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Evolutionary radiations generate most of Earth’s biodiversity, but are there common ecomorphological traits among the progenitors of radiations? In Synapsida (the mammalian total group), ‘small-bodied faunivore’ has been hypothesized as the ancestral state of most major radiating clades, but this has not been quantitatively assessed across multiple radiations. To examine macroevolutionary patterns in a phylogenetic context, we generated a time-calibrated metaphylogeny (‘metatree’) comprising 1,888 synapsid species from the Carboniferous through the Eocene (305–34 Ma) based on 269 published character matrices. We used comparative methods to investigate body size and dietary evolution during successive synapsid radiations. Faunivory is the ancestral dietary regime of each major synapsid radiation, but relatively small body size is only established as the common ancestral state of radiations near the origin of Mammaliaformes in the Late Triassic. The faunivorous ancestors of synapsid radiations typically have numerous novel characters compared with their contemporaries, and these derived traits may have helped them to survive faunal turnover events and subsequently radiate.
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Survival of the novel: derived faunivores are the forerunners of major synapsid radiations
Evolutionary radiations generate most of Earth’s biodiversity, but are there common ecomorphological traits among the progenitors of radiations? In Synapsida (mammalian total group), ‘small-bodied faunivore’ has been hypothesized as the ancestral state of most major radiating clades. To quantitatively test this hypothesis across multiple radiations, we used a meta-phylogeny (‘metatree’) of Carboniferous through Eocene (305–34 Ma) species in conjunction with jaw lengths (as a proxy for body size) and diet reconstructions for 404 synapsid species. We focus primarily on five major radiations: (i) non-therapsid pelycosaurs, (ii) non-cynodont therapsids, (iii) non-mammaliaform cynodonts, (iv) non-therian mammaliaforms, and (v) therians. Contrary to our expectations, we did not find universal support for the hypothesis that ‘small-bodied faunivore’ is the ancestral state of radiating synapsid groups. Although faunivory was the typical ancestral diet of each major ecological radiation, the radiation forerunners were not relatively small-bodied in many non-mammaliaform synapsid groups. Instead, the small-to-large trend in body-size within radiations does not become common until the end-Triassic size bottleneck near the base of Mammaliaformes. We also find that ecomorphological diversification was often preceded by the extinction of contemporary clades. As a potential causal mechanism for the observed macroevolutionary patterns, it is tempting to assume that the forerunners of major radiations were relatively unspecialized faunivores with reduced extinction risk. However, ‘survival of the unspecialized’ does not fully explain our results. Many of the progenitors of major synapsid radiations may appear to be unspecialized faunivores, but this is likely due to observational bias: the early lineages of each radiation were ‘unspecialized’ relative to many of their later descendant lineages, but, compared to their contemporaries, they exhibit numerous novel characters. These characters were likely important in promoting their long-term survival and diversification, but it appears that mass extinctions and other faunal turnovers were necessary for the lineages that possessed these characters to reach their full evolutionary potential. Therefore, ‘survival of the novel’ appears to be a persistent macroevolutionary pattern throughout synapsid history.
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- Award ID(s):
- 1754502
- PAR ID:
- 10405929
- Date Published:
- Journal Name:
- Journal of vertebrate paleontology
- Issue:
- Program and Abstracts, 2022
- ISSN:
- 0272-4634
- Page Range / eLocation ID:
- 163
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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