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Previous work has shown increased morphological variance within the forelimbs of the Permian synapsid group known as Therapsida over that of their Carboniferous and early Permian forerunners (“pelycosaurs”). Considering that disparity trends have been known to point to underlying macroevolutionary transitions, here we analyzed morphological variance alongside several additional macroevolutionary metrics to better isolate possible evolutionary mechanisms. Shape data was collected on a sample of 119 humeri and 99 ulnae comprising three major synapsid radiations with a temporal range from the Carboniferous into the Triassic. Taxonomic sample included all major groups of pelycosaur-grade synapsids, all five recognized non-cynodontian therapsid clades, and a sample of pre-prozostrodontian cynodonts. Procrustes variance - a multivariate quantification of morphospace occupation - was the chosen disparity metric for the study. Rate of phenotypic change, which considers the amount of shape change that would be necessary to achieve observed morphologies given the shape of the closely related taxa, was analyzed as the metric for evolutionary rate. Both metrics were considered through-time upon genera present in sequential 5 million year time bins. Our results expand upon previous findings that disparity increases throughout the earliest stages of the Permian, coincident with the diversification of pelycosaurs and the emergence of Therapsida. This expanded dataset further shows that disparity approaches an asymptote around 270 million years ago and only increases marginally through the late Permian, remaining between 0.018–0.021 from 275-245 mya. In contrast, evolutionary rate does not appear to asymptote during this same interval, starting at a low of 6.17e-6 (300 mya) and increasing to a peak of 1.78e-5 right before the End Permian Mass Extinction Event (252 mya). The continuing increase of evolutionary rate shows that morphological change continues across taxa, but the plateauing of morphological disparity suggests that morphospace is not expanding concurrent with this. The incongruence between these two metrics suggests a critical change in evolutionary mode, wherein morphological change continues rapidly but does not result in the evolution of novel morphologies. These results provide some of the strongest quantitative data yet of an evolutionary constraint acting upon the morphology of the synapsid forelimb through deep time. 

Lystrosaurus was one of the few tetrapods to survive the Permo-Triassic mass extinction, the most profound biotic crisis in Earth’s history. The wide paleolatitudinal range and high abundance of Lystrosaurus during the Early Triassic provide a unique opportunity to investigate changes in growth dynamics and longevity following the mass extinction, yet most studies have focused only on species that lived in the southern hemisphere. Here, we present the long bone histology from twenty Lystrosaurus skeletal elements spanning a range of sizes that were collected in the Jiucaiyuan Formation of northwestern China. In addition, we compare the average body size of northern and southern Pangean Triassic-aged species and conduct cranial geometric morphometric analyses of southern and northern taxa to begin investigating whether specimens from China are likely to be taxonomically distinct from South African specimens. We demonstrate that Lystrosaurus from China have larger average body sizes than their southern Pangean relatives and that their cranial morphologies are distinctive. The osteohistological examination revealed sustained, rapid osteogenesis punctuated by growth marks in some, but not all, immature individuals from China. We find that the osteohistology of Chinese Lystrosaurus shares a similar growth pattern with South African species that show sustained growth until death. However, bone growth arrests more frequently in the Chinese sample. Nevertheless, none of the long bones sam pled here indicate that maximum or asymptotic size was reached, suggesting that the maxi mum size of Lystrosaurus from the Jiucaiyuan Formation remains unknown. 

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. 

Lystrosaurus was one of the few tetrapods to survive the Permo-Triassic mass extinction, the most profound biotic crisis in Earth’s history. The wide paleolatitudinal range and high abundance of Lystrosaurus during the Early Triassic provide a unique opportunity to investigate changes in growth dynamics and longevity following the mass extinction, yet most studies have focused only on species that lived in the southern hemisphere. Here, we present the long bone histology from twenty Lystrosaurus skeletal elements spanning a range of sizes that were collected in the Jiucaiyuan Formation of northwestern China. In addition, we compare the average body size of northern and southern Pangean Triassic-aged species and conduct cranial geometric morphometric analyses of southern and northern taxa to begin investigating whether specimens from China are likely to be taxonomically distinct from South African specimens. We demonstrate that Lystrosaurus from China have larger average body sizes than their southern Pangean relatives and that their cranial morphologies are distinctive. The osteohistological examination revealed sustained, rapid osteogenesis punctuated by growth marks in some, but not all, immature individuals from China. We find that the osteohistology of Chinese Lystrosaurus shares a similar growth pattern with South African species that show sustained growth until death. However, bone growth arrests more frequently in the Chinese sample. Nevertheless, none of the long bones sampled here indicate that maximum or asymptotic size was reached, suggesting that the maximum size of Lystrosaurus from the Jiucaiyuan Formation remains unknown. 

