The development of the skull in Acrochordus granulatus (Schneider) (Reptilia: Serpentes), with special consideration of the otico-occipital complex

The skull of Acrochordus has been characterized by the absence of a crista circumfenestralis (a synapomorphy shared by all snakes), and by the absence of a recessus scalae tympani that in other squamates forms by subdivision of the embryonic metotic fissure. These traits have variably been identified as either plesiomorphic or paedomorphic. The study of the development of the osteocranium in a series of cleared and stained embryos of Acrochordus shows a close correspondence to the development of the skull in other snakes. The exception is the formation of the postorbital from two ossification centers. The significance of this observation, which might suggest the embryonic fusion of a postfrontal with a postorbital, remains enigmatic, as it is based on one side only of the skull of a single specimen. By contrast, a rudimentary and modified crista circumfenestralis can be identified in the skull of Acrochordus. Furthermore, absence of a recessus scalae tympani is not due to an undivided fissura metotica, but results from the obliteration of the anterior part of the metotic fissure instead. With respect to this character, Acrochordus is neither plesiomorphic, nor paedomorphic, but autapomorphic. This interpretation of the skull of Acrochordus is compatible with the sister-group relationships this genus shares with colubroid snakes.