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

Abstract

BackgroundPrevious studies have demonstrated that the clear morphological differences among vertebrae across the presacral column are accompanied by heterogeneous functional signals in vertebral shape. Further, several lines of evidence suggest that the mammalian axial skeleton is a highly modular structure. These include its composition of serial units, a trade-off between high shape variance and strong conservation of vertebral count, and direct association of regions with anterior expression sites of Hox genes. Here we investigate the modular organisation of the presacral vertebral column of modern cats (Felidae, Carnivora, Mammalia) with pairwise comparisons of vertebral shape covariation (i.e. integration) and evaluate our results against hypotheses of developmental and functional modularity. We used three-dimensional geometric morphometrics to quantify vertebral shape and then assessed integration between pairs of vertebrae with phylogenetic two-block partial least square analysis (PLS).ResultsSix modules were identified in the pairwise analyses (vertebrae included are designated as ‘C’ for cervical, ‘T’ for thoracic, and ‘L’ for lumbar): an anterior module (C1 to T1); a transitional module situated between the last cervicals and first thoracics (C6 to T2); an anterior to middle thoracic set (T4 to T8); an anticlinal module (T10 and T11); a posterior set composed of the last two thoracics and lumbars (T12 to L7); and a module showing covariation between the cervicals and the posterior set (T12 to L7). These modules reflect shared developmental pathways, ossification timing, and observed ecological shape diversification in living species of felids.ConclusionsWe show here that patterns of shape integration reflect modular organisation of the vertebral column of felids. Whereas this pattern corresponds with hypotheses of developmental and functional regionalisation in the axial skeleton, it does not simply reflect major vertebral regions. This modularity may also have permitted vertebral partitions, specifically in the posterior vertebral column, to be more responsive to selection and achieve higher morphological disparity than other vertebral regions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-017-0975-2) contains supplementary material, which is available to authorized users.

Phenotypic integration of the cervical vertebrae in the Hominoidea (Primates)

“…(). Dashed lines indicate relationships that were hypothesized by Randau and Goswami () but not tested in that study.…”

“…Three recent studies have shown that phenotypic integration patterns of the cervical vertebral column are both highly conserved and highly predictable in felids and canids (Arnold et al. ; Randau and Goswami , b), generally in line with the predicted genetic and functional modules (Kessel et al. ; Kessel and Gruss ; Buchholtz et al.…”

“…, b). Randau and Goswami () report a generalized cervical module with a C6 through T2 transitional zone and no clear distinction of C1–C2. Although they did not include vertebrae C3 and C5, they estimate that there would not be a distinguishable C3 through C5 module (Randau and Goswami, ).…”

“…Randau and Goswami () report a generalized cervical module with a C6 through T2 transitional zone and no clear distinction of C1–C2. Although they did not include vertebrae C3 and C5, they estimate that there would not be a distinguishable C3 through C5 module (Randau and Goswami, ). They also found that generally vertebrae can be separated into vertebral body and vertebral spine modules, but that this was not the case for the C4 (Randau and Goswami , b).…”

“…). Randau and Goswami (, b), thus suggest a module that possibly includes C1 through C5, with a C6–C7 transitional zone to the thoracic vertebrae, and ventral and dorsal aspects to each cervical vertebra, whereas Arnold et al. () suggest a C3–C4–C5 module and a C6–C7 module, with each vertebra composed of a single unit rather than ventral and dorsal aspects.…”