A comparative study of cranial allometry reveals common patterns in all Cingulata

Abstract

Allometry represents a pervasive pattern in morphological evolution. Recognition of a common allometric pattern across species requires comparative studies because analyses of size-related shape changes within a species are not sufficient to infer allometric patterns within other species or in an entire clade. A recent work used 3D geometric morphometric methods to study allometric patterns of the entire skull and cranial units at the ontogenetic and static levels in the armadillo Dasypus novemcinctus which we compare to two phylogenetically distant armadillo species in the genera Cabassous and Zaedyus, to identify common patterns of allometry. Here we propose to compare these intraspecific patterns to an evolutionary analysis of allometry, gathering most extant cingulate species and most of the emblematic fossil groups (with a focus on glyptodonts), using the same approach. Our results reveal a widespread craniofacial allometry, i.e., relative skull lengthening and reduction of braincase proportions as size increases, in all cingulates. Our study also demonstrates that an increase in skull size in cingulates is generally accompanied by a relatively greater postorbital constriction, more protruding nuchal crests, broader temporal fossae, and a flatter cranial roof. The analyses conducted on cranial subunits show that widespread allometric patterns are also found more locally, such as for the relative position of the hypoglossal foramen, the proportions of the foramen magnum, and the protrusion of the posterior root of the zygomatic arch and the mastoid process. This analysis also highlights an effect of size on shape variation for internal cranial structures, with a shallower fossa subarcuata and a thickening of the frontal bone resulting from size increases. These results evidence strong and widespread allometric patterns affecting cranial shape variation in cingulates, and represent a solid basis for the establishment of a mapping of strong covariation patterns to be discussed with respect to morphological variation. In addition, this study highlights a particular allometric component of glyptodonts compared to other cingulates, a promising path to explore for future studies on face development in large mammals.