Squamate fauna from the Denver Basin shows major ecosystem disruption across K/Pg boundary.

Abstract

The Cretaceous/Palaeogene (K/Pg) boundary marks the most recent and the second-most severe mass extinction in Earth's history. Marine K/Pg boundary sections that preserve uninterrupted sedimentation are abundant, and thus the global marine response to this extinction event is well understood. Similar terrestrial sedimentary sequences that record the before and after of the extinction and that are also geochronologically well constrained are rare. We describe and compare the non-marine squamate diversity immediately before (ca 57 kyr) and after (ca 128 kyr) the K/Pg mass extinction from two geographically and chronostratigraphically well constrained localities in the Denver Basin, Colorado. The latest Cretaceous squamate record there consists of 235 isolated fossils and is extraordinarily diverse, with 27 operational taxonomic units, whereas the earliest Palaeocene record is depauperate in both richness (3 taxa) and abundance (20 isolated fossils)-a ca 91% drop in specimen abundance and a 93% extinction rate. Integrating the Denver Basin squamate fossil record with that of the Williston and Powder River basins of Montana, North and South Dakota, and Wyoming, provides a regional extinction rate of between 75 and 84% for squamates in the Western Interior of North America and suggests this group was more severely impacted across the K/Pg boundary than other small-bodied vertebrates.