Ontogenetic and static allometry in the skull and cranial units of nine-banded armadillos (Cingulata: Dasypodidae:Dasypus novemcinctus)

Abstract

A large part of extant and past mammalian morphological diversity is related to variation in size through allometric effects. Previous studies suggested that craniofacial allometry is the dominant pattern underlying mammalian skull shape variation, but cranial allometries were rarely characterized within cranial units such as individual bones. Here, we used 3D geometric morphometric methods to study allometric patterns of the whole skull (global) and of cranial units (local) in a postnatal developmental series of nine-banded armadillos (Dasypus novemcinctus ssp.). Analyses were conducted at the ontogenetic and static levels, and for successive developmental stages. Our results support craniofacial allometry as the global pattern along with more local allometric trends, such as the relative posterior elongation of the infraorbital canal, the tooth row reduction on the maxillary, and the marked development of nuchal crests on the supraoccipital with increasing skull size. Our study also reports allometric proportions of shape variation varying substantially among cranial units and across ontogenetic stages. The multi-scale approach advocated here allowed unveiling previously unnoticed allometric variations, indicating an untapped complexity of cranial allometric patterns to further explain mammalian morphological evolution.

Common Patterns of Skull Bone Fusion and Their Potential to Discriminate Different Ontogenetic Stages in Extant Birds

“…This is different from the situation in mammals, where ontogenetic fusion of bones is common but varies both on the interspecific and intraspecific level and cannot be used as general proxy for ontogeny-only as specific proxy for particular subclades (Rager et al, 2013). Only the bones from the occipital region seem to share a common pattern among mammals (Rager et al, 2013) even if for instance in Cingulata some adults can still have visible sutures (Le Verger et al, 2020). Thus, although all three amniote groups show bone fusion in the skull, the strong ontogenetic signal detected in this study are most likely linked to high degree of fusion in adult crown birds (Plateau and Foth, 2020), which correlates with a general decreasing trend of sutural disparity.…”

Exceptional Changes in Skeletal Anatomy under Domestication: The Case of Brachycephaly

“…Second, facial length scales positively allometrically with total cranial length or body size, both within domestic species, e.g., dogs ( Lumer 1940 and references therein) and among closely related species of various mammalian clades, sharing a similar cranial bauplan (cranial evolutionary allometry hypothesis or “CREA”; Radinsky 1985 ; Emerson and Bramble 1993 ; Cardini and Polly 2013 ; Cardini et al 2015 ; Tamagnini, Meloro and Cardini 2017 ; Cardini 2019 ; Le Verger et al 2020 ) and birds ( Bright et al 2016 ; Linde-Medina 2016 ; Tokita et al. 2017 ).…”

Comparative anatomy and phylogenetic contribution of intracranial osseous canals and cavities in armadillos and glyptodonts (Xenarthra, Cingulata)

“…Mammals generally show an allometric elongation of the rostrum as size increases, known as craniofacial allometry (Cardini & Polly, 2013;Cardini, 2019). In Dasypus, this allometric pattern mostly affects the proportions of the rostrum horizontally, much less vertically (Le Verger et al, 2020). The possible allometric increase in the height of the dentition in cingulates thus seems unrelated to craniofacial allometry but this certainly requires further testing.…”

“…The course of the nasolacrimal canal barely changes during ontogeny in our sample (Figure 3). Its length changes and follows the lengthening of the snout that accompanies the growth of the cranium (Le Verger et al, 2020). In addition, Cabassous and, to a lesser degree, the adult stage of Zaedyus show a more medial orientation of the course of the nasolacrimal canal in its most posterior part which differs slightly from Dasypus and other stages of Zaedyus, where it runs more parallel to the external surface of the cranium (Figure 3).…”