Cretaceous Hell Creek Formation Research Articles

Updated cranial and mandibular description of the Late Cretaceous (Maastrichtian) baenid turtle Saxochelys gilberti based on micro-computed tomography scans and new information on the holotype-shell association

Saxochelys gilberti is a baenid turtle from the Late Cretaceous Hell Creek Formation of the United States of America known from cranial, shell, and other postcranial material. Baenid turtles are taxonomically diverse and common fossil elements within Late Cretaceous through Eocene faunas. Detailed anatomical knowledge is critical to understanding the systematics and morphological evolution of the group. This is particularly important as baenids represent an important group of continental vertebrates that survived the mass extinction event associated with the Cretaceous/Paleogene boundary. High-resolution micro-computed tomography scanning of the holotype skull reveals additional anatomical details for the already well-known Saxochelys gilberti. This includes the revision of some anatomical statements from the original description, but also detailed knowledge on internal anatomical features of the braincase and the description of a well-preserved axis (cervical vertebra 2). Our new detailed description and previous work on the shell and postcrania make Saxochelys one of the best-described, nearly complete baenid turtles, which are often only known from either isolated shell or cranial material. A revised phylogenetic analysis confirms the position of Saxochelys gilberti as a derived baenid (Eubaeninae) more closely related to Baena arenosa than to Eubaena cephalica.

New paddlefishes (Acipenseriformes, Polyodontidae) from the Late Cretaceous Tanis Site of the Hell Creek Formation in North Dakota, USA

AbstractThe recently discovered mass mortality of fishes from the Tanis Site in the North Dakota portion of the Late Cretaceous Hell Creek Formation contains many well-preserved, three-dimensional skeletons. Among these are representatives of two acipenseriform families, Acipenseridae (sturgeons) and Polyodontidae (paddlefishes). This paper describes two new monotypic polyodontid genera, expanding our knowledge of polyodontid diversity. The first of the new species described here is †Parapsephurus willybemisi n. gen. n. sp. It is distinguished from all other known species by having a combination of posteriorly elongate parietals, the middle fenestra longitudinalis bordered medially by the parietal and frontal and laterally by the dermopterotic, slender and numerous dorsal caudal fulcra, an elongate hyomandibula that is not hourglass shaped, and gill rakers that are short and widely spaced. The second polyodontid species described here is †Pugiopsephurus inundatus n. gen. n. sp. It is diagnosed by a combination of having stellate bones that are exceptionally poorly developed or absent and having a dermopalatine with a medial expansion and lacking an ectopterygoid process. The two species of paddlefishes described in this paper add to the morphological and taxonomic diversity of Polyodontidae. The presence of these taxa within the Hell Creek Formation hints at substantial diversity of polyodontids at this stage of their evolutionary history.UUID: http://zoobank.org/08cbf8c5-59ba-41fa-9265-8cb762cf7b13

A new pachycephalosaurid from the Hell Creek Formation, Garfield County, Montana, U.S.A.

ABSTRACT A partial skull of a pachycephalosaurid from the Upper Cretaceous Hell Creek Formation, Montana, is interpreted as a new taxon, Platytholus clemensi gen. et sp. nov. MOR 2915 does not fit into an ontogenetic continuum of known pachycephalosaurids from the Hell Creek Formation, Montana, and contemporaneous sediments from the Western Interior. Comparisons to known ontogimorphs of Sphaerotholus and Pachycephalosaurus preclude including this specimen into an ontogenetic series of either taxon. We hypothesize that MOR 2915 is a new species based on a relatively low, broad dome at this advanced ontogenetic age that is neither round nor oval in dorsal view, distinct but fused lateral cranial elements fully incorporated into the dome without any dorsal lobe differentiation, and individual tab-like tubercle ornamentation dorsolaterally. Phylogenetic analysis posits that Platytholus clemensi is a Prenocephale-grade taxon deeply nested within Pachycephalosaurinae, but it is not a member of Pachycephalosaurini. Platytholus clemensi is intermediate in size between the other contemporaneous pachycephalosaurids in the Hell Creek Formation and suggests a diverse set of taxa-partitioned ecological niches by body size. We confirm a well organized, major internal vascular network using high resolution computed tomography. Foramina present on the orbital roofs indicate these canals penetrated the entire ceiling of the orbits within the frontal and supraorbital bones. Abundant neurovascular canals passing through the dome to the ectocranial surface indicate a keratinous structure of some kind, possibly with a vertical structural framework, was present on the dome. We review the history of the head-butting hypothesis and associated behavioral implications.

