Clint A Boyd | North Dakota Geological Survey (original) (raw)

Papers by Clint A Boyd

Journal of Vertebrate Paleontology, 2024

The vertebrate assemblages of the Albian to Cenomanian Wayan Formation of southeastern Idaho and ... more The vertebrate assemblages of the Albian to Cenomanian Wayan Formation of southeastern Idaho and southwestern Montana's coeval Vaughn Member of the Blackleaf Formation are dominated by the small, burrowing orodromine dinosaur Oryctodromeus cubicularis. Here, we describe in detail the osteology of Oryctodromeus based on new specimens from Idaho and Montana that add substantially to the preliminary description of the types from Montana, and provide a suite of additional diagnostic characters for the taxon: ilium with elongate preacetabular process; elongate cervical vertebra centra with an anteroposterior length 1.6 times the dorsoventral height; elongate dorsal vertebra centra with an anteroposterior length 1.4 times the dorsoventral height; more than 55 elongate caudal vertebrae enveloped in hypaxial and epaxial ossified tendons; and a femoral head on an elongate neck-similar to that of Koreanosaurusprojecting from the greater trochanter at about 35°. The tail, comprising two-thirds of the animal's roughly 3 meters length, and associated tendon sheaths in the axial column indicate greater flexibility than previously supposed for ossified tendons or, alternatively, suggest that the Oryctodromeus burrows had separate, or multiple entrances and exits. The elongated and angled femoral head likely facilitated digging via a braced splayed-leg posture. Our phylogenetic analysis incorporates new characteristics and supports the monophyly of Orodrominae, a clade of neornithischian dinosaurs from the middle to Late Cretaceous of Asia and western North America that utilized burrowing.

Bulletin of the American Museum of Natural History, 2023

Mosasaurs are large, carnivorous aquatic lizards with a global distribution that lived during the... more Mosasaurs are large, carnivorous aquatic lizards with a global distribution that lived during the Late Cretaceous. After 200 years of scientific study, new mosasaur species are still being discovered as new localities are explored and specimens collected long ago are reevaluated using modern standards of species delimitation. Even so, the phylogenetic positions of many key taxa are unresolved and therefore our understanding of mosasaur macroevolution is muddled. Here, we describe a new genus and species of mosasaurine mosasaur comprising a partial skull and skeleton from the Pembina Member of the Pierre Shale Formation in Cavalier County, North Dakota. The lower bound on the age of the specimen is 80.04 ±0.11 Ma, provided by the underlying bentonite bed. Its skull and jaws are nearly complete, and the postcranial skeleton preserves seven cervical vertebrae with three hypapophyseal peduncles, 11 ribs, and five anterior dorsal vertebrae. The new specimen was scored into a modified version of an existing phylogenetic matrix of Mosasauroidea and was recovered in a polytomy with Clidastes; however, given that its morphology is significantly different from that of Clidastes, we refer it to a new genus and species, Jormungandr walhallaensis. Notably, this new taxon shares a mosaic of features seen in both basal (e.g., Clidastes; high dental counts) and derived (e.g., Mosasaurus; subrectangular quadrate) mosasaurines, in addition to possessing its own unique suite of autapomorphies. Given that it possesses morphology intermediate between Clidastes and Plotosaurini, we suspect that future analyses of mosasaur phylogeny, following the addition of new characters and taxa, will recover Jormungandr as transitional between them. Its occurrence increases the known diversity of mosasaurs from the Pembina Member and is the earliest mosasaur to possess autapomorphies of Plotosaurini. Finally, we also analyzed the matrix using different outgroups to test their effect on tree topology and resolution, and found that including multiple nonmosasauroid anguimorphs increased resolution, but not support, of mosasaurid ingroup relationships.

Paludicola, 2023

Several recently published studies documented much of the micromammal portion of the Oligocene fa... more Several recently published studies documented much of the micromammal portion of the Oligocene faunae recovered from the Fitterer Ranch and Obritsch Ranch localities in Stark County, North Dakota. That work identified four stratigraphically segregated faunae that range in age from latest Orellan to late Whitneyan. This study reviews the remaining micromammals from those localities, the leptictids and lipotyphlans, recognizing the presence of ten taxa. One species, Mystipterus austinae (Talpidae, Uropsilinae), is new and represents the earliest occurrence of the genus. In addition to Leptictis dakotensis, the presence of a new leptictid taxon is documented but is not named pending a detailed revision of that group. The recognition of the erinaceid Amphechinus horncloudi represents the earliest occurrence of this species, which was previously restricted to the Arikareean. Four other taxa previously known from older faunae are identified, extending their known ranges into the Whitneyan (Centetodon marginalis, Cryptoryctes sp., cf. C. kayi, Domnina gradata, and Oligoscalops galbreathi). With the incorporation of the data from this study, the well-sampled micromammal faunae from Fitterer Ranch and Obritsch Ranch provide important information for evaluating broader patterns of micromammal evolution, dispersal, and extinction within the Great Plains region of North America.

Paludicola, 2022

Paludicola is published under the auspices of the Rochester Institute of Vertebrate Paleontology ... more Paludicola is published under the auspices of the Rochester Institute of Vertebrate Paleontology (RIVP), Rochester, NY. It is intended to serve as an "in-house" publication for students and professional paleontologists who are not affiliated with institutions that provide such a publication. All manuscripts considered for publication in Paludicola are expected to be original and have not been published elsewhere in any other form or are not being considered for publication anywhere else. This journal is intended as an outlet for papers on any aspect of Vertebrate Paleontology. Paludicola is published twice annually (Fall and Spring).

