| Streszczenie: | Adaptive radiations are hypothesised as a generating mechanism for much of the
morphological diversity of extant species 1,2,3,4,5,6,7 . The Cenozoic radiation of
placental mammals, the foundational example of this concept 8,9 , 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 dinosaurs 10,11,12,13 .
However, study of this and other radiations has focussed on variation in evolutionary
rates 4,5,7,14 , leaving the extent to which relaxation of constraints enabled the origin
of novel phenotypes less well-characterised 15,16,17 . 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, postdating 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.
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