DERMOPHIIDAE, Taylor, 1969

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 : 168-170

DERMOPHIIDAE ( FIGS 3 , S 4–S 6 )

In dermophiids the main body of the sphenethmoid accounts for 30% or less of the total length of the sphenethmoid ( Fig. 3A ). The short lateral wall of its main body is capped by a broad sutural surface (less broad in Geotrypetes seraphini ) that receives the anterolateral lappet of the parietal ( Fig. S5B ). The anterolateral corner bears an ossified anterolateral process ( Fig. 3B ), except for Geo. seraphini in which only a small anterolateral expansion is present ( Fig. S5B ). The dorsomedial process is thick and long, extending well beyond the anterior limit of the lateral walls of the os basale ( Fig. 3A ). The process is variably exposed dorsally in specimens of Dermophis mexicanus ( Fig. S4B ), a portion is exposed in Geo. seraphini ( Fig. S5B ) and Schistometopum thomense ( Fig. S6B ), and the dorsal exposure of the sphenethmoid in Gymnopis multiplicata corresponds to a region over the nasal septum ( Fig. 3B ). The posterior margin of the lateral wall of the sphenethmoid is deeply incised by the optic foramen in all dermophiids. The posterior margin of the floor of the main body of the sphenethmoid is deeply incised at the midline, but otherwise the floor is quite broad and only slightly concave ( Fig. 3B ). Geotrypetes seraphini is distinct amongst dermophiids in the presence of a deeply concave posterior margin of the floor ( Fig. S5B ).

The ossified portion of the nasal septum is long, and reaches the posterior limit of the external naris. The dorsal surface of the nasal septum is broad near its base (less broad in Geo. seraphini ), and tapers to a thin ridge towards the tip ( Fig. 3B ). The height of the nasal septum remains roughly uniform throughout its length ( Fig. 3A ). Long, tubular sola nasi extend anteriorly from the main body of the sphenethmoid ( Fig. 3B, C ); those in Geo. seraphini are less robust ( Fig. S5C ) and those in Sc. thomense are connected to the nasal septum via a thin ventral sheet of bone ( Fig. S6C ).

The anterior wall contains paired dorsal and ventral foramina ( Fig. 3D, E ). The foramina of the dorsal pair are located close to the midline, and those of the ventral pair are offset laterally from the midline. The dorsal foramina lead to short canals before exiting onto the anterior surface of the sphenethmoid. The ventral pair leads to the tubular sola nasi. A single anterolateral foramen is present in the base of each anterolateral process ( Fig. 3D, E ) in dermophiids. An additional small foramen is also present in the anterolateral expansion of Geo. seraphini ( Fig. S5D ).

The antotic wall of the os basale is orientated parallel (angled slightly anteromedially in Geo. seraphini ) to its contralateral wall in dermophiids ( Fig. 3B ). In anterior view the wall is vertical ( Fig. 3F ), except for Geo. seraphini in which it is angled slightly dorsolaterally. The dorsal sutural surface is narrow anteriorly and expands dramatically at the level of the antotic foramina into a broad surface in all species, except for Geo. seraphini ( Fig. S5B ) in which it remains narrow. The surface is continuous with the sutural surface along the anterior margin of the otic capsule. The anterior margin of the antotic wall is deeply incised by the posterior margin of the oval optic foramen; in Geo. seraphini it is posteroventrally obliquely so ( Fig. S5B ). Five foramina are present in the antotic wall of all dermophiids ( Fig. 3H ), except for Geo. seraphini in which there are three ( Fig. S5H ). The difference occurs because the two trunks of the trigeminal and the ventral vein all exit a large common foramen in Geo. seraphini (referred to here as Pattern 3), but discrete foramina are present in the other dermophiid species (referred to here as Pattern 1).

