Publications by Colleges and Departments (MSU - Bozeman)
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Item A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana(PeerJ, 2022-11) Warshaw, Elías A.; Fowler, Denver W.Here we describe a new derived tyrannosaurine, Daspletosaurus wilsoni sp. nov., from Judithian strata (~76.5 Ma) intermediate in age between either of the previously described species of this genus. D. wilsoni displays a unique combination of ancestral and derived characteristics, including a cornual process of the lacrimal reduced in height relative to D. torosus and more basal tyrannosaurines, and a prefrontal with a long axis oriented more rostrally than in D. horneri and more derived tyrannosaurines. The description of this taxon provides insight into evolutionary mode in Tyrannosaurinae, lending strength to previous hypotheses of anagenesis within Daspletosaurus and increasing the resolution with which the evolution of this lineage can be reconstructed. Cladistic phylogenetic methods, stratigraphy, and qualitative analysis of the morphology of relevant taxa supports an anagenetic model for the origin of morphological novelty in this genus, highlighting the predominance of anagenetic evolution among contemporary dinosaur lineages.Item Vertebrate microfossils from the Upper Freshwater Molasse in the Swiss Molasse Basin: Implications for the evolution of the North Alpine Foreland Basin during the Miocene Climate Optimum(2015-05) Jost, Jürg; Kälin, Daniel; Börner, Saskia; Vasilyan, Davit; Lawver, Daniel R.; Reichenbacher, BettinaThe older part of the Upper Freshwater Molasse (OSM) in the Swiss and South German Molasse Basin records the extended warm period known as the Miocene Climate Optimum. However, dating and global correlation of fossils and palaeoclimatic data from OSM sediments remains challenging, because sections are often incomplete and biostratigraphic data sometimes ambiguous. Here we present the rare case of a fossiliferous OSM section that can be securely dated to the late Early Miocene and early Middle Miocene (c. 16.1–15.7 Ma). Vertebrate microfossils have been recovered from three levels in superposition. Fish teeth document primary freshwater fishes (Cyprinidae, Channidae), but otoliths found in the middle level indicate dominance of euryhaline fishes (Cyprinodontiformes, Gobiiformes). The herpetofaunal assemblages largely consist of taxa that were widely distributed in Central Europe during the Miocene Climate Optimum and fragments of turtle eggshells assignable to the Oofamily Testudoolithidae Hirsch, 1996. The small-mammal fauna is dominated by the cricetid Megacricetodon bavaricus Fahlbusch, 1964. The fossil biota implies that the lowermost level (late Early Miocene, c. 16.1 Ma) represents a palaeo-soil that formed under humid conditions, while the levels above it (early Middle Miocene, c. 15.7–15.8 Ma) record a warm freshwater pond subject to evaporation (middle level), and mean annual temperature ≥ 17 °C in the vicinity of a river with an open hinterland (upper level). Our results, together with previous data, suggest that the palaeoclimate of the North Alpine Foreland Basin of Switzerland and Southwest Germany was humid during the late Early Miocene and earliest Middle Miocene, and that the Middle Miocene onset of seasonality and low mean annual precipitation occurred by c. 15.7–15.8 Ma. We conclude that global climate change and the 100-kyr orbital eccentricity minimum at 15.75 Ma may have triggered the decrease in humidity in the North Alpine Foreland Basin.Item From dinosaurs to birds: a tail of evolution(2014-05) Rashid, Dana J.; Chapman, Susan C.; Larsson, Hans C. E.; Organ, Chris L.; Merzdorf, Christa; Bradley, Roger S.; Horner, John R.A particularly critical event in avian evolution was the transition from long- to short-tailed birds. Primitive bird tails underwent significant alteration, most notably reduction of the number of caudal vertebrae and fusion of the distal caudal vertebrae into an ossified pygostyle. These changes, among others, occurred over a very short evolutionary interval, which brings into focus the underlying mechanisms behind those changes. Despite the wealth of studies delving into avian evolution, virtually nothing is understood about the genetic and developmental events responsible for the emergence of short, fused tails. In this review, we summarize the current understanding of the signaling pathways and morphological events that contribute to tail extension and termination and examine how mutations affecting the genes that control these pathways might influence the evolution of the avian tail. To generate a list of candidate genes that may have been modulated in the transition to short-tailed birds, we analyzed a comprehensive set of mouse mutants. Interestingly, a prevalent pleiotropic effect of mutations that cause fused caudal vertebral bodies (as in the pygostyles of birds) is tail truncation. We identified 23 mutations in this class, and these were primarily restricted to genes involved in axial extension. At least half of the mutations that cause short, fused tails lie in the Notch/Wnt pathway of somite boundary formation or differentiation, leading to changes in somite number or size. Several of the mutations also cause additional bone fusions in the trunk skeleton, reminiscent of those observed in primitive and modern birds. All of our findings were correlated to the fossil record. An open question is whether the relatively sudden appearance of short-tailed birds in the fossil record could be accounted for, at least in part, by the pleiotropic effects generated by a relatively small number of mutational events.Item An integrative approach to understanding bird origins(2014-12) Xu, Xing; Zhou, Zhonghe; Dudley, Robert; Mackem, Susan; Chuong, Cheng-Ming; Erickson, Gregory M.; Varricchio, David J.Recent discoveries of spectacular dinosaur fossils overwhelmingly support the hypothesis that birds are descended from maniraptoran theropod dinosaurs, and furthermore, demonstrate that distinctive bird characteristics such as feathers, flight, endothermic physiology, unique strategies for reproduction and growth, and a novel pulmonary system originated among Mesozoic terrestrial dinosaurs. The transition from ground-living to flight-capable theropod dinosaurs now probably represents one of the best-documented major evolutionary transitions in life history. Recent studies in developmental biology and other disciplines provide additional insights into how bird characteristics originated and evolved. The iconic features of extant birds for the most part evolved in a gradual and stepwise fashion throughout archosaur evolution. However, new data also highlight occasional bursts of morphological novelty at certain stages particularly close to the origin of birds and an unavoidable complex, mosaic evolutionary distribution of major bird characteristics on the theropod tree. Research into bird origins provides a premier example of how paleontological and neontological data can interact to reveal the complexity of major innovations, to answer key evolutionary questions, and to lead to new research directions. A better understanding of bird origins requires multifaceted and integrative approaches, yet fossils necessarily provide the final test of any evolutionary model.