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1. Survival of the novel: derived faunivores are the forerunners of major synapsid radiations

   Grossnickle, David; Hellert, Spencer; Kammerer, Christian; Angielczyk, Kenneth D.; Lloyd, Graeme (October 2022, Journal of vertebrate paleontology)

   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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2. The influence of ancestral body size on ecomorphological trebds in synapsid radiations

   Hellert, S.; Lloyd, G.; Grossnickle, D.; Angielczyk, K. D. (November 2021, The Society of Vertebrate Paleontology Virtual Meeting Conference Program. 81st Annual Meeting.)

   ‘Small-bodied faunivore’ is the dominant ancestral ecomorphotype early in crown mammalian radiations, but it is unknown how far back this trend extends within Synapsida (the mammalian total group). To examine synapsid macroevolutionary patterns in a phylogenetic context, we built a time-calibrated meta-phylogeny of 2,128 synapsid species from the Carboniferous through Eocene (305–34 Ma), based on 211 published character matrices, each weighted according to their dependence on ‘parent’ matrices. All published character matrices focusing on non-mammalian synapsids were included, making the meta-phylogeny comprehensive for non-mammaliaform synapsids. Further, we used the most comprehensive early mammaliaform matrices. We then collected jaw length measurements (as a proxy for body size) and dietary information for 408 synapsid species (37 non-therapsid pelycosaurs, 134 non-cynodont therapsids, 46 non-mammaliaform cynodonts, 80 non-therian mammaliaforms, and 178 therians). We used the meta-phylogeny in conjunction with jaw length measurements to investigate patterns of body-size and dietary evolution during radiations of synapsid subclades. The results show that faunivory is the typical ancestral diet of each major radiation within Synapsida, but the small-to-large trend in body-size within radiations does not become established until the end-Triassic size bottleneck near the base of Mammaliaformes. Instead, ‘pelycosaur’, ‘therapsid’, and ‘cynodont’ subclades have ancestral jaw lengths that are considerably larger than non-therian mammaliaforms and therians. The shift to small ancestral body sizes is one of several aspects of the mammalian phenotype to emerge in the Late Triassic. Furthermore, by placing ‘mammaliaforms’ and mammals near their likely lower size limit, this change forced most subsequent body-size diversification to consist of trends toward larger sizes, altering macroevolutionary dynamics for the remainder of synapsid history. 

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3. Phylogeny, function and ecology in the deep evolutionary history of the mammalian forelimb

   https://doi.org/10.1098/rspb.2021.0494  

   Lungmus, Jacqueline K.; Angielczyk, Kenneth D. (April 2021, Proceedings of the Royal Society B: Biological Sciences)

   Mammals are the only living members of the larger clade Synapsida, which has a fossil record spanning 320 Ma. Despite the fact that much of the ecological diversity of mammals has been considered in the light of limb morphology, the ecological comparability of mammals to their fossil forerunners has not been critically assessed. Because of the wide use of limb morphology in testing ecomorphological hypothesis about extinct tetrapods, we sought: (i) to estimate when in synapsid history, modern mammals become analogues for predicting fossil ecologies; (ii) to document examples of ecomorphological convergence; and (iii) to compare the functional solutions of distinct synapsid radiations. We quantitatively compared the forelimb shapes of the multiple fossil synapsid radiations to a broad sample of extant Mammalia representing a variety of divergent locomotor ecologies. Our results indicate that each synapsid radiation explored different areas of morphospace and arrived at functional solutions that reflected their distinctive ancestral morphologies. This work counters the narrative of non-mammalian synapsid forelimb evolution as a linear progression towards more mammalian morphologies. Instead, a disparate array of early-evolving shapes subsequently contracted towards more mammal-like forms. 

