New dates for a long-debated New Mexico fossil site reveal that dinosaurs were thriving and regionally diverse until the end-Cretaceous asteroid strike 66 million years ago, according to a new study. The disappearance of non-avian dinosaurs 66 million years ago ended the final chapter of the Cretaceous Period, yet scientists still debate whether their extinction was sudden – triggered by the asteroid impact – or the culmination of a gradual decline that left them vulnerable to catastrophe. According to Andrew Flynn and colleagues, this uncertainty stems from gaps and biases in the fossil record from this period. To date, the best dated paleontological evidence of faunas straddling the Cretaceous-Paleogene (K-Pg) boundary comes from the Hell Creek and Fort Union Formations in the northern Great Plains of North America. However, investigations into these and other nearby fossil assemblages have produced conflicting conclusions about whether the fossil record reflects a sudden dinosaur extinction at the end of the Cretaceous or a more gradual, Late Cretaceous decline. Here, Flynn et al. present new geochronological data that precisely date the Naashoibito Member – a fossil-rich rock layer of the Kirtland Formation of northern New Mexico – to ~66.4 to 66 million years ago, making its dinosaur fossils contemporaneous with those of the Hell Creek Formation farther north. These dinosaurs, diverse in size, diet, and species, lived within roughly 340,000 years of the Cretaceous–Paleogene boundary, showing no signs of ecological decline before the mass extinction. Ecological analyses incorporating these new findings indicate that, contrary to the long-held idea of uniform Late Cretaceous ecosystems, distinct regional differentiation of dinosaur species persisted across western North America until the very end of the Cretaceous. Evidence from other continents, though less precisely dated, suggests similarly robust and diverse dinosaur faunas surviving until the final moments of the Cretaceous. According to the authors, taken together, these findings suggest that non-avian dinosaurs were not in long-term decline but were instead abruptly wiped out by the end-Cretaceous asteroid impact. In an accompanying perspective, Lindsay Zanno discusses the study’s findings in greater detail.

New paleontological findings offer insights into Wyoming’s “dinosaur mummies,” revealing that the stunningly preserved skin, spikes, and hooves of duck-billed dinosaurs are not fossilized flesh at all, but delicate clay molds formed by microbes as the creatures decayed, researchers report. Soft-tissue preservation in fossils usually occurs in fine-grained, oxygen-poor environments such as lagoons or seabeds, which enable the fossilization of delicate structures like feathers and skin. However, the so-called “dinosaur mummies” of duck-billed dinosaurs (Edmontosaurus annectens), which were discovered in the early 1900s in eastern Wyoming with what appeared to be fossilized skin texture and fleshy body parts, were found in coarse, oxygenated river deposits, making the nature of their remarkable preservation a long-standing mystery. Using historical documents, Paul Sereno and colleagues approximated the location where these original specimens were discovered more than a century ago and report on new findings from the site, including newly discovered juvenile and adult Edmontosaurus mummies. Notably, the late juvenile represents the first subadult dinosaur mummy and the first large-bodied dinosaur with a fully preserved fleshy outline, including a neck and trunk crest. The adult is the first hadrosaurid to retain the full tail spike row and the earliest known hooves in any tetrapod, marking the first reptile with hooved feet. According to the authors, the midline crest and tail spikes of E. annectens reveal a far more complex hide than historical restorations have suggested. Comparisons to living reptiles suggest functional and morphological similarities to some modern squamates. Sereno et al. also show that these features are preserved as thin (<1 millimeter) clay layers bounded by sandstone, rather than as true fossilized soft tissue. Detailed analyses using optical, CT, and electron microscopy, and X-ray spectroscopy revealed no evidence of original organics within the clay or surrounding matrix. The uniform clay layer likely formed as a surface template over the decaying carcass, aided by biofilms, rather than replicating original tissue. This clay templating mechanism preserves external integumental forms in three dimensions, even in coarse, oxygenated fluvial deposits, expanding the range of depositional settings in which soft-tissue features can fossilize.