrading fluvial and lacustrine systems in a volcanically active area (Lorenz and Gavin, 1984; 
Rogers et al., 1993). 
 
2.1. Egg Mountain locality 
 The Egg Mountain locality, which forms the focus of this study, occurs at the Willow Creek 
Anticline near Choteau, Montana (47° 48.370’ N, 112° 25.850’ W) (Fig. 1). It consists of a small hill 
that rises around 15 meters above the surrounding topography. 40Ar/39Ar dating of a bentonite horizon of 
the Willow Creek Anticline suggests strata equivalent to or just stratigraphically lower than Egg 
Mountain are 75.53 ± 0.32 Ma (Varricchio et al., 2010). During this time, the area was positioned in 
geographically proximal to the emerging Rocky Mountains to the west, and distal to the Interior Seaway 
(Lorenz and Gavin, 1984; Horner, 1987).  
The Egg Mountain site is famous for producing dinosaur egg clutches of Troodon formosus and 
the oogenus Continuoolithus (Horner, 1984; Horner, 1987; Hirsch and Quinn, 1990; Varricchio et al., 
1999). Isolated eggshell including some attributable to Maiasaura peeblesorum (Oser, 2014) is common 
throughout the section. Recent excavations from 2010-2016 have produced an abundance of associated 
to articulated skeletons of the metatherian Alphadon halleyi, the multituberculate Cimexomys judithae, 
both previously described from the site (Montellano, 1988; Montellano et al., 2000), and an 
iguanomorph lizard, Magnuviator ovimonsensis (DeMar et al., 2017). Concentrations of unknown thin 
 
eggshell have been excavated at different horizons, representing nesting events. Isolated and associated 
elements of the small ornithopod Orodromeus occur throughout the section. Isolated hadrosaur teeth are 
common, and dromaeosaurid, Troodon, and tyrannosaurid teeth are less common. In addition to the 
abundant pupal cases, rare isolated planispiral terrestrial snails have been found. An Adocus turtle shell 
fragment and an isolated frog frontal are the only aquatic vertebrate forms known. 
 
2.2. Paleoenvironments 
Deposition at the Egg Mountain locality has been interpreted as the product of crevasse splays in 
a fluvial upland facies (Horner, 1984; Lorenz and Gavin, 1984) or an up-dip alluvial plain (Fricke et al., 
2010) and Horner (1987) interpreted the locality as a lakeside dinosaur nesting ground. The stratigraphy 
~100 meters to the north is marked by lacustrine limestone and interbedded mudstone and siltstone at a 
similar or slightly lower stratigraphic level. Similar lacustrine sediments are present to the south of the 
locality. These sediments, which contain charophytes and well-preserved pterodactyloid pterosaur 
(Padian, 1984), are interpreted to represent a low energy alkaline lake (Lorenz and Gavin, 1984). Lorenz 
and Gavin (1984) suggest the lake may have been seasonal, as evidenced by subaerial mud cracks; this 
is parsimonious with other inferences of seasonality throughout Two Medicine deposition (e.g., Rogers, 
1990; Falcon-Lang, 2003). To the north and east, facies change to interbedded sandstones deposited in 
shallow braided streams (Lorenz and Gavin, 1984). The Egg Mountain locality lies at the interface of the 
lake and braided stream lithofacies (Horner, 1987). 
 Paleozoic limestones to the west of the locality are the likely source of the calcareous sediments 
(Lorenz and Gavin, 1984). Large amounts of calcite are present in both primary features (sediment) and 
secondary features (infill of tectonic fractures, diagenetic crystalline infill of some fossil cocoons and 
eggshell). High levels of calcium limestone favor the preservation of eggshell and egg material. 
 
Varricchio et al. (1999) argued for a pedogenic origin for the limestones based on the presence of root 
traces, burrows, nodules, and low rare earth element concentrations, but distinct paleosol horizons are 
lacking. However, isotopic analysis favors groundwater cementation (Moore et al., pers. comm., 2016). 
A fluctuating water table is parsimonious with inferred seasonality throughout Egg Mountain deposition 
and may explain the irregularity of the limestone horizons at the locality (Lorenz and Gavin, 1984). 
 
2.3. Sedimentary facies of study section  
 Three lithologic units comprise the top 1.5 meters of the studied section in an artificial quarry 
(Fig. 2). The lowermost interval (Unit 1) is defined by a >50 cm thick micritic limestone that is laterally 
continuous across the 7 m by 11 m quarry. The unit is thickest at the north end of the quarry and thins to 
the south (Fig. 3a). Some medium- to coarse-grained sand, pebbles, and small, centimeter-scale mud rip 
up clasts are present in the thickest portion of this unit. It is massive with no distinct internal structure. 
The limestone has a very irregular upper surface with decimeter scale relief that extends into the 
siltstone unit above (Fig. 3b). Where exposed, it is characterized by a surficial rust color. The upper 
contact of the limestone and siltstone is usually sharp. The dominant lithology of Unit 2 is a ~1 m thick 
grey calcareous siltstone. The siltstone weathers brittlely and blocky with an irregular pattern. Nodules 
(diameters 4.6-17.4 mm) occur infrequently in the siltstone units. Indurated, irregular micritic limestones 
inter-finger with the siltstone to form irregular discontinuous beds (Fig. 3c). These limestones are 
similar in composition to those in Unit 1 but lack larger pebbles and mud rip up clasts. Unit 3 comprises 
a relatively thin irregular micritic limestone similar to those in Unit 2. The limestone is variable in 
thickness and topographic relief of the upper surface. Unit 3 preserved a Troodon nest structure and egg 
clutch (Varricchio et al., 1999). The unit preserves infrequent root traces, particularly within the nest 
structure (Varricchio et al., 1999). Overall, the sedimentary succession is homogenous and displays no 
 
distinct bedding markers or paleosol horizons. The only marked difference between the lithologic units 
is the varying limestone induration. 
  
3. Material and methods 
 Horner (1987) described three nest-bearing horizons at Egg Mountain in a vertical section of 
roughly three meters. A new 7x11 meter main quarry was opened in 2010 to the east of and adjacent to 
Horner’s quarry and was worked each year through 2016. Excavations worked through the upper 1.5 
meters of Horner’s (1987) section. Another adjacent upper quarry was opened in 2015 (Fig. 4a). The 
trace fossil data used here have been collected from the main quarry from 2010 to 2016. The 1.5 meters 
of vertical section through the main quarry was lowered systematically in a series of 12 jackhammer 
passes (JHPs), with an average thickness of 12.5 cm for each pass (Fig. 2). Quarry depths were 
measured for each pass using a sight level. In the absence of primary bedding structures, evenly spaced 
jackhammer passes act as artificial markers with which trace fossil abundance and diversity can be 
measured. Rubble created in the wake of the jackhammer was picked through and specimens were 
collected and sorted into bags, their abundances recorded for each jackhammer pass and used 
quantitatively to describe sedimentation rate and general depositional processes. Lateral location of 
specimens was recorded throughout the section using a meter by meter grid system (Fig. 4b). In total, 
3661 fossil cocoons were collected throughout the section. An additional 1031 cocoons were collected 
but unassignable to any JHP in the main quarry. 
 Abundances and diversity data may be influenced by collection bias. Some samples are badly 
fractured, making identification based on size nearly impossible. Data from the 2010 field season are 
incomplete; only a sample of traces were collected. Thus, although the two jackhammer passes from 
 
2010 are included in sedimentation analyses, total abundances for the two passes are underrepresented. 
Trace fossil data used to infer general sedimentation rates were restricted to the main quarry.  
 We measured the x, y, and z axes (i.e. length, maximum diameter, and diameter perpendicular to 
maximum) of all complete specimens at the Egg Mountain locality (N=1155) to quantifiably distinguish 
trace morphologies. Specimens were measured and included in morphological analysis only if all three 
dimensions accessible for measurements. The remaining traces (N=3537) were assigned to a size 
morphology based on measurements of accessible dimensions (i.e. measurement of one or two 
identifiable axes) and qualitative features such as surface texture. Minimum or maximum dimensions of 
traces included in length and diameter ranges overlap size categories, but those outlier specimens were 
qualitatively assigned to a group based on the best-preserved dimensional characteristics and 
observations of surface texture. The few traces that were poorly preserved, fractured, or encased in 
sediment and unable to be measured were not assigned to a morphology and not used in abundance 
counts.  
 All specimens are deposited in the Invertebrate Collections of Museum of the Rockies (MOR-
IV) under the catalog MOR-IV-2804. Individual field bags retain their field identification numbers. 
Figured specimens were separated from their individual bag and given their own unique specimen 
identification number in addition to the field identification number.  
 
