Collaborative Research: Evolving the mammalian forelimb: modeling musculoskeletal transformation in the forerunners of mammals

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A unique feature of mammals is their many different forelimb shapes and functions, including highly distinct forelimbs for running, climbing, digging, swimming, and flying. No other group of animals has such diverse forelimbs, and the flexibility of mammal forelimbs seems to be a key reason for the group's success. The first mammal relatives found in the fossil record are over 300 million years old, and are a striking contrast to living mammals: their forelimbs show a very limited range of shapes and functions. How did the evolutionarily-flexible forelimbs of mammals evolve from such seemingly constrained ancestors, and did changes to the forelimb increase the ability of mammals to explore new ways of life? This research project will use state-of-the-art techniques, and extensive data from fossils and modern animals, to reconstruct forelimb structure and function in ancient mammal relatives, and to test whether changes to the forelimb correlate with increases in their ecological diversity. The results will deepen our understanding of the evolution of a quintessential characteristic of mammals, and its role in the evolution of animals as distinct as bats, moles, whales, horses, and humans. Dissemination of the research will occur through a series of educational online videos produced by the award-winning YouTube Channel "The Brain Scoop". The series will include three episodes describing different stages of the research, and two episodes that describe the results and their significance. Audience responses to the videos will be used to evaluate the effectiveness of online videos as a tool for science education.Modern mammals display remarkable ecomorphological diversity, which is underpinned by exaptation of the forelimb to serve novel functions (e.g. flying, running, swimming, and digging). In contrast, the ancestors of mammals - the non-mammalian synapsids - are generalized terrestrial tetrapods with seemingly limited ecological scope. The mammalian-style forelimb, including a fully mobile scapula, ventrally oriented ball-and-socket shoulder joint, and "upright" limb posture and movement, is the result of a profound reorganization of the ancestral synapsid musculoskeletal system. Drawing from the extensive synapsid fossil record, this project integrates a diverse set of methodological techniques to pinpoint how morphofunctional transformation of the forelimb influenced the tempo and mode of phenotypic evolution of the synapsid clade. Rigorous morphometric analyses will quantify shape disparity in forelimb skeletal elements and identify major shifts in morphology. Cutting-edge biomechanical analyses will correlate morphological shifts with functional transformation. And, advanced phylogenetic modeling techniques will determine whether shifts in morphology and function resulted in increases in diversification rates. Key products include a new comprehensive metatree of Synapsida, an extensive geometric morphometric dataset, and 3D musculoskeletal models of ten taxa at phylogenetically informative stages of synapsid evolution. Combined, this project will build a robust comparative framework to quantify musculoskeletal transformations in extinct animals and provide a unique evolutionary perspective on an integrated anatomical module of significant adaptive value.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.