Cephalic fossae and pits in the parietal shields of caenophidian snakes and their correspondence in the braincase

Abstract

Anatomical and osteological analyses in several species of ophidians belonging to the clade Caenophidia have led to the identification of differentstructures present in the cephalic shields of these reptiles. These structures are observed expressed mainly in the parietal shields, manifesting themselves in different forms. “Major parietal pits,” generally symmetrical, occur in the center of the parietal shields, more or less medially distanced to each other. These may appear circular, linear, separated, conjunct, or in contact with the lateral margin of the parietal shield by a slit. Together with the pits, a new distinct trait has been observed in the parietal shields of different species of colubroids, here named “parietal fossae.” Parietal pits and fossae are situated in correspondence with the paired foramina on the parietal bone’s dorsal plane, which in turn correspond ventrally with the epiphysis, and the contact between the cephalic hemispheres and the optic lobes. Other fossae may be present in certain species, occurring more posteriorly in the parietal shields, or in the supraocular shields, in correspondence with neurovascular foramina and fossae present in the underlying cranial bones. A second category of pits has also been found, here named “minor pits,” which appear randomly within the cephalic shields, and are also linked to neurovascular foramina present in the corresponding cranial bones. A third type of pit, distinct from the others, is the “medial posterior pit”, which has been found in aquatic snakes. This pit occurs in the posterior half of the medial contact of the parietal shields, and corresponds to a large central foramen in the posterior apex of the parietal bone’s plane, ventrally opening within a canal between the two fossae covering the optic lobes. All of these traits have been observed in a large number of snakes belonging to the families Colubridae, Natricidae, Psammophiidae, Atractaspididae, Pareidae, Xenodermidae, Micrelapidae and Elapidae. These do not occur in all individuals within the examined taxa, but rather affect part of the specimens of a given species. Fossae and pits appear to be sporadic surface structures that correspond to neurovascular foramina present in the underlaying cranial bones.