Thomas E Williamson

New Mexico Museum of Natural History and Science · Research and Collections

Non-avian dinosaurs went extinct 66 million years ago, geologically coincident with the impact of a large bolide (comet or asteroid) during an interval of massive volcanic eruptions and changes in temperature and sea level. There has long been fervent debate about how these events affected dinosaurs. We review a wealth of new data accumulated over...

Metatherians, which comprise marsupials and their closest fossil relatives, were one of the most dominant clades of mammals during the Cretaceous and are the most diverse clade of living mammals after Placentalia. Our understanding of this group has increased greatly over the past 20 years, with the discovery of new specimens and the application of...

Mammals are the most encephalized vertebrates, with the largest brains relative to body size. Placental mammals have particularly enlarged brains, with expanded neocortices for sensory integration, the origins of which are unclear. We used computed tomography scans of newly discovered Paleocene fossils to show that contrary to the convention that m...

Mammals exhibit vast ecological diversity, including a panoply of locomotor behaviours. The foundations of this diversity were established in the Mesozoic, but it was only after the end-Cretaceous mass extinction that mammals began to increase in body size, diversify into many new species and establish the extant orders. Little is known about the p...

Significance Molecular (DNA) studies suggest that birds radiated rapidly in the wake of the Cretaceous–Paleogene mass extinction (66 Ma), diversifying into nearly all the major groups we recognize today. However, fossil evidence for this pattern has been difficult to find because of the poor fossilization potential of small, delicate-boned birds. W...

Following the end-Cretaceous mass extinction, mammals underwent an increase in body size, taxonomic diversity and ecological specialization throughout the Paleocene, exemplifying their adaptability. One especially enigmatic group is the “Triisodontidae”, medium- to large-sized ungulate-like placentals from the Paleocene which are best known from th...

Conoryctes comma is a member of the enigmatic group Taeniodonta, Paleogene mammals that have been found only in North America. Taeniodonts were part of the first wave of placental mammal diversification after the end-Cretaceous extinction. The lack of postcranial elements has limited the understanding of the anatomy and locomotion of Conoryctes, an...

Ever since the discovery of Macrauchenia patachonica by Charles Darwin in 1834, the affinities of litopterns—a group of extinct South American Native Ungulates (SANUs)—have been elusive. In particular, the interfamilial relationships and timing of the familial diversification within the order Litopterna have not been addressed with adequate taxon a...

A new, exceptionally well-preserved articulated skull roof and braincase of Melvius chauliodous, from the Kirtland Formation of New Mexico, reveals new features that provide detailed osteological information for Melvius, thus allowing for a reassessment of characters that defines the genus, its two recognized species M. chauliodous and M. thomasi,...

After successfully diversifying during the Paleocene, the descendants of the first wave of mammals that survived the end-Cretaceous mass extinction waned throughout the Eocene. Competition with modern crown clades and intense climate fluctuations may have been part of the factors leading to the extinction of these archaic groups. Why these taxa wen...

It has long been debated why groups such as non-avian dinosaurs became extinct whereas mammals and other lineages survived the Cretaceous/Paleogene mass extinction 66 million years ago. We used Markov networks, ecological niche partitioning, and Earth System models to reconstruct North American food webs and simulate ecospace occupancy before and a...

The Late Cretaceous dinosaur Tyrannosaurus rex was recently split into three species based on the premise that variation in the T. rex hypodigm is exceptional, indicating cryptic species and “robust” and “gracile” morphs. The morphs are based on proportional ratios throughout the skeleton. The species are claimed to be stratigraphically separate, w...

After the end-Cretaceous extinction, placental mammals quickly diversified1, occupied key ecological niches2,3 and increased in size4,5, but this last was not true of other therians6. The uniquely extended gestation of placental young7 may have factored into their success and size increase8, but reproduction style in early placentals remains unknow...

Tyrannosaurid theropods topped the terrestrial food chain in North America and Asia during the latest Cretaceous. Most tyrannosaurids, exemplified by Tyrannosaurus rex, had deep snouts, thick teeth, and large jaw muscles that could generate high bite forces. They coexisted in Asia with a morphologically divergent group of long-snouted relatives, ca...