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4. Phylogenomic reconstruction of Solenogastres (Mollusca, Aplacophora) informs hypotheses on body size evolution

   https://doi.org/10.1016/j.ympev.2024.108029  

   Yap-Chiongco, Meghan K; Bergmeier, Franziska S; Roberts, Nickellaus G; Jörger, Katharina M; Kocot, Kevin M (May 2024, Molecular Phylogenetics and Evolution)

   Body size is a fundamental characteristic of animals that impacts every aspect of their biology from anatomical complexity to ecology. In Mollusca, Solenogastres has been considered important to understanding the group’s early evolution as most morphology-based phylogenetic reconstructions placed it as an early branching molluscan lineage. Under this scenario, molluscs were thought to have evolved from a small, turbellarian-like ancestor and small (i.e., macrofaunal) body size was inferred to be plesiomorphic for Solenogastres. More recently, phylogenomic studies have shown that aplacophorans (Solenogastres + Caudofoveata) form a clade with chitons (Polyplacophora), which is sister to all other molluscs, suggesting a relatively large-bodied (i.e., megafaunal) ancestor for Mollusca. Meanwhile, recent investigations into aplacophoran phylogeny have called the assumption that the last common ancestor of Solenogastres was small-bodied into question, but sampling of meiofaunal species was limited, biasing these studies towards large-bodied taxa and leaving fundamental questions about solenogaster body size evolution unanswered. Here, we supplemented available data with transcriptomes from eight diverse meiofaunal species of Solenogastres and conducted phylogenomic analyses on datasets of up to 949 genes. Maximum likelihood analyses support the meiofaunal family Meiomeniidae as the sister group to all other solenogasters, congruent with earlier ideas of a small-bodied ancestor of Solenogastres. In contrast, Bayesian Inference analyses support the large-bodied family Amphimeniidae as the sister group to all other solenogasters. Investigation of phylogenetic signal by comparing site-wise likelihood scores for the two competing hypotheses support the Meiomeniidae-first topology. In light of these results, we performed ancestral character state reconstruction to explore the implications of both hypotheses on understanding of Solenogaster evolution and review previous hypotheses about body size evolution and its potential consequences for solenogaster biology. Both hypotheses imply that body size evolution has been highly dynamic over the course of solenogaster evolution and that their relatively static body plan has successfully allowed for evolutionary transitions between meio-, macro- and megafaunal size ranges. 

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5. Morphological disparity across the synapsid forelimb: sub-order-level patterns across 80 million years of synapsid evolution

   Lungmus, Jacqueline A. Angielczyk (October 2018, Society of Vertebrate Paleontology 2018 Meeting Program and Abstracts)

   Synapsid evolution can be characterized by three successive radiations: the Permo– Carboniferous pelycosaurs, the Permo–Triassic Therapsida, and the Triassic Eucynodontia. Previous geometric morphometric research at the clade level revealed a continuous increase in humeral morphological disparity in Therapsida, in contrast to their pelycosaur forebearers. Here we present associated data on ulnar morphological disparity, as well an an overall taxonomic expansion of the analyses. This increase in sample size brings the dataset to 765 specimens from which functional units across the forelimb were analyzed. Further, it allows for a more detailed discussion of variance within nearly every major group of early Synapsida, as well as across 80 million years of geologic history. Groups were analyzed for Procrustes variance in 5 million year time bins from 305–225 Mya (Carboniferous–Triassic). In all analyzed functional units—the proximal humerus, distal humerus, and proximal elbow—within group disparity is higher in therapsid families than in pelycosaur families. In addition, therapsid family level disparity is much more variable between groups and across time. Ulnar variance values are higher than humeral values for the entire study period. Procrustes variance for the forelimb decreases across the End Permian Mass Extinction Event in the major therapsid groups that survived it— Anomodontia and Cynodontia. Macroevolutionary changes observed in Synapsida have historically been associated with ecological diversification. Cynodontia and Anomodontia have the highest variance in Therapsida, while Gorgonopsia has the lowest. The high values in Anomodontia, as one of the most taxonomically and ecological diverse clades of Therapsida, suggests that forelimb variance is linked to aspects of ecological diversification. Further, within pelycosaurs Sphenacodontidae has the lowest variance through time, while Ophiacodontidae has the highest. The finding of uniquely high variance levels in Ophiacodontidae, hypothesized by some to be semi-acquatic, is suggestive of a potentially unique forelimb ecomorphology. This research provides evidence that along with major shifts in forelimb morphology, within-family disparity dynamics may have been critical to the evolutionary success of individual synapsid sub-orders. 

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