Origins of forelimb diversity in mammals

Researchers report the evolutionary origins of morphological diversity in mammal forelimbs. Mammals exhibit diverse forelimb structures that reflect a range of ecological, behavioral, and locomotor functions. The closest fossil relatives of mammals exhibit similar forelimb diversity, but the evolutionary time of origin of such diversity is unclear. Jacqueline K. Lungmus and Kenneth D. Angielczyk quantified morphological disparity in forelimb bones from two groups: pelycosaurs, the earliest members of the synapsid clade that includes mammals, and therapsids, the more advanced synapsids that replaced the pelycosaurs in the late Permian. The authors analyzed the shapes of 284 distal ends and 309 proximal ends of humerus, or upper arm bone, specimens representing 73 genera spanning 305-235 million years ago. The authors found that the variance in both proximal and distal end shapes began to increase approximately 270 million years ago, concurrent with the emergence of Permian therapsids. The increase coincides with the evolution of therapsid ecologies previously unoccupied by pelycosaurs. Forelimb morphological variance declined following the Permo-Triassic mass extinction, 252 million years ago, but subsequently rebounded. The results suggest that the diversity of forelimbs seen in present-day mammals represents a unique synapsid characteristic that originated with the emergence of therapsids in the middle Permian. According to the authors, increased diversity in therapsid forelimb morphology may have facilitated increased locomotor capabilities and allowed novel forelimb functions to evolve.

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Article #18-02543: "Antiquity of forelimb ecomorphological diversity in the mammalian stem lineage (Synapsida)," by Jacqueline K. Lungmus and Kenneth D. Angielczyk.

MEDIA CONTACT: Jacqueline K. Lungmus, University of Chicago, IL; tel: 847-708-1916; e-mail: jlungmus@uchicago.edu

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