The dorsal surface of the otic-occipital complex of the os basale is posteroventrally tilted, tapered towards the midline ( Fig. 3A, B ). The anterior portion bears the sutural surface that receives the parietal, which is broad, except for Geo. seraphini . A laterally extending shelf, continuous with the dorsal surface, is present dorsal to the fenestra vestibuli in all dermophiids except Geo. seraphini ( Fig. S5B ). The fenestra vestibuli is elongate anteroposteriorly. Its ventral margin is thick in D. mexicanus and Gy. multiplicata ( Fig. 3A ). It opens in the lateral direction and only gently incises the lateral margin of the otic-occipital complex when viewed posteriorly ( Fig. 3G ), except for D. mexicanus in which it does so very deeply ( Fig. S4G ). In lateral view the occipital condyle projects far beyond the posterior limit of the otic capsule ( Fig. 3A ), and the jugular foramen is visible in its base. The foramen magnum is large and subcircular ( Fig. 3G ).

The medial wall of the otic capsule is perforated by six to eight foramina. Four of these are common to those of all species (endo- and perilymphatic foramina, and anterior and posterior vestibulocochlear nerve foramina; Maddin, 2011). Variably within the family two to four additional foramina transmit portions of the medial branch of the vestibulocochlear nerve ( Fig. 3H ).

The anterior margin of the floor of the os basale tapers dramatically to a thin, rod-like tip in D. mexicanus ( Fig. S4C ) and Gy. multiplicata ( Fig. 3C ), but not the other species ( Figs S5C, S 6C ). The floor extends far anteriorly, terminating in the anterior half of the nasal septum ( Fig. 3C ), except for Geo. seraphini in which it just barely underlaps the nasal septum ( Fig. S5C ). The floor of the os basale is smooth and slightly concave dorsally, and a well-defined median depression is present in the posterior region of the floor ( Fig. 3B ). A well-developed basicranial articulation, complete with an ovoid sutural surface, is present in all dermophiids ( Fig. 3B ); however, those in Geo. seraphini are distinctive in their tab-like appearance ( Fig. S5C ). In ventral view the lateral margin of the floor, just posterior to the basicranial articulation, is slightly constricted (arrowhead; Fig. 3C ), except in Sc. thomense in which it is strongly constricted (arrowhead; Fig. S6C ). A well-developed ventral winglike projection is present on the ventral surface of the otic capsule, marking the lateral limit of the ventral muscle attachment site ( Fig. 3A, C ). In D. mexicanus and Gy. multiplicata the posterior margin of the floor terminates at transverse ridge that does not closely approach the foramen magnum ( Fig. 3C ). In Geo. seraphini and Sc. thomense the floor terminates at a rounded point, the apex of which closely approaches the foramen magnum ( Fig. S5C ).

The foramen for the carotid artery that leads to a canal in the floor of the os basale is located just anterior to the otic capsule ( Fig. 3C ). Its lateral and medial exit points are located ventral to the midpoint of the antotic foramina, on either side of the antotic wall.

The footplate of the stapes is elongate anteroposteriorly ( Fig. 3I, J ), and is fairly tall in dermophiid species, except for Geo. seraphini ( Fig. S5I, J ). Its long axis is orientated horizontally to slightly anteroventrally inclined. The anterior margin of the footplate is straight and closely applied to the margin of the fenestra vestibuli, and the rest closely approach its margins, except for Geo. seraphini in which the footplate is somewhat inset from the margin of the fenestra. The medial surface of the footplate is strongly concave. The columellar process is short and robust. It extends anteriorly from the footplate in roughly the horizontal plane. There is no foramen in the base of the columellar process ( Fig. 3L ).

Figures

Figure 3. The morphology of the braincase (sphenethmoid and os basale) and stapes, as revealed through microcomputed tomography, of Gymnopis multiplicata (BM 1907.10.9.10). 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 4. The morphology of the braincase (sphenethmoid and os basale) and stapes, as revealed through microcomputed tomography, of Boulengerula boulengeri (BM 2002.776). 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 5. The morphology of the braincase (sphenethmoid and os basale) and stapes, as revealed through microcomputed tomography, of Ichthyophis kohtaoensis (ZMH A08981). 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 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.