Late Cretaceous sturgeons (Acipenseridae) from North America, with two new species from the Tanis site in the Hell Creek Formation of North Dakota

AbstractAlthough the Cretaceous is widely regarded as a time of great evolutionary transition for the freshwater fish fauna of North America, the fossil record of this period is notoriously poor, consisting mostly of fragments and isolated skeletal elements. Exceptions include the acipenseriforms, discussed in this paper, and some exceedingly rare teleosts. Here we describe two new species of well-preserved sturgeons (Acipenseridae) from the Tanis site in the Late Cretaceous Hell Creek Formation of North Dakota. The type and referred materials were preserved in a loosely consolidated matrix. †Acipenser praeparatorum n. sp. is represented by multiple body fossils (including the head and relatively complete postcranial remains) and a specimen of an intact, three dimensionally preserved skull and pectoral girdle. This taxon can be diagnosed based on features of the opercular elements (exceptionally tall and narrow branchiostegal). The second species, †Acipenser anisinferos n. sp., is represented by a partially preserved skull, and can be diagnosed by a relatively elongate preorbital region (i.e., snout) and the absence of thorn-like spines on the skull roofing bones. Most known sturgeon fossils from the Cretaceous are represented only by undiagnosable fragmentary remains (i.e., scutes and pectoral-fin spines) or poorly preserved partial skeletons (e.g., †Protoscaphirhynchus), with †Priscosturion and †Anchiacipenser (both monotypic) being rare exceptions. Therefore, the newly discovered Tanis fossils give a rare glimpse into the evolution of Acipenseridae at a critical time in the phylogenetic history of acipenseriforms, and suggest significant morphological and taxonomic diversity early in the evolution of this group.UUID: http://zoobank.org/375b586a-2dd8-4a31-b6c4-42151e6e8fc8

A chasmosaurine ceratopsid premaxilla from the basal sandstone of the Hell Creek Formation, Montana

A well-preserved large chasmosaurine ceratopsid premaxilla (MOR 1122 7-22-00-1) collected from the basal sandstone of the Cretaceous Hell Creek Formation (HCF) represents one of the stratigraphically lowest ceratopsid occurrences in the formation. The specimen was discovered in 2000, during the excavation of a large Torosaurus latus skull (MOR 1122) which was later hypothesized to represent an advanced growth stage of the more commonly recovered HCF ceratopsid Triceratops. MOR 1122 7-22-00-1 compares favorably with the incomplete premaxillae of the MOR 1122 skull and reveals details of premaxilla morphology from this stratigraphic zone. It preserves large, closely spaced ventromedial foramina, a narrow triangular process, and a thin septal flange at the base of the narial strut. The nasal process is narrow, caudally inclined and has a forked dorsal surface which appears to represent an intermediate between the morphology expressed in the slightly stratigraphically lower ceratopsid Eotriceratops xerinsularis from the Horseshoe Canyon Formation of Alberta and specimens recovered higher in the HCF. MOR 1122 7-22-00-1 expresses a deep recess extending medial to the strut of the triangular process, a feature shared with other HCF ceratopsid specimens but not Eotriceratops or other earlier occurring triceratopsin taxa. The morphology of MOR 1122 7-22-00-1 is consistent with noted stratigraphic trends in HCF ceratopsids and highlights the increased complexity of the narial region in uppermost Cretaceous triceratopsins.