PeerJ, 2021

Ornithischians form a large clade of globally distributed Mesozoic dinosaurs, and represent one o... more Ornithischians form a large clade of globally distributed Mesozoic dinosaurs, and represent one of their three major radiations. Throughout their evolutionary history, exceeding 134 million years, ornithischians evolved considerable morphological disparity, expressed especially through the cranial and osteodermal features of their most distinguishable representatives. The nearly two-century-long research history on ornithischians has resulted in the recognition of numerous diverse lineages, many of which have been named. Following the formative publications establishing the theoretical foundation of phylogenetic nomenclature throughout the 1980s and 1990s, many of the proposed names of ornithischian clades were provided with phylogenetic definitions. Some of these definitions have proven useful and have not been changed, beyond the way they were formulated, since their introduction. Some names, however, have multiple definitions, making their application ambiguous. Recent implementation of the International Code of Phylogenetic Nomenclature (ICPN, or PhyloCode) offers the opportunity to explore the utility of previously proposed definitions of established taxon names. Since the Articles of the ICPN are not to be applied retroactively, all phylogenetic definitions published prior to its implementation remain informal (and ineffective) in the light of the Code. Here, we revise the nomenclature of ornithischian dinosaur clades; we revisit 76 preexisting ornithischian clade names, review their recent and historical use, and formally establish their phylogenetic definitions. Additionally, we introduce five new clade names: two for robustly supported clades of later-diverging hadrosaurids and ceratopsians, one uniting heterodontosaurids and genasaurs, and two for clades of nodosaurids. Our study marks a key step towards a formal phylogenetic nomenclature of ornithischian dinosaurs.

Paludicola, 2019

In a recent review of the North American rodent fossil record during the Whitneyan biozone (32.0 ... more In a recent review of the North American rodent fossil record during the Whitneyan biozone (32.0 to 30.0 Ma) a publication error resulted in the table containing the primary data on reported occurrences of rodent taxa not being published in its entirety. This report publishes the full Whitneyan data table used in that study, along with additional tables documenting Duchesnean, Chadronian, Orellan, and Arikareean occurrences of rodent taxa within North America. These data serve as an updated resource for facilitating biostratigraphic correlations and studying patterns in rodent evolution during the late Paleogene and early Neogene.

Proceedings of the South Dakota Academy of Sciences, 2018

An astragalus belonging to an indeterminate amynodontid, within the tribe Metamynodontini, is ide... more An astragalus belonging to an indeterminate amynodontid, within the tribe Metamynodontini, is identified within collections from the Cedar Pass local fauna from the North Unit of Badlands National Park, South Dakota. This new specimen represents the youngest occurrence of Amynodontidae in North America. Prior to this study, Metamynodon makes its highest verified occurrence in the underlying Scenic Member of the Brule Formation (Orellan North American Land Mammal Age). The Cedar Pass local fauna originates within the lower Poleslide Member of the Brule Formation and is considered to contain a medial Oligocene fauna (early Whitneyan North American Land Mammal Age). However, matrix affixed to th e new sp ecimen is primarily composed of sandstone and gravel consistent with the river channel deposits within the Sharps Formation, which overlies the Poleslide Member. Despite recent stratigraphic reclassification of underlying sediments from the basal Sharps Formation to the upper Poleslide Member, the associated Sharps fauna is distinctly characterized as Arikareean North American Land Mammal Age in character. Taxa previously recovered within the Arikaree channel deposits include the entelodont Daeodon, the canid Sunkahetanka, and the rodents Tamias, Proheteromys, and Hitonkala. Specimens of the castorids Capacikala and Palaeocastor were recovered from the underlying Poleslide siltstones. Prior referrals of the channel deposits at the top of Cedar Pass to the classic "Protoceras channels," along with associated faunal components, confound our understanding of the Whitneyan-Arikareean transition and highlight the need for further research into the fauna of the Sharps Formation in the North Unit of Badlands National Park.

Journal of Vertebrate Paleontology, 2023

An exquisitely preserved male skeleton of an early arctoid, Eoarctos vorax new genus and species,... more An exquisitely preserved male skeleton of an early arctoid, Eoarctos vorax new genus and species, provides a unique window into the origin and early divergence of Carnivora. Recovered from the Fitterer Ranch locality in the early Oligocene (Orellan to Whitneyan North American Land Mammal ages) Brule Formation of southwestern North Dakota (∼32 Ma), the new arctoid offers an opportunity to evaluate the fundamental relationships of the caniform (dog-like) carnivorans. Eoarctos vorax possesses a suite of plesiomorphic characters inherited from its miacid ancestors, making it an ideal model for ancestral arctoids. We present a comprehensive treatment of E. vorax, combining traditional description with photographic documentation, microCT, laser scans, and photogrammetry. Showing its plesiomorphic morphology, Eoarctos vorax is scansorial, somewhat like a modern raccoon, retaining the ability to climb trees and lacking cursorial adaptations present in the early canid Hesperocyon. However, E. vorax shows clear signs of durophagous cranio-dental morphology, presumably for an obligatory diet of mollusks, with frequent damage to shell-crushing premolars, plus associated dental infections. We review several other key North American early arctoids and present total-evidence (nuclear DNA and discrete morphological characters) Bayesian and parsimony analyses of Caniformia phylogeny, including extinct stem taxa plus a living member of all modern families. We recognize an endemic North American ursoid clade, Subparictidae, which includes Eoarctos vorax. We demonstrate the importance of North America as an early cradle of evolution for caniform carnivorans, including early precursors of Canidae, Amphicyonidae, Ursidae, and Pinnipedia.

PeerJ, 2021

PLOS ONE

Removal or protection from biostratinomic agents of decomposition, such as predators and scavenge... more Removal or protection from biostratinomic agents of decomposition, such as predators and scavengers, is widely seen as a requirement for high-quality preservation of soft tissues in the fossil record. In this context, extremely rapid burial is an oft-cited mechanism for shielding remains from degradation, but not all fossils fit nicely into this paradigm. Dinosaurian mummies in particular seemingly require two mutually exclusive taphonomic processes to preserve under that framework: desiccation and rapid burial. Here we present a recently prepared Edmontosaurus mummy that reveals an alternate fossilization pathway for resistant soft tissues (e.g., skin and nails). While the skin on this specimen is well-preserved in three dimensions and contains biomarkers, it is deflated and marked by the first documented examples of injuries consistent with carnivore activity on dinosaurian soft tissue during the perimortem interval. Incomplete scavenging of the carcass provided a route for the ga...

