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1. Morphological integration and cranial modularity in six genera of echimyid rodents (Rodentia: Echimyidae)

   https://doi.org/10.1093/jmammal/gyab151  

   Raidan, Carolline; de Andrade Costa, Bárbara Maria; Marroig, Gabriel; Aprígio Assis, Ana Paula; Paresque, Roberta (December 2021, Journal of Mammalogy)

   Rowe, Kevin (Ed.)

   Abstract Rodents of the family Echimyidae present a wide variety of life histories and ecomorphological adaptations. This study evaluated morphological integration patterns, modularity, and evolutionary flexibility in six Echimyid genera representing ecomorphological extremes within the family. The relationships between traits were evaluated by comparing estimated covariance and correlations matrices of populations. The presence of modules was investigated by comparing the patterns of integration between traits and using hypothetical matrices based on shared development/function and masticatory stress. The results point to a common covariance and correlation pattern among the six echimyid genera, suggesting a conserved pattern of covariation (associations among traits) throughout the evolution of this group. The overall magnitude of integration, however, varied greatly. We also found a high degree of modularity in all six echimyid genera. Finally, we observed a clear association between flexibility, i.e., the ability of a species to respond to the direction of selection, with the overall magnitude of integration and degree of modularization. The results of this study provide hypotheses concerning the underlying effects of the association among traits, which may have facilitated or constrained the evolution of morphological variation in the diverse family Echimyidae. 

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2. Simultaneous integration and modularity underlie the exceptional body shape diversification of characiform fishes

   https://doi.org/10.1093/evolut/qpac070  

   Burns, Michael D; Collyer, Michael L; Sidlauskas, Brian L (December 2022, Evolution)

   Abstract Evolutionary biology has long striven to understand why some lineages diversify exceptionally while others do not. Most studies have focused on how extrinsic factors can promote differences in diversification dynamics, but a clade’s intrinsic modularity and integration can also catalyze or restrict its evolution. Here, we integrate geometric morphometrics, phylogenetic comparative methods and visualizations of covariance to infer the presence of distinct modules in the body plan of Characiformes, an ecomorphologically diverse fish radiation. Strong covariances reveal a cranial module, and more subtle patterns support a statistically significant subdivision of the postcranium into anterior (precaudal) and posterior (caudal) modules. We uncover substantial covariation among cranial and postcranial landmarks, indicating body-wide evolutionary integration as lineages transition between compressiform and fusiform body shapes. A novel method of matrix subdivision reveals that within- and among-module covariation contributes substantially to the overall eigenstructure of characiform morphospace, and that both phenomena led to biologically important divergence among characiform lineages. Functional integration between the cranium and post-cranial skeleton appears to have allowed lineages to optimize the aspect ratio of their bodies for locomotion, while the capacity for independent change in the head, body and tail likely eased adaptation to diverse dietary and hydrological regimes. These results reinforce a growing consensus that modularity and integration synergize to promote diversification. 

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3. 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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4. Evolutionary Integration and Modularity in the Archosaur Cranium

   https://doi.org/10.1093/icb/icz052  

   Felice, Ryan N.; Watanabe, Akinobu; Cuff, Andrew R.; Noirault, Eve; Pol, Diego; Witmer, Lawrence M.; Norell, Mark A.; O'Connor, Patrick M.; Goswami, Anjali (May 2019, Integrative and Comparative Biology)

   Abstract Complex structures, like the vertebrate skull, are composed of numerous elements or traits that must develop and evolve in a coordinated manner to achieve multiple functions. The strength of association among phenotypic traits (i.e., integration), and their organization into highly-correlated, semi-independent subunits termed modules, is a result of the pleiotropic and genetic correlations that generate traits. As such, patterns of integration and modularity are thought to be key factors constraining or facilitating the evolution of phenotypic disparity by influencing the patterns of variation upon which selection can act. It is often hypothesized that selection can reshape patterns of integration, parceling single structures into multiple modules or merging ancestrally semi-independent traits into a strongly correlated unit. However, evolutionary shifts in patterns of trait integration are seldom assessed in a unified quantitative framework. Here, we quantify patterns of evolutionary integration among regions of the archosaur skull to investigate whether patterns of cranial integration are conserved or variable across this diverse group. Using high-dimensional geometric morphometric data from 3D surface scans and computed tomography scans of modern birds (n = 352), fossil non-avian dinosaurs (n = 27), and modern and fossil mesoeucrocodylians (n = 38), we demonstrate that some aspects of cranial integration are conserved across these taxonomic groups, despite their major differences in cranial form, function, and development. All three groups are highly modular and consistently exhibit high integration within the occipital region. However, there are also substantial divergences in correlation patterns. Birds uniquely exhibit high correlation between the pterygoid and quadrate, components of the cranial kinesis apparatus, whereas the non-avian dinosaur quadrate is more closely associated with the jugal and quadratojugal. Mesoeucrocodylians exhibit a slightly more integrated facial skeleton overall than the other grades. Overall, patterns of trait integration are shown to be stable among archosaurs, which is surprising given the cranial diversity exhibited by the clade. At the same time, evolutionary innovations such as cranial kinesis that reorganize the structure and function of complex traits can result in modifications of trait correlations and modularity. 

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5. Convergence and contingency in the evolution of a specialized mode of life: multiple origins and high disparity of rock-boring bivalves

   https://doi.org/10.1098/rspb.2022.1907  

   Collins, Katie S.; Edie, Stewart M.; Jablonski, David (February 2023, Proceedings of the Royal Society B: Biological Sciences)

   Evolutionary adaptation to novel, specialized modes of life is often associated with a close mapping of form to the new function, resulting in narrow morphological disparity. For bivalve molluscs, endolithy (rock-boring) has biomechanical requirements thought to diverge strongly from those of ancestral functions. However, endolithy in bivalves has originated at least eight times. Three-dimensional morphometric data representing 75 species from approximately 94% of extant endolithic genera and families, along with 310 non-endolithic species in those families, show that endolithy is evolutionarily accessible from many different morphological starting points. Although some endoliths appear to converge on certain shell morphologies, the range of endolith shell form is as broad as that belonging to any other bivalve substrate use. Nevertheless, endolithy is a taxon-poor function in Bivalvia today. This limited richness does not derive from origination within source clades having significantly low origination or high extinction rates, and today's endoliths are not confined to low-diversity biogeographic regions. Instead, endolithy may be limited by habitat availability. Both determinism (as reflected by convergence among distantly related taxa) and contingency (as reflected by the endoliths that remain close to the disparate morphologies of their source clades) underlie the occupation of endolith morphospace. 

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