Molecular tests support the viability of rare earth elements as proxies for fossil biomolecule preservation

The rare earth element (REE) composition of a fossil bone reflects its chemical alteration during diagenesis. Consequently, fossils presenting low REE concentrations and/or REE profiles indicative of simple diffusion, signifying minimal alteration, have been proposed as ideal candidates for paleomolecular investigation. We directly tested this prediction by conducting multiple biomolecular assays on a well-preserved fibula of the dinosaur Edmontosaurus from the Cretaceous Hell Creek Formation previously found to exhibit low REE concentrations and steeply-declining REE profiles. Gel electrophoresis identified the presence of organic material in this specimen, and subsequent immunofluorescence and enzyme-linked immunosorbant assays identified preservation of epitopes of the structural protein collagen I. Our results thereby support the utility of REE profiles as proxies for soft tissue and biomolecular preservation in fossil bones. Based on considerations of trace element taphonomy, we also draw predictions as to the biomolecular recovery potential of additional REE profile types exhibited by fossil bones.

Euselachian diversity through the uppermost Cretaceous Hell Creek Formation of Garfield County, Montana, USA, with implications for the Cretaceous-Paleogene mass extinction in freshwater environments

Temporal changes in the diversity of euselachians (e.g., sharks and rays) across the Cretaceous-Paleogene (K-Pg) boundary are not well understood, particularly from freshwater ecosystems. Here, we quantitatively analyze euselachian diversity during the last ca. 2 Ma of the Cretaceous using 1518 teeth from 40 vertebrate microfossil localities within the nonmarine facies of the Hell Creek Formation, northeastern Montana, USA. We identify 10 euselachians including one hybodont, five orectolobiforms, one lamniform, one sclerorhynchiform, and two rajiforms. Among these, two are novel and described herein. Diversity metrics reveal an increase in species richness and heterogeneity from the lower to middle portions of the Hell Creek Formation. Thereafter, diversity remained elevated and stable with no turnover until ~5 m below the K-Pg boundary. Above this horizon, including the last ~50 kyr of the Cretaceous, raw species richnesses dropped precipitously, and all euselachians (except possibly Myledaphus) went locally extinct across the boundary. Preceding this drop in richness, changes in euselachian community structure occurred, including steady declines in the relative abundances of M. pustulosus. These patterns do not support regression of the Western Interior Seaway as the single proximal cause of euselachian extinctions across the K-Pg boundary. Rather, euselachian local extinctions likely were the result of the multiple environmental perturbations occurring just before (volcanism, climate change) and at the K-Pg boundary (bolide impact). This high-resolution temporal pattern of euselachian diversity adds to those from other local taxa (e.g., mammals, lissamphibians) to present a more complex view of the K-Pg mass extinction of the continental biota.

What factors influence our reconstructions of Morrison Formation sauropod diversity?

The Upper Jurassic Morrison Formation classically represents the “Golden Age” of sauropods, and the Morrison Formation is reported to have yielded 13 genera and 24 species of sauropods. This incredible diversity has produced numerous theories attempting to reconcile the co-occurrence of such large, and similar taxa. Previously, a comparably high diversity has been proposed for the Late Cretaceous Hell Creek Formation of North America – possibly comprising nearly three dozen species from over 20 genera of ankylosaura, caenagnathids, ceratopsians, dromaeosaurids, hadrosaurs, ornithomimids, pachycephalosaurs, thescelosaurs, and tyrannosaurs. However, much of the morphologic variation previously ascribed to taxonomic differences has recently been shown to be a result of stratigraphy and/or ontogeny – resulting in this rich assemblage being downsized to 13 genera and 16 species. Whereas still rich in diversity, such factors have an immediate effect towards our reconstruction of true richness.Following the example of the Hell Creek Formation, we can investigate the ontogenetic and strati-graphic origin of possible diversity inflation in other formations, and within this study, apply it to the Morrison Formation. New dating techniques are resulting in finer temporal resolution, and are changing the temporal position of well-known quarries. Differences in body size and ontogenetic stages can also affect diversity estimates. Plotting body size stratigraphically, it initially appears that larger specimens (interpreted as different species) occur higher in the section. An increase in average body size may be a legitimate trend, but there are several specimens that counter this “rule” for many genera. Likewise, dramatic allometric ontogenetic trajectories have led to the erection of at least three diplodocid genera – Amphicoelias, Seismosaurus, and Suuwassea – and it is suspected that many more Morrison Formation “species” could alternatively be explained as ontogimorphs. We have a long way to go towards revealing the true nature of Morrison Formation sauropod diversity. Although dietary partitioning undoubtedly occurred at the level of both the species (e.g., Brachiosaurus vs. Diplodocus) and between ontogenetic stages, a base of 24 levels of co-occurring divisions seems unlikely. The Morrison Formation may have exhibited a sauropod-rich assemblage unlike any other in North America, and the implications of stratigraphy, ontogeny, and variation may be minor, yet these factors alter perceived “diversity.” True diversity will not be fully understood unless these factors are considered.