An anomalous specimen of Dinictis felina (Carnivora: Nimravidae) from the Scenic Member of the Br... more An anomalous specimen of Dinictis felina (Carnivora: Nimravidae) from the Scenic Member of the Brule Formation in South Dakota was recently identified possessing bilaterally symmetrically split carinae on the anterior surfaces of the upper canines. The anterior carinae display normal serration patterns from the apex until they bifurcate towards the base. The primary carinae maintain their normal positions while the accessory carinae diverge laterally before curving dorsally towards the tooth base. Though other types of mammalian, particularly human, tooth abnormalities are thoroughly documented, split carinae are previously unreported in mammalian taxa. However, these anomalies are well documented in disparate families of theropod dinosaurs (Tyrannosauridae, Dromaeosauridae, and Carcharodontosauridae). The aforementioned morphology of the carinae of the D. felina specimen is similar to that previously noted in theropods (e.g., split carinae on the anterior face of the tooth), sugges...

PaleoBios, 2021

The upper Maastrichtian Breien Member situated within the lower portion of the Hell Creek Formati... more The upper Maastrichtian Breien Member situated within the lower portion of the Hell Creek Formation in south-central North Dakota records one of the last transgressions of the Western Interior Seaway (WIS) during the terminal Cretaceous. A fragmentary articular-prearticular complex and isolated vertebra belonging to a mosasauroid were recovered in 2016 from sandstones and mudstones deposited in a nearshore marine paleoenvironment within the southern arm of the bisected WIS. The medially-rotated retroarticular process on the articular-prearticular complex, the shape of the glenoid fossa, along with the morphology of the isolated vertebra, facilitate a conservative referral to a large-bodied mosasaurine such as Mosasaurus or Prognathodon. The rocks of the Breien Member provide paleontologists a unique glimpse of intracontinental marine ecosystems immediately prior to the end of the Cretaceous Period. This discovery provides additional evidence that the latest Maastrichtian marine fauna is a continuation of the fauna preserved in the underlying Fox Hills Formation and that the marine faunal turnover that gave rise to the subsequent Cannonball Sea fauna recorded in Paleocene rocks in North Dakota occurred at the Cretaceous-Paleogene boundary.

Journal of Paleontology, 2020

Five marsupial species are recognized from the Brule Formation at two localities in southwestern ... more Five marsupial species are recognized from the Brule Formation at two localities in southwestern North Dakota: Fitterer Ranch and Obritsch Ranch (middle Oligocene; Whitneyan North American Land Mammal Age [NALMA]). The herpetotheriids Herpetotherium fugax Cope, 1873a, Copedelphys superstes new species, and the per-adectid Nanodelphys hunti (Cope, 1873b) are represented at both localities. A fourth species is H. sp., cf. H. merriami (Stock and Furlong, 1922), represented by a single specimen from Fitterer Ranch, being limited elsewhere to the later Arikareean NALMA. A fifth species is represented by two isolated lower cheek teeth, interpreted as m1s, from Fitterer Ranch that are unique in lacking a trigonid (only two cusps present) while having a well-developed talonid. These specimens are referred to an indeterminate herpetotheriine species. The new species of Copedelphys is distinct from other species of the genus in that the anterior two lower molars are enlarged relative to the posterior molars. Overall, this new species is more similar in proportions to the latest Eocene (Chadronian) C. titanelix (Matthew, 1903) than the Oligocene (Orellan and Whitneyan) C. stevensoni (Cope, 1873b). This study adds a third and fourth Whitneyan marsupial fauna from the Great Plains region of North America, increases the known diversity of Whitneyan marsupials, and provides further evidence that marsupial diversity during the late Paleogene in North America was relatively stable until the late early Arikareean NALMA. UUID: http://zoobank.org/b8534802-b9ed-4120-baca-fc72917f7d6a

Annals of the Carnegie Museum, 2019

Until recently, few well-described rodent faunae from the Whitneyan North American Land Mammal Ag... more Until recently, few well-described rodent faunae from the Whitneyan North American Land Mammal Age [NALMA] were known, hindering studies of rodent diversity, biogeography, and evolutionary patterns during the Oligocene. This study describes a new Whitneyan rodent assemblage from the Obritsch Ranch paleontological locality in the Little Badlands region of North Dakota. Specimens were collected from three stratigraphi-cally restricted sampling intervals within the middle to upper Brule Formation, resulting in the recognition of fourteen rodent species, five of which are elsewhere known to first appear in Whitneyan faunae. Described is one new species, the eomyid Paradjidaumo obritschorum, and the first cranial material of the heteromyid rodent Proharrymys Korth and Branciforte, 2007. The rodent fauna from the upper two sampling intervals at Obritsch Ranch and the uppermost fauna recently described from the nearby Fitterer Ranch paleontological locality share four taxa in common with the late Whitneyan Blue Ash local fauna from southwestern South Dakota, indicating these two North Dakota rodent faunae are also from the late Whitneyan. Increasing knowledge of Whitneyan rodent faunae in North America reveals unusually high survivorship of rodent species from the older Orellan NALMA into the Whitneyan NALMA and much geographic variation in the diversity, distribution, and relative abundance of different rodent families between individual Whitneyan rodent faunae. Those factors help explain prior difficulties in differentiating Orellan and Whitneyan rodent faunae and in identifying biostratigraphically useful rodent taxa for the Whitneyan. Overall, Whitneyan rodent faunae from North America display an increase in the diversity of aplodontiids, cricetids, and sciurids and a decrease in eomyid and ischyromyid diversity relative to the Orellan.

Smithsonian Contributions to Paleobiology, 2019

Eighteen rodent species are recognized from the Fitterer Ranch fauna of North Dakota on the basis... more Eighteen rodent species are recognized from the Fitterer Ranch fauna of North Dakota on the basis of more than a thousand collected specimens. Of the species recognized, four are new: the prosciurine aplodontiids Prosciurus hogansoni and Altasciurus leonardi, the heliscomyid Heliscomys borealis, and the cricetid Eumys lammersi. A previously described castorid from this fauna, Oligotheriomys primus Korth, 1998, is considered a synonym of “Eutypomys” magnus Wood, 1937, on the basis of the recovery of lower dentitions but is retained in the genus Oligotheriomys. A single specimen is questionably referred to Microparamys, a genus elsewhere limited to the Eocene (Clarkforkian-Chadronian land mammal ages). The rodent fauna appears to be a combination of predominantly Orellan and Whitneyan species (early Oligocene), suggesting that the section might transcend the Orellan-Whitneyan boundary. However, the rodent fauna does not alter significantly from the lowest to the highest horizons.