4. Trace fossil types 
 
4.1 Ichnofamily Pallichnidae Genise 2004 
Ichnogenus Fictovichnus Johnston et al., 1996 
Fictovichnus sciuttoi (Genise et al., 2007) 
 
Figure 7 
Material. – MOR-IV-2804; 1155 specimens. 
Occurrence. – Massive grey calcareous siltstone and micritic limestone at the Egg Mountain locality 
(TM-006), Two Medicine Formation, Montana, USA. 
Description. – Fictovichnus sciuttoi is an ovoid, capsule-shaped structure showing fine lobed textures 
with helically arranged ridges, unlike the more gracile and tapered F. aragon and the externally smooth 
F. gobiensis (Genise et al., 2007; Genise, 2016). Original material is described as Rebuffoichnus sciuttoi 
by Genise et al. (2007) from the Upper Cretaceous (Coniacian-Santonian) Bajo Barreal Formation of 
Argentina, but the original description was amended to exclude interpretive and micromorphological 
characteristics (Alonso-Zarza et al., 2014). From the Argentinian material, 27 specimens range from 
26—33 millimeters long and 12—16 millimeters wide and were interpreted as cocoons of aculeate 
wasps (Genise et al., 2007). The best-preserved Egg Mountain specimens display helical ornamentation 
and some show scarring, a flattened surface oriented obliquely to the long axis of the structure, and/or 
truncation, a flat surface at one extreme of the structure with perpendicular orientation to the long axis.  
 Four distinct morphologies are displayed (Fig. 5, Table 1). Three ellipsoidal to ovoidal 
morphologies of F. sciuttoi—large, medium, and small—are present throughout the section at Egg 
Mountain (Fig. 6). Specimens of the large morphology are on average 31.5—37.7 mm x 13.0—16.0 mm 
in diameter (N=62) (Fig. 6A-C). They display a greater length to diameter ratio (2.4), average volume 
(3.02 cm3), and average length (34.6 mm) than small and medium classes, though they display an 
average diameter comparable to medium morphology (14.5 mm and 12.5 mm, respectively). Medium-
sized F. sciuttoi average 19.7—23.4 mm in length and 11.5—13.6 mm in diameter (N=674) (Fig. 6D-
G). They can be distinguished from the large, more elongate morphology by a lower length to diameter 
ratio (1.7). F. sciuttoi of the small morphology also display a lower length to diameter ratio equivalent to 
 
that of the medium morphology (1.7). Small specimens average 10.9—13.0 mm in length and 6.4—7.5 
mm in diameter (N=364) (Fig. 6H-K). Despite dimensional differences, examples in each of the three 
size classes preserve a helical ridge ornamentation that resembles modern hymenopteran cocoons (Fig. 
7). Very infrequently, some specimens display dark, dendritic markings on their exterior, interpreted as 
rootlet traces (Fig. 6I). This is common in extant and fossil insect traces (Johnston et al., 1996; Alonso-
Zarza et al., 2014). 
 We analyzed the micromorphology of three specimens (MOR-IV-2804-B1.14.small, -
B11.13.small, -B7.12.large). In all specimens, the differences between the inner filling and outer matrix 
are negligible; both consist of a tan and grey micrite with sparse calcite and plagioclase clasts and clay 
material. The boundary between the outer matrix and the inner filling is largely discontinuous. MOR-IV-
2804-B11.13.small, a small F. sciuttoi specimen, exhibits a thin (20 µm) discrete calcite wall (Fig. 8A, 
8B). Along the outer edge of the micritic wall (external to the cast) is a discontinuous, poorly defined 
layer consisting of packed grey clay material with scattered larger clasts (Fig. 8A). This layer varies in 
thickness from 300 µm to 600 µm. The same clay-rich zone occurs in MOR-IV-2804-B1.14.small, a 
small morphology of F. sciuttoi (Fig. 8C). This specimen does not preserve the discrete micritic wall, 
and is instead detached from the surrounding matrix, separated by a gap. MOR-IV-2804-B7.12.large, a 
specimen of large morphology isolated from the matrix, preserves a thin, discontinuous dark clay rim 
along the outer edge of the cast (Fig. 8D). 
 An additional morphology of F. sciuttoi displays a characteristically wider morphology than 
other representatives at the Egg Mountain locality and elsewhere. These specimens are designated here 
by the wide category (Table 1, Fig. 5). Average length is typically 25.6-33.8 mm, and average diameter 
is 20.0-24.3 mm (N=55). They are characterized by a low length to diameter ratio (1.3) and have the 
largest average volume (7.26 cm3) and average diameter (22.1 mm) amongst pupation structures of the 
 
locality. Some specimens preserve a bumpy linear to oblique helical texture, similar to but coarser and 
more irregular than that observed on other more elongate ellipsoidal cocoons of F. sciuttoi (Fig. 9A). 
Four specimens were found in situ, sitting vertically with the long axis perpendicular with respect to 
bedding (Fig. 9B). This differs from the oblique to horizontal orientation typical of F. sciuttoi. 
 The micromorphology of one wide specimen (MOR-IV-2804-B16.11.FB) was analyzed. An 
undulating calcite boundary is present and consistent along the outer edge of the trace, separating outer 
and inner matrices. The outer matrix (i.e., grey indurated limestone of unit 1) and inner matrix are 
similar in composition and grain size; however, the outer matrix is a greyish color, while the inner 
matrix is tan to brown. The boundary is composed primarily of tan crystalline calcite, 500-700 µm in 
thickness. The clasts do not show any alignment, as in Rebuffoichnus casamiquelai (Genise et al., 2007). 
Intermittent sections of the boundary display a fine-grained matrix with sparse calcite crystals. These 
matrix-supported sections interrupt the coarse-grained, crystalline sections of the boundary and 
occasionally interfinger.  
 While an outer structure provided rigidity and texture to the trace and allows it to be easily 
distinguished from the surrounding matrix, the boundary cannot be seen with the unaided eye, a 
characteristic of Fictovichnus. Combined with the apparent absence of Rebuffoichnus from the locality, 
this confirms that these specimens are true Fictovichnus and not internal casts of Rebuffoichnus (Genise, 
2016). Despite the consistently stouter morphology compared to other material at the locality, some 
wide specimens preserve an external helical ornamentation. This character is diagnostic of F. sciuttoi; as 
such, we treat these specimens as an additional morph of F. sciuttoi. The lower aspect ratio of the wide 
morphology of F. sciuttoi is aberrant from other F. sciuttoi here and elsewhere, though differences in 
size and aspect ratio do not justify resurrecting a new ichnospecies (Bertling et al., 2006).  
 
 
4.2 Ichnofamily Lazaichnidae Genise, 2016 
Ichnogenus Tombownichnus Mikuláš and Genise, 2003 
Tombownichnus cf. parabolicus Mikuláš and Genise, 2003 
Figure 11a 
Material. – MOR-IV-2804; Two specimens (MOR-IV-2804-B7.13-1, MOR-IV-2804-B7.14-1) from the 
Egg Mountain locality of the Upper Cretaceous Two Medicine Formation. 
Occurrence. – Massive grey calcareous siltstone and micritic limestone at the Egg Mountain locality 
(TM-006), Two Medicine Formation, Montana, USA. 
Description. – Tombownichnus is represented by an elliptical or sub-cylindrical pit in the midsection of 
insect chambers. In T. parabolicus, the singular pit does not breach the external surface, leaving a 
distinctive depression. This differs from T. plenus, which is represented by multiple complete borings 
through the interior of the chamber. These traces are also not assignable to T. pepei, which occur in the 
internal face of the discrete wall of Coprinisphaera specimens. 
 Tombownichnus cf. parabolicus is interpreted based on 5-7 mm long indents in the midsection of 
two medium-sized specimens of F. sciuttoi from Egg Mountain (Fig. 10A). In both specimens, the 
depression occurs about one third of the way along the total length of the host cocoon. The shape and 
size of the depressions in F. sciuttoi described here matches that described and figured by Martin and 
Varricchio (2011, Fig. 5D). Specimens are recovered from JHPs 6 and 9. 
 
4.3 Unassignable trace fossils 
 Several instances of associated or cross-cutting cocoons occur throughout the measured section 
(Fig. 10B, 10C). Two specimens of small morphology cocoons intersecting, fused, or embedded with 
large morph cocoons are noted (Fig. 10B). Both the large and small morphologies are assignable to F. 
sciuttoi based on the presence of an external raised helical texture (Fig. 10B). Specimens are recovered 
 
from JHPs 1 and 7, respectively. Fictovichnus of similar sizes are also preserved intersecting or cross-
cutting one another. At least four instances of similarly sized intersecting cocoons are observed 
throughout the section, and all instances are represented by the large morphology. They intersect one 
another at angles ranging from 90-150 (Fig. 10C). Specimens are recovered from JHPs 6, 7, and 10.  
 
4.4 Cylindrical burrows 
 Burrows collected throughout the section vary in size, but most are 1.0–2.6 cm diameter, 10–15 
cm long (N=138). They are rarely associated directly with pupation chambers and are often truncated 
and highly fractured, in part due to the excavation process. Similar sized burrows are associated with 
pupation chambers in a nearby outcrop (Martin and Varricchio, 2011). Smaller burrows (3-5 mm 
diameter) are less common. Most are badly fractured and have been described out of geologic context, 
making ichnological identification difficult. However, small burrows here resemble those seen in nearby 
outcrop tentatively assigned to Skolithos by Martin and Varricchio (2011). Distinguishing burrows from 
root traces is difficult due to truncation and fracture in most specimens. Those specimens that are better 
preserved do not display tapering or branching characteristic of root traces. Other rhizoliths are present 
but scarce at the locality; some root traces occur on fossil pupation structures (Fig. 6I). Burrow 
morphologies are consistent with those of extant sand wasp species (Bembicinae). Smaller species, such 
as Microbembix, construct burrows with a diameter of 5 mm, while burrows of larger species such as 
Stictia carolina can be up to 2 cm in diameter (Evans, 1966). Wasps typically do not line the burrow and 
only some species backfill selected portions; this may contribute to the lack of preservation. 
 
4.5 Pallichnidae indet. 
 
 Members of this unknown ichnotaxon are identified as small half-spherical structures with a 
distinctive brown coating, colloquially dubbed “Brownies” by Egg Mountain field crew. Specimens are 
mostly found emplaced in the sediment with the half-spherical structure extending down into the 
substrate. This preservation resembles that of Feoichnus (Krause et al., 2008), though the discrete wall is 
not observable macroscopically and the characteristic root notch is absent, among other differences. The 
defining characteristic of these traces is a brown outer rind. Specimens are not used in sedimentation 
rate analysis. A detailed description is in preparation (Panascí et al., 2016; Panascí, pers. comm., 2019). 
 