Geochemical taphonomy of the Standing Rock Hadrosaur Site: Exploring links between rare earth elements and cellular and soft tissue preservation

The Standing Rock Hadrosaur Site (SRHS) in South Dakota preserves a mass-death assemblage of the hadrosaurine dinosaur Edmontosaurus annectens in the Upper Cretaceous Hell Creek Formation. A previous study of SRHS bones found many of them to yield microstructures morphologically consistent with vertebrate osteocytes, blood vessels, and proteinaceous matrix upon demineralization with weak acid. To examine the geochemical history of these specimens in detail, we herein document the intra-bone distribution of rare earth elements (REE) within nine of these previously-examined Edmontosaurus limb bones from SRHS. Laser ablation-inductively coupled plasma mass spectrometry was used to determine the trace element composition of the specimens, which revealed the bones to possess unusually low REE concentrations for bones of Mesozoic age, consistent and steep declines in light REE concentrations with increasing cortical depth, and uniformly low concentrations of middle REE and elements with moderate and low diffusivities through the middle cortex. These attributes imply: (1) at least partial preservation of early-diagenetic trace element signatures; (2) that trace element uptake occurred primarily during a single, early-diagenetic phase, and; (3) that the duration of trace element diffusion was brief in these bones. As we previously reported, low permeability of the fine-grain host matrix and early-diagenetic cementation of portions of the host matrix by precipitation of siderite concretions impeded pore fluid replenishment; this partial hindrance of fluid flow was also likely responsible for the minimal trace element uptake observed in the nine SRHS specimens further studied herein. From a paleontological perspective, minimal trace element uptake reflects minimal chemical alteration, meaning bones from this site are predicted to be ideal candidates for preservation of original organics according to hypotheses previously advanced by geochemical studies. Because low REE concentrations reflect limited interaction with groundwaters during diagenesis, we propose that late-diagenetic overprinting of early-diagenetic REE profiles was minimal and that hydrolytic degradation of soft tissues was limited in the burial environment at SRHS. Mitigation of biomolecular decay by hydrolysis would favor ultrastructure stabilization and is therefore advanced as a partial explanation for the preservation of cells and soft tissue microstructures in these fossil bones.

Tooth replacement in a specimen of Tyrannosaurus rex (Dinosauria, Theropoda) from the Hell Creek Formation (Maastrichtian), Montana

ABSTRACT A well-preserved skull of Tyrannosaurus rex from the Late Cretaceous (Maastrichtian) Hell Creek Formation of Montana is used to document its tooth replacement with the help of high-resolution medical CT scanning. A pattern of retaining only one single replacement tooth before replacing the functional tooth is reconstructed for most alveoli of the jaws, with exception of the second alveolus of the left maxilla, and the fourth alveolus of the right dentary, where another, second replacement tooth can be seen. The youngest replacement teeth have only a thin layer of enamel around the tooth crown, the larger replacement teeth have thicker enamel, fortified at the apex as in the functional teeth. The Zahnreihen approach allows to reconstruct a z-spacing of 2 in the upper jaws, whereas the right dentary displays a z-spacing of 1 in the rostral and distal third of the tooth row and a z-spacing of 2 in its medial part. In MB.R.91216, teeth of the upper jaws and the medial part of the dentary were probably replaced in a regular alternating pattern between odd and even teeth, whereas the mesial and distal teeth of the dentary were replaced simultaneously.

