Abstract
Accurate knowledge of skeletal ontogeny in extant organisms is crucial in understanding important morpho-functional systems and in enabling inferences of the ontogenetic stage of fossil specimens. However, detailed knowledge of skeletal ontogeny is lacking for most squamates, including snakes. Very few studies have discussed postnatal development in snakes, with none incorporating data from all three major ontogenetic stages—embryonic, juvenile and adult. Here, we provide the first analysis encompassing these three ontogenetic stages for any squamate, using the first complete micro-computed tomography (micro-CT)-based segmentations of any non-adult snake, based on fresh specimens of Thamnophis radix. The most significant ontogenetic changes involve the feeding apparatus, with major elongation of the tooth-bearing elements and jaw suspensorium causing a posterior shift in the jaw articulation. This shift enables macrostomy (large-gaped feeding in snakes) and occurs in T. radix via a different developmental trajectory than in most other macrostomatans, indicating that the evolution of macrostomy is more complex than previously thought. The braincase of T. radix is also evolutionarily unique among derived snakes in lacking a crista circumfenestralis, a phenomenon considered herein to represent paedomorphic retention of the embryonic condition. We thus present numerous important challenges to current paradigms regarding snake cranial evolution.
Highlights
Though many studies have examined embryonic cranial development in snakes (e.g. [1,2,3]), comparatively few have analysed the postnatal development of the snake skull (e.g. [4,5,6])
We present a full description of the cranial ontogeny of each bone in the T. radix skull, with the bones grouped according to the general skull region
A summary of the most significant ontogenetic changes to the T. radix skull is presented in table 1
Summary
Though many studies have examined embryonic cranial development in snakes (e.g. [1,2,3]), comparatively few have analysed the postnatal development of the snake skull (e.g. [4,5,6]). Considered to have evolved only once [10], recent analyses of the postnatal cranial development of various major snake groups have revealed the macrostomatan condition to be achieved via different developmental pathways in different groups of snakes, suggesting that this feature is homoplastic [6]. This conclusion is supported by recent phylogenies showing macrostomatan snakes to be non-monophyletic This conclusion is supported by recent phylogenies showing macrostomatan snakes to be non-monophyletic (e.g. [11])
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