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Mammaliaform extinctions as a driver of the morphological radiation of Cenozoic mammals

Citation

Brocklehurst, Neil; Panciroli, Elsa; Benevento, Gemma; Benson, Roger B.J. (2021), Mammaliaform extinctions as a driver of the morphological radiation of Cenozoic mammals, Dryad, Dataset, https://doi.org/10.5061/dryad.bk3j9kdbt

Abstract

Adaptive radiations are hypothesised as a generating mechanism for much of the morphological diversity of extant species1,2,3,4,5,6,7. The Cenozoic radiation of placental mammals, the foundational example of this concept8,9, gave rise to much of the morphological disparity of extant mammals, and is generally attributed to relaxed evolutionary constraints following the extinction of non-avian dinosaurs10,11,12,13. However, study of this and other radiations has focussed on variation in evolutionary rates4,5,7,14, leaving the extent to which relaxation of constraints enabled the origin of novel phenotypes less well-characterised15-17. We evaluate constraints on morphological evolution among mammaliaforms (mammals and their closest relatives) using a new method that quantifies the capacity of evolutionary change to generate phenotypic novelty. We find that Mesozoic crown-group therians, which include the ancestors of placental mammals, were significantly more constrained than other mammaliaforms. Relaxation of these constraints occurred in the mid-Paleocene, post-dating the extinction of non-avian dinosaurs at the K/Pg boundary, instead coinciding with important environmental shifts and with declining ecomorphological diversity in non-theriimorph mammaliaforms. This relaxation occurred even in small-bodied Cenozoic mammals weighing <100g, which are unlikely to have competed with dinosaurs. Instead, our findings support a more complex model whereby Mesozoic crown therian evolution was in-part constrained by co-occurrence with disparate mammaliaforms, as well as by presence of dinosaurs, within-lineage incumbency effects and environmental factors. Our results demonstrate that variation in evolutionary constraints can occur independently of variation in evolutionary rate; and that both make important contributions to the understanding of adaptive radiations.

Funding

Palaeontological Association

H2020 European Research Council

Deutsche Forschungsgemeinschaft