A new baenid turtle, Saxochelys gilberti, gen. et sp. nov., from the uppermost cretaceous (Maastrichtian) Hell Creek Formation: sexual dimorphism and spatial niche partitioning within the most speciose group of Late Cretaceous turtles

ABSTRACTThe uppermost Cretaceous (Maastrichtian) Hell Creek Formation in North America has yielded a diverse assemblage of fossil turtles dominated by baenids. A population of over 30 individual skeletons from the Hell Creek Formation of North Dakota comprises a new baenid turtle, Saxochelys gilberti, increasing the number of recognized Hell Creek turtles to at least 26, 11 of which are baenids. Saxochelys gilberti is differentiated from all other baenids based on the presence of a nuchal scale, finely sculptured shell, absence of an omega-shaped femoral-anal sulcus, presence of an omega-shaped extragular-humeral sulcus, a cavum tympani that is relatively smaller than the diameter of the orbit, presence of a robust single scale on the posteromedial portion of the parietals, and jugal exclusion from the orbital margin. Two distinct plastral morphologies, concave (N = 8) and flat (N = 9), are interpreted as representing male and female individuals, respectively. A ca. 10% size difference between the sexes indicates that S. gilberti is a sexually size-dimorphic turtle with larger females. This suggests female mate choice in this baenid turtle. A referred S. gilberti skeleton from above the Cretaceous–Paleogene boundary provides evidence for the survival of yet another species of turtle across this extinction event. The type locality of S. gilberti is close both stratigraphically and geographically to another locality consisting of a completely different assortment of baenid species, suggesting a fine degree of spatial niche partitioning in baenid turtles that may help explain the extremely high diversity of the group within the Hell Creek Formation.

NO LARGE BIAS WITHIN SPECIES BETWEEN THE RECONSTRUCTED AREAS OF COMPLETE AND FRAGMENTED FOSSIL LEAVES

Research Article| January 21, 2019 NO LARGE BIAS WITHIN SPECIES BETWEEN THE RECONSTRUCTED AREAS OF COMPLETE AND FRAGMENTED FOSSIL LEAVES ERIC R. HAGEN; ERIC R. HAGEN 1Wesleyan University, Department of Earth and Environmental Sciences, Middletown, Connecticut 06459, USA Search for other works by this author on: GSW Google Scholar DANA L. ROYER; DANA L. ROYER 1Wesleyan University, Department of Earth and Environmental Sciences, Middletown, Connecticut 06459, USA Search for other works by this author on: GSW Google Scholar RYAN A. MOYE; RYAN A. MOYE 1Wesleyan University, Department of Earth and Environmental Sciences, Middletown, Connecticut 06459, USA Search for other works by this author on: GSW Google Scholar KIRK R. JOHNSON KIRK R. JOHNSON 2Smithsonian Institution, National Museum of Natural History, Washington, D.C. 20013, USA Search for other works by this author on: GSW Google Scholar Author and Article Information ERIC R. HAGEN 1Wesleyan University, Department of Earth and Environmental Sciences, Middletown, Connecticut 06459, USA DANA L. ROYER 1Wesleyan University, Department of Earth and Environmental Sciences, Middletown, Connecticut 06459, USA RYAN A. MOYE 1Wesleyan University, Department of Earth and Environmental Sciences, Middletown, Connecticut 06459, USA KIRK R. JOHNSON 2Smithsonian Institution, National Museum of Natural History, Washington, D.C. 20013, USA email: ehagen@wesleyan.edu Publisher: SEPM Society for Sedimentary Geology First Online: 21 Jan 2019 Online Issn: 1938-5323 Print Issn: 0883-1351 Copyright © 2019, SEPM (Society for Sedimentary Geology) PALAIOS (2019) 34 (1): 43–48. https://doi.org/10.2110/palo.2018.091 Article history First Online: 21 Jan 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ERIC R. HAGEN, DANA L. ROYER, RYAN A. MOYE, KIRK R. JOHNSON; NO LARGE BIAS WITHIN SPECIES BETWEEN THE RECONSTRUCTED AREAS OF COMPLETE AND FRAGMENTED FOSSIL LEAVES. PALAIOS 2019;; 34 (1): 43–48. doi: https://doi.org/10.2110/palo.2018.091 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract Taphonomic processes may filter in a biased manner the tiny fraction of leaves preserved as fossils. A common perception is that large leaves are underrepresented; this is based both on intuition (large leaves are more likely to break apart) and some observations of extant vegetation. Characterizing leaf area correctly is critical for reconstructing climate and for studying evolutionary and biogeographic patterns. In extant dicotyledonous angiosperms, leaf area generally scales with the inverse of second-order vein density. This scaling offers the potential to test if fossil leaf fragments were derived from leaves that were larger than complete (or nearly complete) fossil leaves of the same species. Here we test vein scaling on 573 complete leaves from the latest Cretaceous Hell Creek Formation and earliest Paleocene Fort Union Formation in the Williston Basin of western North and South Dakota. We find a strong scaling similar to extant vegetation, with a somewhat shallower slope (1.67 vs. 2.04) and lower r2 (0.64 vs. 0.80). We apply these two scalings to 41 species-site pairs from the Williston Basin that are each represented by complete (n = 355) and fragmented (n = 387) leaves. With both scalings, the reconstructed leaf areas of fragments are on average 10% larger (±36% 1σ) than their complete companions. This small but noisy signal means that the underrepresentation of large leaves, as captured by our study design, is probably not critical for most fossil applications. Comparing directly the reconstructed areas of complete and fragmented leaves appears reasonable, thus expanding the usefulness of fossil leaf fragments. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dental Structure of the Late Cretaceous (Maastrichtian) Guitarfish (Neoselachii: Batoidea)Myledaphus pustulosusfrom the Hell Creek Formation of Garfield County, Montana