During their long evolutionary history, neornithischian dinosaurs diverged into several clades wi... more During their long evolutionary history, neornithischian dinosaurs diverged into several clades with distinctive adaptations. However, the early evolution within Neornithischia and the resolution of the phylogenetic relationships of taxa situated near the base of the clade remain problematic. This is especially true for those taxa traditionally placed at the base of Ornithopoda, either as 'hypsilophodontids' or at the base of the diverse clade Iguanodontia. Recent studies are improving our understanding of the anatomy and relationships of these taxa, with discoveries of several new non-ankylopollexian ornithopods from South America and Europe providing key insights into early ornithopod evolution and palaeobiogeography. Here, we describe a new basal ornithopod, Burianosaurus augustai gen. et sp. nov., based on a well-preserved femur from the upper Cenomanian strata (Korycany Beds of the Peruc-Korycany Formation) of the Czech Republic. The new taxon is diagnosed by a unique suite of characters and represents the only occurrence of a Cenomanian non-avian dinosaur in Central Europe north of the Alpine Tethyan areas. Histological examination of the type specimen reveals the presence of a loosely packed Haversian system which suggests relatively mature bone from a possible young adult. Phylogenetic analyses of two different data sets, selected to test the placement of B. augustai in various parts of the neornithischian tree, reconstruct B. augustai as a basal ornithopod, firmly nested outside Ankylopollexia. These results also support a diverse Elasmaria as a basal clade within Ornithopoda and reconstruct Hypsilophodon outside Ornithopoda as the sister taxon to Cerapoda. However, the relationships of 'hypsilophodontids' within Neornithischia remain contentious.

Avian remains are rare from Paleogene sediments in the Great Plains region of North America, espe... more Avian remains are rare from Paleogene sediments in the Great Plains region of North America, especially those that can be identified taxonomically. Here, we describe avian eggshell fragments from the Fitterer Ranch locality within the Oligocene Brule Formation of North Dakota and assign them to ootaxonomy. The specimen examined (NDGS 1934) exhibits low tubercles that form undulating surface ornamentation; 650–900-mm-thick eggshell composed of three structural layers of calcite; abrupt transition between mammillary and continuous layers; prisms visible in continuous layer; squamatic texture primarily visible in lower half of continuous layer; and mammillary-to-total shell thickness ratio of 1:2.70–2.75. Due to this unique combination of characters, we establish the new oospecies Metoolithus jacksonae, oosp. nov. This new oospecies extends both the geographic and temporal ranges of the oogenus Metoolithus and demonstrates the importance of oological research in revealing the diversity of Oligocene avifauna in this region.

This report re-evaluates the depositional history, biostratigraphic “age,” and regional correlat... more This report re-evaluates the depositional history, biostratigraphic “age,” and regional correlates of Chadron Formation rocks in North Dakota using a variety of data sources. A series of well-developed paleosols are herein recognized for the first time at the top of the Chalky Buttes Member across much of southwestern North Dakota. Those paleosols are contemporaneous with the Weta Paleosol Series of southern South Dakota developed on top of the Chamberlain Pass Formation, and together these paleosols indicate a period of nondeposition and geomorphic stability throughout the Great Plains region following a first phase of late Eocene deposition. Within the Williston Basin, that first phase of deposition begins with rocks dominated by group A heavy minerals (zircon > staurolite > aluminosilicates, tourmaline) that are overlain by rocks dominated by group B heavy minerals (epidote > garnet > zircon). Deposition of these group A and group B rocks would have occurred sometime in the early Chadronian (Ch1-Ch2: 36.9-35.8 Ma).

The second phase of late Eocene deposition in North Dakota consisted of local downcutting of stream channels that were infilled with rocks dominated by group D heavy minerals (hornblende > diopside > epidote). Two faunas are now known from these group D rocks: the late early Chadronian (Ch2: 36.6-35.8 Ma) Medicine Pole Hills local fauna, and the newly reported middle Chadronian (Ch3: 35.8-34.8 Ma) Stover Site local fauna (Adams County, North Dakota). This second phase of deposition occurred after the onset of paleosol development in North Dakota as indicated by the common presence of ferruginous aggregate grains within the opaque heavy minerals from the Stover Site sample that appear to be derived from erosion of paleosols developed on older Chalky Buttes Member rocks that contained group B heavy minerals. Deposition of group D rocks in North Dakota was contemporaneous with portions of those rocks infilling the base of the Red River Paleovalley in southern South Dakota (Ahearn and Crazy Johnson Members).

Deposition of the South Heart Member began gradually during the development of the paleosols within the Williston Basin, indicating those rocks are younger than the group A and group B rocks of the Chalky Buttes Member. Given the lack of biostratigraphic data from the South Heart Member and absence of those rocks within Bowman and Adams Counties where group D rocks of the Chalky Buttes Member are exposed, insufficient evidence is currently available to determine the relative timing of deposition of group D Chalky Buttes Member rocks and those of the South Heart Member. Overall, this study demonstrates that late Eocene deposition was more complicated than previously reported, though these patterns match those found elsewhere within the Great Plains region.

