Therian mammals experience an ecomorphological radiation during the Late Cretaceous and selective extinction at the K–Pg boundary

Abstract

It is often postulated that mammalian diversity was suppressed during the Mesozoic Era and increased rapidly after the Cretaceous-Palaeogene (KPg) extinction event. We test this hypothesis by examining macroevolutionary patterns in early therian mammals, the group that gave rise to modern placentals and marsupials. We assess morphological disparity and dietary trends using morphometric analyses of lower molars, and we evaluate generic level taxonomic diversity patterns using techniques that account for sampling biases. In contrast with the suppression hypothesis, our results suggest that an ecomorphological diversification of therians began 10-20 Myr prior to the K-Pg extinction event, led by disparate metatherians and Eurasian faunas. This diversification is concurrent with ecomorphological radiations of multituberculate mammals and flowering plants, suggesting that mammals as a whole benefitted from the ecological rise of angiosperms. In further contrast with the suppression hypothesis, therian disparity decreased immediately after the K-Pg boundary, probably due to selective extinction against ecological specialists and metatherians. However, taxonomic diversity trends appear to have been decoupled from disparity patterns, remaining low in the Cretaceous and substantially increasing immediately after the K-Pg extinction event. The conflicting diversity and disparity patterns suggest that earliest Palaeocene extinction survivors, especially eutherian dietary generalists, underwent rapid taxonomic diversification without considerable morphological diversification.

A large carnivorous mammal from the Late Cretaceous and the North American origin of marsupials

“…Accompanying this taxonomic radiation, and consistent with other recent findings30, were increases in marsupialiform body size (up to 5.2 kg) and dietary diversity, from insectivory to omnivory, frugivory and carnivory, consuming a range of soft to hard food items5. This ecomorphological expansion was broadly coincident with and perhaps related to the onset of adaptive radiations in multituberculates and flowering plants31.…”

“…This recasting of mammals from the Age of Dinosaurs, however, has previously focused on ‘dead-end' non-therian lineages18. Until recently30, there has been little evidence that therians, which comprise 99.9% of all living mammals, underwent significant ecomorphological radiation during their more than 100 Myr of Mesozoic evolution. D. vorax highlights a previously under-appreciated radiation of Late Cretaceous NA stem marsupialiforms and its direct link to the origin and evolution of living marsupials.…”

Virtual endocranial and inner ear endocasts of the Paleocene ‘condylarth’ Chriacus: new insight into the neurosensory system and evolution of early placental mammals

“…The end‐Cretaceous cataclysm that killed off non‐avian dinosaurs and many other vertebrates about 66 million years ago provided placental mammals with a unique opportunity to thrive (Wible et al., ; O'Leary et al., ). Some 10 million years into the Paleogene, placental mammals had already displayed a clear increase in taxonomic and ecological diversity, as well as in evolutionary rates, in contrast to their end‐Cretaceous relatives (Alroy, ; Slater, ; Grossnickle & Newham, ; Halliday & Goswami, ,b; Halliday et al., ). This extrinsic environmental change definitively opened new ecological opportunities for placental mammals; however, intrinsic aspects of placental mammals likely played a role in their success as well (Wilson, ).…”

Mammalian distal humerus fossils from eastern Montana, USA with implications for the Cretaceous-Paleogene mass extinction and the adaptive radiation of placentals

“…The lower locomotor diversity of the latest Cretaceous assemblage could thus reflect the emerging picture that non-therian mammals, for unknown reasons, were able to ecologically diversify in the Mesozoic, whereas therians were more ecologically constrained until after the K-Pg mass extinction event (Wilson et al, 2012;Grossnickle and Polly, 2013;Wilson, 2013; but see Grossnickle and Newham, 2016;Wilson et al, 2016 for pre-K-Pg ecological diversification among therians).…”