In this study, we examine surface collected teeth of Myledaphus pustulosus from the Upper Cretaceous (Maastrichtian) Hell Creek Formation of Garfield County, Montana. Macroscopic analysis of teeth shows that 54% of the teeth have pronounced attritional wear due to a durophagous diet. A higher percentage of unworn teeth than worn teeth (64% vs 27%) have acid damage due to predation or scavenging. Ultrastructural analysis of whole and sectioned teeth show that the enameloid of the teeth is 100–125 µm thick and is composed of a single crystallite enameloid in which apatite crystallites show no preferred orientation. Orthodentine in the tooth crown is composed of dense material with numerous canals for odontoblast processes which extend nearly to the tooth surface. Dentinal tubules surround odontoblasts most of the way through the enameloid. The enameloid microstructure of M. pustulosus is consistent with the pattern seen in other batoids.

The shell morphology of the latest Cretaceous (Maastrichtian) trionychid turtle Helopanoplia distincta.

BackgroundHelopanoplia distincta is an extinct soft-shelled turtle (Pan-Trionychidae) for which the type specimen is a fragmentary costal and the inguinal notch portion of the left hypoplastron from the Late Cretaceous (Maastrichtian) Lance Formation of Wyoming, USA that bear a distinct surface sculpture pattern consisting of raised tubercles. Over the course of the past few decades, a number of additional, fragmentary specimens from the Late Cretaceous (Maastrichtian) Hell Creek Formation of Montana and North Dakota have been referred to this taxon based on the presence of these tubercles, but a more complete understanding of the anatomy and phylogenetic relationships of this distinctive soft-shelled turtle is still outstanding.MethodsWe here figure and describe shell remains of eight fossils referable to Helopanoplia distincta from the Hell Creek Formation of Montana and North Dakota that, in combination, document nearly all aspects of the shell morphology of this taxon. We furthermore explore the relationships of this fossil turtle by inserting it into a modified phylogenetic analysis of pan-trionychid relationships.ResultsThe new fossil material thoroughly supports the validity of Helopanoplia distincta. In addition to its unique surface sculpture pattern, this turtle can be diagnosed relative to all other named pan-trionychids by the presence of a distinct corner along the margin of costals II, the complete covering of costal ribs I–VI by metaplastic bone, midline contact of the main plastral elements, hyoplastral shoulder, presence of a lateral, upturned margin on the hyo/hypoplastron that is covered dorsally and laterally by sculptured metaplastic bone, a single, lateral hyoplastral process, and the apomorphic presence of fine scallops along the margin of costals VIII, formation of a laterally embraced, rounded nuchal, anteriorly rounded costals I, distally expanded costals II, and narrow costals VII. A phylogenetic analysis places Helopanoplia distincta as sister to the clade formed by Plastomenus thomasii and Hutchemys spp., thereby confirming its identity as a plastomenid. The vast majority of Helopanoplia distincta material has been recovered from fine-grained overbank deposits, thereby supporting the hypothesis that this turtle favored ponded waters.