5. Trace abundance and diversity 
  The overall cocoon abundance per JHP is presented in Figure 12. Abundances from JHP 1 and 
JHP 2 were not fully sampled and are therefore underrepresented and not useful in comparing JHP 
abundances. Traces are present throughout all jackhammer passes; there are no passes without traces 
(Fig. 11). Trace abundance is relatively low in the top 60 cm of the section, representing 21% of all 
traces. The remaining 79% of all traces are concentrated in the bottom 90 cm of the section. JHP 9 
records the highest abundance of all passes with 628 traces representing 17.2% of all traces in the 
section. In JHP 9, 288 medium pupae were found in one 2x8 m section. Similarly, 224 medium pupae 
were collected from the same area of the quarry, but stratigraphically higher in JHP 8. These horizons 
are contained in a lateral area of 4x8 m with a vertical section of 0.6 m. In this section, a total of 1228 
cocoons occur, or 34% of all traces from the main quarry. 
 The abundance of each cocoon morphology throughout the section is presented in Figure 12 and 
Table 2. Medium F. sciuttoi are the most common, with specimens present in all passes and comprising 
53% of all traces. They are the dominant trace in all passes with the exception of JHP 1 and JHP 5 where 
small F. sciuttoi are most abundant. The medium morphology ranges in abundance from N=22 in JHP 5 
 
to N=467 in JHP 9 (Table 2). The minimum relative abundance of the medium morphology is 20% in 
JHP 1, the maximum 76% in JHP 8. Small F. sciuttoi are consistently the second most common trace 
except in JHP 2. The small morphology is present in all passes, making up 32% of all traces. Their 
minimum abundance is in JHP 4 (N=29) and maximum abundance is in JHP 12 (N=203). Minimum 
relative abundance is in JHP 2 (20%) and maximum is in JHP 1 (55%). Large, elongate F. sciuttoi are 
present in all passes and are consistently less abundant than medium and small morphologies, 
representing 8% of the total traces in the section. The wide morphology shows the most sporadic trend 
in abundance. They are concentrated towards the top half of the section, with their greatest abundance in 
JHP 5 (N=54) and a maximum relative abundance of 21% (JHP 4). They are noticeably absent in JHPs 8 
through 11, until the very bottom of the measured section where they constitute just 3% of traces (Table 
2). 
 Trace diversity generally decreases down section (Fig. 13). From the top of the section through 
JHP 6, all size categories are represented. JHPs 2, 5, and 6 record the most equal distribution of trace 
categories (Fig. 13). Beginning at JHP 7, there is a noticeable decline in abundance of the two largest 
traces, the elongate and wide morphologies of F. sciuttoi with a corresponding increase in the relative 
abundance of medium F. sciuttoi. JHPs 8 and 9 display the highest abundances of medium F. sciuttoi 
which correspond to passes of lowest diversity (Table 2). 
 
6. Discussion 
6.1. Background on burrowing wasps 
 Modern sand wasps (Hymenoptera: Bembicinae) provide model organisms for equating 
ethological inferences of the insect traces at Egg Mountain. Sand wasps are holometabolous, meaning 
they undergo full metamorphosis. A larva hatches from an egg, feeds on provisions in the cell, and 
 
undergoes an inactive pupal stage in a cocoon, typically woven from silk and occasionally constructed 
with sand grains (some Bembiciniae and Dryinidae) before emerging as an adult. Most species of sand 
wasps overwinter—they undergo the seasonal pupal stage and are univoltine (have one brood per year) 
(Evans, 1966). Females construct burrows and lay their eggs on flies, weevils, or other prey items. A cell 
is created for the prey item at the terminus of a burrow, typically at depths of 10-30 cm (Evans, 1966). 
However, cells have been found as shallow as 0.8 cm and as deep as 60 cm (Evans and Evans, 2007). 
Cells are often constructed horizontally, and burrows are typically oriented obliquely with respect to the 
ground surface but can vary from sub-horizontal to sub-vertical (Evans, 1966) (Fig. 14). Some species 
close off the cell from the main burrow or cap the burrow entrance. In general, larger wasp species tend 
to construct larger pupation structures and, amongst sand wasps, burrow at greater depths (Evans, 1966). 
However, additional environmental factors also play a role in cocoon size and burrow depth.  
 Most sand wasps prefer bare, dry, friable soils in open habitats often devoid of extensive 
vegetation cover (Genise, 2000; Evans and O’Neill, 2009), but burrows have been found in a variety of 
substrates (Evans and Evans, 2007). While substrate preference varies widely between different species, 
suitable substrate with ample prey or food sources is probably a limiting factor in the distribution of 
many fossorial wasps (O’Neill, 2001, p. 160). Nesting site selection is key for offspring ability to 
thermoregulate and control water balance (O’Neill 2001, p. 167). Burrow depth is a factor of the 
compactness and moisture in the soil; loose, dry soil enables deeper burrowing (Kurczweski, 2003; 
Evans and Evans, 2007). Additionally, burrow depth likely reflects the subsurface temperature and 
moisture; moist (but not saturated) horizons at depth probably provide a temperature buffer on the order 
of 25-30°C for wasp offspring (Willmer, 1982; O’Neill, 2001). Pupa size varies amongst species and 
may also be related to sexual dimorphism of the same species. Female larvae of some taxa receive 
several prey items in the cell while males receive fewer; this dimorphism is reflected in both cell size 
 
and cocoon size, where longer and larger larvae receive more provisions and produce females (Evans et 
al., 1980; Kurczweski, 2003; Evans and Evans, 2007; Martin, 2013), though size may also simply reflect 
different sized prey items in provisioning the nest (e.g., Evans, 1966; Evans and O’Neill, 2009). A single 
species of sand wasp (e.g., Bembix) may prey on insects of up to 28 different orders (Evans and O’Neill, 
2009). 
 Of the structures in a typical sand wasp nest (Fig. 14), cocoons constructed with silk and soil 
materials have the most potential for preservation, while simple, unlined excavations such as burrows 
and cells are usually not preserved (Genise and Bown, 1994; Genise and Cladera, 2004). This trend is 
reflected in the Egg Mountain assemblage, as internal casts of cocoons are most abundant. 
 
6.2 Fictovichnus trace-makers 
 The size, shape, surface texture, and micromorphology of F. sciuttoi specimens found at Egg 
Mountain are comparable to those found elsewhere, and as such are interpreted as the cocoons of 
burrowing sphechid, pompilid, or crabronid wasps (Genise et al., 2007; Genise, 2016). They are most 
similar to those described by Genise et al. (2007) from the Upper Cretaceous Bajo Barreal Formation 
(Cenomanian; Suárez et al., 2014) and Allen Formation (Campanian-Maastrichtian; e.g., Leanza and 
Hugo, 2001) of Argentina and the Paleocene-Eocene Claron Formation of the United States (Bown et 
al., 1997). Argentinian specimens similar to the large morphology described here were associated 
directly with sauropod dinosaur eggs (Genise and Sarzetti, 2011). F. sciuttoi have also been described 
from the Eocene of France, where three different size morphologies are described alongside reptilian 
eggs (Kuntz, 2012). Fossil cocoons bearing similar external ornamentation (i.e., helical ridges and lobes) 
are described from the Eocene of England (Edwards et al., 1998) and the Cretaceous-Paleogene of 
Uruguay (Alonso-Zarza et al., 2011). Two of the morphologies described by Edwards et al. (1998) 
 
match the proportions of medium and large F. sciuttoi from Egg Mountain. Scarring is interpreted as the 
constricted area that connects the chamber to other chambers or tunnels, as seen in F. gobiensis (Alonso-
Zarza et al., 2014). Truncation is interpreted here and by others (Genise et al., 2007) as an adult 
emergence trace, where the cap, typical of an enclosed cocoon, is not preserved.  
 The thin, discrete external boundary observed in thin section of some F. sciuttoi here is similar to 
that of the best-preserved specimens from the Cretaceous of Argentina (Genise et al., 2007, Fig. 6A; 
Genise and Sarzetti, 2011, Fig. 3B) (Fig. 8A, 8B, 8D). However, unlike the Argentinian material, this 
structure is not continuous through the entirety of the cross section (Fig. 8D). The clay-rich zone 
observed in some specimens is analogous to that described by Johnston et al. (1996) in the original 
description of Fictovichnus and interpreted as mechanical artifact of the pupation chamber, formed by 
the trace-maker as clay and silt became densely packed into open pore spaces in the sediment (Fig 9a, 
9c). Additionally, some specimens here are detached from the host matrix, a mode of preservation seen 
in some specimens of F. gobiensis from the Miocene of Spain (Alonso-Zarza et al., 2014, Fig. 3B) (Fig. 
8C). 
 While the more elongate morphologies (i.e., small, medium, large) more closely resemble wasp 
cocoons, the trace-maker of the wide morphology cocoons is less certain, and interpretations here are 
speculative. The helical ornamentation on some specimens is strongly suggestive of wasp cocoons. 
However, most hymenopteran cocoons are slender and tapering, though some exhibit wider dimensions 
(Genise, pers. comm., 2018). Some soil-dwelling coleopteran pupation chambers may be stout, 
particularly those of some Scarabaeids. The thickness of the boundary casts doubt on a hymenopterous 
trace-maker, as the wall of wasp cocoons is typically much thinner (e.g., Genise et al., 2007). The 
morphology of some Fictovichnus gobiensis from Laetoli described by Genise and Harrison (2018) is 
similar to Fictovichnus described here, and the authors tentatively assign the trace-makers to 
 
Tenebrionid, Curculionid and/or Scarabaeid beetles (Genise and Harrison, 2018, Fig. 1E, K). Some 
members of these groups, including some weevils and chafers, favor similar environments as burrowing 
wasps, as their trace fossils (Rebuffoichnus, F. gobiensis) are consistently associated with F. sciuttoi as 
components of the Celliforma ichnofacies (Genise, 2016). If the wide morphology Fictovichnus 
represents one of these, they still may suggest similar broad paleonenvironmental interpretations 
(Genise, 2016). Back-filled burrows in herbivorous dinosaur coprolites from nearby deposits of the 
Willow Creek Anticline are attributable to coleopterans (Chin and Gill, 1996; Chin, 2007) and offer 
another line of evidence for the presence of coleopteran trace-makers. 
 
