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1. Morphological disparity across the synapsid forelimb: sub-order-level patterns across 80 million years of synapsid evolution

   Lungmus, Jacqueline A. Angielczyk (October 2018, Society of Vertebrate Paleontology 2018 Meeting Program and Abstracts)

   Synapsid evolution can be characterized by three successive radiations: the Permo– Carboniferous pelycosaurs, the Permo–Triassic Therapsida, and the Triassic Eucynodontia. Previous geometric morphometric research at the clade level revealed a continuous increase in humeral morphological disparity in Therapsida, in contrast to their pelycosaur forebearers. Here we present associated data on ulnar morphological disparity, as well an an overall taxonomic expansion of the analyses. This increase in sample size brings the dataset to 765 specimens from which functional units across the forelimb were analyzed. Further, it allows for a more detailed discussion of variance within nearly every major group of early Synapsida, as well as across 80 million years of geologic history. Groups were analyzed for Procrustes variance in 5 million year time bins from 305–225 Mya (Carboniferous–Triassic). In all analyzed functional units—the proximal humerus, distal humerus, and proximal elbow—within group disparity is higher in therapsid families than in pelycosaur families. In addition, therapsid family level disparity is much more variable between groups and across time. Ulnar variance values are higher than humeral values for the entire study period. Procrustes variance for the forelimb decreases across the End Permian Mass Extinction Event in the major therapsid groups that survived it— Anomodontia and Cynodontia. Macroevolutionary changes observed in Synapsida have historically been associated with ecological diversification. Cynodontia and Anomodontia have the highest variance in Therapsida, while Gorgonopsia has the lowest. The high values in Anomodontia, as one of the most taxonomically and ecological diverse clades of Therapsida, suggests that forelimb variance is linked to aspects of ecological diversification. Further, within pelycosaurs Sphenacodontidae has the lowest variance through time, while Ophiacodontidae has the highest. The finding of uniquely high variance levels in Ophiacodontidae, hypothesized by some to be semi-acquatic, is suggestive of a potentially unique forelimb ecomorphology. This research provides evidence that along with major shifts in forelimb morphology, within-family disparity dynamics may have been critical to the evolutionary success of individual synapsid sub-orders. 

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2. Incongruence of morphological disparity and evolutionary rate in the forelimbs of Paleozoic synapsids

   Lungmus, Jacqueline K.; Angielczyk, Kenneth D. (October 2022, Journal of vertebrate paleontology)

   Previous work has shown increased morphological variance within the forelimbs of the Permian synapsid group known as Therapsida over that of their Carboniferous and early Permian forerunners (“pelycosaurs”). Considering that disparity trends have been known to point to underlying macroevolutionary transitions, here we analyzed morphological variance alongside several additional macroevolutionary metrics to better isolate possible evolutionary mechanisms. Shape data was collected on a sample of 119 humeri and 99 ulnae comprising three major synapsid radiations with a temporal range from the Carboniferous into the Triassic. Taxonomic sample included all major groups of pelycosaur-grade synapsids, all five recognized non-cynodontian therapsid clades, and a sample of pre-prozostrodontian cynodonts. Procrustes variance - a multivariate quantification of morphospace occupation - was the chosen disparity metric for the study. Rate of phenotypic change, which considers the amount of shape change that would be necessary to achieve observed morphologies given the shape of the closely related taxa, was analyzed as the metric for evolutionary rate. Both metrics were considered through-time upon genera present in sequential 5 million year time bins. Our results expand upon previous findings that disparity increases throughout the earliest stages of the Permian, coincident with the diversification of pelycosaurs and the emergence of Therapsida. This expanded dataset further shows that disparity approaches an asymptote around 270 million years ago and only increases marginally through the late Permian, remaining between 0.018–0.021 from 275-245 mya. In contrast, evolutionary rate does not appear to asymptote during this same interval, starting at a low of 6.17e-6 (300 mya) and increasing to a peak of 1.78e-5 right before the End Permian Mass Extinction Event (252 mya). The continuing increase of evolutionary rate shows that morphological change continues across taxa, but the plateauing of morphological disparity suggests that morphospace is not expanding concurrent with this. The incongruence between these two metrics suggests a critical change in evolutionary mode, wherein morphological change continues rapidly but does not result in the evolution of novel morphologies. These results provide some of the strongest quantitative data yet of an evolutionary constraint acting upon the morphology of the synapsid forelimb through deep time. 

