Unravelling intravertebral integration, modularity and disparity in Felidae (Mammalia)

Abstract

SUMMARY Morphological integration and modularity, which describe the relationships among morphological attributes and reflect genetic, developmental, and functional interactions, have been hypothesized to be major influences on trait responses to selection and thus morphological evolution. The mammalian presacral vertebral column shows little variation in vertebral count and therefore specialization for function occurs primarily through modification of vertebral shape. However, vertebral shape has been suggested to be under strong control from developmental canalization, although this has never been explicitly tested. Here, we assess hypotheses of developmental modules in the vertebrae of felids to determine whether developmental interactions are a primary influence on vertebral modularity. Additionally, we analyze the magnitudes of both intravertebral integration and disparity to evaluate if level of integration varies along the vertebral column and, if so, whether integration and disparity are associated. Our results confirm the hypothesis of vertebral developmental modularity, with most presacral vertebrae displaying two modules. Exceptions are concentrated in the boundaries among traditional and functional regions, suggesting that intravertebral modularity may reflect larger‐scale modularity of the felid vertebral column. We further demonstrate that overall integration and disparity are highest in posterior vertebrae, thus providing an empirical example of integration potentially promoting greater morphological responses to selection.

Morphological modularity in the vertebral column of Felidae (Mammalia, Carnivora)

“…[4, 29, 40]). One example of a hypothesised vertebral module is composed of the mid-cervicals C3 to C5.…”

“…The observation of regionalised patterns of allometric scaling in a linear morphometric study both supported the mid-cervical vertebral module and suggested the presence of three additional modules: an anterior cervicothoracic module, a lumbar module, and a functional ‘anticlinality module’ composed of the T10-T12 vertebrae [29]. Additionally, we have previously demonstrated that presacral vertebral shape in felids is driven by the developmental origins of vertebral components, with two morphological modules found in adult vertebral shape: the ‘centrum’ and the ‘neural spine-related’ modules (referred to as the ‘developmental two-module model’ therein to reflect the different somitic origins of these modules; [27, 40, 42]). Interestingly, this model of modularity, although widespread through most the presacral column, was not supported in vertebrae that are positioned immediately at or adjacent to the borders of morphological vertebral column regions: specifically, C4, T1, T8, L6 and L7.…”

“…Interestingly, this model of modularity, although widespread through most the presacral column, was not supported in vertebrae that are positioned immediately at or adjacent to the borders of morphological vertebral column regions: specifically, C4, T1, T8, L6 and L7. This observation led to the suggestion of a disruption of developmental modularity – or a functional overprint – in order to maintain the larger modular organisation of the vertebral column [40]. …”

“…Although there have been recent additions to the literature on the morphological, biomechanical and developmental changes to the vertebral column in mammals or across vertebrates in general [6, 10, 40, 43–47], much is yet unknown on its evolution and how patterns of trait integration or modularity may affect its response to selection [48]. Here we analyse patterns of shape covariation across the presacral vertebral column in order to quantify the modular organisation of the axial skeleton in felids.…”

“…cervical, thoracic and lumbar) in the felid vertebral column form discrete morphological modules [4, 6, 21, 23, 30]; 2) the ‘cervicothoracic and lumbar modules’ hypothesis: Two modules composed of multiple vertebrae that share a common allometric pattern [29] can be found in the presacral axial skeleton: an anterior cervicothoracic module (where vertebrae show positive allometry related to centrum and neural spine dimensions) and a lumbar module (with positive allometry of traits related to the neural spine lever arm) [29]; 3) the ‘thoracolumbar’ hypothesis: Thoracic and lumbar vertebrae show high covariation [4, 11, 21, 23, 41]; 4) the ‘anticlinality’ hypothesis: Vertebrae T10 to T12 compose an ‘anticlinality module’ [29]; and 5) the ‘developmental model disruption’ hypothesis: Boundaries of modular organisation of the vertebral column match vertebral positions where the intravertebral developmental two-module (centrum and neural spine) model is not supported, specifically at the edges of the C3 – C5 cervical module, between cervicals and thoracics (i.e. at T1), the division of the vertebral column into pre- and postdiaphragmatic regions at T8, and at the last two presacral vertebrae L6 and L7 [40].

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