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 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 a number of important challenges to current paradigms regarding snake cranial evolution.
S1. Video of segmented adult T. radix skull
This supplementary video depicts the skull of an adult T. radix individual (see figure captions in main text for labelling of bones). This specimen is housed in the University of Alberta Museum of Zoology (UAMZ R636), and was scanned using a Bruker-SkyScan 1076 micro-CT scanner (see article text for details regarding methods). The micro-CT scans were visualized and segmented in Dragonfly 1.0, and the video was created using Dragonfly 3.0.
Adult.mp4
S2. Video of segmented juvenile T. radix skull
This supplementary video depicts the skull of a juvenile T. radix individual (see figure captions in main text for labelling of bones). This specimen is housed in the University of Alberta Museum of Zoology (UAMZ R950), and was scanned using a Bruker-SkyScan 1076 micro-CT scanner (see article text for details regarding methods). The micro-CT scans were visualized and segmented in Dragonfly 1.0, and the video was created using Dragonfly 3.0.
Juvenile.mp4
S3. Video of segmented embryonic T. radix skull
This supplementary video depicts the skull of an embryonic T. radix individual (see figure captions in main text for labelling of bones). This specimen is housed in the University of Alberta Museum of Zoology (UAMZ R620), and was scanned using a Bruker-SkyScan 1076 micro-CT scanner (see article text for details regarding methods). The micro-CT scans were visualized and segmented in Dragonfly 1.0, and the video was created using Dragonfly 3.0.
Embryo.mp4
S4. 3D mesh files of all segmented skull bones of adult T. radix
This folder contains 3D mesh (.stl) files of all skull bones of an adult T. radix individual. This specimen is housed in the University of Alberta Museum of Zoology (UAMZ R636), and was scanned using a Bruker-SkyScan 1076 micro-CT scanner (see article text for details regarding methods). The micro-CT scans were visualized and segmented in Dragonfly 1.0. The mesh (.stl) files can be viewed in many 3D visualization programs (e.g., Avizo, Adobe Photoshop, etc).
Mesh Files (Adult).zip
S5. 3D mesh files of all segmented skull bones of juvenile T. radix
This folder contains 3D mesh (.stl) files of all skull bones of a juvenile T. radix individual. This specimen is housed in the University of Alberta Museum of Zoology (UAMZ R950), and was scanned using a Bruker-SkyScan 1076 micro-CT scanner (see article text for details regarding methods). The micro-CT scans were visualized and segmented in Dragonfly 1.0. The mesh (.stl) files can be viewed in many 3D visualization programs (e.g., Avizo, Adobe Photoshop, etc).
Mesh Files (Juvenile).zip
S6. 3D mesh files of all segmented skull bones of embryonic T. radix
This folder contains 3D mesh (.stl) files of all skull bones of an embryonic T. radix individual. This specimen is housed in the University of Alberta Museum of Zoology (UAMZ R620), and was scanned using a Bruker-SkyScan 1076 micro-CT scanner (see article text for details regarding methods). The micro-CT scans were visualized and segmented in Dragonfly 1.0. The mesh (.stl) files can be viewed in many 3D visualization programs (e.g., Avizo, Adobe Photoshop, etc).
Mesh Files (Embryo).zip