6.3 Tombownichnus and unassignable traces 
 The ichnogenus Tombownichnus (represented by two ichnospecies, T. parabolicus and T. plenus) 
is interpreted as borings of cleptoparasitic insects occurring in the walls of another chambered insect 
trace fossil. T. parabolicus shares a similar interpretation as T. plenus; the latter is a complete boring 
through the external surface of the host chamber, while the former is a failed perforation attempt 
(Mikuláš and Genise, 2003). Trace-makers have been attributed to parasitic wasps or wasp larvae 
(Mutillidae, Sapygidae, Chrysididae, Scoliidae, or Tiphiidae), blister beetles (Meloidea), or flies 
(Bombyliidae) (Mikuláš and Genise, 2003; Genise, 2016, p. 372-375). Tombownichnus occurs in several 
insect trace fossils, including Rebuffoichnus (pupation chambers of chafers and weevils), 
Coprinisphaera (dung beetle brood balls), and Teisseirei (moth pupation chambers). Most notably, T. 
parabolicus occurs in nearby outcrops in specimens of Fictovichnus sciuttoi and Rebuffoichnus (Martin 
and Varricchio, 2011). In the same outcrops, small diameter burrows are connected to F. sciuttoi and 
Rebuffoichnus specimens, which may also represent traces of cleptoparasites (Martin and Varricchio, 
2011).   
 
 Along with Tombownichnus observed at the locality, associations and cross-cutting of small and 
large cocoons may represent parasitic behavior (Fig. 10B). Both cleptoparisitism (stealing of food or 
prey items) and parasitoid behavior (consuming of host) on burrowing wasp nests has been observed in 
many holometabolous insects, including flies and other parasitoid wasps. Predatory holometabolous 
insects attack the wasp during its larval or pupating (cocoon-bearing) stage, usually to deposit an egg. 
The egg will hatch into a larva that feeds on the provisions of the host wasp (Evans and O’Neil, 2009). 
Some parasitoids of the family Chrysididae (cuckoo wasps) lay eggs in host wasp cells; the parasitoid 
larvae wait until the host larvae spins a cocoon before feeding on the host (Evans and Eberhard, 1970; 
Genise, 2016). In other members of Chrysididae, the parasitic larva consumes the host pupa and spins a 
cocoon inside the cocoon of the host (Evans and Eberhard, 1970; Genise, 2016). This behavior has been 
observed in the parasitic sand wasp Nysson, where a parasitic larva will spin a cocoon virtually identical 
to the host but often smaller (Evans, 1966). Similarly, bee-flies of the genus Hyperalonia (Bombyliidae) 
will wait for the host larva to pupate before consuming the host and pupating within their cocoon (Evans 
and Eberhard, 1970, p. 230). Thus, the intersecting small and larger cocoons more likely records 
parasitoid behavior, where the parasitic taxon consumes the host pupa within its cocoon, rather than 
simply stealing provisions of the cell before the host larva has a chance to feed and spin a cocoon. 
Similar associations of small cocoons inside large cocoons have been documented from the Paleocene-
Eocene Claron Formation (Bown et al., 1997). Cocoons ranging from 3.5 to 7.5 mm in length are found 
within 18 to 32 mm long cocoons. The larger morphs display a helical external pattern, which the 
authors cite as evidence for wasps, while the smaller cocoons show a smooth exterior. The trace-makers 
of the small cocoons are interpreted to be wasp parasites, which could be other wasps, hymenopterans, 
or dipterans (Bown et al., 1997). Additionally, Martin and Varricchio (2011) describe similar co-
occurrences of small cocoons within larger cocoons from nearby outcrop in the Two Medicine 
 
Formation, which they attribute to parasitoid insects, the oldest such behavior recorded in the fossil 
record.  
 Cross-cutting cocoons of similar size and morphology of pupal structures likely represent the 
same trace-maker. Therefore, this most likely implies multiple generations of pupation on the same 
horizon. These behaviors have been documented in extant wasps and may represent multivoltine 
patterns (more than one generation per year) that is observed some in extant wasps (Evans and Eberhard, 
1970), though overlapping of traces more likely suggest multiple generations of nesting over extended 
periods (Genise et al., 2013). Alternatively, cross-cutting or closely associated cocoons of similar sizes 
may reflect usurpation of a nest by different females. Usurpation may be noncompetitive, as when a 
wasp takes over the space remaining in a nest after the original occupant vacates permanently 
(Krombein, 1976), or competitive and lead to fighting amongst females, as when one female takes over 
after the brood of the female that constructed the nest has emerged (O’Neill, 2001, p. 181 and references 
therein). Nest usurpation is often intraspecific (Field, 1992; O’Neill, 2001). Thus, associated cocoons of 
similar sizes may reflect nest usurpation. Similar specimens are described from a nearby outcrop (Martin 
and Varricchio, 2011).  
 
6.4 A depauperate Celliforma Ichnofacies? 
 The Egg Mountain locality preserves an ichnoassemblage consisting of four (possibly five) 
morphologies of insect pupation structures, corresponding to different species of soil-nesting insects. 
The assemblage is dominated by Fictovichnus sciuttoi, interpreted to be burrowing wasp cocoons 
(though the wide morph may represent coleopterans). Notably, there is a distinct absence of traces 
definitively referable to bees (e.g., Celliforma, Palmiraichnus), beetles (Rebuffoichnus), and moths 
(Teisseirei), unlike other deposits of the Two Medicine Formation that represent the Celliforma 
 
ichnofacies (Martin and Varricchio, 2011). The observed assemblage may represent a depauperate 
Celliforma ichnofacies, with only one dominant ichnotaxon and several secondary components, 
including burrows, rhizoliths, vertebrate coprolites, pellet masses, and evidence of vertebrate nesting 
consisting of partial to complete eggs and eggshell concentrations (Genise et al., 2010). Alternatively, 
the monospecific assemblage may not represent the Celliforma ichnofacies; nonetheless, potentially five 
different insect taxa are represented. If indeed the majority of cocoons represent those of solitary wasps, 
the paleoenvironmental conditions favored by wasps are the same conditions proposed for the 
Celliforma ichnofacies (Genise, et al., 2010; Genise, 2016). Further, a single trace has a variety of 
implications. Wasps prefer to burrow in well-drained, friable soils with little vegetation cover. Cell 
emplacement is largely controlled by subsurface conditions, independent of size of the wasp, though 
larger wasp species tend to construct larger cocoons, and in Bembicinae, larger taxa tend to burrow 
deeper (Evans, 1966). 
 Some specimens here assigned to F. sciuttoi do not show the characteristic external helical 
weave or lobed pattern (Fig. 6K). The lack of helical ornamentation on some Egg Mountain specimens 
may reflect differences related to preservation (e.g., adhering matrix). However, these specimens may 
represent the ichnospecies F. gobiensis, where the only definitive distinguishing characteristic is a 
smooth external texture compared to the helically ridged pattern of F. sciuttoi. A similar taxonomical 
issue is encountered at the Pliocene hominid site, Laetoli; Genise and Harrison (2018) describe 
thousands of F. gobiensis, identifying a select few specimens with helical texture, normally a 
characteristic of F. sciuttoi. Based on the scarcity of these textured specimens, the authors conserve the 
F. gobiensis designation. Similarly, we do not disregard the importance of surface texture, but instead 
elect not to formally treat a small subset of traces that do not display texture, which could have been 
altered by taphonomic processes. Regardless, designating these traces as F. sciuttoi (wasps) or F. 
 
gobiensis (beetles, chafers, or weevils) yield broadly the same paleoenvironmental implications and are 
found within the Celliforma ichnofacies (Genise, 2016). 
 
6.5 Paleoenvironment 
 The suite of trace fossils at Egg Mountain suggests the presence of three or four wasp species 
and potentially a fifth unknown insect species. The abundance of fossorial wasp pupation structures 
prescribes the nature of the local environment: friable substrate well above the water table with suitable 
exposure to the substrate surface. The abundance of traces in all jackhammer passes also indicates that a 
suitable environment for insect nesting persisted throughout the time of deposition. The environment 
persisted through at least the measured 1.5 meters (and likely more) of stratigraphy fairly continuously. 
Higher concentration of stout and large pupation structures may reflect subtle changes in substrate, 
water table depth, or other environmental nuances. The absence of the stout and elongate F. sciuttoi 
from particular horizons (e.g., JHP 7 through 11) may reflect unfavorable conditions for larger insect 
species, as exemplified in Bembicinae, where larger wasps tend to burrow deeper (Evans, 1966). Subtle 
changes in relative pupae abundance may thus be due to local environmental changes favoring different 
taxa over others. High nest density in a localized area, such as the 2x8 meter extent in JHP 8 and 9, has 
been documented in extant wasp populations, where the number of cells exceeds 30 per square meter 
(Evans, 1966). Other nest aggregations exceed 300 cells in 0.75 x 1.5-meter area (O’Neill, 2001). Evans 
(1970) documented several species occupying the same area in Jackson Hole, Wyoming, with up to four 
different species in the same 10-meter plot (O’Neill, 2001, Fig. 6-7). Similar nesting densities have been 
observed in other apocritans; alkali bees (of the genus Nomia) are native to deserts and semiarid 
environments with well-drained soils and have been observed nesting in aggregations of 400 nests per 
square meter (Linsley, 1958; Cane, 2007). High abundances in certain passes (e.g., JHP 9) may suggest 
 
multiple brooding seasons or high population densities during one short span, such as during the dry 
season (Martin and Varricchio, 2011; Genise, 2016, p. 581). 
 Abundant insect (primarily wasp) cocoons and nesting traces suggests abundant insect prey items 
for nest provisions, as modern wasps seem to be limited to nesting locales based on prey availability (in 
addition to abiotic factors) (O’Neill, 2001). In this sense, the abundance of dinosaur nesting events and 
eggshell material may explain the abundance of wasp cocoons; insects feeding on decaying eggs and 
other organic material (first posited by Horner, 1987) would have attracted wasps that provision their 
nests with insects (Genise and Sarzetti, 2011). Another possibility is that both fossorial wasps and 
oviparous organisms were drawn to similar soil conditions. A relatively stable depositional environment 
with friable soil conditions that allow for reworking would be conducive not only to nest construction, 
but also for rearing of vertebrate eggs. Most insects prefer to construct burrows and brooding chambers 
in well-drained soils that are perennially or seasonally above the water table (Genise, 2000; Genise, 
2004; Genise et al., 2010). These environmental parameters would be favorable to egg-laying 
organisms, especially those that partially bury eggs in sediment such as Troodon formosus (Varricchio et 
al., 1999) or with fully buried clutches as in crocodilians, squamates, and most dinosaurs (Deeming, 
2006). The suitability of the environment for wasps and potentially other species of burrowing insects 
may help explain the presence of the potentially insectivorous iguanamorph lizard Magnuviator 
ovimonsensis (DeMar et al., 2017), and the small metatherian Alphadon, and multituberculate 
Cimexomys (Montellano, 1988; Montellano et al., 2000), which may have preyed upon the wasps, their 
larvae, or other insect prey items the wasps would have used in provisioning their nests. The abundance 
of insect nests may also explain the presence of dinosaur nesting structures and abundant eggshell 
material.  
 
