College of Letters & Science

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The College of Letters and Science, the largest center for learning, teaching and research at Montana State University, offers students an excellent liberal arts and sciences education in nearly 50 majors, 25 minors and over 25 graduate degrees within the four areas of the humanities, natural sciences, mathematics and social sciences.

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Chthonic severance: dinosaur eggs of the Mesozoic, the significance of partially buried eggs and contact incubation precursors
(The Royal Society, 2023-07) Hogan, Jason D.; Varricchio, David J.
For most dinosaurs, clutches consisted of a single layer of spherical to sub-spherical, highly porous eggs that were probably fully buried. Both eggs and clutch form change drastically with pennaraptoran theropods, the clade that includes birds. Here, far less porous, more elongate eggs are arranged with additional complexity, and only partially buried. While partial egg burial seems to be effective for an extremely small group of modern birds, the behaviour's overall rarity complicates our understanding of Mesozoic analogies. Recent experimental examination of pennaraptoran nesting thermodynamics suggests that partial egg burial, combined with contact incubation, may be more efficacious than has been presumed. We propose that nest guarding behaviour by endothermic archosaurs may have led to an indirect form of contact incubation using metabolic energy to affect temperature change in a buried clutch through a barrier of sediment, which in turn may have selected for shallower clutch burial to increasingly benefit from adult-generated energy until partial egg exposure. Once partially exposed, continued selection pressure may have aided a transition to fully subaerial eggs. This hypothesis connects the presence of partially buried dinosaurian clutches with the transition from basal, crocodile-like nesting (buried clutches guarded by adults) to the dominant avian habit of contact incubating fully exposed eggs.
Microstructural and crystallographic evolution of palaeognath (Aves) eggshells
(eLife Sciences Publications, Ltd, 2023-01) Choi, Seung; Hauber, Mark E.; Legendre, Lucas J.; Kim, Noe-Heon; Lee, Yuong-Nam; Varricchio, David J.
The avian palaeognath phylogeny has been recently revised significantly due to the advancement of genome-wide comparative analyses and provides the opportunity to trace the evolution of the microstructure and crystallography of modern dinosaur eggshells. Here, eggshells of all major clades of Palaeognathae (including extinct taxa) and selected eggshells of Neognathae and non-avian dinosaurs are analysed with electron backscatter diffraction. Our results show the detailed microstructures and crystallographies of (previously) loosely categorized ostrich-, rhea-, and tinamou-style morphotypes of palaeognath eggshells. All rhea-style eggshell appears homologous, while respective ostrich-style and tinamou-style morphotypes are best interpreted as homoplastic morphologies (independently acquired). Ancestral state reconstruction and parsimony analysis additionally show that rhea-style eggshell represents the ancestral state of palaeognath eggshells both in microstructure and crystallography. The ornithological and palaeontological implications of the current study are not only helpful for the understanding of evolution of modern and extinct dinosaur eggshells, but also aid other disciplines where palaeognath eggshells provide useful archive for comparative contrasts (e.g. palaeoenvironmental reconstructions, geochronology, and zooarchaeology).
Tiny, ornamented eggs and eggshell from the Upper Cretaceous of Utah represent a new ootaxon with theropod affinities
(Springer Science and Business Media LLC, 2021-05) Oser, Sara E.; Chin, Karen; Sertich, Sertich; Varricchio, David J.; Choi, Seung; Rifkin, Jeffrey
A new Cretaceous ootaxon (eggshell type) from the Kaiparowits Formation of Grand Staircase-Escalante National Monument is among a growing number of very small eggs described from the Mesozoic. Analyses of two partial eggs (~ 17.7 mm in diameter) and 29 eggshell fragments reveal that this new ootaxon exhibits nodose ornamentation with distinctive branching pore canals that open atop the nodes. Its two-layered microstructure consists of a mammillary layer and a continuous layer with rugged grain boundaries between calcite grains. Although the exact identity of the egg producer is unknown, the eggshell microstructure and small size is consistent with a small-bodied avian or non-avian theropod. The specific combination of small egg size, branching pores, two-layered microstructure, and dispersituberculate ornamentation preserved in this new ootaxon is unique among theropod eggs. This underscores that both eggshell and skeletal fossils of Cretaceous theropods can display a mosaic of transitional morphological and behavioural features characteristic of both avian and non-avian taxa. As such, this new ootaxon increases the diversity of Cretaceous eggs and informs our understanding of the evolution of theropod eggshell microstructure and morphology.
