EXTRAORDINARY NEW FOSSIL LOCALITY CASTS LIGHT ON CENTRAL PATAGONIA’S LAST CRETACEOUS CONTINENTAL VERTEBRATES

Thanks to numerous fossil discoveries, knowledge of continental vertebrate faunas approaching the Cretaceous/Paleogene (K/Pg) mass extinction event has increased exponentially in recent decades. Most of these finds have come from the Laurasian (i.e., Northern Hemisphere) continents. In Gondwana, by contrast, far fewer fossils of end-Mesozoic continental vertebrates have been recovered, leading to a pronounced ‘Laurasian bias’ in studies of extinction patterns of these animals at the K/Pg boundary.

Discovered in 2021, a new fossil locality in southern Chubut Province, Argentina is proving crucial to a greater understanding of the terminal Cretaceous continental vertebrate faunas of southern South America. Informally known as the “Cañadón Tomás Quarry” (CTQ), the site exposes the uppermost part of the Upper Cretaceous (Coniacian–Maastrichtian) Lago Colhué Huapi Formation within the Golfo San Jorge Basin (GSJB) of central Patagonia. Based on palynomorphs recovered from nearby, stratigraphically equivalent horizons, CTQ exposures are thought to date to the upper Maastrichtian.

The most common CTQ fossils are those of hadrosauroids, large-bodied ornithopods that were abundant in Laurasia during the Late Cretaceous but that are rare and poorly known in Gondwana. CTQ hadrosauroid elements pertain to multiple individuals of differing sizes, suggesting that the site may represent a mixed age bonebed. The only other fossil of a large-bodied vertebrate collected from the CTQ to date is a non-avian theropod tooth, probably that of an abelisaurid.

Most remarkably, the CTQ produces isolated fossils of rare and small-bodied vertebrates, the most significant of which are those of a ?madtsoiid snake, a probable noasaurid theropod, and a reigitheriid mammal. The snake and mammal represent the first Cretaceous records of these clades from the GSJB. Importantly, only limited microvertebrate sampling has occurred at the CTQ to date, indicating that the site will likely produce other noteworthy small vertebrate material in the future. In so doing, it will offer key data on central Patagonian end-Mesozoic vertebrates, thereby facilitating comparison with better known assemblages from northern Patagonia and providing insight into faunal dynamics in southern South America leading up to the K/Pg extinction.