North American stegosaur paleobiology

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Montana – Stegosaur plate

My undergraduate thesis involved a great deal of paleontology independent of Princeton. My project required field work in Montana helping to excavate new stegosaur specimens as well as visits to museums throughout the US and in Switzerland. I was interested in the biology of North American stegosaurs, focusing mainly on the question of plate variation and function.

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Montana – Stegosaur plate

 

 

Collections I visited to measure and photograph stegosaur fossils were: Smithsonian National Museum of Natural History, American Museum of Natural History, Yale Peabody Museum, Carnegie Museum of Natural History, Dinosaur National Monument, Denver Museum of Nature and Science, Sauriermuseum in Aathal (Switzerland), Natural History Museum of Utah, Wyoming Dinosaur Center in Thermopolis, Brigham Young University Museum of Paleontology, Museum of Western Colorado, Virginia Museum of Natural History, Black Hills Institute of Geologic Research, and Utah Field House of Natural History State Park Museum. In researching my thesis, I personally examined approximately 130 of plates and a large number of related bones

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Montana – Stegosaur plate CT scanning

As I observed more specimens, it became clear to me that I should test for sexual dimorphism in the species Stegosaurus (or Hesperosaurus) mjosi. Doing so required additional methodologies including basic sedimentological and taphonomic interpretations of the Montana quarry, CT scanning, and histological thin sectioning/microscopy.

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Montana – Stegosaur plate histological sampling

The topic of sexual dimorphism came to dominate the focus of the project with the completed thesis consisting of four chapters: “The taxonomic status of Stegosaurus  (=Hesperosaurus ) mjosi”, “Evidence for sexual dimorphism in Stegosaurus mjosi”, “Modeling selection pressures and inferring thermoregulation in Stegosaurus”, and “An examination of inter- and intraspecific variation in Stegosaurus”.

The project also attracted quite a lot of media attention prior to publication of one of the chapters from the Billings GazettePrinceton Alumni Weekly, and the Princeton Office of Communications, as well as local news stations KULR8 and Q2.

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Museum of Ancient Life, Utah

The full thesis is available through Princeton, as well as through my Researchgate page. The poster for Princeton’s Department of Ecology and Evolutionary Biology senior thesis presentation is also available through Researchgate. I plan to publish more from my undergraduate thesis in the future.

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Dinosaur National Monument, Utah

Abstract

The discovery in central Montana of the first ever multi-individual, monotaxic stegosaur site in North America prompted a revaluation of previous Stegosaurus material. Of particular interest was the plate morphology and function, as the plates of these specimens come in two morphs. This study examined a majority of known Stegosaurus material in collections across the USA and in Switzerland using a combination of quantitative techniques, histological analysis, and CT scanning. Several major insights into the biology of North American stegosaurs were made. First, in agreement with previous research, the taxa “Hesperosaurus mjosi” was found to be similar enough to specimens of Stegosaurus such that it should be considered a species within this genus. Second, dimorphism in the plates of S. mjosi was confirmed, with one morph being wide and oval and the other being tall and narrow. The wide morph plates reach sizes 45% larger in surface area than do the tall morph plates. This variation was determined to be sexual dimorphism by the elimination of alternate hypotheses. These included individual, ontogenetic, and interspecific variation as well as the possibility that one individual possessed both morphs of plates. There appear to be no intermediate morphs. Both morphs are known from sexually mature, young adults and fully-grown, old adults. Taphonomy of the Montana site suggests that these two morphs comprised a social group and the only dimorphic feature is plate shape. Isolated specimens possess only one morph of plate or the other and plates of both morphs can be identified along the entire plate series from neck to tail. This is the strongest evidence for sexual dimorphism in dinosaurs yet presented and the first to rigorously test all alternate hypotheses for the observed dimorphism. Third, a simple model was developed to estimate the relative intensities of predation risk to sexual selection in a species based on the degree of dimorphism observed in their horny structures. By applying this to S. mjosi and available data on extant bovids, it was found that modern analogs for S. mjosi include the mountain goat (Oreamnos americanus), American bison (Bison bison), African or Cape buffalo (Syncerus caffer), and the genus of antelope Gazella. The comparison to extant bovids was further explored. A tail spike found at the central Montana quarry was preserved with fossilized keratin extending off of the tip. By developing a proxy for heat flow into or out of horny structures based on the surface areas of bony core and keratin sheath, it was found that Stegosaurus likely thermoregulated to the same extent as would be expected for an extant, tropical bovid of the same body size. Fourth, an examination of some of the variation within Stegosaurus suggests that the genus likely exhibited high species richness and varying degrees of sexual dimorphism between species within the genus. Overall these results provide evidence for multiple functions of plates including inter- and intraspecific display, predator deterrence, and thermoregulation. The relative importance of these different functions probably varied between species of Stegosaurus. Future paleontological and stratigraphic work on Morrison Formation stegosaurs, in combination with comparative approaches to extant species, will likely provide new insights into the details of the paleobiology of Stegosaurus.

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Princeton University – Stegosaur plate thin section