Do paleontologists dream of electric dinosaurs? Investigating the presumed inefficiency of dinosaurs contact incubating partially buried eggs
(Cambridge University Press, 2021-01) Hogan, Jason D.; Varricchio, David J.
Troodon formosus, a theropod from the Late Cretaceous, is one of the few species of dinosaurs with multiple nest sites uncovered. It has been consistently demonstrated that eggs within these nests would have been partially buried in life—an exceedingly rare state in modern vertebrates. There has been debate over Troodon's capacity to engage in thermoregulatory contact incubation, especially regarding an adult's ability to efficiently supply partially buried eggs with energy. An actualistic investigation was undertaken to determine the thermodynamic efficiency of contact incubating partially buried eggs. An efficient system would keep eggs at temperatures closer to the surrogate parent than the ambient, without prohibitively high energy input. For the experiment, a surrogate dinosaur was created and used in both indoor controlled ambient temperature trials and in an outdoor variant. Even with ambient temperatures that were likely cooler than Cretaceous averages, the results showed that contact incubating partially buried eggs did seem to confer an energetic advantage; egg temperatures remained closer to the surrogate than ambient in both indoor and outdoor tests. Still, critics of contact incubating partially buried eggs are correct in that there is a depth at which adult energy would fail to make much of an impact—perhaps more relevant to buried eggs, as partially buried eggs would be in contact with an adult and likely above the thermal input threshold. Additionally, results from this experiment provide evidence for a possible evolutionary path from guarding behavior to thermoregulatory contact incubation.
Do paleontologists dream of electric dinosaurs? Investigating the presumed inefficiency of dinosaurs contact incubating partially buried eggs
(2020-10) Hogan, Jason D.; Varricchio, David J.
Troodon formosus, a theropod from the Late Cretaceous, is one of the few species of dinosaurs with multiple nest sites uncovered. It has been consistently demonstrated that eggs within these nests would have been partially buried in life—an exceedingly rare state in modern vertebrates. There has been debate over Troodon's capacity to engage in thermoregulatory contact incubation, especially regarding an adult's ability to efficiently supply partially buried eggs with energy. An actualistic investigation was undertaken to determine the thermodynamic efficiency of contact incubating partially buried eggs. An efficient system would keep eggs at temperatures closer to the surrogate parent than the ambient, without prohibitively high energy input. For the experiment, a surrogate dinosaur was created and used in both indoor controlled ambient temperature trials and in an outdoor variant. Even with ambient temperatures that were likely cooler than Cretaceous averages, the results showed that contact incubating partially buried eggs did seem to confer an energetic advantage; egg temperatures remained closer to the surrogate than ambient in both indoor and outdoor tests. Still, critics of contact incubating partially buried eggs are correct in that there is a depth at which adult energy would fail to make much of an impact—perhaps more relevant to buried eggs, as partially buried eggs would be in contact with an adult and likely above the thermal input threshold. Additionally, results from this experiment provide evidence for a possible evolutionary path from guarding behavior to thermoregulatory contact incubation.
A new terrestrial trace fossil Feoichnus martini n. isp. from the Upper Cretaceous Two Medicine Formation (USA)
(2020-09) Panasci, Giulio; Varricchio, David J.
A new trace fossil, Feoichnus martini new ichnospecies, from the Two Medicine Formation is here described. This ichnotaxon is reported from the upper Campanian deposits of the Egg Mountain locality (Montana) and consists of a hemispherical to hemiellipsoidal structure with a truncated upper edge, and a regular, rounded lower edge marked by a lined border composed of stained layers. The trace maker likely impregnated the border using organic fluids. The simple lined wall observed in F. martini n. isp. suggests that the structure was produced by an invertebrate soil-dwelling organism, likely an insect. Specimens are preserved as casts in calcitic Inseptisols alongside an abundant vertebrate fossil record composed by dinosaurian and nondinosaurian fossil remains, dinosaur eggs and nest structures, and pervasive insect bioturbation. Feoichnus martini n. isp. represents an additional, minor component of the impoverished Celliforma ichnofacies reported at Egg Mountain and expands the paleogeographical distribution of the ichnogenus Feoichnus Krause et al., 2008 to the Upper Cretaceous deposits of Montana, USA.
