New Mexico Museum of Natural History and Science

The Tyrannosauridae emerged as the dominant large predators in Laurasia during the Late Cretaceous. Their evolution in North America culminated with the replacement of Albertosaurinae, Daspletosaurinae, and Teratophonei, with masses of 2-3 tonnes, by the giant Tyrannosaurus in the late Maastrichtian, with a mass approaching or exceeding 10 tonnes. The origin of Tyrannosaurus remains enigmatic, but fossils suggest an origin in the Campanian-Maastrichtian of southern Laramidia. Here we describe an unusually large and robust tyrannosaurid tibia from the late Campanian aged Hunter Wash Member of the Kirtland Formation, New Mexico, ~ 74 Ma. The tibia measures 960 mm in length and 128 mm in diameter, ~84% and 78% the dimensions of the largest known Tyrannosaurus, suggesting a mass approaching 5 tonnes, larger than any contemporary tyrannosaur. The Hunter Wash tyrannosaur could potentially represent (i) an unusually large and robust Bistahieversor, (ii) a previously unknown lineage of giant tyrannosaur, or (iii) an early representative of Tyrannosaurini. The tibia's large size, robust proportions, and shape of the distal shaft are most consistent with referral to Tyrannosaurini. The Hunter Wash tyrannosaur emphasizes the marked endemicity of Laramidian dinosaurs; while smaller Albertosaurinae and Daspletosaurini inhabited the north, giant tyrannosaurins occurred in the south.

Abstract Numerous early Cenozoic transient warming events (“hyperthermals”) have been recognized in the marine record, but few have been located in fossiliferous terrestrial strata. Locating warming events is critical for understanding how terrestrial animals and plants responded to past climate change. We recognize two carbon isotope excursions (CIEs) in lower Paleocene (Danian) strata of the San Juan Basin, New Mexico, United States, based on sustained decreases in carbon isotope values in bulk organic carbon. The upper excursion correlates closely with the Latest Danian Event (LDE) hyperthermal, previously known with confidence only from the marine record. We correlate the lower excursion to a smaller CIE in the marine record tied to a 405,000-year eccentricity cycle, and modest warming. Both excursions in the San Juan Basin occur in red bed intervals bounded by drab-colored rocks, suggesting transient seasonal drying. Both are also associated with marked decreases in percent total organic carbon, possibly representing decreased primary productivity. The lower excursion is associated with a strong pulse of mammalian faunal turnover. Strata above the LDE are unfossiliferous, precluding local turnover analysis, but a strong, regional pulse of turnover occurs at or near the LDE. Our results suggest warming and/or drying during these events resulted in ecological perturbations, accelerating mammalian origination and extinction rates. Additionally, the first representatives of two North American mammalian families (Arctocyonidae, Plesiadapidae) reached Europe around the time of the LDE. We hypothesize that this was in response to short-term warming, which opened a previously impassable high-latitude dispersal route connecting the continents.

Compsemys is an enigmatic turtle first described over 160 years ago. Its taxonomic position has been uncertain, as many of the species placed in the genus have since been removed from it. For the last 30+ years, Compsemys victa has been the only recognized North American fossil species. New fossil specimens collected from the Paleocene in the United States necessitate a re-evaluation of the taxonomy of these turtles and call into question the long-held belief that only one species of Compsemys is valid in North America. Specimens from the Paleocene are far more complete than the holotype of C. victa, and all have been referred to this species based on a similar surface texture. Differences between material from the Cretaceous and Paleocene suggest multiple species of Compsemys are valid. In addition to the genotypic species Compsemys victa, C. puercensis, and C. torrejonensis are both valid species from the Paleocene. Beyond surface sculpturing and texture, features of the carapace and plastron can be used to help differentiate the three species. North American Compsemys survived the Cretaceous–Paleogene mass extinction. Rather than being a monospecific genus, Compsemys diversified in the Paleocene. Not only does this suggest an increase in the diversity of North American compsemydids, but it also leads to the possibility of asymmetric or budding cladogenesis among Compsemys species during the Late Cretaceous and Paleocene, at least in the American Southwest.

An isolated left frontal from the Upper Cretaceous (upper Campanian) strata of the San Juan Basin in northwestern New Mexico, USA, was originally identified as a species of the dromaeosaurid (Theropoda: Dromaeosauridae) Saurornitholestes (“S.” robustus) and was subsequently identified as a troodontid (Troodontidae). A reassessment of this holotype specimen shows its distinct nature among troodontids and is here named Dinevenator, gen. nov. Dinevenator robustus differs from other troodontids, including the thick-skulled species Xenovenator espinosai from the upper Campanian Cerro del Pueblo Formation, Coahuila, Mexico. These differences include: dorsoventrally thick and robust frontals; frontals thicker posteriorly; relatively flat frontals, lacking a well-developed parasagittal trough and sagittal ridge; a posteromedial nasal projection between the frontals that is wider than long; a more laterally oriented posterior portion of the lateral endocranial wall; and a mediolaterally wider anteromedial portion of the frontal that houses the orbital fossa. A phylogenetic analysis was also run based on a modified dataset, recovering Dinevenator robustus in a clade of robust troodontins (Troodontini) as the basal-most taxon and as the sister taxon to (Albertavenator curriei + Xenovenator espinosai). The recognition of a distinct, large, robust-skulled troodontid also increases the known diversity of theropod dinosaurs from the southern United States and provides further evidence for dinosaurs that are unique to the Kirtlandian faunas of New Mexico during the Late Cretaceous.

Reported trace fossils from the Clifton Formation, New Brunswick, Canada, are limited to a single occurrence of a temnospondyl trackway and invertebrate trackways identified in the Pennsylvanian continental strata. Our study builds on this with a description of a horizontal burrow interpreted to have been constructed by a fossorial tetrapod. Historically, limited focus has been placed on tetrapod burrow activity and its ichnotaxonomy, so our findings also have broader implications. We report on the first horizontal burrow collected from the Clifton Formation near the township of Stonehaven, with unique features that include transversely oriented angular mudstone intraclasts without a meniscate backfill. We propose a new ichnotaxon, Thrasmaichnus hindsi igen. et isp. nov., and interpret the trace as having been made by a fossorial tetrapod. Thrasmaichnus hindsi is the first described Pennsylvanian-age tetrapod burrow, whereas Pennsylvanian-age fossorial tetrapods are currently only known from body fossils.