Journal of Vertebrate Paleontology, 2024

Bulletin of the American Museum of Natural History, 2023

Paludicola, 2023

Paludicola, 2022

PeerJ, 2021

Paludicola, 2019

Proceedings of the South Dakota Academy of Sciences, 2018

Journal of Vertebrate Paleontology, 2023

PeerJ, 2021

PLOS ONE

PaleoBios, 2021

Journal of Paleontology, 2020

Annals of the Carnegie Museum, 2019

Smithsonian Contributions to Paleobiology, 2019

At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah , Oct 2013

The systematic relationships of basal ornithischian dinosaurs remain contentious, especially the ... more The systematic relationships of basal ornithischian dinosaurs remain contentious, especially the position of basal neornithischians (i.e., ‘hypsilophodontids’). Prior analyses of basal ornithischian relationships have been hampered by the fact that the hypodigm material of many basal neornithischian taxa is fragmentary, denying access to character data crucial to resolving their relationships. The recent discovery of several new basal neornithischian taxa and the referral of more complete specimens to known taxa provide important new data pertinent to resolving these relationships. The results of this study supplement those recent advances by improving our understanding of the anatomy and systematic relationships of basal neornithischian taxa from the Late Cretaceous of North America. These new insights are accomplished through a taxonomic revision of the Maastrichtian taxa Bugenasaura and Thescelosaurus, a detailed anatomical description of the cranial anatomy of Thescelosaurus neglectus based on the referral of a specimen that includes a nearly complete skull (NCSM 15728), and description of a new basal neornithischian taxon from the Kaiparowits Formation (Campanian) of Utah. All of these new data are compiled into a dataset composed of 255 characters for 65 terminal taxa (all species exemplars) focused on assessing basal ornithischian relationships. The recovered strict consensus topology is the most highly resolved, stratigraphically congruent phylogenetic hypothesis of basal ornithischian relationships yet proposed. This analysis places all basal neornithischians except Hypsilophodon foxii outside of Cerapoda, substantially reducing the taxonomic contents of Ornithopoda. A new clade containing fourteen basal neornithischian taxa is recovered as the sister taxon to Cerapoda and includes all North American basal neornithischians from the Cretaceous. The historical biogeography of Ornithischia is also reconstructed using a method that incorporates time calibrated branch lengths that represent the implied missing fossil record of each taxon. The results of this analysis support two dispersals of neornithischian taxa into South America during the Cretaceous: one consisting of basal iguanodontians dispersing from Australia (possibly via Antarctica) and a second consisting of basal neornithischians dispersing from Asia through North America.

The Quarterly Review of Biology, 2015

… Society of America Abstracts with Programs …, 2010

North-Central Section (44th Annual) and South-Central Section (44th Annual) Joint Meeting (1113 ... more North-Central Section (44th Annual) and South-Central Section (44th Annual) Joint Meeting (1113 April 2010). Paper No. 29-16, Presentation Time: 1:30 PM-5:00 PM. ASSESSING THE PRESENCE OF BASAL NEORNITHISCHIAN ...

A. Skeletal reconstruction of the undescribed ‘hypsilophodontid’ from the Kaiparowits Fo... more A. Skeletal reconstruction of the undescribed ‘hypsilophodontid’ from the Kaiparowits Formation with known material shown in white (modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057605#pone.0057605-Boyd1" target="_blank">[65]</a>). B. Partial left scapula (UMNH VP 21104) with feeding traces collected from UMNH locality 303. C. Outline drawing of left scapula (UMNH VP 21104) with feeding traces highlighted and colored boxes showing the locations of figure parts D, F, and G (colors match the respective figure parts). D. Bisected pit on the left scapula (UMNH VP 21104). E. Bisected pit on a modern cow femur produced by Alligator mississippiensis during actualistic experiments <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057605#pone.0057605-DrumhellerHorton1" target="_blank">[20]</a>. F. Small pit (highlighted by white arrow) on the proximal portion of the left scapula (UMNH VP 21104). G. Series of small scores present along the ventral margin of the neck of the left scapula (UMNH VP 21104). H. Distal portion of a right femur (UMNH VP 21107) with feeding traces collected from UMNH locality 303. I. Outline of right femur (UMNH VP 21107) with feeding traces highlighted and colored box showing the location of figure part J. J. Puncture containing an embedded tooth present on the right femur (UMNH VP 21107) and a small pit (highlighted by white arrow) just ventral to the puncture. K. Puncture present on a modern cow femur produced by A. mississippiensis during actualistic experiments <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057605#pone.0057605-DrumhellerHorton1" target="_blank">[20]</a>. L. Reconstruction of the hypothesized impact of the crocodyliform tooth with the right femur, creating the puncture observed in UMNH VP 21107. M. Reconstruction of the hypothesized fracturing of the damaged crocodyliform tooth crown, resulting in the embedded tooth observed in UMNH VP 21107. Scale bar equals one meter in A, 10 mm in B, E, H, and K, 2 mm in D, F, G, and J. Abbreviations: cort, cortical bone; dis, distal; dor, dorsal; f, tooth fragment; lat, lateral; med, medial; pr, proximal; t, tooth crown.</p

The Cedar Pass Local Fauna comprises of a substantial and comprehensive fauna from stratigraphica... more The Cedar Pass Local Fauna comprises of a substantial and comprehensive fauna from stratigraphically restricted localities in the Poleslide Member of the Brule Formation in Badlands National Park. The fossiliferous horizon is situated 14-20 meters below the traditional Rockyford Ash. This zone contains the first abundant occurrences of the taxon Leptauchenia, previously distinguished as the “Leptauchenia beds”, and one of the archetypical Whitneyan horizons. However, the analysis of the corresponding fauna prompts an inquiry to a potentially erroneous assignment to the Whitneyan North American Land Mammal Age. Characteristic Arikareean taxa are represented in this fauna, while noting a deficit of the characteristic White River chronofauanal counterparts. The Arikareean canids Phlaocyon and Cynarctoides make their appearance with a borophagine dominant ecosystem, along with no known or documented occurrence of Hesperocyon within the interval. All collected specimens of Hypertragulus have been erroneously identified, revealing the lowest stratigraphic occurrence of Nanotragulus. However, not all constituents of the White River Chronofauna are missing with the observation of numerous specimens of Merycoidodon and Hyracodon. This study exhibits the paucity of comprehensive Whitneyan faunas and the vague transition from the Whitneyan to the Arikareean.

A diverse herpetofauna is also noted, not reavealing any refined biostratigraphic zonation, but still siginificant since it is the only “Whitneyan” fauna outside of the I-75 Local Fauna of Florida. This herpetofauna includes anurans, anguids, amphisbaenids, iguanids, and boids. One of the most significant finds is the presence of an anuran skull closely resembling the genus Rana. This specimens would mark the first Oligocene occurrence of true frogs in North America since the Hemingfordian, with the only other Oligocene occurrence in Germany. This taxon can be compared to Rana pipiens, with a few morphologic differences. We elect to not include this in the questionable “Rana pipiens complex” that is more typical within the Miocene.