 The assemblage at Egg Mountain records a possible association of the Celliforma ichnofacies 
with vertebrate nesting events. Additional associations of vertebrate eggs and single or associated 
ichnotaxa common to the Celliforma ichnofacies include the Campanian-Maastrichtian Allen Formation 
(Genise and Sarzetti, 2011), the Campanian-Maastrichtian Djakhota Formation (Johnston et al., 1996), 
and Eocene deposits of France (Kuntz, 2012). Aside from the Two Medicine Formation, the only 
additional association of the Celliforma ichnofacies and vertebrate eggs are recorded from the 
Campanian-Maastrichtian Mercedes Formation of Uruguay where bee traces (Celliforma spirifer, C. 
germanica) dominate (Faccio, 1994; Alonso-Zarza et al., 2011). In the Two Medicine Formation, the 
ichnoassemblage is dominated by traces attributed to wasps (Fictovichnus sciuttoi), while bee traces are 
notably absent (though bee traces may occur in nearby strata; see Martin and Varricchio, 2011). The 
differences in trace fossil and vertebrate egg assemblages between the Two Medicine Formation and the 
Mercedes Formation may suggest different climatic factors or environmental preferences for nesting 
between maniraptoran and sauropod dinosaurs, respectively. Associations of vertebrate nesting and 
insect trace fossils (which may represent the Celliforma ichnofacies) are represented elsewhere. These 
include Fictovichnus sciuttoi and sauropod eggs from the Campanian-Maastrichtian of South America 
(Genise and Sarzetti, 2011), F. gobiensis and dinosaur eggs from the Campanian-Maastrichtian of Asia 
(Mikhanilov et al., 1994; Johnston et al., 1996), and cf. Fictovichnus and reptilian eggs from the Eocene 
of France (Kuntz, 2012). These examples provide a framework for future work that may enlighten 
environmental context of the evolution of vertebrate reproductive strategies. 
 
6.6. Depositional processes 
 Egg Mountain stratigraphy is devoid of primary sedimentary structures (e.g., bedding, 
depositional contacts). Limestone horizons generally are lenticular and not laterally continuous, display 
 
sharp upper contacts and decimeter-scale relief on the upper surface, all characteristics of pedogenic 
carbonates (Retallack, 1988). Peds, cutans, and other hallmarks of developed paleosols are lacking while 
root traces and nodules are rare, suggesting incipient paleosol development (Retallack, 1988; Varricchio 
et al., 1999). However, abundant insect pupation structures and burrows attest unequivocally to the 
presence of a paleosol (Genise, 2016). The absence of distinct, continuous soil horizons suggests 
paleosol development was weak, representing an inceptisol (Retallack, 1988). Additionally, the 
interfingering of pedogenic carbonates with detrital siltstones suggests deposition was frequent enough 
to disrupt paleosol development (Alonso-Zarza, 1998; Kraus, 1999). The abundance of calcite and 
infrequent nodules suggests well-drained, alkaline conditions (Retallack, 1988) and corroborates 
previous interpretations of the paleoenvironment of the area (Lorenz and Gavin, 1984; Horner, 1987; 
Retallack, 1997; Martin and Varricchio, 2011). While the presence of paleosols is unequivocal, isotopic 
evidence suggests phreatic processes were involved in limestone formation and in overprinting original 
structures in the form of crystalline calcite infill of some sediment pore space, ichnofossils, root traces, 
and eggshell (Moore et al., pers. comm., 2016). 
 In the absence of well-developed paleosols, it is likely that bioturbation is the primary factor in 
the destruction of original bedding. Traces and burrows occur throughout the measured section 
consistently and as discrete structures, and overlap is infrequent (Bioturbation Index 3, or moderate; 
Taylor and Goldring, 1993; Genise et al., 2004). Abundant traces suggest an absence of major sediment 
pulses sufficiently large enough to prohibit thorough colonization. With no distinct omission in trace 
abundance, deposition events were certainly less than burrows lengths (10-30 cm on average for extant 
wasps, maximum 14 cm based on preserved burrows), and less than the 12.5 cm jackhammer passes. If 
deposition events were greater than the jackhammer pass depth and burrow depth, a sediment band 
absent of traces would be expected throughout the section. Additionally, we observe cocoons of similar 
 
sizes (probably of the same producer) at very different depths. This variation in cocoon emplacement is 
not expected in extant wasps, as the same species at the same site emplaces its cells at approximately the 
same depth; there are no cases at the same site where cocoons would be separated by 1.5 meters, since 
soil conditions vary at depth clines (Retallack, 2001; Genise, 2016). Wasps place cells at approximately 
the same depth relative to the soil surface and the water table, so cocoons at different horizons track the 
accumulation of sediments over time. Thus, abundant traces throughout the section and lack of erosional 
contacts are indicative of relatively slow, consistent deposition and cumulative paleosols (Kraus, 1999). 
This relationship is seen more classically in marine settings, where low sedimentation enables thorough 
bioturbation of a substrate, and lack of bedding planes suggests continuous deposition (e.g., Howard, 
1975). Particularly high abundances on a single horizon (e.g., JHP 9) may reflect otherwise 
unrecognizable hiatuses or periods of low sedimentation. Conversely, a relatively low abundance of 
traces in a JHP (e.g., JHP 4 and 5) may reflect a relatively higher rate of sedimentation during that 
interval. Three examples of cross-cutting cocoons occur in passes with relatively high abundance of 
traces (JHP 6, JHP 7). It is likely those passes represent extended periods of little deposition, thus 
increasing the potential for multiple brooding seasons on the same horizon and possibly explaining the 
examples of cross-cutting traces in these passes. 
 The preservation of cylindrical burrows and pupation structures as internal casts suggests passive 
filling. Additionally, the presence and abundance of fragile insect pupation chambers, particularly with 
minute ornamentation, reflects a largely autochthonous or parautochthonous assemblage. This is 
supported by the presence of nesting horizons and egg clutches (Horner, 1987; Varricchio et al., 1999), 
unabraded eggshell (Oser and Jackson, 2014), angularity of volcanoclastic sediments (Varricchio et al., 
1999), and the extreme representation of terrestrial organisms and paucity of aquatic forms. Egg 
Mountain deposition and infilling of insect nests may have been brought on by seasonal flooding of the 
 
area. Destruction of nests caused by flooding has been documented in extant burrowing apocritan 
populations (Fellendorf et al., 2004). 
 
7. Conclusions 
1) The trace fossil assemblage at Egg Mountain is comprised of small, medium, large, and wide 
morphologies of Fictovichnus sciuttoi, interpreted as the cocoons of fossorial wasps and potentially 
beetles, and represent a depauperate Celliforma ichnofacies. Associated and cross-cutting cocoons of 
different sizes along with Tombownichnus may further represent the oldest record of insect 
parasitism. 
2) Hymenopteran trace fossils support a semi-arid climate and seasonality inferred during Egg 
Mountain deposition. Abundant wasp trace fossils indicate a sparsely vegetated subaerial 
environment with workable, well-drained soil conditions favorable for both fossorial insect pupation 
and oviparous vertebrate nesting. Pervasive insect traces suggest ample prey opportunities for 
abundant mammals and lizards at the locality.  
3) In the absence of primary sedimentary structures and definitive paleosol horizons, trace fossil 
abundance and emplacement reflect sedimentological processes; pervasive insect nesting structures 
in bioturbated floodplain overbank deposits suggest the cumulative development of paleosols and 
relatively slow, continuous deposition. The continuous record of insect traces represents an absence 
of major depositional events, while horizons with greater abundances of traces may suggest more 
extended periods of subaerial exposure. 
4) The continued and abundant presence of insect trace fossils together with mammals, articulated 
lizards, and a variety of vertebrate nesting events through the section highlight Egg Mountain as an 
exceptional window into terrestrial ecosystems of the Late Cretaceous of North America.  
 
5) Components of the Celliforma ichnofacies here and elsewhere occur in association with vertebrate 
nesting events. Differences in the taxonomic diversity of the Celliforma ichnofacies representing 
different trace-makers in different egg-bearing localities may reflect subtle environmental 
preferences and provide environmental context to the evolution of reproductive behaviors in 
different oviparous groups.  
 
Acknowledgments 
 We thank Jack Horner and the Museum of the Rockies for permission and support to reopen the 
Egg Mountain Quarry. WJF would like to thank Dr. Jorge Genise for constructive feedback at various 
points throughout this project. Thanks to Dr. Clint Cowan of Carleton College for his support with 
“comps” and his patience and advice throughout WJF’s undergraduate career. Thanks to Giulio Panascí 
for helpful conversations about this study. We thank Dr. Tony J. Martin and an anonymous reviewer for 
great improvement of the manuscript. Many thanks to Egg Mountain crews of 2010-2016 for their 
diligence and laborious rubble-picking. Funding was provided by the Woodside Endowed Fund for 
Career Exploration and Shackel Internship Fund of the Carleton College Career Center, and the Carleton 
College Geology Department to WJF. Work was funded by the National Science Foundation (EAR) 
grant nos. 0847777 and 1325674 to DJV. 
 
