Digital Endocasts of the Cranial Cavity and Osseous Labyrinth of the Burrowing Snake Uropeltis woodmasoni (Alethinophidia: Uropeltidae)
The endocranial space, nerve pattern, and inner ear of a specimen of Uropeltis woodmasoni are described from digital endocasts rendered from high-resolution X-ray computed tomography scans. The cast of the endocranial space reveals short, stout olfactory bulbs as well as general shortening of the brain along the longitudinal axis. The pattern of innervation agrees with descriptions reported for other uropeltid snakes, although this individual exhibits skull asymmetry in the pathway of the glossopharyngeal nerve and the bifurcation of the jugular canal. There are no internal connections between the perilymphatic canal leading to the foramen pseudorotundum, the recessus scala tympani, and the jugular canal, a situation that differs from those of other alethinophidian snakes. The inner ear is compactly packaged with smoothly curved semicircular canals located in close proximity to the vestibule. Semicircular canal size is comparable to those of larger snakes, and the small size of the skull (9.4 mm) coupled with relative enlargement of the sensory regions of the brain and otic capsule is consistent with the morphological pattern of miniaturized vertebrates, although compounding factors, including phylogeny, ontogeny, and variation, are currently not well understood.Abstract
Diagram of semicircular canal measurements. (A) Frontal section through right vestibule and lateral semicircular canal. (B) Dorsal view of ear endocast, anterior up. Abbreviations: ed, endolymphatic duct; h, height; lsc, lateral semicircular canal; stm, statolithic mass; w, width.
Skull of Uropeltis woodmasoni, M-10006, with overlying soft tissue removed. Anterior to the left. Scale bar equals 1 mm. (A) Dorsal view. (B) Lateral view. Abbreviations: bc, braincase complex; fr, frontal; jsr, juxtastapedial recess; lsf, laterosphenoid foramen for cid nerve; oc, occipital condyle; of, optic foramen; p, parietal; q, quadrate; V2f, foramen for maxillary branch of the trigeminal nerve; V3f, foramen for mandibular branch of the trigeminal nerve.
Skull with overlying soft tissue removed and bone rendered transparent to reveal the endocasts. Anterior to the left unless noted. Scale bar equals 1 mm. (A) Dorsal view of brain endocast. (B) Left lateral view of brain endocast. (C) Dorsal view of ear endocast. (D) Right lateral view of ear endocast, anterior to the right.
Brain endocast. Anterior to the left. Scale bar equals 1 mm. (A) Dorsal view. (B) Ventral view. (C) Left lateral view. Abbreviations: avc, anterior portion of Vidian nerve; bs, brain stem; cbl, cerebellum; cer, cerebrum; cid, cid nerve and internal secondary anastomotic vein; fm, foramen magnum; fpj, position of frontal-parietal joint; ic, internal carotid artery; jug, jugular vein; olb, olfactory bulb; olt, olfactory tract; opl, optic lobe; opn, optic nerve (optic tract); pvc, posterior portion of Vidian nerve; st, scala tympani; VII, facial nerve seven; VIIIc, cochlear branch of auditory nerve; VIIIv, vestibular branch of auditory nerve; V2, maxillary branch of the trigeminal nerve; V3, mandibular branch of the trigeminal nerve; X, vagus nerve; XII, hypoglossal nerve.
Scale bar equals 1 mm. (A) Frontal slice of CT scan showing outline of endocranial space and relatively large otic capsules. Anterior to the left. (B) Axial cross-section through the posterior otic region of the three-dimensional model of the skull showing statolithic mass and constriction of endocranial space. Anterior toward viewer. Abbreviations: apm, medial aperture of the recessus scala tympani; elf, endolymphatic foramen; fp, footplate of stapes; iam, internal auditory meatus; ihg, internal hypoglossal foramen; jsr, juxtastapedial recess; lsc, lateral semicircular canal; oc, occipital condyle; p, parietal; pf, perilymphatic foramen; psc, posterior semicircular canal; q, quadrate; stm, statolithic mass.
Orthogonal views of right inner ear endocast with stapes included. Scale bar equals 1 mm. Arrows mark approximate locations of apl and apm. (A) Dorsal view, anterior to the right. (B) Ventral view, anterior to the left. (C) Lateral view, anterior to the right. (D) Medial view, anterior to the left. (E) Anterior view, lateral to the left. (F) Posterior view, lateral to the right. Abbreviations: aa, anterior ampulla; apl, lateral aperture of the recessus scala tympani; apm, medial aperture of the recessus scala tympani; asc, anterior semicircular canal; cc, common crus; col, osseous portion of columella; ed, endolymphatic duct; fp, footplate of stapes; h, helicotrema; IX, glossopharyngeal nerve; js, juxtastapedial sinus; lag, lagena (cochlea); lsc, lateral semicircular canal; pa, posterior ampulla; psc, posterior semicircular canal; sv, scala vestibuli portion of the perilymphatic cistern; u, utricle; vb, vestibule; VIIIc, cochlear branch of auditory nerve; VIIIv, vestibular branch of auditory nerve.
Oblique views of right inner ear endocast with stapes removed. Scale bar equals 1 mm. Arrows mark approximate locations of apl and apm. (A) Dorsolateral view, anterior to the right. (B) Posteromedial view, lateral to the right. Abbreviations: apl, lateral aperture of the recessus scala tympani; apm, medial aperture of the recessus scala tympani; asc, anterior semicircular canal; cc, common crus; ed, endolymphatic duct; h, helicotrema; IX, glossopharyngeal nerve; js, juxtastapedial sinus; lag, lagena (cochlea); lsc, lateral semicircular canal; psc, posterior semicircular canal; sv, scala vestibuli portion of the perilymphatic cistern; vb, vestibule; VIIIc, cochlear branch of auditory nerve; VIIIv, vestibular branch of auditory nerve.
Contributor Notes
Associate Editor: D. Kizirian.