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1. 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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2. Phylogeny, function and ecology in the deep evolutionary history of the mammalian forelimb

   https://doi.org/10.1098/rspb.2021.0494  

   Lungmus, Jacqueline K.; Angielczyk, Kenneth D. (April 2021, Proceedings of the Royal Society B: Biological Sciences)

   Mammals are the only living members of the larger clade Synapsida, which has a fossil record spanning 320 Ma. Despite the fact that much of the ecological diversity of mammals has been considered in the light of limb morphology, the ecological comparability of mammals to their fossil forerunners has not been critically assessed. Because of the wide use of limb morphology in testing ecomorphological hypothesis about extinct tetrapods, we sought: (i) to estimate when in synapsid history, modern mammals become analogues for predicting fossil ecologies; (ii) to document examples of ecomorphological convergence; and (iii) to compare the functional solutions of distinct synapsid radiations. We quantitatively compared the forelimb shapes of the multiple fossil synapsid radiations to a broad sample of extant Mammalia representing a variety of divergent locomotor ecologies. Our results indicate that each synapsid radiation explored different areas of morphospace and arrived at functional solutions that reflected their distinctive ancestral morphologies. This work counters the narrative of non-mammalian synapsid forelimb evolution as a linear progression towards more mammalian morphologies. Instead, a disparate array of early-evolving shapes subsequently contracted towards more mammal-like forms. 

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3. Derived faunivores are the forerunners of major synapsid radiations

   https://doi.org/10.1038/s41559-023-02200-y  

   Hellert, Spencer M; Grossnickle, David M; Lloyd, Graeme T; Kammerer, Christian F; Angielczyk, Kenneth D (October 2023, Nature Ecology & Evolution)

   Evolutionary radiations generate most of Earth’s biodiversity, but are there common ecomorphological traits among the progenitors of radiations? In Synapsida (the mammalian total group), ‘small-bodied faunivore’ has been hypothesized as the ancestral state of most major radiating clades, but this has not been quantitatively assessed across multiple radiations. To examine macroevolutionary patterns in a phylogenetic context, we generated a time-calibrated metaphylogeny (‘metatree’) comprising 1,888 synapsid species from the Carboniferous through the Eocene (305–34 Ma) based on 269 published character matrices. We used comparative methods to investigate body size and dietary evolution during successive synapsid radiations. Faunivory is the ancestral dietary regime of each major synapsid radiation, but relatively small body size is only established as the common ancestral state of radiations near the origin of Mammaliaformes in the Late Triassic. The faunivorous ancestors of synapsid radiations typically have numerous novel characters compared with their contemporaries, and these derived traits may have helped them to survive faunal turnover events and subsequently radiate. 

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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. 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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