A new fossil locality, here termed the Bloom Basin Local Fauna, discovered within the Bloom Basin... more A new fossil locality, here termed the Bloom Basin Local Fauna, discovered within the Bloom Basin at Badlands National Park is providing important insights into the nature of the Chadron-Brule contact in this area of South Dakota. The fossil-bearing horizon consists of a brown mudstone bed situated within three meters of the base of the Scenic Member (Brule Formation), and is positioned on the north flank of a paleotopographic hill that was once an erosional remnant of the Chadron Formation. At this location, the base of the Scenic Member is deposited within an erosional surface cut into the underlying Chadron Formation. As a result, the fossil-bearing horizon is situated within one of the lowest deposits of the Scenic Member in the local area, 28 meters below a widespread, originally horizontal marker layer: the Hay Butte marker. The early Orellan age of this fauna is confirmed by the presence of the characteristic Orellan artiodactyls Hypertragulus calcaratus and Leptomeryx evansi. A second Leptomeryx morphotype is also present that displays both the diagnostic ‘Palaeomeryx fold’ of L. evansi and the ectostylid that diagnoses the older Late Chadronian species Leptomeryx speciosus. This latter form may represent a transitional morphotype intermediate between L. speciosus and L. evansi, and its presence may indicate that the Bloom Basin Local Fauna is positioned close to the Chadronian-Orellan transition. This hypothesis is supported by fact that the rodent fauna at this locality includes two taxa, Eutypomys parvus and Adjidaumo lophatus, which are otherwise only reported from the Chadronian of Saskatchewan, Montana, Wyoming, and North Dakota.

The contact between the Chadron and Brule Formations within the Bloom Basin was previously placed coincident with a series of four freshwater limestone beds collectively referred to as the Bloom Basin Limestone Bed. However, the Bloom Basin Limestone Bed directly overlies this new locality, indicating that the localized limestone is positioned stratigraphically higher within the Scenic Member than previously proposed. Thus, a re-evaluation of the inferred age and depositional history of the Bloom Basin Limestone Bed is needed.

Evidence of intraclade combat among nimravids was first reported in 1936 based on a single speci... more Evidence of intraclade combat among nimravids was first reported in 1936 based on a single specimen of Nimravus (SDSM 348) that preserves a prominent puncture mark on the frontal. Since that report, the topic of intraclade combat amongst nimravids received little attention. Examination of relatively complete crania preserved in the collections at the South Dakota School of Mines and Technology and at Badlands National Park revealed an additional four specimens (two of Dinictis and two of Hoplophoneus) that display bite marks referable to other nimravids. Comparison of these bite marks reveals upper canine punctures are concentrated around the orbital region, with four of the specimens exhibiting damage to the medial wall of the orbit. Additionally, most upper canine punctures present within the frontals and parietals on these specimens are aligned so that the other canine would have inserted into the orbit and are often paired with punctures/extensive damage on the medial wall of the orbit. Relatively conical punctures consistent with the morphology of the lower canines are positioned more posteriorly on the skulls, most of which are present on the posterior surface of the nuchal crest or the anterior corner of the sagittal crest. The distribution of these bite marks indicates a consistent pattern of attack that was oriented from behind with a primary goal of blinding the target individual. This pattern of attack is consistent with the widely hypothesized need for saber-toothed feliforms to minimize impacting dense bone with their delicate upper canines, which could result in breakage. Only the shallow frontal puncture originally reported on SDSM 348 shows evidence of healing, indicating that in most instances these attacks were either immediately fatal or resulted in death soon afterwards. Additionally, recent preparation work on SDSM 348 reveals a second set of bite marks along the right side of the skull that show no evidence of healing, suggesting this individual was subjected to a second, fatal attack. The fact that similar punctures are currently unknown from the crania of potential prey species suggests that relatively risky bites to the crania were restricted to cases of intraclade combat amongst nimravids where the risk of breaking their delicate upper canines was outweighed by the need to defend territory, social standing, or oneself against a competitor. Furthermore, the recognition of these additional specimens and the presence of bite marks on individuals of at least three different taxa indicates that intraclade combat resulting in death was not an uncommon occurrence within Nimravidae.

Few ontogenetically immature specimens are known for most nimravid taxa, impeding identification... more Few ontogenetically immature specimens are known for most nimravid taxa, impeding identification of important developmental and evolutionary trends in their dental anatomy. Examination of multiple Nimravus specimens representing various ontogenetic stages provides insight into the pattern of tooth eruption and spacing in this taxon. The only deciduous upper premolars are dP4 and dP3. Eruption of M1 begins before the loss of dP4, and P4 erupts just prior to P3. As the diastema between dC and dP3 elongates, but before P3 fully erupts, the alveolus for P2 opens in this space close to the posteromedial margin of the dC alveolus. As growth continues, a diastema opens anterior to the posterolaterally migrating P2. Eventually the alveolus for P1 (when present) opens in the diastema posteromedial to the dC alveolus. The positions of P1 and P2 continue to migrate posterolaterally relative to the dC alveolus as the muzzle elongates, changing the maxillary tooth row from curved to straight. The position of P3 also migrates from its original position situated tightly between the anterior roots of P4 to a more anterolateral position in line with the crown of P4. Similar tooth eruption and migration patterns are also noted in Dinictis, though fewer stages were observed. Finally, the variable presence of P1 in both Hoplophoneus (CSC-41-42) and Dinictis (UNSM 25524) further highlights that the presence of upper and lower anterior premolars is highly variable in nimravids.
Nimravus and Hoplophoneus deciduous upper canines are similar in morphology to their respective adult upper canines in both general dimensions (‘scimitar-tooth’ versus ‘dirk-tooth’) and in average serration density (~2/mm vs. >4/mm). Adult and deciduous upper canines of Dinictis are intermediate in morphology between these two taxa; however, the serration density of dC resembles the ‘dirk-tooth’ condition seen in more derived taxa (>4/mm), while the adult upper canines possess a lower serration density (~3/mm). The current phylogenetic position of the dwarf nimravid Nanosmilus between Dinictis and the ‘dirk-tooth’ nimravids (e.g, Hoplophoneus) suggests a mechanism for the evolution of ‘dirk-tooth’ canines in nimravids. The dC morphology of Dinictis could have been paedomorphically retained in adults of Nanosmilus, and later accentuated in other hoplophonins.

