 |
| large Gobiconodon outcompeted the ancestors of modern mammals in the time of dinosaurs. in Brocklehurst, Panciroli, Benevento & Benson, 2021. Illustration: Corbin Rainbolt. twitter.com/CorbinRainbolt |
Highlights
• The therian mammal radiation is usually associated with extinctions among dinosaurs
• Mesozoic therians show greater morphological constraint than their close relatives
• The release of this constraint occurred later than the extinction of the dinosaurs
• The therian radiation was in part driven by extinctions among other mammaliaforms
Summary
Adaptive radiations are hypothesized as a generating mechanism for much of the morphological diversity of extant species. The Cenozoic radiation of placental mammals, the foundational example of this concept, 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 dinosaurs. However, study of this and other radiations has focused on variation in evolutionary rates, leaving the extent to which relaxation of constraints enabled the origin of novel phenotypes less well characterized. 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 <100 g, 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 the 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.
Keywords: adaptive radiation, constraint, mammal, Mesozoic, K/Pg mass extinction
 |
| Early lineages of mammal like this large Gobiconodon from Mongolia outcompeted the ancestors of modern mammals in the time of dinosaurs. Illustration: Corbin Rainbolt. |
Neil Brocklehurst, Elsa Panciroli, Gemma Louise Benevento and Roger B.J. Benson. 2021. Mammaliaform Extinctions as A Driver of the Morphological Radiation of Cenozoic Mammals. Current Biology. DOI: 10.1016/j.cub.2021.04.044
In brief: Brocklehurst et al. report that Mesozoic therians evolved under greater morphological constraint than contemporary mammaliaforms. This constraint was released later than the extinction of non-avian dinosaurs, including in therians less than 100 g. They suggest the therian radiation was in part driven by extinctions among non-therian mammaliaforms.
Mammals in the time of dinosaurs held each other back
