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1. Ontogenetic allometry and scaling in catarrhine crania

   https://doi.org/10.1111/joa.13331  

   Simons, Evan A. ; Frost, Stephen R. ( October 2020 , Journal of Anatomy)

   In studies of ontogenetic allometry, ontogenetic scaling has often been invoked to explain cranial morphological differences between smaller and larger forms of closely related taxa. These scaled variants in shape have been hypothesized to be the result of the extension or truncation of common growth allometries. In this scenario, change in size is the determining factor, perhaps under direct selection, and changes in cranial shapes are byproducts, not under direct selection themselves. However, many of these conclusions are based on studies that used bivariate generalizations of shape. Even among multivariate analyses of growth allometries, there are discrepancies as to the prevalence of ontogenetic scaling among primates, how shared the trajectories need to be, and which taxa evince properties of scaled variants. In this investigation, we use a large, comparative ontogenetic sample, geometric morphometric methods, and multivariate statistical tests to examine ontogenetic allometry and evaluate if differences in cranial shape among closely related catarrhines of varying sizes are primarily driven by size divergence, that is, ontogenetic scaling. We then evaluate the hypothesis of size as a line of least evolutionary resistance in catarrhine cranial evolution. We found that patterns of ontogenetic allometry vary among taxa, indicating that ontogenetic scalingsensu strictodoes not often account for most morphological differences and that large and small taxa within clades are generally not scaled variants. The presence of a variety of ontogenetic pathways for the evolution of cranial shapes provides indirect evidence for selection acting directly on the cranial shape, rather than on size alone.

    

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2. Ontogenetic diversity buffers communities against consequences of species loss

   https://doi.org/10.1111/1365-2656.13470  

   Rudolf, Volker H. W. ; Eveland, Lauren ( March 2021 , Journal of Animal Ecology)

   Biodiversity can be measured at multiple organizational scales. While traditional studies have focused at taxonomic diversity, recent studies have emphasized the ecological importance of diversity within populations. However, it is unclear how these different scales of diversity interact to determine the consequence of species loss.

   Here we asked how predator diversity and presence of ontogenetic diversity within predator populations influences community structure. Ontogenetic diversity arises from shifts in the traits and ecology of individuals during ontogeny and it is one of the biggest sources of intraspecific diversity. However, whether it dampens or strengthens the negative consequences of with species loss is poorly understood.

   To study the interaction of species diversity and ontogenetic diversity, we experimentally manipulated predator species diversity and diversity of developmental stages within focal predator species and analysed their joint effect on predator and prey survival, biomass and prey community structure in experimental pond systems.

   While individual effects of ontogenetic diversity were often species specific, losing predator species from the community often had a much smaller or no effect on prey survival, biomass or community structure when all predator populations had high ontogenetic diversity. Thus, ontogenetic diversity within populations buffered against some of the consequences of biodiversity loss at higher organizational levels. Because the experiment controlled mean per capita size and biomass across structured versus unstructured populations, this pattern was not driven by differences in biomass of predators. Instead, results suggest that effects were driven by changes in the functional roles and indirect interactions across and within species. This indicates that even if all environmental conditions are similar, differences in the intrinsic structure of populations can modify the consequences of biodiversity loss.

   Together, these results revealed the importance of ontogenetic diversity within species for strengthening the resilience of natural communities to consequences of biodiversity loss and emphasize the need to integrate biodiversity patterns across organizational scales.

    

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3. Comparative digital reconstruction of Pica pica and Struthio camelus and their cranial suture ontogenies

   https://doi.org/10.1002/ar.25275  

   Plateau, Olivia ; Green, Todd L. ; Gignac, Paul M. ; Foth, Christian ( June 2023 , The Anatomical Record)

   To date, several studies describe post‐hatching ontogenetic variation in birds; however, none of these studies document and compare ontogenetic variation of the entire skull in multiple avian species. Therefore, we studied ontogenetic skull variation of two bird species with very different ecologies,Pica pica, andStruthio camelus, using μCT based 3D reconstructions. For each specimen, we performed bone‐by‐bone segmentation in order to visualize and describe the morphological variation of each bone during ontogeny and estimated the average sutural closure of the skulls to identify different ontogenetic stages. Although bone fusion ofP.picaoccurs more rapidly than that ofS.camelusthe general sequence of bone fusion follows a similar trend from posterior to anterior, but a more detailed analysis reveals some interspecific variation in the fusion patterns. Although growth persists over a longer period inS.camelusthan inP.picaand adults of the former species are significantly larger, the skull of the most matureS.camelusis still less fused than that ofP.pica. Different growth and fusion patterns of the two species indicate that the interspecific ontogenetic variation could be related to heterochronic developments. Nevertheless, this hypothesis needs to be tested in a broader phylogenetic framework in order to detect the evolutionary direction of the potential heterochronic transformations.