Data Availability 
Supplementary Information 1 – Measurements used size distribution of traces and descriptive statistics. 
Supplementary Information 2 – Distribution of trace fossils from field collections assigned to 
jackhammer passes.   
 
References 
 
Alonso-Zarza, A.M., Silva, P.G., Goy, J.L. and Zazo, C., 1998, Fan-surface dynamics and biogenic 
calcrete development: interactions during ultimate phases of fan evolution in the semiarid SE 
Spain (Murcia): Geomorphology, v. 24, no. 2-3, p. 147-167. 
 
Alonso-Zarza, A. M., Genise, J. F., and Verde, M., 2011, Sedimentology, diagenesis and ichnology of 
Cretaceous and Palaeogene calcretes and palustrine limestones from Uruguay: Sedimentary 
Geology, v. 236, no. 1-2, p. 45-61. 
 
Alonso-Zarza, A. M., Genise, J. F., and Verde, M., Paleoenvironments and ichnotaxonomy of insect 
trace fossils in continental mudflat deposits of the Miocene Calatayud–Daroca Basin, Zaragoza, 
Spain: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 414, p. 342-351. 
 
Bertling, M., Braddy, S.J., Bromley, R.G., Demathieu, G.R., Genise, J., Mikuláš, R., Nielsen, J.K., 
Nielsen, K.S., Rindsberg, A.K., Schlirf, M. and Uchman, A., 2006, Names for trace fossils: a 
uniform approach: Lethaia, v. 39, no. 3, p. 265-286. 
 
Bown, T. M., Hasiotis, S. T., Genise, J. F., Maldonado, F., and Brouwers, E. M., 1997, Trace fossils of 
Hymenoptera and other insects, and paleoenvironments of the Claron Formation (Paleocene and 
Eocene), southwestern Utah: US Geological Survey Bulletin v. 2153, p. 41-58. 
 
Cane, J. H., 2007, Alkali Bee in Shimanuki, H., Flottum, K., Harman, A. (editors), ABC & XYZ of Bee 
Culture: A.I. Root Company, Medina, Ohio, v. 41, p. 11-12. 
 
Chin, K., 2007, The paleobiological implications of herbivorous dinosaur coprolites from the Upper 
Cretaceous Two Medicine Formation of Montana: why eat wood?: Palaios, v. 22, no. 5, p. 554-
566. 
 
Chin, K., and Gill, B. D., 1996, Dinosaurs, dung beetles, and conifers: participants in a Cretaceous food 
web: Palaios, v. 11, no.3, p. 280-285. 
 
Dashtgard, S. E., 2011, Linking invertebrate burrow distributions (neoichnology) to physicochemical 
stresses on a sandy tidal flat: implications for the rock record: Sedimentology, v. 58, no. 6, p. 
1303-1325. 
 
Deeming, D. C., 2006, Ultrastructural and functional morphology of eggshells supports the idea that 
dinosaur eggs were incubated buried in a substrate: Palaeontology, v. 49, no. 1, p. 171-185. 
 
DeMar, D. G., Conrad, J. L., Head, J. J., Varricchio, D. J., and Wilson, G. P., A new Late Cretaceous 
iguanomorph from North America and the origin of New World Pleurodonta (Squamata, 
Iguania), in Proceedings Proc. R. Soc. B2017, Volume 284, The Royal Society, p. 20161902. 
 
Edwards, N., Jarzembowski, E. A., Pain, T., & Daley, B. (1998). Cocoon-like trace fossils from the 
lacustrine-palustrine Bembridge Limestone Formation (Late Eocene), southern 
England: Proceedings of the Geologists' Association, v. 109, no. 1, p. 25-32. 
 
 
Evans, H. E., 1966, The comparative ethology and evolution of the sand wasps. 
 
Evans, H. E., 1970, Ecological-behavioral studies of the wasps of Jackson Hole, Wyoming: Bulletin of 
the Museum of Comparative Zoology, v. 140, p. 451-511. 
 
Evans, H. E., and West-Eberhard, M., 1970, The Wasps. Univ. Michigan: Ann Arbor, 265 p. 
 
Evans, H. E., and O'Neill, K. M., 2007, The sand wasps: Natural history and behavior, 406 p. 
 
Evans, H. E., Kurczewski, F. E., & Alcock, J., 1980, Observations on the nesting behaviour of seven 
species of Crabro (Hymenoptera, Sphecidae): Journal of Natural History, v. 14, no. 6, p. 865-
882. 
 
Evans, H. E., O'Neill, K. M., and Evans, H. E., 2009, The sand wasps: natural history and behavior, 
Harvard University Press, 330 p. 
 
Faccio, G., 1994, Dinosaurian eggs from the Upper Cretaceous of Uruguay in Dinosaur eggs and babies, 
Carpenter, K., Hirsch, K.F. and Horner, J.R., p. 47-55. 
 
Falcon-Lang, H. J., 2003, Growth interruptions in silicified conifer woods from the Upper Cretaceous 
Two Medicine Formation, Montana, USA: implications for palaeoclimate and dinosaur 
palaeoecology: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 199, no. 3, p. 299-314. 
 
Falcon-Lang, H.J., Labandeira, C. and Kirk, R., 2015, Herbivorous and detritivorous arthropod trace 
fossils associated with subhumid vegetation in the Middle Pennsylvanian of southern Britain: 
Palaios, v. 30, no. 3, p. 192-206. 
 
Fellendorf, M., Mohra, C., and Paxton, R. J., 2004, Devasting effects of river flooding to the ground-
nesting bee, Andrena vaga (Hymenoptera: Andrenidae), and its associated fauna: Journal of 
Insect Conservation, v. 8, no. 4, p. 311-312. 
 
Field, J., 1992, Intraspecific parasitism as an alternative reproductive tactic in nest‐building wasps and 
bees: Biological Reviews, v. 67, no. 1, p. 79-126. 
 
Fricke, H.C., Foreman, B.Z. and Sewall, J.O., 2010, Integrated climate model-oxygen isotope evidence 
for a North American monsoon during the Late Cretaceous: Earth and Planetary Science Letters, 
v. 289, no. 1-2, p. 11-21. 
 
Genise, J. F., 2000, The ichnofamily Celliformidae for Celliforma and allied ichnogenera: Ichnos: An 
International Journal of Plant & Animal, v. 7, no. 4, p. 267-282. 
 
Genise, J. F., 2016, Ichnoentomology: Insect Traces in Soils and Paleosols, Springer, 695 p. 
 
 
Genise, J.F. and Bown, T.M., 1994. New Miocene scarabeid and hymenopterous nests and Early 
Miocene (Santacrucian) paleoenvironments, Patagonian Argentina: Ichnos: An International 
Journal of Plant & Animal, v. 3, no. 2, p. 107-117. 
 
Genise, J. F., and Sarzetti, L. C., 2011, Fossil cocoons associated with a dinosaur egg from Patagonia, 
Argentina: Palaeontology, v. 54, no. 4, p. 815-823. 
 
Genise, J. F., and Cladera, G., 2004, Chubutolithes gaimanensis and other wasp trace fossils: breaking 
through the taphonomic barrier: Journal of the Kansas Entomological Society, v. 77, no. 4, p. 
626-638. 
 
Genise, J. F., Bellosi, E., and Gonzalez, M., 2004, An approach to the description and interpretation of 
ichnofabrics in palaeosols: Geological Society, London, Special Publications, v. 228, no. 1, p. 
355-382. 
 
Genise, J. F., Melchor, R. N., Bellosi, E. S., González, M. G., and Krause, M., 2007, New insect 
pupation chambers (Pupichnia) from the Upper Cretaceous of Patagonia, Argentina: Cretaceous 
Research, v. 28, no. 3, p. 545-559. 
 
Genise, J. F., Melchor, R. N., Bellosi, E. S., and Verde, M., 2010, Invertebrate and vertebrate trace 
fossils from continental limestones: Developments in Sedimentology, v. 61, p. 319-369. 
 
Genise J.F., Alonso-Zarza A.M., Verde M., Melendez A., 2013, Insect trace fossils in aeolian deposits 
 and calcretes from the Canary Islands: their ichnotaxonomy, producers, and palaeoenvironmental 
 significance: Palaeogeography Palaeoclimatology Palaeoecology, v. 377, no. 1, p. 110–124. 
 
Gingras, M. K., MacEachern, J. A., and Dashtgard, S. E., 2011, Process ichnology and the elucidation of 
physico-chemical stress: Sedimentary Geology, v. 237, no. 3, p. 115-134. 
 
Gingras, M. K., Pemberton, S. G., Saunders, T., and Clifton, H. E., 1999, The ichnology of modern and 
Pleistocene brackish-water deposits at Willapa Bay, Washington; variability in estuarine settings: 
Palaios, v. 14, no. 4, p. 352-374. 
 
Hasiotis, S. T., 2003, Complex ichnofossils of solitary and social soil organisms: understanding their 
evolution and roles in terrestrial paleoecosystems: Palaeogeography, Palaeoclimatology, 
Palaeoecology, v. 192, no. 1, p. 259-320. 
 
Hasiotis, S.T. and Bown, T.M., 1992. Invertebrate trace fossils: The backbone of continental ichnology: 
Short Courses in Paleontology, no. 5, p. 64-104. 
 
Hasiotis, S. T., Van Wagoner, J., Demko, T., Wellner, R., Jones, C., Hill, R., McCrimmon, G., Feldman, 
H., Drzewiecki, P., and Patterson, P., 2002, Continental ichnology: using terrestrial and 
freshwater trace fossils for environmental and climatic interpretations: SEPM Short Course, v. 
51. 
 
