SIPHONOPIDAE, Bonaparte, 1850

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 : 181-183

SIPHONOPIDAE ( FIGS 9 , S18)

The sphenethmoid is substantially more robust in Siphonops annulatus ( Fig. 9A ) in comparison to Microcaecilia albiceps (Fig. S18A). The main body of the sphenethmoid is relatively short and broad in siphonopids ( Fig. 9A ), and accounts for less than half of the total length of the sphenethmoid. An expansion is present representing the only ossified portion of the anterolateral processes, but it is weakly developed ( Fig. 9B ). The dorsomedial process is thin and covered dorsally in M. albiceps (Fig. S18B) and robust and exposed in Si. annulatus ( Fig. 9B ). In both species the process extends beyond the anterior limit of the lateral walls. The posterior margin of the lateral wall is deeply incised by the anterior margin of the optic foramen ( Fig. 9A ). The posterior margin of the floor is incised at the midline and concave in outline ( Fig. 9B ), more so in M. albiceps (Fig. S18B).

The nasal region is divided by a moderately long nasal septum, which is tall in M. albiceps (Fig. S18A) and short in Si. annulatus ( Fig. 9A ). It terminates as a tall process just posterior to the external naris. The dorsal sutural surface tapers towards the tip, and for the majority of its length the dorsal margin is thin and blade-like ( Fig. 9B ). The ventral surface is similarly thin for most of its length. A pair of well-developed, tubular sola nasi extends from the anterior surface of the main body ( Fig. 9C ).

The anterior wall is pierced by two pairs of foramina ( Fig. 9E ). The ventral pair is displaced somewhat laterally from the midline ( Fig. 9E ). The dorsal foramina lead to short canals before exiting onto the anterior surface of the sphenethmoid. The anterolateral foramina are incomplete laterally ( Fig. 9D ).

The antotic wall of the os basale is slightly angled towards the midline anteriorly when viewed dorsally ( Fig. 9B ), more so in M. albiceps (Fig. S18B). In anterior view the wall is vertical in Si. annulatus ( Fig. 9F ), but it is dorsolaterally inclined in M. albiceps (Fig. S18F). The dorsal sutural surface is very narrow; however, there is a large expansion over the antotic region in Si. annulatus ( Fig. 9B ). The anterior margin of the antotic wall is moderately incised by the posterior margin of the oval optic foramen ( Fig. 9A ). Five foramina perforate the antotic wall ( Fig. 9H ), and these have the same configuration and transmit the same structures as those of rhinatrematids and dermophiids (referred to here as Pattern 1; Maddin, 2011). The two large foramina that transmit the two trunks of the trigeminal nerve are incompletely differentiated in the specimens of M. albiceps examined (Fig. S18H), possibly indicating immaturity.

The dorsal surface of the otic-occipital complex is tilted posteroventrally and remains broad in Si. annulatus ( Fig. 9B ), but tapers in width towards the midline to a narrow surface in M. albiceps (Fig. S18B). The fenestra vestibuli is elongate anteroposteriorly ( Fig. 9A ), and is especially small in Si. annulatus ( Fig. 9A ). In posterior view the fenestra vestibuli only slightly incises the lateral margin of the otic-occipital complex ( Fig. 9G ). The occipital condyle projects far beyond the posterior limit of the otic capsule and the large jugular foramen is visible in lateral view ( Fig. 9A ). The foramen magnum is subcircular in outline ( Fig. 9G ).

The medial wall of the otic capsule is pierced by four foramina (endo- and perilymphatic foramina, anterior and posterior vestibulocochlear nerve foramina), in the common locations found in all species examined here ( Fig. 9H ). Variably within the family two or three additional foramina are present along the ventral margin of the otic capsule for the transmission of parts of the medial branch of the vestibulocochlear nerve ( Maddin, 2011).

The anterior margin of the floor of the os basale tapers continuously to a point towards the tip, anterior to an abrupt constriction in Si. annulatus ( Fig. 9C ). The tip extends to contact the ventral surface of the nasal septum. A median posterior depression is present and well defined ( Fig. 9F ). Lateral to the antotic wall is the well-developed basicranial articulation ( Fig. 9A ). In ventral view the lateral margin of the floor of the os basale is slightly constricted medially, towards the level of the carotid foramina (arrowhead; Fig. 9C ), and slightly more so in M. albiceps (Fig. S18C). A weak ventral projection is present on the ventral surface of each otic capsule, although this is very poorly developed in M. albiceps (Fig. S18C). The posterior margin of the floor of the os basale terminates at a rounded point, the apex of which closely approaches the ventral margin of the foramen magnum.

The foramen leading to the canal for the carotid artery enters the floor of the os basale anterior to the otic capsule ( Fig. 9C ) and bifurcates, resulting in lateral and medial canals. The canals terminate in foramina located ventral to the antotic foramina.

The footplate of the stapes is elongate anteroposteriorly, and its long axis is orientated slightly anteroventrally as in Si. annulatus ( Fig. 9I, J ) to horizontally (Fig. S18I, J). It is also very small and bears a large anterior articular surface in Si. annulatus . The columellar process of the stapes extends roughly horizontally from the lateral surface of the footplate. A weak ridge is present on the process of M. albiceps towards its distal tip (Fig. S18I). There is no foramen present in the base of the columellar process.

Figures

Figure 9. The morphology of the braincase (sphenethmoid and os basale) and stapes, as revealed through microcomputed tomography, of Siphonops annulatus (UMMZ 150624). 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.