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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)

   null (Ed.)

   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. Musculoskeletal modelling untangles the origins of mammal forelimb function and posture; Society of Experimental Biology Centenary Conference Abstract Book

   Brocklehurst, R_J; Pierce, S_E (July 2023, Society of Experimental Biology)

   The ‘sprawling-parasagittal’ transition was a major postural shift in the ancestors of mammals, resulting in musculoskeletal reorganization of the forelimbs that underpins modern mammal locomotor diversity. However, ‘when’ and ‘how’ this important postural shift occurred is unknown. While the anatomical changes characterizing this transition can be traced through the fossil record, how these relate to functional changes, and the acquisition of parasagittal posture, remains poorly understood. We produced three-dimensional musculoskeletal models of the forelimbs of extant (n=3) and fossil (n=8) taxa that phylogenetically and functionally span the sprawling–parasagittal transition. We calculated joint range of motion (ROM) to determine a 3D pose-space, using the novel APSE algorithm (Accelerated Pose Searching with Electrostatics). We then estimated muscle moment arms (MMAs) across the entire pose space for all muscles crossing the shoulder and elbow joints. Models of extant species were validated against empirical measures of ROM and MMA derived from ex vivo XROMM (X-ray reconstruction of moving morphology). Among extant species, in both models and experiments, our parasagittal taxon occupied a distinct region of pose-space, with more retracted and depressed shoulder joint angles. MMA data show increased emphasis on shoulder elevation associated with a parasagittal posture, but greater shoulder depression in sprawlers. We hypothesised the fossil taxa would follow trends in these postural variables – e.g., increasing shoulder retraction and elevation MMAs through time – but they instead showed complex, non-linear patterns of forelimb transformation. We demonstrate that the ‘sprawling-parasagittal’ transition is characterized by considerable homoplasy and continuous postural variation throughout mammalian evolution. 

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3. Musculoskeletal modeling untangles the origins of mammal forelimb function and posture; 2023 International Congress of Vertebrate Morphology Abstracts

   Brocklehurst, R_J; Pierce, S_E (July 2023, The Anatomical Record)

   The ‘sprawling–parasagittal’ transition was a major postural shift that occurred in the ancestors of mammals, [PSE1] underpinned by musculoskeletal reorganization of the limbs. However, ‘when’ and ‘how’ this important postural shift occurred is unknown. While the anatomical changes characterizing this transition can be traced through the fossil record, how these relate to functional changes, and the acquisition of parasagittal posture, remains poorly understood. Here, we produced three-dimensional musculoskeletal models of the forelimbs of extant (n=3) and fossil (n=8) taxa that phylogenetically and functionally span the sprawling–parasagittal transition. We calculated joint range of motion (ROM) to determine a 3D pose space, using the novel APSE algorithm (Accelerated Pose Searching with Electrostatics). We then estimated muscle moment arms (MMAs) across the entire pose space for all muscles crossing the shoulder and elbow joints. Models of extant species were validated against empirical measures of ROM and MMA derived from ex vivo XROMM (X-ray reconstruction of moving morphology). Among extant species, our parasagittal taxon occupied a distinct region of pose-space, with more retracted and depressed shoulder joint angles. MMA data show increased emphasis on shoulder elevation associated with a parasagittal posture, but greater shoulder depression in sprawlers. Results from the fossil species show complex, non-linear patterns of forelimb transformation, demonstrating that the ‘sprawling-parasagittal’ transition is characterized by homoplasy and postural variation within the mammalian lineage. 

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4. Adaptive landscapes reveal complex evolution of forelimb posture in stem mammals (Synapsida)

   Brocklehurst, Robert; Mercado, Magdalen; Pierce, Stephanie (January 2023, Integrative and comparative biology)

   The ‘sprawling–parasagittal’ transition was a major postural shift during mammal evolution, but ‘when’ and ‘how’ it occurred has been debated for decades. Previous work focused on a few exceptional fossils from discrete points in time, but broader studies of individual limb elements may provide a more comprehensive evolutionary perspective. Here we address when and how parasagittal forelimb posture evolved in the ancestors of mammals, the non-mammalian synapsids (NMS), using functional adaptive landscape analysis of the humerus bone, incorporating data from morphology, function, and phylogeny, to assess forelimb evolution in deep time. The humerus is subjected to different functional stresses in parasagittal vs. sprawling limbs, and so its morphology is expected to reflect postural differences. We measured humerus shape and various functional traits on a large sample of NMS (n = 61), with a diverse array of extant taxa (n = 140) serving as a robust comparative dataset. We recover distinct adaptive landscapes for extant sprawling and parasagittal taxa, highlighting functional specialization of the humerus associated with different postures. The landscapes for NMS had distinct adaptive peaks from extant sprawlers. While there is repeated evolution of humeri representing ‘transitional’ postures in NMS, humeri consistent with parasagittal posture do not appear until the crown group. Our data reveal the complexity of postural evolution within Synapsida, with the ‘sprawling-parasagittal’ transition typified by considerable homoplasy, and postural variation within individual synapsid clades. 

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5. Shoulder Muscle Architecture in the Echidna (Monotremata: Tachyglossus aculeatus) Indicates Conserved Functional Properties

   https://doi.org/10.1007/s10914-020-09498-6  

   Regnault, Sophie; Fahn-Lai, Philip; Norris, Rachel M.; Pierce, Stephanie E. (March 2020, Journal of Mammalian Evolution)

   Monotremes are a group of egg-laying mammals, possessing a mosaic of ancestral and derived anatomical features. Despite much interest in monotremes from phylogenetic, morphological, and ecological perspectives, they have been the subject of relatively few biomechanical studies. In this study, we examined shoulder and proximal forelimb muscle anatomy and architecture in the short-beaked echidna, Tachyglossus aculeatus, through contrast-enhanced computed tomography and gross dissection. Muscle architecture is a major determinant of muscle function and can indicate specialized muscle roles, such as the capacity for generating large forces (through large physiological cross-sectional area, PCSA) or working ranges (through long fascicle lengths). We hypothesized that some muscles would exhibit architectural specializations convergent with other fossorial and/or sprawling animals, and that other muscles would reflect the echidna’s unusual anatomy and locomotor style. Instead, we found the shoulder and proximal forelimb muscles in echidna to have little variation in their architecture. The muscles generally had long fascicles and small-to-intermediate PCSAs, consistent with force production over a wide working range. Further, muscles did not show overt differences in architecture that, in therian mammals, have been linked to increased forelimb mobility and the transition from sprawling to parasagittal posture. Our measures of architectural disparity placed the echidna closer to the tegu lizard than other sprawling fossorial mammals (e.g., mole). The low architectural diversity found in the echidna’s shoulder and proximal forelimb muscles is interpreted as a lack of functional specialization into distinct roles. We hope our study will contribute to greater understanding of monotreme anatomy and biomechanical function, and to the reconstruction of musculoskeletal evolution in mammals. 

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