    

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4. The Forearm Musculature of the Gray Mouse Lemur ( Microcebus murinus ): An Ontogenetic Study

   https://doi.org/10.1002/ar.24258  

   Boettcher, Marissa L. ; Leonard, Kaitlyn C. ; Dickinson, Edwin ; Aujard, Fabienne ; Herrel, Anthony ; Hartstone‐Rose, Adam ( October 2019 , The Anatomical Record)

   Although studies have sought to characterize variation in forearm muscular anatomy across the primate order, none have attempted to quantify ontogenetic changes in forearm myology within a single taxon. Herein, we present muscle architecture data for the forearm musculature (flexors and extensors of the wrist and digits) ofMicrocebus murinus, a small Lemuroid that has been the focus of several developmental studies. A quadratic curvilinear model described ontogenetic changes in muscle mass and fascicle length; however, fascicle lengths reached peak levels at an earlier age and showed a stronger decline during senescence. Conversely, physiological cross‐sectional area followed a more linear trend, increasing steadily throughout life. As previous studies into the functional role of the primate forelimb emphasize the importance of long muscle fascicles within arboreal taxa in order to maximize mobility and flexibility, the early attainment of peak fascicle lengths may consequently reflect the importance of agility within this mobile and highly arboreal species. Similarly, observed myological trends in forearm strength are supported by previousin vivodata on grip strength withinM. murinusin which senescent individuals showed no decline in forearm force relative to prime age individuals. This trend is interpreted to reflect compensation for the previously reported decline in hind limb grip strength in the hind limb with age, such that older individuals are able to maintain arboreal stability. Interestingly, the ontogenetic trajectory of each architectural variable mirrored previous observations of the masticatory musculature inM. murinus, suggesting that ontogenetic trends are relatively conserved between anatomical regions. Anat Rec, 303:1354–1363, 2020. © 2019 American Association for Anatomy

    

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5. Are endocasts good proxies for brain size and shape in archosaurs throughout ontogeny?

   https://doi.org/10.1111/joa.12918  

   Watanabe, Akinobu ; Gignac, Paul M. ; Balanoff, Amy M. ; Green, Todd L. ; Kley, Nathan J. ; Norell, Mark A. ( December 2018 , Journal of Anatomy)

   Cranial endocasts, or the internal molds of the braincase, are a crucial correlate for investigating the neuroanatomy of extinct vertebrates and tracking brain evolution through deep time. Nevertheless, the validity of such studies pivots on the reliability of endocasts as a proxy for brain morphology. Here, we employ micro‐computed tomography imaging, including diffusible iodine‐based contrast‐enhancedCT, and a three‐dimensional geometric morphometric framework to examine both size and shape differences between brains and endocasts of two exemplar archosaur taxa – the American alligator (Alligator mississippiensis) and the domestic chicken (Gallus gallus). With ontogenetic sampling, we quantitatively evaluate how endocasts differ from brains and whether this deviation changes during development. We find strong size and shape correlations between brains and endocasts, divergent ontogenetic trends in the brain‐to‐endocast correspondence between alligators and chickens, and a comparable magnitude between brain–endocast shape differences and intraspecific neuroanatomical variation. The results have important implications for paleoneurological studies in archosaurs. Notably, we demonstrate that the pattern of endocranial shape variation closely reflects brain shape variation. Therefore, analyses of endocranial morphology are unlikely to generate spurious conclusions about large‐scale trends in brain size and shape. To mitigate any artifacts, however, paleoneurological studies should consider the lower brain–endocast correspondence in the hindbrain relative to the forebrain; higher size and shape correspondences in chickens than alligators throughout postnatal ontogeny; artificially ‘pedomorphic’ shape of endocasts relative to their corresponding brains; and potential biases in both size and shape data due to the lack of control for ontogenetic stages in endocranial sampling.

    

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