An oviraptorosaur adult-egg association from the Cretaceous of Jiangxi Province, China
(2019-11) Jin, Xingsheng; Varricchio, David J.; Poust, Ashley W.; He, Tao
With abundant well-preserved clutches and several adult-clutch associations, oviraptorids provide some of the most detailed information on reproduction for dinosaurs. Here, we describe an oviraptorosaur closely associated with two eggs from the Upper Cretaceous Nanxiong Formation of Jiangxi Province, China, and discuss its implications for various reproductive hypotheses. The specimen consists of a partial skeleton (gastralia, pelvis, portions of both hind limbs, and tail), with one egg within the pelvic canal and the other just posterior to it, ventral to the anterior caudal vertebrae. Several geopetal features indicate that the individual came to rest on its left side, with the eggs likely extruded during buildup of abdominal gases during decomposition. Similarity of pubis, caudal vertebrae, and pes dimensions to recently described material from the formation, e.g., Tongtianlong and Jiangxisaurus, suggests oviraptorid affinities. The specimen provides additional association of elongatoolithid eggs and the oogenus Macroolithus with oviraptorosaurs and further evidence for monoautochronic ovulation, i.e., iterative laying of two eggs at daily or greater intervals. With each egg 36–48% the size predicted for a modern bird of the same adult mass, total egg production would be slightly lower to similar between this non-avian maniraptoran and Neornithes. Histological tissues and open neurocentral sutures indicate that this reproductively active individual was several years old but still growing at the time of death, a pattern observed in other non-avian maniraptorans. The complete absence of medullary bone in this egg-bearing individual may challenge the identification of this tissue in other dinosaurs more distantly related to birds.
Horseshoe crab trace fossils from the Upper Cretaceous Two Medicine Formation of Montana, USA, and a brief review of the xiphosurid ichnological record
(2020-05) Shibata, Masateru; Varricchio, David J.
A locality in the Upper Cretaceous Two Medicine Formation of Montana preserves abundant and variable horseshoe crab tracks and trails of the ichnotaxon Kouphichnium isp. These specimens span six morphologies differing in track form and trail configuration. These differences likely reflect variations in track-maker locomotion and behavior, substrate consistency, epichnial versus hypichnial preservation, and undertrack versus true tracks. Several tracks preserve the first clear appendage impressions for an extinct horseshoe crab. This discovery adds new information to the fossil horseshoe crab diversity in the Cretaceous Period. Trackway dimensions, such as the external width across the pusher legs or of the prosomal drag mark, provide information on the track-maker size. Most trackways correspond with crabs 9–14 cm wide; the abundance but limited size range of the traces suggests the large assemblage corresponds to a mating aggregation. The trace fossil record of xiphosurids indicates that throughout their history, horseshoe crabs inhabited both marine and nonmarine settings. They were definitively present in freshwater habitats from the lower Carboniferous through at least the Paleogene. Horseshoe crab trace abundance is highest from the upper Carboniferous through the Jurassic and likely reflects two factors: true upper Carboniferous taxonomic diversity and a preponderance of suitable sites for trackway preservation in the Late Triassic and Jurassic. Cretaceous traces are uncommon, and this Two Medicine locality is the first occurrence of horseshoe crab traces in the Late Cretaceous worldwide. Overall, track abundance and diversity would seem to correspond well with the reported horseshoe crab body fossil record.
Insect trace fossils elucidate depositional environments and sedimentation at a dinosaur nesting site from the Cretaceous (Campanian) Two Medicine Formation of Montana
(2019-11) Freimuth, William J.; Varricchio, David J.
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.
New occurrence of Neurankylus sp. (Testudines: Paracryptodiria: Baenidae) in the Upper Cretaceous Frontier Formation of south western Montana (USA)
(2020-04) Panasci, Giulio; Varricchio, David J.; Hirayama, Ren
An incomplete specimen assigned to the genus Neurankylus sp. was collected from Coniacian deposits of the Frontier Formation of Montana (USA). The material consists of anterior plastron, a costal plate and neural plate fragment, and several undetermined skeletal elements. The identification is based on the relatively large size of the specimens, scute elements joining along the mid-line of the plastron with the intergulars reaching the humerals, and complete fusion of the sutures. The specimen represents the first baenid turtle reported from non-marine Frontier Formation deposits and is particularly relevant for understanding stratigraphical and paleogeographical distribution of the Family Baenide. This will have important implications for phylogenetic studies and paleoenvironmental interpretation of non-marine Frontier Formation deposits of south-western Montana.

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