 
Hirsch, K. F., and Quinn, B., 1990, Eggs and eggshell fragments from the Upper Cretaceous Two 
Medicine Formation of Montana: Journal of Vertebrate Paleontology, v. 10, no. 4, p. 491-511. 
 
Horner, J. R., 1982, Evidence of colonial nesting and ‘site fidelity’among ornithischian dinosaurs: 
Nature, v. 297, no. 5868, p. 675-676. 
 
Horner, J. R., 1984, The nesting behavior of dinosaurs: Scientific American, v. 250, no. 4, p. 130-137. 
 
Horner, J. R., 1987, Ecological and behavioral implications derived from a dinosaur nesting site: 
Dinosaurs past and present, v. 2, p. 50-63. 
 
Horner, J. R., and Makela, R., 1979, Nest of juveniles provides evidence of family structure among 
dinosaurs: Nature, v. 282, no. 5736, p. 296-298. 
 
Horner, J. R., Schmitt, J. G., Jackson, F., and Hanna, R., 2001, Bones and rocks of the Upper Cretaceous 
Two Medicine-Judith River clastic wedge complex, Montana, in Proceedings Field trip 
guidebook, Society of Vertebrate Paleontology 61st Annual Meeting: Mesozoic and Cenozoic 
Paleontology in the Western Plains and Rocky Mountains. Museum of the Rockies Occasional 
Paper, v. 3, p. 3-14. 
 
Howard, J. D., 1975, The sedimentological significance of trace fossils, The study of trace fossils, 
Springer, p. 131-146. 
 
Johnston, P. A., Eberth, D., and Anderson, P. K., 1996, Alleged vertebrate eggs from Upper Cretaceous 
redbeds, Gobi Desert, are fossil insect (Coleoptera) pupal chambers: Fictovichnus new 
ichnogenus: Canadian Journal of Earth Sciences, v. 33, no. 4, p. 511-525. 
 
Kraus, M.J., 1999, Paleosols in clastic sedimentary rocks: their geologic applications: Earth-Science 
Reviews, v. 47, no. 1-2, p. 41-70. 
 
Krause, J., Bown, T. M., Bellosi, E. S., and Genise, J. F., 2008, Trace fossils of cicadas in the Cenozoic 
of Central Patagonia, Argentina: Palaeontology, v. 51, no. 2, p. 405-418. 
 
Krombein, K.V., 1967, Trap-nesting Wasps and Bees: Life Histories and Nest Associates: Smithsonian, 
Washington, D. C. 
 
Kuntz, P., 2012, Révision des ovoïdes du Lutétien de Bouxwiller en Alsace: Gazette de la Association 
Strasbourgeoise des Amis de la Minéralogie. ASAM Bulletin, v. 12, p. 42-59. 
 
Kurczweski, F. E., 2003, Comparative nesting behavior of Crabro monticola (Hymenoptera: 
Spechidae): Northeastern Naturalist, v. 10, no. 2, p. 425-450. 
 
Labandeira, C.C., 2006, The four phases of plant-arthropod associations in deep time: Geologica Acta, 
v. 4, no. 4, p. 409-438. 
 
 
Leanza, H.A., and Hugo, C.A., 2001, Cretaceous red beds from southern Neuquén Basin (Argentina): 
age, distribution and stratigraphic discontinuities: Publicación Electrónica de la Asociación 
Paleontológica Argentina, v. 7, no. 1, p. 117-122. 
 
Linsley, E., 1958, The ecology of solitary bees in Hilgardia, v. 27, no. 19, p. 543-599. 
 
Lorenz, J. C., and Gavin, W., 1984, Geology of the Two Medicine Formation and the sedimentology of 
a dinosaur nesting ground, in Proceedings Field Conference Northwestern Montana. Montana 
Geological Society, p. 175-187. 
 
Martin, A. J., 2001, Trace fossil evidence for insect-on-insect parasitoid behavior in the Upper 
Cretaceous Two Medicine Formation of Choteau, Montana. 2001 GSA Annual Meeting in 
Boston, Massachusetts, USA (5-8 November 2001), Paper No. 112-0. 
 
Martin, A. J., 2013, Life traces of the Georgia coast: Revealing the unseen lives of plants and animals, 
Indiana University Press. 
 
Martin, A. J., and Varricchio, D. J., 2011, Paleoecological utility of insect trace fossils in dinosaur 
nesting sites of the Two Medicine Formation (Campanian), Choteau, Montana: Historical 
Biology, v. 23, no. 01, p. 15-25. 
 
Martin, A. J. and Varricchio, D. J. 2015.  Insects using one another during the Late Cretaceous: Trace 
fossil evidence of parasitoid (or was it cleptoparasite?) behavior in the Two Medicine Formation, 
Montana. 2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015), Paper 
No. 70-12. 
 
Melchor, R. N., Genise, J. F., Buatois, L. A., and Umazano, A. M., 2012, Fluvial environments: Trace 
fossils as indicators of sedimentary environments, Developments in Sedimentology, v. 64, p. 
329-378. 
 
Melchor, R. N., Genise, J. F., Miquel, S. E., 2002, Ichnology, sedimentology and paleontology of 
Eocene calcareous paleosols from a palustrine sequence, Argentina: Palaios, v. 17, p. 16-35.  
 
Mikulás, R., and Genise, J. F., 2003, Traces within traces: holes, pits and galleries in walls and filling of 
insect trace fossils in paleosols: Geologica Acta, v. 1, no. 4, p. 339-348. 
 
Mikhailov, K.E., Sabath, K. and Kurzanov, S., 1994, Eggs and nests from the Cretaceous of Mongolia in 
Dinosaur eggs and babies, Carpenter, K., Hirsch, K.F. and Horner, J.R., p. 88-115. 
 
Montellano, M., 1988, Alphadon halleyi (Didelphidae, Marsupialia) from the Two Medicine Formation 
(Late Cretaceous, Judithian) of Montana: Journal of Vertebrate Paleontology, v. 8, no. 4, p. 378-
382. 
 
Montellano, M., Weil, A., Clemens, W. A., 2000, An exceptional specimen of Cimexomys judithae 
(Mammalia: Multituberculata) from the Campanian Two Medicine Formation of Montana, and 
 
the phylogenetic status of Cimexomys: Journal of Vertebrate Paleontology v. 20, no. 2, p. 333-
340. 
 
O'Neill, K. M., 2001, Solitary wasps: behavior and natural history, Cornell University Press. 
 
Oser, S. E., 2014, Fossil eggs and perinatal remains from the upper Cretaceous Two Medicine 
Formation of Montana: description and implications [Master’s thesis]: Montana State University-
Bozeman, College of Letters & Science, 144 p. 
 
Oser, S.E., and Jackson, F. D., 2014, Sediment and eggshell interactions: using abrasion to assess 
transport in fossil eggshell accumulations: Historical Biology, v. 26, no. 2, p. 165-172. 
 
Panascí, G., Varricchio, D. J., and Martin, A. J., 2016 First Occurrence of Pallichnus dakotensis in a 
Dinosaur Nesting Site from the Two Medicine Formation (Campanian, Upper Cretaceous) of 
North America. Ichnia 2016 in Lisbon, Portugal (6-9 May 2016), p. 88. 
 
Retallack, G.J., 1988, Field recognition of paleosols: Geological Society of America Special Paper, no. 
216, p. 1-20. 
 
Retallack, G.J., 1997, Dinosaurs and dirt: DinoFest International (eds DL Wolberg, E. Stump & GD 
Rosenberg), p. 345-359. 
 
Retallack, G.J., 2001, Soils of the past: an introduction to paleopedology, John Wiley & Sons, 404 p.  
 
Ritchie, J. M., 1987, Trace fossils from burrowing Hymenoptera from Laetoli. In Leakey, M. D., and 
Harris, J. MM (editors), Laetoli-A Pliocene site in northern Tanzania: Clarendon Press, Oxford, 
p. 433-438. 
 
Rogers, R. R., 1990, Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two Medicine 
Formation of northwestern Montana: evidence for drought-related mortality: Palaios, p. 394-413. 
 
Rogers, R. R., Swisher III, C. C., and Horner, J. R., 1993, 40Ar/39Ar age and correlation of the 
nonmarine Two Medicine Formation (Upper Cretaceous), northwestern Montana, USA: 
Canadian Journal of Earth Sciences, v. 30, no. 5, p. 1066-1075. 
 
Rogers, R.R., Fricke, H.C., Addona, V., Canavan, R.R., Dwyer, C.N., Harwood, C.L., Koenig, A.E., 
Murray, R., Thole, J.T. and Williams, J., 2010, Using laser ablation-inductively coupled plasma-
mass spectrometry (LA-ICP-MS) to explore geochemical taphonomy of vertebrate fossils in the 
Upper Cretaceous Two Medicine and Judith River formations of Montana: Palaios, v. 25, no. 3, 
p. 183-195. 
 
Sánchez, M.V., Laza, J.H., Bellosi, E.S. and Genise, J.F., 2010, Ichnostratigraphy of middle Cenozoic 
Coprinisphaera from central Patagonia: Insights into the evolution of dung beetles, herbivores 
and grass-dominated habitats: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 297, no 3-
4, p. 633-648. 
 
 
Suárez, M., Márquez, M., De La Cruz, R., Navarrete, C. and Fanning, M., 2014, Cenomanian-? early 
Turonian minimum age of the Chubut Group, Argentina: SHRIMP U–Pb geochronology: Journal 
of South American Earth Sciences, v. 50, p. 67-74. 
 
Taylor, A.M. and Goldring, R., 1993, Description and analysis of bioturbation and ichnofabric: Journal 
of the Geological Society, v. 150, no. 1, p. 141-148. 
 
Varricchio, D. J., and Horner, J. R., 1993, Hadrosaurid and lambeosaurid bone beds from the Upper 
Cretaceous Two Medicine Formation of Montana: taphonomic and biologic implications: 
Canadian Journal of Earth Sciences, v. 30, no. 5, p. 997-1006. 
 
