INDOTYPHLIDAE, Lescure, Renous & Gasc, 1986

Maddin, Hillary C., Russell, Anthony P. & Anderson, Jason S., 2012, Phylogenetic implications of the morphology of the braincase of caecilian amphibians (Gymnophiona), Zoological Journal of the Linnean Society 166 (5), pp. 160-201 : 175-177

INDOTYPHLIDAE ( FIGS 6 , S 12–S 14)

The main body accounts for roughly one third or less than the total length of the sphenethmoid ( Fig. 6A ). The lateral wall of the main body is short and is capped by a narrow to moderately broad dorsal sutural surface ( Fig. 6B ). An ossified anterolateral process is present in all indotyphlid species except for Id. russeli (Fig. S14B). The dorsomedial process is broad and long, reaching beyond the level of the antotic wall, in all species except for Id. russeli . In all species the process is not exposed dorsally (dorsal exposure in Id. russeli corresponds to the nasal septum region). The posterior margin of the lateral wall is deeply incised by the anterior margin of the oval optic foramen ( Fig. 6A ). The posterior margin of the floor of the sphenethmoid is shallowly concave (more so in Grandisonia alternans ) and deeply incised by a slit-like opening at the midline.

The nasal region is divided by a long nasal septum that is short dorsoventrally. The nasal septum is tall near its base, but tapers in height to a relatively short profile at its tip. The dorsal sutural surface tapers from being very broad over the anterior portion of the main body to being a thin ridge near its tip, resulting in an overall very triangular shape to the surface ( Fig. 6B ). The ventral surface also tapers towards the tip and forms a narrow shelf below the openings for the ventral branch of the olfactory nerve ( Fig. 6C ). The shelf is especially broad in Gr. alternans ( Fig. S12C ) and no shelf is present in Id. russeli (Fig. S14C). Sola nasi are absent.

The anterior wall is perforated by the paired dorsal and ventral foramina ( Fig. 6D, E ). The ventral pair is located close to the midline in Id. russeli (Fig. S14E), further away from the midline in Hypogeophis rostratus ( Fig. S13E ) and Gegeneophis ramaswami ( Fig. 6E ), and even further in Gr. alternans ( Fig. S12E ). Both the dorsal and ventral foramina lead to short canals before exiting on the anterior surface of the sphenethmoid. The anterolateral foramen pierces the anterolateral corner ( Fig. 6D ).

The antotic wall of the os basale is orientated towards the midline anteriorly when viewed dorsally ( Fig. 6B ). It is also vertical to subvertical amongst indotyphlid species when viewed anteriorly ( Fig. 6F ). The dorsal sutural surface is very narrow ( Id. russeli ) to very broad ( Hyp. rostratus ), and it is continuous posteriorly with the sutural surface along the anterior margin of the otic capsule, except for Id. russeli in whichthe parietal sutural surface is lacking (Fig. S14B). A well-developed sutural surface for the quadrate is present in the posterodorsal region of the antotic wall of all indotyphlid species ( Fig. 6A ). The anterior margin of the antotic wall is moderately incised by the posterior margin of the oval optic foramen ( Fig. 6A ). Five foramina with a pattern like that of dermophiids, plus one unidentified, are present in Id. russeli (referred to here as Pattern 1; Fig. S14H). Four foramina are present in Geg. ramaswami ( Fig. 6H ), resembling the condition of dermophiids except for the common exit of the two trunks of the trigeminal (referred to here as Pattern 2). A single foramen for all antotic structures is present in the antotic wall of Gr. alternans ( Fig. S12H ) and Hyp. rostratus (referred to here as Pattern 8; Fig. S13H ).

The dorsal surface of the otic-occipital complex of the os basale is strongly tilted posteroventrally, and tapers in width towards the midline ( Fig. 6A ). A broad portion of this surface is exposed dorsally in Gr. alternans and Hyp. rostratus ( Fig. S12B ). A short diagonal ridge extends anterodorsally just dorsal to the fenestra vestibuli and the fenestra is strongly anteriorly facing ( Fig. 6A, G ). In lateral view the occipital condyle projects well beyond the posterior limit of the otic capsule, and the large jugular foramen is visible in lateral view ( Fig. 6A ). The foramen magnum is circular to diamond-shaped.

