Comparative morphology of snake (Squamata) endocasts: evidence of phylogenetic and ecological signals

Abstract

Brain endocasts obtained from computed tomography (CT) are now widely used in the field of comparative neuroanatomy. They provide an overview of the morphology of the brain and associated tissues located in the cranial cavity. Through anatomical comparisons between species, insights on the senses, the behavior, and the lifestyle can be gained. Although there are many studies dealing with mammal and bird endocasts, those performed on the brain endocasts of squamates are comparatively rare, thus limiting our understanding of their morphological variability and interpretations. Here, we provide the first comparative study of snake brain endocasts in order to bring new information about the morphology of these structures. Additionally, we test if the snake brain endocast encompasses a phylogenetic and/or an ecological signal. For this purpose, the digital endocasts of 45 snake specimens, including a wide diversity in terms of phylogeny and ecology, were digitized using CT, and compared both qualitatively and quantitatively. Snake endocasts exhibit a great variability. The different methods performed from descriptive characters, linear measurements and the outline curves provided complementary information. All these methods have shown that the shape of the snake brain endocast contains, as in mammals and birds, a phylogenetic signal but also an ecological one. Although phylogenetically related taxa share several similarities between each other, the brain endocast morphology reflects some notable ecological trends: e.g. (i) fossorial species possess both reduced optic tectum and pituitary gland; (ii) both fossorial and marine species have cerebral hemispheres poorly developed laterally; (iii) cerebral hemispheres and optic tectum are more developed in arboreal and terrestrial species.

Endocranial anatomy of plesiosaurians (Reptilia, Plesiosauria) from the Late Cretaceous (Turonian) of Goulmima (southern Morocco)

“…Plesiosaurian olfactory tracts, elongated anteroposteriorly and not separated along their length (Fig. 3), are similar to those reported in mosasauroids and nothosaurians (Camp, 1942;Voeten et al, 2018) but differ from the short olfactory tracts observed in turtles (Paulina-Carabajal et al, 2013) and the more or less separated olfactory tracts found in snakes (Allemand et al, 2017c), thalattosuchians (e.g., Pierce et al, 2017) and ichthyosaurs (e.g., Marek et al, 2015). Plesiosaurian endocasts are nearly linear (Fig.…”

“…3), both the anterior and posterior parts being at the same dorsoventral level, similarly as reported in snakes (Allemand et al, 2017c), thalattosuchians (e.g., Pierce et al, 2017), nothosaurians (Voeten et al, 2018) and turtles (Paulina-Carabajal et al, 2013), a condition differing from that of Placodus gigas (Neenan and Scheyer, 2012), in which the structure appears anteroposteriorly inclined and sigmoidal. The horizontal projection of the plesiosaurian pituitary differs from the tilted one observed in snakes (Allemand et al, 2017c) and thalattosuchians (e.g., Pierce et al, 2017). In plesiosaurians, the cerebellum seems strongly developed and easily discernible (Fig.…”

“…data 2017b), ichthyosaurians (Marek et al, 2015) and Triassic sauropterygians (Neenan and Scheyer, 2012;Voeten et al, 2018). On the contrary, although a pineal complex occurs in extant turtles and snakes (Wyneken, 2003(Wyneken, , 2007, the structure is covered by the dorsal longitudinal vein (Aurboonyawat et al, 2008) and not visible from the endocast (Paulina-Carabajal et al, 2013;Allemand et al, 2017c). In this study, we hypothesize that the bulge visible on the dorsal surface of the endocast of the three plesiosaurian specimens (Fig.…”

Endocranial shape variation in the squirrel‐related clade and their fossil relatives using 3D geometric morphometrics: contributions of locomotion and phylogeny to brain shape

“…Nevertheless, the addition of earlier aplodontid rodents will be needed to test this hypothesis further. Allemand et al (2017) analyzed the endocranial shape of snakes. Based on their PC analysis, they found that fossorial species had distinct endocranial shapes but arboreal and terrestrial species mostly overlap.…”

Exploring the functional meaning of head shape disparity in aquatic snakes

“…water or soil). A resemblance between fossorial and aquatic species has been highlighted previously in snakes at the cranial (Savitzky, 1983) and endocranial level (Allemand et al, 2017), with C. ruffus noticeably grouping with the aquatic species in terms of its endocranial shape. Aquatic and fossorial species share a similar constraint: the high density of the medium in which they live.…”