Late Cretaceous and Paleocene Lissamphibia and Squamata of Montana and the end-Cretaceous mass extinction

Abstract

Late Cretaceous and Paleocene lissamphibians (e.g., salamanders and albanerpetontids) and squamates (e.g., lizards and snakes) are common components of the nonmarine vertebrate fossil record of North America. However, within the context of the Cretaceous-Paleogene (K-Pg) mass extinction (ca. 66 million years ago) those clades have received little attention relative to other aspects of the continental biota (e.g., dinosaurs, mammals). This dissertation represents the first comprehensive study of salamander (Caudata) and salamander-like lissamphibians (Allocaudata: Albanerpetontidae) and nonmarine squamates leading up to and across the K-Pg boundary. The primary impetus behind these studies was to better determine the timing, mode, and severity of lissamphibian and squamate extinctions and to address the proposed causal mechanisms of the mass extinction and their ancillary effects prior to (e.g., climate change, Deccan Traps volcanism, acid rain) and at the K-Pg boundary (e.g., bolide impact, thermal pulse). To assess lissamphibian and squamate diversity patterns leading up to and across the K-Pg boundary, my coauthors and I documented temporal species richness, taxonomic composition, and turnover. The larger lissamphibian dataset (2021 specimens versus 200 in squamates) allowed for additional quantitative measures of diversity to be calculated including faunal evenness, heterogeneity indices, and relative abundance distributions. Species-level diversity was recorded from a succession of ≥ 45 temporally constrained vertebrate microfossil localities of the uppermost Cretaceous Hell Creek and lowermost Paleocene Tullock formations of Garfield County, northeastern Montana, USA. Results of the caudate and allocaudate study revealed a stepwise pattern of species loss during the last ca. 200 k.y. of the Cretaceous (based on new and revised age determinations of the study area). Five of nine species were either extirpated (33%) from the local area or went extinct (22%) at or near the K-Pg boundary. Declines in species diversity and significant changes in community structure coincided with those species losses. Combined, these results suggest growing ecological stress in the local caudate and allocaudate faunas prior to the bolide impact at the K-Pg boundary. Squamates suffered high species extinctions (81%) during the last ca. 200 k.y. of the Cretaceous which were concentrated nearer the K-Pg boundary. Though species extinctions were highest across the K-Pg boundary, low to high levels of turnover (appearances and disappearances) in the squamate faunas of northeastern Montana occurred throughout most of the depositional duration of the Hell Creek Formation (ca. 1.9 Ma). Coincident with a moderate level of local squamate turnover that occurred more than ca. 300 k.y. before the K-Pg boundary was the loss of chamopsiids and platynotans possessing monocuspid and fang-like teeth, respectively, from the lower to upper halves of the Hell Creek Formation. A similar but less pronounced biostratigraphic pattern has been observed in other aspects of the local vertebrate faunas (euselachians, lissamphibians, turtles, and mammals). The caudate fauna of the earliest Paleocene satisfies several expectations of early post-extinction recovery faunas in being species depauperate (five species), predominated by a “bloom” taxon (Opisthotriton kayi), and invaded by immigrants (Proamphiuma cretacea). A Lazarus taxon (the caudate Prodesmodon copei) reappeared as early as the Pu2/3 of the local section. The squamate fauna during that interval possessed two chamopsiid lizards. Chamopsiids are part of a clade thought to have gone extinct during the K-Pg mass extinction. These new records suggest that chamopsiids were a “dead clade walking” as they failed to recover in abundance or diversity following the extinction. Several new fossil lissamphibians and squamates were documented in this dissertation including a new fossil salamander (proteid) and several lizards (indeterminate iguanomorph, six scincomorphs, and five anguimorphs) from the Hell Creek Formation. Results of my systematic and phylogenetic work demonstrate that the Hell Creek Formation of the study area contains the taxonomically richest caudate and squamate assemblages known from the Mesozoic of North America. My coauthored description and phylogenetic analysis of a new stem iguanian lizard from the slightly older deposits of the Two Medicine Formation (Campanian) of northwestern Montana has yielded important insights into the evolution of Iguanomorpha during the Late Cretaceous. The new taxon, Magnuviator ovimonsensis gen. et sp. nov., represents the oldest unequivocal iguanomorph from North America and is the sister taxon to a clade of paracontemporaneous iguanomorphs (Temujiniidae) from the Gobi Desert of Mongolia. Its phylogenetic relationships imply that crown iguanians were likely absent from North America prior to the K-Pg boundary despite previous claims. Combined, these systematic and phylogenetic studies add to the growing taxonomic inventory of the lissamphibian and squamate fossil record and enhance our understanding of their evolution within Montana, across the Western Interior of North America, and across the northern landmasses of Laurasia during the Late Cretaceous and early Paleocene.