New cranial material of Gilmoremys lancensis (Testudines, Trionychidae) from the Hell Creek Formation of southeastern Montana, U.S.A.

ABSTRACTPlastomenidae is a speciose clade of soft-shelled turtles (Trionychidae) known from Campanian to Eocene deposits throughout western North America. We here describe two large skulls from the Upper Cretaceous (Maastrichtian) Hell Creek Formation of Carter County, Montana, that document the adult morphology of the plastomenid Gilmoremys lancensis. Whereas juveniles of this species, as previously documented by five subadult skulls, have narrow skulls, a narrow processus trochlearis oticum, a deep and narrow median palatal groove, low accessory ridges, and a secondary palate fully formed by the maxilla, skeletally mature individuals have notably broad skulls, a broad processus trochlearis oticum, a shallow but broad median palatal groove, high accessory ridges, and a substantial contribution of the vomer to the secondary palate. An expanded phylogenetic analysis reveals that the Campanian Aspideretoides foveatus and the Paleocene Aspideretoides superstes, nov. comb., are situated within Plastomenidae as sister to all previously identified plastomenid turtles, despite their general resemblance to trionychine soft-shelled turtles. The name Aspideretoides should therefore not be used as a taxonomic wastebasket for fossil trionychids with unclear phylogenetic relationships.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Joyce, W. G., T. R. Lyson, and S. Williams. 2016. New cranial material of Gilmoremys lancensis (Testudines, Trionychidae) from the Hell Creek Formation of southeastern Montana, U.S.A. Journal of Vertebrate Paleontology, e1225748. DOI: 10.1080/02724634.2016.1225748.

Fossil egg and eggshells from the Upper Cretaceous Hell Creek Formation, Montana

ABSTRACTDespite over a hundred years of intense paleontological exploration, the terrestrial rocks of the Upper Cretaceous (Maastrichtian) Hell Creek Formation of eastern Montana are remarkable for the absence of fossil eggs. Here, we describe the first fossil egg and additional eggshell fragments from the formation. The two-layered structure of the smooth 1600-µm-thick eggshell of Belonoolithus garbani, oogen. et oosp. nov., permits assignment of the 6 cm × 8 cm egg to Theropoda. Additional theropod eggshells, Dimorphoolithus bennetti, oogen. et oosp. nov., closely resemble Tubercuoolithus choteauensis (incertae sedis) from the Campanian Two Medicine Formation of Montana, differing only in ornamentation. Therefore, we establish Tubercuoolithidae, oofam. nov., to accommodate both eggshell types. Other eggshell types include two varieties of Spheroolithus, as well as Testudoolithus and Krokolithes. Whereas B. garbani and D. bennetti are unique to the Hell Creek Formation, the other three oogenera also occur in the stratigraphically older Two Medicine and Judith River formations of Montana, and the Oldman Formation of southern Alberta, Canada.Citation for this article: Jackson, F. D., and D. J. Varricchio. 2016. Fossil egg and eggshells from the Upper Cretaceous Hell Creek Formation, Montana. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1185432.

The furculae of the dromaeosaurid dinosaur Dakotaraptor steini are trionychid turtle entoplastra.

Dakotaraptor steini is a recently described dromaeosaurid dinosaur from the Upper Cretaceous (Maastrichtian) Hell Creek Formation of South Dakota. Included within the D. steini hypodigm are three elements originally identified as furculae, one of which was made part of the holotype specimen. We show that the elements described as D. steini ‘furculae’ are not theropod dinosaur furculae, but are rather trionychid turtle entoplastra referable to cf. Axestemys splendida. The hypodigm of D. steini should be adjusted accordingly.