It has long been recognized that “dirk-tooth” and “scimitar-tooth” nimravids display relatively ... more It has long been recognized that “dirk-tooth” and “scimitar-tooth” nimravids display relatively fine and coarse serrations on their canine teeth, respectively. However, little work has been done to determine if differences in canine serration density between nimravid taxa are taxonomically informative. If such differences are present, they would provide powerful tools for referring fragmentary, taxonomically ambiguous material to specific clades, enhancing our ability to assess the geographic and biostratigraphic ranges of these taxa. The present study focused on evaluating patterns of canine serration density among the nimravid taxa Nimravus (n=7), Dinictis (n=12), and the hoplophonins Hoplophoneus (n=19) and Nanosmilus (n=2). Serration density was measured on the upper and lower canines over a length of at least five millimeters and then averaged to obtain the serration count per millimeter (SPM). Significant differences (p-value > 0.001) were noted between the mean values obtained for the upper canines of Nimravus (2.14 SPM), Dinictis (3.39 SPM), and the clade Hoplophonini (4.48 SPM) and no overlap was observed in the values recorded for these three taxa, though the 95% confidence intervals for the latter two taxa do slightly overlap. Alternatively, values of SPM were found to be relatively consistent along the carinae of individual teeth, between left and right canines in individual specimens, and between deciduous and adult canines from the same taxon (the latter tested in Nimravus and Hoplophoneus). Similar, taxonomically significant trends in SPM value are seen in the lower canines of the sampled taxa (though sample sizes were smaller) despite the fact that lower canines show less morphological variation within Nimravidae than upper canines. Preliminary examination of some specimens of other “saber-tooth” feliform carnivores (i.e., the barbourofelid Barbourofelis and the felids Smilodon, Nimravides, and Pseudalurus) reveals that these "scimitar-tooth" and "dirk-tooth" taxa also display low and high SPM values, respectively. These preliminary results suggest that canine serration size in “saber-tooth” feliform carnivores is influenced by the morphology (but not overall size) of the upper canines. Though broader sampling of nimravid taxa is needed, these preliminary results suggest that SPM values are a useful tool for identifying fragmentary specimens to specific clades, though resolution to the genus or species level may not always be possible.

Over the past two decades stratigraphic consistency metrics were used to analyze a diverse array ... more Over the past two decades stratigraphic consistency metrics were used to analyze a diverse array of clades covering nearly the entire tree of life, yet misconceptions regarding their calculation and applicability persist. Calculating these metrics requires two sources of data directly linked to the included set of terminal taxa: temporal data and a tree topology. Accurate comparison of stratigraphic consistency values between analyses requires that the included set of terminal taxa remains fixed, and it has long been assumed by most researchers that each terminal taxon has only a single source of temporal data, the stratigraphic record. However, terminal taxa actually possess three potential sources of temporal data: current stratigraphic data (i.e., the fossil record as known today), historical stratigraphic data (e.g., the fossil record as known in 1910), and molecular-based data (i.e., molecular divergence dates). Recognition of these additional sources of temporal data allows stratigraphic consistency metrics to be applied in situations where the tree topology is held constant while different sets of temporal data are assessed. As a result, stratigraphic congruence metrics can now be used to quantify the temporal disparity between molecular divergence estimates and the fossil record. Additionally, these metrics can be used to compare current and historical stratigraphic data to elucidate patterns in fossil data acquisition (i.e., new fossil discoveries) and interpretation (i.e., taxonomic referral and revision) through time. These applications are facilitated by a new methodology that allows multiple sources of temporal data to be evaluated simultaneously by inserting ‘anchor taxa’ into the tree topology to adjust the ages of the specific nodes under study. Finally, a new program script is introduced that greatly simplifies the calculation of several stratigraphic congruence metrics. Together these advances transform stratigraphic consistency metrics from narrowly-focused descriptive statistics into powerful tools for evaluating molecular divergence estimates and understanding historical trends in fossil data acquisition and interpretation.

The Chadron Formation (Eocene) in South Dakota produces numerous fossils of extinct taxa, includi... more The Chadron Formation (Eocene) in South Dakota produces numerous fossils of extinct taxa, including brontotheriids, nimravids, and alligatorines. The majority of these fossils were historically found on private lands, national grasslands, or the Pine Ridge Reservation. Despite the fact that over 250 fossil localities were documented within the North Unit of Badlands National Park between 1979, when the park was designated, and 2009, only a single fossil locality was created within the Chadron Formation (BADL-LOC-0217: discovered in 2005). Material collected from this locality consisted mainly of fragmentary remains of brontotheriids, equids, and testudines. In 2010, two new fossil localities (BADL-LOC-0260 & 0261) were discovered within the Chadron Formation. Only 5-8 meters of the Peanut Peak Member of the Chadron Formation is exposed at each of the localities, representing a wide range of depositional environments. The lithologies at BADL-LOC-0217 and 0260 are predominantly claystones and mudstones interpreted as floodplain deposits. At BADL-LOC-0261 the lithologies consist of sandstone meandering channel fills with prominent crossbedding, and claystones to mudstones interpreted as floodplain deposits and possible oxbow lake deposits. All three fossil localities preserve numerous identifiable fossil remains. In addition to more common taxa (e.g. Eumys, Palaeolagus, Merycoidodon, Archaeotherium and Mesohippus), these localities also contain material from some of the rarer taxa including Brontops, Trigonias, Colodon, cf. Elomeryx, and Alligator. The majority of recovered specimens were in situ and well preserved, ranging from complete tooth rows to a complete cranium of Brontops. Based on the data collected from these three fossil localities, the Chadron Formation within the North Unit is shown to produce well-preserved specimens of high scientific importance. These new discoveries highlight the need for a comprehensive survey of the Chadron Formation within the North Unit similar to those previously conducted for the Brule Formation (Oligocene). Additionally, further investigation of these localities could provide important information relevant to addressing questions concerning biogeography, biostratigraphy, and the terrestrial faunal Eocene/Oligocene transition.

