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1. Evolutionary rate analysis reveals dynamic and variable patterning of forelimb evolution across the deep evolutionary Synapsida.

   Lungmus, Jacqueline K. (July 2020, Journal of Vertebrate Paleontology, Program and Abstracts, 2020)

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   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. 

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2. Can Geometric Morphometric Analyses of Limb Shape Reveal Ecomorphological Patterns Across the Evolutionary History of Synapsida?

   Lungmus, J. K.; Angielczyk, K. D. (April 2019, Journal of morphology)

   Extant Mammalia are the only living representatives of the larger clade known as Synapsida, which has a continuous fossil record from around 320 million years ago to today. Despite the fact that much of the ecological diversity of mammals has been considered in light of limb morphology, the deep time origin of synapsid limb diversity and its influence on ecological diversity has received less attention. Here, we present shape analyses focusing on the forelimbs of the two earliest synapsid radiations (“pelycosaurs”, and pre-mammaliaforme Therapsida) in comparison to a broad sample of extant Mammalia. Using an expansive geometric morphometric data set, comprised of 384 fossil specimens and 148 extant mammalian specimens, we sought evidence for ecomorphological signals that could provide insight on the ecology of the earliest synapsids. Collecting shape data of humeral and ulnar elements from an extant sample representing multiple known eco morphologies provided the framework for a comparative exploration of extinct ecomorphologies, associated specifically with locomotion. Our results show that distal humeral shape is not informative of broad locomotor ecomorphologies in early fossil Synapsida. In contrast, proximal humeral shape shows a more complex pattern that suggests shape similarity between basal synapsids and members of extant Perissodactyla, and certain highly derived fully fossorial mammals, as just two examples. Overall, however, our findings suggest general shape analyses may have limited utility when analyzing for ecological-signal across deep time. Considering skeletal morphology in a holistic framework that considers unique combinations of shapes, as well as the use of biomechanically focused indices (such are functional proportions), may help to elucidate the more nuanced ways that locomotor ecology influenced limb shape in some of the earliest amniote radiations. 

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3. Limb Ecometrics Show Limited Applicability for Quantifying Ecological Novelty in the Deep Evolution of Synapsida

   Lungmus, J. K.; Angielczyk, K. D.; Luo, Z. X. (January 2020, Scoiety for Integrative aand Comparative Biology 2020 Abstract Book)

   Mammalia are the only living members of the larger clade known as Synapsida, which has a fossil record spanning from 320mya to today. Despite the fact that much of the ecological diversity of mammals has been considered in light of limb morphology, the origin of broader synapsid limb diversity and its influence on ecological diversity has received less attention. Here we present shape analyses of the forelimbs of the multiple fossil synapsid radiations in comparison to a broad sample of extant Mammalia. Previous work by the authors has shown that shape broadly is not informative of specific locomotor ecomorphologies in earliest fossil Synapsida. Considering the broader scientific use of limb morphology in testing for fossil ecomorphologies, we sought to better understand at what juncture in synapsid evolutionary history do limb metrics begin to show utility in ecomorphological analyses. Shape data on humeri and ulnae elements from an extant sample representing known ecomorphologies provided the framework for a comparative study of extinct ecomorphologies, associated specifically with locomotion. We conducted linear and geometric morphometric comparisons between the extant sample and five taxonomic subsampled radiations moving crown-ward along the synapsid lineage. Taxonomic designations were the PermoCarboniferous "pelycosaurs”, both Permian and Triassic therapsids, "Non-mammaliaforme cynodonts”, and "Mammaliaformes”. Results show that many limb ecomorphological metrics commonly used are not effective designators until close to the origin of crown Mammalia, as late as the Jurassic. This brings into question the overall utility of using extant analogues to test for ecological signal in a given tetrapod group's deepest fossil ancestors. 

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4. Antiquity of forelimb ecomorphological diversity in the mammalian stem lineage (Synapsida)

   https://doi.org/10.1073/pnas.1802543116  

   Lungmus, Jacqueline K.; Angielczyk, Kenneth D. (March 2019, Proceedings of the National Academy of Sciences)

   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. 

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5. Broad similarities in shoulder muscle architecture and organization across two amniotes: implications for reconstructing non-mammalian synapsids

   https://doi.org/10.7717/peerj.8556  

   Fahn-Lai, Philip; Biewener, Andrew A.; Pierce, Stephanie E. (January 2020, PeerJ)

   The evolution of upright limb posture in mammals may have enabled modifications of the forelimb for diverse locomotor ecologies. A rich fossil record of non-mammalian synapsids holds the key to unraveling the transition from “sprawling” to “erect” limb function in the precursors to mammals, but a detailed understanding of muscle functional anatomy is a necessary prerequisite to reconstructing postural evolution in fossils. Here we characterize the gross morphology and internal architecture of muscles crossing the shoulder joint in two morphologically-conservative extant amniotes that form a phylogenetic and morpho-functional bracket for non-mammalian synapsids: the Argentine black and white tegu Salvator merianae and the Virginia opossum Didelphis virginiana . By combining traditional physical dissection of cadavers with nondestructive three-dimensional digital dissection, we find striking similarities in muscle organization and architectural parameters. Despite the wide phylogenetic gap between our study species, distal muscle attachments are notably similar, while differences in proximal muscle attachments are driven by modifications to the skeletal anatomy of the pectoral girdle that are well-documented in transitional synapsid fossils. Further, correlates for force production, physiological cross-sectional area (PCSA), muscle gearing (pennation), and working range (fascicle length) are statistically indistinguishable for an unexpected number of muscles. Functional tradeoffs between force production and working range reveal muscle specializations that may facilitate increased girdle mobility, weight support, and active stabilization of the shoulder in the opossum—a possible signal of postural transformation. Together, these results create a foundation for reconstructing the musculoskeletal anatomy of the non-mammalian synapsid pectoral girdle with greater confidence, as we demonstrate by inferring shoulder muscle PCSAs in the fossil non-mammalian cynodont Massetognathus pascuali . 

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