The medial wall contains the four foramina in their common locations (endo- and perilymphatic foramina, anterior and posterior vestibulocochlear nerve foramina) that are present in all species examined ( Fig. 6H ). Variably within the family two or three additional foramina that are located along the ventral margin of the otic capsule transmit the medial branch of the vestibulocochlear nerve ( Maddin, 2011).

The anterior portion of the floor of the os basale is triangular in outline ( Fig. 6C ). It extends roughly halfway along the length of the nasal septum, except for Id. russeli in which it extends to within the anterior half of the nasal septum (Fig. S14C). The median posterior depression is moderately well defined ( Fig. 6B, F ). Lateral to the antotic wall, a relatively well-developed basicranial articulation is present, complete with an ovoid sutural surface ( Fig. 6A, B ). In ventral view the lateral margin of the floor of the os basale is strongly constricted posterior to the basicranial articulation (arrowhead; Fig. 6C ). The constriction reaches to the level of the foramen that transmits the carotid artery. A ventral wing-like projection is present on the ventral surface of the otic capsule, except for Id. russeli (Fig. S14C). The posterior margin of the floor of the os basale terminates at a sharp or rounded point that closely approaches the ventral margin of the foramen magnum ( Fig. 6C ).

The foramen that leads to a canal in the floor of the os basale for the carotid artery is located just anterior to the otic capsule ( Fig. 6C ). It bifurcates anteriorly into lateral and medial canals, which terminate in foramina located at roughly the same level, ventral to the large antotic foramen, on the lateral and medial sides of the antotic walls ( Fig. 6H ).

The footplate of the stapes is subcircular in Geg. ramaswami ( Fig. 6I, J ) and Id. russeli (Fig. S14I, J) and more elongate in Gr. alternans ( Fig. S12I, J ) and Hyp. rostratus ( Fig. S13I, J ). The long axis is roughly horizontally orientated. It is closely applied to the anterior margin of the fenestra vestibuli, and the remaining margins closely approach the margins of the fenestra vestibuli. The medial surface is strongly concave. The columellar process is very short with a lateral ridge, which is absent in Id. russeli . There is no foramen in the base of the columellar process.

Figures

Figure 6. The morphology of the braincase (sphenethmoid and os basale) and stapes, as revealed through microcomputed tomography, of Gegeneophis ramaswami (KU 203038). A, the braincase in lateral view; B, dorsal view; and C, ventral view (arrowhead indicates location of the constriction of floor of the os basale, if present). D, the sphenethmoid in anterior view; and E, posterior view. F, the os basale in anterior view; and G, posterior view (asterisk indicates the location of the incision of the margin of the otic capsule by the fenestra vestibuli). H, the right medial surface of the os basale revealing the foramina of the antotic wall and of the medial wall of the otic capsule. I–L, the left stapes in natural position. I, lateral view; J, medial view; K, anterior view; and L, posterior view. Scale bars = 1 cm. See Figure 2 for abbreviations.

Figure 12. The 50% majority-rule trees resulting from the parsimony (A–C) and 95% maximum clade credibility tree resulting from the Bayesian phylogenetic analyses of morphology (D) including the new braincase and stapes characters. A, all characters, all taxa; B, exclusion of certain eye characters (see text for details), yielding strong congruence with family level systematics; C, exclusion of certain eye characters and Sylvacaecilia grandisonae, which yields improved resolution to the family level with little discrepancy with molecular analyses (bootstrap values greater than 50% shown on the nodes); D, Bayesian analysis excluding certain eye characters and Sy. grandisonae, and the resolution of the nine families under this topology (Dermophiidae, Herpelidae, and Siphonopidae are paraphyletic). Node letters correspond to the text. 1, Rhinatrematidae; 2, Ichthyophiidae; 3, Scolecomorphidae; 4, Herpelidae; 5, Typhlonectidae; 6, Caeciliidae; 7, Indotyphlidae; 8, Siphonopidae; and 9, Dermophiidae.

Figure 13. The 50% majority-rule tree resulting from the Bayesian analysis of the combined morphological and molecular data sets. Node letters correspond to the text, and node values are posterior probabilities.