Mammalian stem lineage (Synapsida)

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Abstract

Mammals and their closest fossil relatives are unique amon tetrapods in expressing a high degree of pectoral girdle an forelimb functional diversity associated with fully pelagic, curso rial, subterranean, volant, and other lifestyles. However, the earlies members of the mammalian stem lineage, the “pelycosaur”-grad synapsids, present a far more limited range of morphologies an inferred functions. The more crownward nonmammaliaform therap sids display novel forelimb morphologies that have been linked t expanded functional diversity, suggesting that the roots of this quin tessentially mammalian phenotype can be traced to the pelyco saur–therapsid transition in the Permian period. We quantifie morphological disparity of the humerus in pelycosaur-grad synapsids and therapsids using geometric morphometrics. W found that disparity begins to increase concurrently with th emergence of Therapsida, and that it continues to rise until th Permo-Triassic mass extinction. Further, therapsid exploration o new regions of morphospace is correlated with the evolution o novel ecomorphologies, some of which are characterized by change to overall limb morphology. This evolutionary pattern confirms tha nonmammaliaform therapsid forelimbs underwent ecomorphologica diversification throughout the Permian, with functional elaboratio initially being more strongly expressed in the proximal end of th humerus than the distal end. The role of the forelimbs in the func tional diversification of therapsids foreshadows the deployment o forelimb morphofunctional diversity in the evolutionary radiatio of mammals.

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Lungmus, J. K., & Angielczyk, K. D. (2019). Mammalian stem lineage (Synapsida). Proceedings of the National Academy of Sciences of the United States of America, 116(14), 6903–6907. https://doi.org/10.1073/pnas.1802543116

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