biostudies-literatureUnknown22(1)Brownstein CD<h4>Background</h4>The consequences of the K-Pg mass extinction are reflected across present biodiversity, but many faunas that appeared immediately after the extinction event were very different from current ones. Choristodera is a clade of reptiles of uncertain phylogenetic placement that have an extremely poor fossil record throughout their 150-million-year history. Yet, choristoderes survived the K-Pg event and persisted until the Miocene.<h4>Results</h4>I describe the skulls and skeletons of two new choristoderes from a single Paleocene ecosystem in western North America that reveal the hidden Cenozoic diversity of this reptile clade. Despite their similar size, the new species deviate dramatically in morphology. Kosmodraco magnicornis gen. et sp. nov. possesses an extremely short snoBMC ecology and evolution34https://www.ebi.ac.uk/biostudies/studies/S-EPMC8935759biostudies-literatureHigh morphological disparity in a bizarre Paleocene fauna of predatory freshwater reptiles.PMC8935759Brownstein CDfalseHigh morphological disparity in a bizarre Paleocene fauna of predatory freshwater reptiles.<h4>Background</h4>The consequences of the K-Pg mass extinction are reflected across present biodiversity, but many faunas that appeared immediately after the extinction event were very different from current ones. Choristodera is a clade of reptiles of uncertain phylogenetic placement that have an extremely poor fossil record throughout their 150-million-year history. Yet, choristoderes survived the K-Pg event and persisted until the Miocene.<h4>Results</h4>I describe the skulls and skeletons of two new choristoderes from a single Paleocene ecosystem in western North America that reveal the hidden Cenozoic diversity of this reptile clade. Despite their similar size, the new species deviate dramatically in morphology. Kosmodraco magnicornis gen. et sp. nov. possesses an extremely short sno2022-01-01T00:00:00Z2022 Mar2026-03-27T16:30:22.95Z2025-04-05T20:49:07.566ZS-EPMC89357593531382210.1186/s12862-022-01985-z