We describe the tympanic anatomy of the petrosal of Deltatherium fundaminis, an enigmatic Paleocene mammal based on cranial specimens recovered from New Mexico, U.S.A. Although the ear region of Deltatherium has previously been described, there has not been a comprehensive, well-illustrated contribution using current anatomical terminology. The den...

The end-Cretaceous mass extinction, 66 million years ago, profoundly reshaped the biodiversity of our planet. After likely originating in the Cretaceous, placental mammals (species giving live birth to well-developed young) survived the extinction and quickly diversified in the ensuing Paleocene. Compared to Mesozoic species, extant placentals have...

Conference talk

Colbert Poster Prize, Mesozoic & Early Cenozoic Mammalian Evolution

Colbert Poster Prize, Mesozoic & Early Cenozoic Mammalian Evolution

If we imagine walking through Mesozoic lands, we would be able to observe vertebrates with peculiar combinations of morphological traits, some of which would seem to be intermediary to animals seen today. We would witness a terrestrial vertebrate fauna dominated by dinosaurs of various sizes and diversity, accompanied by many other animal groups th...

The lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation from the San Juan Basin (SJB) in northwestern New Mexico preserve arguably the best early Paleocene mammalian record in North America and is the type location for the Puercan (Pu) and Torrejonian (To) North American land mammal ages (NALMA). However, the lack of precise depositional a...

Tyrannosaurus rex and other tyrannosaurid dinosaurs were apex predators during the latest Cretaceous, which combined giant size and advanced neurosensory systems. Computed tomography (CT) data have shown that tyrannosaurids had a trademark system of a large brain, large olfactory bulbs, elongate cochlear ducts, and expansive endocranial sinuses sur...

The end‐Cretaceous mass extinction allowed placental mammals to diversify ecologically and taxonomically as they filled ecological niches once occupied by non‐avian dinosaurs and more basal mammals. Little is known, however, about how the neurosensory systems of mammals changed after the extinction, and what role these systems played in mammalian d...

SIZE VARIATION IN POPULATIONS OF TETRACLAENODON (MAMMALIA, ‘CONDYLARTHRA’), FROM THE TORREJONIAN NALMA OF THE SAN JUAN BASIN (NEW MEXICO) REVEALS NEW INSIGHTS INTO THEIR EVOLUTION AND PALEOENVIRONMENT HOLPIN, Sofia, University of Edinburgh, Edinburgh, Scotland; WILLIAMSON, Thomas E., New Mexico Museum of Natural History, Albuquerque, NM, United St...

Colbert Prize Poster Session

Conference talk

The lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation from the San Juan Basin (SJB) in northwestern New Mexico preserve arguably the best early Paleocene mammalian record in North America and is the type location for the Puercan (Pu) and Torrejonian (To) North American Land Mammal ages (NALMA). However, the lack of precise depositional a...

Understanding the tectonic and landscape evolution of the Colorado Plateau−southern Rocky Mountains area requires knowledge of the Laramide stratigraphic development of the San Juan Basin. Laramide sediment-transport vectors within the San Juan Basin are relatively well understood, except for those of the Nacimiento and Animas formations. Throughou...

Mammals underwent a profound diversification after the end‐Cretaceous mass extinction, with placentals rapidly expanding in body size and diversity to fill new niches vacated by dinosaurs. Little is known, however, about the brains and senses of these earliest placentals, and how neurosensory features may have promoted their survival and diversific...

Periptychus is the archetypal genus of Periptychidae, a clade of prolific Paleocene ‘condylarth’ mammals from North America that were among the first placental mammals to radiate after the end-Cretaceous extinction, remarkable for their distinctive dental anatomy. A comprehensive understanding of the anatomy of Periptychus has been hindered by a la...

Periptychus carinidens raw anatomical measurements. This file is formatted as an excel file and includes raw measurements for the Periptychus specimens described in this paper. Individual bones are listed in separate tabs. (XLSX)

We analyzed samples for paleomagnetism, ⁴⁰Ar/³⁹Ar detrital sanidine ages, and mammalian fauna to produce a precise chronostratigraphic framework for the Upper Cretaceous to Lower Paleocene Dawson Creek section of Big Bend National Park, west Texas. Prior to this work, the absolute ages and durations of the Upper Cretaceous Aguja and Javelina Format...