Varricchio, D. J., Horner, J. R., and Jackson, F. D., 2002, Embryos and eggs for the Cretaceous theropod 
dinosaur Troodon formosus: Journal of Vertebrate Paleontology, v. 22, no. 3, p. 564-576. 
 
Varricchio, D. J., Jackson, F., Borkowski, J. J., and Horner, J. R., 1997, Nest and egg clutches of the 
dinosaur Troodon formosus and the evolution of avian reproductive traits: Nature, v. 385, no. 
6613, p. 247-250. 
 
Varricchio, D. J., Jackson, F., and Trueman, C. N., 1999, A nesting trace with eggs for the Cretaceous 
theropod dinosaur Troodon formosus: Journal of Vertebrate Paleontology, v. 19, no. 1, p. 91-100. 
 
Varricchio, D. J., Koeberl, C., Raven, R. F., Wolbach, W. S., Elsik, W. C., and Miggins, D. P., 2010, 
Tracing the Manson impact event across the Western Interior Cretaceous Seaway: Geological 
Society of America Special Papers, v. 465, p. 269-299. 
 
Willmer, P.G., 1982, Microclimate and the environmental physiology of insects. In Advances in insect 
physiology: Academic Press, v. 16, p. 1-57. 
  
 
Figure 1. Locality map of field area at the Willow Creek Anticline, Two Medicine Formation near 
Choteau, Montana, U.S.A. The Egg Mountain locality (EM) is indicated with arrow. Figure modified 
from Rogers et al. (2010). 
 
Figure 2. Stylized stratigraphic section for the Egg Mountain locality (left) with inset of the measured 
section (right). Measured section is marked by a massive calcareous siltstone and interbedded micritic 
limestone horizons. Schematic jackhammer passes (JHPs) are superimposed and specimens are assigned 
to JHPs to track stratigraphic position. Passes average 12.5 cm in thickness. The level at which fossil 
elements occur is approximate. Fictovichnus sciuttoi represents small, medium, and large size classes, 
and wide Fictovichnus represents the consistently stout morphology. Other fossils not described in the 
key but found throughout the section include Orodromeus isolated and associated elements, isolated 
eggshell, indeterminate bone fragments, hadrosaur teeth, tyrannosaurid teeth, rare Troodon teeth, and 
dromaeosaurid teeth. 
 
Figure 3. Photos of the study site. (a) Unit 1 defined by 50+ cm thick limestone. Photo taken from the 
north end showing the thickest area of the unit. Lower contact of of Unit 1 is not visible in this photo. 
(b) Irregular relief of limestone layer in Unit 1 protruding through siltstone. Topography in photo 
represents the top of JHP 12. Note the competency contrast between the top of the limestone horizon and 
the siltstone. (c) Discontinuous limestone of Unit 2 visible in the east wall of main quarry. 
Abbreviations: Ls, micritic limestone; Sltst, calcareous siltstone. 
 
Figure 4. Photographs of the quarry at the Egg Mountain locality. (A) Photograph of quarry from the 
north. Note the thick rusty limestones in the foreground, which poke through the siltstone in the main 
 
quarry towards the southern quarry wall. A smaller discontinuous limestone lens is visible in the east 
wall of the main quarry (see Figure 4c). (B) Grid system of the main quarry set up 11 m north by 7 m 
west (N1-11/W6-12). Specimens were collected and their lateral position was measured using this 
system. Each quadrant is one square meter. Abbreviations: MQ, main quarry; UQ, upper quarry; Ls, 
micritic limestone; Sltst, calcareous siltstone. 
 
Figure 5. Fossil pupation structure size distribution. Traces sort into four categories based on length and 
diameter measurements. 
 
Figure 6. Exemplar Fictovichnus sciuttoi from the Egg Mountain locality. (A) MOR-IV-2804-B1.10-1, 
Large morphology with ridged ornamentation and subtle curvature. (B) MOR-IV-2804-B1.10-2, Large 
morphology with ornamented texture. Top arrow points to scar, bottom arrow denotes flattened, oval, 
smooth area. (C) MOR-IV-2804-B1.10-3, Large morphology with lobate texture. (D) MOR-IV-2804-
B5.15-1, Medium morphology in top view (left) to show truncation (arrow). Same specimen in lateral 
view (right). Note asymmetry, tapered extreme, and flattened, oval, smooth area (arrow). (E) MOR-IV-
2804-B5.15-2, Medium morphology with subtle helical ornamentation. Arrow denotes truncation, 
interpreted adult emergence trace. (F) MOR-IV-2804-B5.15-3, Medium morphology showing 
asymmetry and tapered end. (G) MOR-IV-2804-B5.15-4, Medium morphology with defined helically 
arranged ridged ornamentation. (H) MOR-IV-2804-B3.12-1, Small morphology with subtle 
ornamentation and crystalline preservation. (I) MOR-IV-2804-B9.12-1, Small morphology with root 
traces. Arrow denotes truncation. (J) MOR-IV-2804-B3.12-2, Small morphology with fine helical 
ornamentation. (K) MOR-IV-2804-B13.16-1, Small morphology cocoon with smooth texture and 
crystalline preservation. A-K all to same scale: 1 cm. 
 
 
Figure 7. MOR-IV-2804-B16.15-1, a medium Fictovichnus sciuttoi (left) showing helically arranged 
ornamentation comparable to a modern insect cocoon (right, Hymenoptera indet.) from the open 
grasslands Willow Creek Anticline near Choteau, MT, USA. Scale 1 cm. 
 
Figure 8. Photomicrographs of Fictovichnus sciuttoi. (A) Cross-polarized image of a small morphotype 
cocoon (MOR-IV-2804-B11.13.small). Dotted line marks the outer extent of the outer micritic 
boundary. Arrows point to the thin external wall. (B) Magnified image of inset box in (A) displaying 
discontinuous thin external wall (arrows). Discontinuity may reflect oblique section of external helical 
ridges. Image under plane-polarized light. (C) Small morph F. sciuttoi (MOR-IV-2804-B1.14.small) that 
is detached from the host matrix. As in (A), dotted line marks the outer extent of the outer micritic 
boundary. Image under cross-polarized light. (D) Large cocoon (MOR-IV-2804-B7.12.large) with 
discontinuous external wall (arrows). Image under plane-polarized light. Scale in (A), (C), and (D) 1 
mm. Scale in (B) 100 µm. 
 
Figure 9. Specimens of the wide morph Fictovichnus from the Egg Mountain locality. (A) MOR-IV-
2804-B1.14-1 with irregular bumpy texture. (B) MOR-IV-2804-B9.11-1 showing coarse ridged 
ornamentation. (C) MOR-IV-2804-B3.12-3, a complete specimen with smooth surface and exemplary 
stout morphology. (D) Wide Fictovichnus display vertical orientation (top of photo is younging 
direction). Structures are outlined in black for emphasis. (E) Photomicrograph of one wide Fictovichnus 
specimen (MOR-IV-2804-B.16.11.FB) under cross-polarized light showing calcite boundary between 
light brown micritic inner matrix (left) and darker outer matrix. Scale 1 cm in (A), (B), (C), and (D), 1 
mm in (E). 
 
 
Figure 10. Traces within traces from Egg Mountain. (A) Two specimens of Tombownichnus cf. 
parabolicus characterized by an ovoid indentation in a medium Fictovichnus sciuttoi (left, MOR-IV-
2804-B7.13-1; right, MOR-IV-2804-B7.14-1). (B) Two examples of small cocoons cross-cutting 
medium morph cocoons (left, MOR-IV-B3.12-4; right, MOR-IV-2804-B1.10-4). Note the helical 
ornamentation on both the small and medium cocoons (arrows). (C) Two large cocoons in close 
association (MOR-IV-B1.10-5). Scale 1 cm. 
 
Figure 11. Trace fossil abundance throughout the main quarry at the Egg Mountain locality. The 
abundances represent the number of all four morphs of Fictovichnus sciuttoi recorded for each JHP. 
Passes 4 and 5 are markedly low, while JHP 9 displays the single highest density of traces of all passes. 
Data labels are presented as number of pupation structures per jackhammer pass, represented by a 
number and the corresponding percentage relative to total traces throughout the section. Note that JHP 1 
and 2 may not reflect actual abundance as those horizons were not fully sampled. 
 
Figure 12. Relative abundance of different fossil cocoon morphologies throughout the main quarry. 
Medium and small Fictovichnus sciuttoi are consistently the most abundant throughout the section and 
are present in all jackhammer passes. Large and wide F. sciuttoi display a notably lower abundance or a 
completely absent from JHP 7 through JHP 11. Note that JHP 1 and 2 may not reflect actual abundance 
as those horizons were not fully sampled. 
 
Figure 13. Relative trace abundance as percentage per jackhammer pass throughout the main quarry. 
Trace diversity generally decreases down the measured section. Medium and small Fictovichnus sciuttoi 
 
are consistently dominant in all JHPs. Large and wide F. sciuttoi are consistently less common and are 
concentrated in the top half of the measured section. Note that JHP 1 and 2 may not reflect actual 
abundance as those horizons were not fully sampled. 
 
Figure 14. Generalized burrow of a solitary wasp. Cell depth and presence of inner and outer closures 
varies by species and substrate conditions. Terminology and burrow scheme based off Evans (1966) for 
sand wasps. 
 
Table 1. Descriptive statistics of fossil cocoon categories at Egg Mountain. Average diameter is 
calculated from measurements of both b and c axes. Volume is calculated using all three dimensions 
(=4/3πabc). 
 
Table 2. Abundance of trace morphologies per jackhammer pass at Egg Mountain. First number 
represents number of specimens. Percentage in parentheses represents the proportion of each trace in the 
jackhammer pass. Italicized numbers correspond to lowest abundance or percentage for each 
morphology, while bold reflects highest. *Abundances in JHP 1 and 2 should not be compared due to 
underrepresented data.