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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. 

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. 

A large phylogenetic tree is a critical component of comparative analyses that examine broad macroevolutionary patterns, such as the tempo and mode of evolution or morphological disparity through time. However, the sample of species included in published phylogenies rarely aligns with the species that researchers wish to examine in comparative analyses. For instance, early synapsid phylogenies often focus on specific subclades, such as pelycosaurs or anomodonts, rather than broadly encompassing all known synapsid lineages, thus hindering analyses that require detailed sampling across synapsid lineages. To address this issue, we generated a time-calibrated meta-phylogeny (‘metatree’) of synapsid species from the Carboniferous through the Eocene (305–34 Ma). The metatree approach uses source character matrices (rather than source trees) and generates complete sets of most parsimonious trees, combining them rather than generating a single consensus tree. We incorporated 269 published morphological character matrices, which includes every non-mammaliaform synapsid character matrix that has ever been published (as of July 2021) and 57 mammaliaform-focused matrices. Due to evolving ideas of relationships and frequent matrix reuse, each of the matrices was weighted according to its publication year and its dependence on ‘parent’ matrices using an established metatree procedure. The metatree approach relies on XML metadata files that reconcile taxon names to valid Paleobiology Database taxa (PBDB). Because the metatree approach utilizes PBDB taxonomy, we vetted the PBDB information and made approximately 500 additions and corrections to taxon information. The resulting metatree includes 2,128 synapsid species, making it one of the largest fossil phylogenies ever produced. Approximately 1600 species are non-mammaliaform synapsids, and the remaining ~525 species are mammaliaforms, including many of the known Mesozoic and early Cenozoic mammaliaforms. The massive taxonomic and temporal breadth of the metatree make it broadly applicable to studies on synapsid macroevolution. The past decade has witnessed a resurgence of research on non-mammaliaform synapsids, and our new, comprehensive metatree provides a rigorous foundation for continuing work on macroevolutionary patterns and processes among the forerunners of mammals.