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3. Evolutionary modularity, integration and disparity in an accretionary skeleton: analysis of venerid Bivalvia

   https://doi.org/10.1098/rspb.2021.1199  

   Edie, Stewart M.; Khouja, Safia C.; Collins, Katie S.; Crouch, Nicholas M.; Jablonski, David (January 2022, Proceedings of the Royal Society B: Biological Sciences)

   Modular evolution, the relatively independent evolution of body parts, may promote high morphological disparity in a clade. Conversely, integrated evolution via stronger covariation of parts may limit disparity. However, integration can also promote high disparity by channelling morphological evolution along lines of least resistance—a process that may be particularly important in the accumulation of disparity in the many invertebrate systems having accretionary growth. We use a time-calibrated phylogenetic hypothesis and high-density, three-dimensional semilandmarking to analyse the relationship between modularity, integration and disparity in the most diverse extant bivalve family: the Veneridae. In general, venerids have a simple, two-module parcellation of their body that is divided into features of the calcium carbonate shell and features of the internal soft anatomy. This division falls more along developmental than functional lines when placed in the context of bivalve anatomy and biomechanics. The venerid body is tightly integrated in absolute terms, but disparity appears to increase with modularity strength among subclades and ecologies. Thus, shifts towards more mosaic evolution beget higher morphological variance in this speciose family. 

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4. Functional diversity of small-mammal postcrania is linked to both substrate preference and body size

   https://doi.org/10.1093/cz/zoaa057  

   Weaver, Lucas N; Grossnickle, David M (September 2020, Current Zoology)

   Muñoz, Martha (Ed.)

   Abstract Selective pressures favor morphologies that are adapted to distinct ecologies, resulting in trait partitioning among ecomorphotypes. However, the effects of these selective pressures vary across taxa, especially because morphology is also influenced by factors such as phylogeny, body size, and functional trade-offs. In this study, we examine how these factors impact functional diversification in mammals. It has been proposed that trait partitioning among mammalian ecomorphotypes is less pronounced at small body sizes due to biomechanical, energetic, and environmental factors that favor a “generalist” body plan, whereas larger taxa exhibit more substantial functional adaptations. We title this the Divergence Hypothesis (DH) because it predicts greater morphological divergence among ecomorphotypes at larger body sizes. We test DH by using phylogenetic comparative methods to examine the postcranial skeletons of 129 species of taxonomically diverse, small-to-medium-sized (<15 kg) mammals, which we categorize as either “tree-dwellers” or “ground-dwellers.” In some analyses, the morphologies of ground-dwellers and tree-dwellers suggest greater between-group differentiation at larger sizes, providing some evidence for DH. However, this trend is neither particularly strong nor supported by all analyses. Instead, a more pronounced pattern emerges that is distinct from the predictions of DH: within-group phenotypic disparity increases with body size in both ground-dwellers and tree-dwellers, driven by morphological outliers among “medium”-sized mammals. Thus, evolutionary increases in body size are more closely linked to increases in within-locomotor-group disparity than to increases in between-group disparity. We discuss biomechanical and ecological factors that may drive these evolutionary patterns, and we emphasize the significant evolutionary influences of ecology and body size on phenotypic diversity. 

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5. Cambrian origin but no early burst in functional disparity for Class Bivalvia

   https://doi.org/10.1098/rsbl.2023.0157  

   Zhou, Sharon; Edie, Stewart M.; Collins, Katie S.; Crouch, Nicholas M.; Jablonski, David (May 2023, Biology Letters)

   Both the Cambrian explosion, more than half a billion years ago, and its Ordovician aftermath some 35 Myr later, are often framed as episodes of widespread ecological opportunity, but not all clades originating during this interval showed prolific rises in morphological or functional disparity. In a direct analysis of functional disparity, instead of the more commonly used proxy of morphological disparity, we find that ecological functions of Class Bivalvia arose concordantly with and even lagged behind taxonomic diversification, rather than the early-burst pattern expected for clades originating in supposedly open ecological landscapes. Unlike several other clades originating in the Cambrian explosion, the bivalves' belated acquisition of key anatomical novelties imposed a macroevolutionary lag, and even when those novelties evolved in the Early Ordovician, functional disparity never surpassed taxonomic diversity. Beyond this early period of animal evolution, the founding and subsequent diversification of new major clades and their functions might be expected to follow the pattern of the early bivalves—one where interactions between highly dynamic environmental and biotic landscapes and evolutionary contingencies need not promote prolific functional innovation. 

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