Microvertebrate sampling methods are often employed to assess various properties of paleoenvironm... more Microvertebrate sampling methods are often employed to assess various properties of paleoenvironments and their associated faunas, including overall taxonomic diversity. Such studies often depend upon the accurate referral of isolated teeth to species or at least higher-level taxa. The referral of isolated dinosaurian teeth to specific clades is generally feasible due to the relative homogeneity of their dentition. However, basal neornithischian taxa (i.e., ‘hypsilophodontids') display marked heterogeneity across their dentition, from their ‘peg-like' premaxillary teeth to their highly modified ‘cheek teeth' (i.e., maxillary and dentary teeth). The plesiomorphic structure of their premaxillary teeth makes them difficult to distinguish them from other archosaurian teeth, especially given the relatively high abundance and diversity of form of crocodylian teeth in many microvertebrate localities. This similarity, combined with the fact that premaxillary teeth in general remain poorly known from most basal neornithischian taxa, could result in the misidentification of isolated premaxillary teeth as crocodylian teeth. To explore this issue, an examination was conducted of fossil material collected from exposures of the Hell Creek Formation within Montana over the past ten years to determine if teeth from the relatively well known basal neornithischian Thescelosaurus were accurately identified and how often premaxillary teeth are recovered. This study revealed that forty percent of the Thescelosaurus teeth collected were premaxillary teeth, and the majority were misidentified as crocodylian teeth or simply not referred to a higher level taxon. Though premaxillary teeth make up around thirteen percent of the total dentition in Thescelosaurus (based on NCSM 15728), they were recovered at a rate three times higher than expected. Thus, the ability to accurately identify and refer isolated premaxillary teeth at least to the clade Neornithischia is critically important, especially when examining material from formations where these taxa were not locally abundant. To address this problem, a list of characteristics is presented that can be used to accurately identify isolated basal neornithischian premaxillary teeth from microvertebrate assemblages.

2009 Portland GSA Annual Meeting, Jan 1, 2009

"The Kaiparowits Formation of southeastern Utah is providing important insights into basal neorni... more "The Kaiparowits Formation of southeastern Utah is providing important insights into basal neornithischian dinosaur evolution within the Western Interior Basin of North America during the Late Cretaceous. Recently recovered specimens provide a wealth of morphological information, facilitating the recognition of a previously undescribed taxon based on three autapomorphic traits present in the skull and forelimb. A phylogenetic analysis places the Kaiparowits taxon in a clade with the North American taxa Orodromeus, Zephyrosaurus, and Oryctodromeus based on the unambiguous presence of two synapomorphies of the sacrum and hindlimb. This new taxon represents the southernmost occurrence of a basal neornithischian during the Late Cretaceous in North America and provides important insights into morphological evolution within this clade, particularly with regards to those traits previously proposed to be linked to fossorial habits.
Material referred to this taxon spans a wide range of sizes (femoral circumference: 31.5 - 77 mm), with smaller material being more common than larger material. At one locality (UMNHVP 12665) material from at least three small individuals is present within a fine sand to silt dominated crevasse splay deposit, as indicated by the presence of three differently sized left femora. Feeding traces are present on two elements recovered from this site: a proximal left scapula and a distal left femur. A tooth crown fragment is embedded within a circular puncture in the anterior surface of the distal femur, the cross-sectional shape of which is consistent with a crocodyliform trace-maker. Teeth are rarely found still embedded in their corresponding feeding traces, and this marks the first time that such tangible evidence linking a crocodyliform to a feeding event has been found. On the lateral surface of the proximal scapula an ovoid bite mark is present that is divided in half about its long axis by a thin subscore that extends slightly margin of the bite mark. This bisected pit is diagnostic of a crocodyliform trace-maker based on actualistic experiments examining the feeding traces of extant taxa. These bite marks provide fascinating insight into the trophic interactions between the small crocodyliforms and dinosaurians of the Kaiparowits Formation.
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Basal ornithischian relationships remain contentious. Further, certain “hypsilophodontid” ornitho... more Basal ornithischian relationships remain contentious. Further, certain “hypsilophodontid” ornithopod taxa appear to disproportionately impact inference of these relationships and, strikingly, also possess the longest ghost lineages within Dinosauria. Whether these long ghost lineages reflect undersampling of the ornithischian fossil record or poor estimation of their phylogenetic relationships has remained unresolved. To address this question, all previously published characters for Ornithischia were evaluated and new characters identified to produce a dataset of more than five-hundred characters, the largest yet assembled to address basal ornithischian relationships. Taxonomic sampling was also significantly increased to include four newly-recognized basal ornithopod taxa from the Cretaceous of North America. Additionally, new information indicates that the terminal taxon “Thescelosaurus” included in prior analyses actually combined data from two distinct species. Phylogenetic analysis of the new dataset recovers novel relationships within and among the major ornithischian taxa and supports the non-monophyly of the Hypsilophodontidae and the Ornithopoda. The latter result is consistent with the findings of several recently published analyses.

The topology recovered from the new analysis, when compared to previously proposed phylogenies of the basal Ornithopoda using measures of stratigraphic fit (i.e., MSM* range, GER range, SCI) is found to be significantly more congruent with the stratigraphic record than preceding phylogenetic hypotheses. A novel method for resolving polytomies when calculating these metrics allowed competing phylogenies containing polytomies to be fully evaluated and the preferred phylogeny accurately selected for the first time. The recognition of exceptionally long ghost lineages and key issues encountered in estimation of the phylogenetic relationships of basal Ornithischia both appear to have been the result of the limited prior taxon sampling and the inclusion of the chimeric terminal taxon to represent Thescelosaurus.

The amphicyonid, or “bear-dog,” Paradaphoenus was a small-bodied, hypercarnivorous carnivoran tha... more The amphicyonid, or “bear-dog,” Paradaphoenus was a small-bodied, hypercarnivorous carnivoran that lived in North America during the Oligocene. Specimens of Paradaphoenus are rare, in part owing to the common inaccurate referral of specimens to the abundant canid Hesperocyon, which was similar in size and morphology. Four new specimens of Paradaphoenus are noted from North Dakota and South Dakota that expand the geographic range of this taxon within the Great Plains region of North America and expand our knowledge of its anatomy. A single specimen from either a transitional Orellan/Whitneyan or early Whitneyan fauna in the Little Badlands area of North Dakota is referred to Paradaphoenus minimus. That taxon is elsewhere restricted to Orellan faunae, making that specimen the youngest occurrence of P. minimus yet recognized. The remaining specimens are from two early Whitneyan faunas, the Cedar Pass local fauna in South Dakota and a new locality within the Little Badlands area of Nort...