The early expression of squamosal horns and parietal ornamentation confirmed by new end-stage juvenile Pachycephalosaurus fossils from the Upper Cretaceous Hell Creek Formation, Montana

ABSTRACTNew end-stage juvenile specimens of Pachycephalosaurus from the Upper Cretaceous Hell Creek Formation, Montana, confirm the earliest expression of squamosal nodes, parietal ornamentation, and jugal morphology in the smallest and presumably youngest individuals yet known. High-resolution computed tomography of the slightly thickened, undomed parietal reveals a dense cortex, a highly cancellous interior of irregularly shaped erosion cavities, and bony trabeculae indicative of primary, fast growing bone. The parietal, with its highly ornamented septum morphology and patent sutures, is nearly identical to the holotype of ‘Dracorex hogwartsia,’ and combined with these new internal histological details, supports the alternative interpretation that ‘D. hogwartsia’ is a juvenile Pachycephalosaurus wyomingensis. The squamosal nodes grow into an array of horns and secondary nodes exemplified by the pachycephalosaurin ‘Stygimoloch spinifer’ considered in this study to be a subadult P. wyomingensis. Unlike the squamosal ornamentation, the hypertrophied midline row of parietal nodes is transient as the frontoparietal dome expands later in ontogeny. We propose the term ‘ontogimorph’ as a substitute for ‘semaphoront’ to describe these taxon-specific morphological variants that grow allometrically and express extreme cranial morphology along a postnatal growth continuum ontogenetically. These juvenile-, sub-adult-, and adult-specific features in the skull of Pachycephalosaurus may have allowed the visual identification of ontogimorphs and signal their changing sociobiological status.Citation for this article: Goodwin, M. B., and D. C. Evans. 2016. The early expression of squamosal horns and parietal ornamentation confirmed by new end-stage juvenile Pachycephalosaurus fossils from the Upper Cretaceous Hell Creek Formation, Montana. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1078343.

Evolutionary trends in Triceratops from the Hell Creek Formation, Montana.

The placement of over 50 skulls of the well-known horned dinosaur Triceratops within a stratigraphic framework for the Upper Cretaceous Hell Creek Formation (HCF) of Montana reveals the evolutionary transformation of this genus. Specimens referable to the two recognized morphospecies of Triceratops, T. horridus and T. prorsus, are stratigraphically separated within the HCF with the T. prorsus morphology recovered in the upper third of the formation and T. horridus found lower in the formation. Hypotheses that these morphospecies represent sexual or ontogenetic variation within a single species are thus untenable. Stratigraphic placement of specimens appears to reveal ancestor-descendant relationships. Transitional morphologies are found in the middle unit of the formation, a finding that is consistent with the evolution of Triceratops being characterized by anagenesis, the transformation of a lineage over time. Variation among specimens from this critical stratigraphic zone may indicate a branching event in the Triceratops lineage. Purely cladogenetic interpretations of the HCF dataset imply greater diversity within the formation. These findings underscore the critical role of stratigraphic data in deciphering evolutionary patterns in the Dinosauria.

A New Large-Bodied Oviraptorosaurian Theropod Dinosaur from the Latest Cretaceous of Western North America

The oviraptorosaurian theropod dinosaur clade Caenagnathidae has long been enigmatic due to the incomplete nature of nearly all described fossils. Here we describe Anzu wyliei gen. et sp. nov., a new taxon of large-bodied caenagnathid based primarily on three well-preserved partial skeletons. The specimens were recovered from the uppermost Cretaceous (upper Maastrichtian) Hell Creek Formation of North and South Dakota, and are therefore among the stratigraphically youngest known oviraptorosaurian remains. Collectively, the fossils include elements from most regions of the skeleton, providing a wealth of information on the osteology and evolutionary relationships of Caenagnathidae. Phylogenetic analysis reaffirms caenagnathid monophyly, and indicates that Anzu is most closely related to Caenagnathus collinsi, a taxon that is definitively known only from a mandible from the Campanian Dinosaur Park Formation of Alberta. The problematic oviraptorosaurs Microvenator and Gigantoraptor are recovered as basal caenagnathids, as has previously been suggested. Anzu and other caenagnathids may have favored well-watered floodplain settings over channel margins, and were probably ecological generalists that fed upon vegetation, small animals, and perhaps eggs.