Variably occurring parietal pits in psammophiid snakes (Squamata: Serpentes): convergent expression of ancestral skin traits?

Amanda M. Cottone* and Aaron M. Bauer

Herpetology Notes, volume 4: 381-385 (2011) (published online on 7 November 2011)

Department of Biology, Villanova University, 800 Lancaster Avenue, Villanova, PA 19085, USA; e-mail: amcottone@gmail.com


“Sense-organ-like” parietal pits (PPs) in snakes have so far been documented in several psammophiid species as well as the Asiatic Atretium schistosum (Natricinae). Hypotheses regarding the function of these structures have been suggested but support for any is weak due to sampling limitations of species exhibiting these, and previous inconclusive histological examinations. Here we present data from additional sampling (N=842 for seven species) on the variable expression of PPs throughout psammophiids and discuss possible explanations for their occurrence pattern in two distantly related caenophidians. We consider the spontaneous atavism hypothesis previously suggested by Miralles and Ineich (2006) in light of our additional data and also suggest that it may be more appropriate to regard PPs as a particular manifestation of a more general underlying developmental mechanism that
produces a wide variety of integumentary sensory structures in snakes.

 Remarks (T.S.)

In the section 'Materials and Methods' the authors regrettably do not explain on what criterium or criteria they decide that a parietal pit is present in the examined specimens and identical to the parietal pits De Haan (2003) describes. They only state that they have closely followed the description by De Haan (2003). There are no illustrations of 'typical' PP's found in their study.

The species in which the authors discovered PP's are: Psammophis schokari (in which species De Haan never found PP's), P. crucifer, P. biseriatus, P. brevisostris, and Psammophylax rhombeatus. No PP's were found on P. punctulatus and P. condanarus. The authors state that it is important to examine large numbers of specimens of each species, as the number of snakes with PP's in each species is relatively small (in their examinations 14,9% of P. schokari and 8,4% in P. crucifer, for instance).

In this study, PP's occurred more often in an asymmetrical condition (71%) than symmetrical, which is strikingly different from the findings of De Haan.