Mammals are noteworthy for their striking ecomorphological diversity in comparison to their non-mammalian synapsid ancestors. However, the lack of a large phenotypic sample coupled with a phylogeny has hindered examination of this characteristic’s acquisition. Did this diversity accumulate at a constant pace, or did evolutionary rate vary between clades and elements? Here I present an analysis of phenotypic evolutionary rate for synapsid forelimbs using 2D geometric morphometric data of 1279 fossil elements, and a time-calibrated composite tree of 160 genera. Rate comparisons were made for five radiations (‘pelycosaurs’, non-cynodont therapsids, non-mammalian cynodonts, Mammaliaformes, Mammalia), and three functional subunits of the forelimb (proximal humerus, distal humerus, ulna). Mammaliaforms were characterized by the highest evolutionary rates for all functional units, followed by therapsids. Both of these groups underwent major ecomorphological diversifications, and the highest rates are found in taxa characterized by specialized forelimb ecologies, such as fossorial dicynodonts (Therapsida) or the semi-aquatic Haldanodon (Mammaliaformes). In all groups the proximal humerus displayed higher rates than the distal humerus, with the highest found in mammaliaforms and therapsids. These groups underwent dramatic morphological change, especially in the gleno-humeral joint. Critically though, the ulna displays the highest evolutionary rates across all groups, highlighting the underappreciated role the ulna played in the morphological and functional transformations of the synapsid forelimb. The simplification of the structure likely increased the possibility for expansion into new morphologies and played a key role in facilitating ecomorphological diversification. Overall, therapsids and mammaliaforms can both be characterized by important functional changes to the forelimb that likely played a role in this dynamic. Phylogenetic signal also varied across the sample, with Pelycosaurs and non-mammalian cynodonts displaying the lowest levels. This in turn reflects these groups’ conservative forelimb morphologies, especially compared with other synapsid clades. Together, these results demonstrate that synapsid forelimb evolution should be characterized as a dynamic and complex accumulation of ‘mammalian’ morphologies. Evolutionary rates varied across taxa and elements as clades adapted their forelimbs in particular ways to accommodate novel ecologies and functions. Funding Sources NSF DEB-1754502, Field Museum Women-in-Science Graduate Fellowship. 

Mammals and their closest fossil relatives are unique among tetrapods in expressing a high degree of pectoral girdle and forelimb functional diversity associated with fully pelagic, cursorial, subterranean, volant, and other lifestyles. However, the earliest members of the mammalian stem lineage, the “pelycosaur”-grade synapsids, present a far more limited range of morphologies and inferred functions. The more crownward nonmammaliaform therapsids display novel forelimb morphologies that have been linked to expanded functional diversity, suggesting that the roots of this quintessentially mammalian phenotype can be traced to the pelycosaur–therapsid transition in the Permian period. We quantified morphological disparity of the humerus in pelycosaur-grade synapsids and therapsids using geometric morphometrics. We found that disparity begins to increase concurrently with the emergence of Therapsida, and that it continues to rise until the Permo-Triassic mass extinction. Further, therapsid exploration of new regions of morphospace is correlated with the evolution of novel ecomorphologies, some of which are characterized by changes to overall limb morphology. This evolutionary pattern confirms that nonmammaliaform therapsid forelimbs underwent ecomorphological diversification throughout the Permian, with functional elaboration initially being more strongly expressed in the proximal end of the humerus than the distal end. The role of the forelimbs in the functional diversification of therapsids foreshadows the deployment of forelimb morphofunctional diversity in the evolutionary radiation of mammals.