Eutherian mammals—placentals and their closest extinct relatives—underwent a major radiation following the end-Cretaceous extinction, during which they evolved disparate anatomy and established new terrestrial ecosystems. Much about the timing, pace, and causes of this radiation remain unclear, in large part because we still know very little about...

Lower Paleocene deposits in the San Juan Basin document one of the best records of mammalian change and turnover following the Cretaceous-Paleogene mass extinction and is the type area for the Puercan (Pu) and Torrejonian (To) North America Land Mammal age (NALMA). One of the largest mammalian turnover events in the early Paleocene occurs between t...

In recent years, neutron radiography and tomography have been applied at different beam lines at Los Alamos Neutron Science Center (LANSCE), covering a very wide neutron energy range. The field of energy-resolved neutron imaging with epi-thermal neutrons, utilizing neutron absorption resonances for contrast as well as quantitative density measureme...

Lower Paleocene deposits in the San Juan Basin document one of the best records of mammalian change and turnover following the Cretaceous-Paleogene extinctions and are the type section for the Puercan (Pu) and Torrejonian (To) North America Land Mammal age biozones (NALMA). One of the largest mammalian turnover events in the early Paleocene occurs...

We analyzed samples for paleomagnetism, 40Ar/39Ar detrital sanidine ages, and mammalian fauna to produce a precise chronostratigraphic framework for the Upper Cretaceous to lower Paleocene Dawson Creek section of Big Bend National Park. Prior to this work, the absolute age and duration of the Upper Cretaceous Aguja and Javelina Formations and Paleo...

Palaechthonid plesiadapiforms from the Palaeocene of western North America have long been recognized as among the oldest and most primitive euarchontan mammals, a group that includes extant primates, colugos and treeshrews. Despite their relatively sparse fossil record, palaechthonids have played an important role in discussions surrounding adaptiv...

Electronic Supplementary Material: Oldest skeleton of a plesiadapiform provides evidence for an exclusively arboreal radiation of stem primates in the Paleocene

Tyrannosaurids — the familiar dinosaur group including Tyrannosaurus, Albertosaurus, Alioramus, and kin-occupied the apex predator role in Asia and North America during the latest Cretaceous. It has long been thought that sophisticated senses and distinctive brain and endocranial morphologies were among the keys to tyrannosaurid success. Little is...

Triisodontids are an archaic group of mammals that possess dental adaptations for carnivory, but are not members of the order Carnivora. They were probably apex mammalian predators for much of the early Paleocene. The wolf-sized species of Triisodon were the largest triisodontids and were also among the largest mammals of their time. A new specimen...

There has been considerable debate about whether the controversial tyrannosauroid dinosaur ‘Nanotyrannus lancensis’ from the uppermost Cretaceous of North America is a valid taxon or a juvenile of the contemporaneous Tyrannosaurus rex. In a recent Cretaceous Research article, Schmerge and Rothschild (2016) brought a new piece of evidence to this di...

Pachycephalosaurs, a group of ornithischian dinosaurs with distinctive cranial ornamentation and skull domes, underwent dramatic changes in cranial morphology during ontogeny. This has caused debate about whether some specimens belong to juveniles or adults, which impacts studies of pachycephalosaur phylogeny and evolution. One such debate concerns...

The San Juan Basin (SJB) of New Mexico contains extensive outcrops of alluvial deposits documenting early Paleocene mammalian evolution and their corresponding paleoclimatic and paleoenvironmental record. Although the early Paleocene mammalian record is well resolved, there has been relatively little work to place it within a detailed paleoclimatic...

To provide paleoenvironmental context for the early Puercan terrestrial ecosystems of the San Juan Basin (SJB), New Mexico, an approximately 25 m thick and 500 m laterally continuous outcrop exposure was studied focusing on sedimentologic, stratigraphic, and paleopedologic attributes. This outcrop is in the basal portion of the lower Paleocene Arro...