RHINATREMATIDAE, Nussbaum, 1977

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 : 177-179

RHINATREMATIDAE ( FIGS 7 , S15)

The main body is relatively long, accounting for just under half the total length of the sphenethmoid ( Fig. 7A ). The lateral wall of the main body is long, and is capped by a broad sutural surface that receives the parietal ( Fig. 7A ). A broad ossified anterolateral process is present, and in E. bicolor it is connected medially to the anterior dorsal sutural surface ( Fig. 7B ). The dorsomedial process is thin and rod-like ( Fig. 7B ), and is completely covered dorsally. The posterior margin of the lateral walls is deeply incised (less so in R. bivittatum ) by the anterior margin of the optic foramen ( Fig. 7A ). The posterior margin of the floor is also deeply concave and U-shaped in outline ( Fig. 7B ).

The nasal septum is moderately long, terminating posterior to the external naris. It remains uniform in height distally ( Fig. 7A ). The dorsal sutural surface is narrow near its base and expands towards the tip ( Fig. 7A ). The ventral surface also maintains a broad sutural surface as far as the tip of the nasal septum. A broad sheet of bone forms a shelf ventral to the exit points of the ventral branch of the olfactory nerve, connecting the sola nasi to the nasal septum ( Fig. 7B,C ).

The foramina in the anterior wall are paired, and the foramina of the ventral pair are located slightly more laterally from the midline than the dorsal pair. A single anterolateral foramen is present in the base of each expanded anterolateral process ( Fig. 7E ).

The wall is parallel to subparallel to the contralateral wall when viewed dorsally ( Fig. 7B ). In anterior view the antotic wall is vertically orientated ( Fig. 7F ). Posteriorly the dorsal sutural surface is expanded slightly, and is continuous with a portion of the surface on 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 ( Fig. 7A ), less so in R. bivittatum (Fig. S15A). Five foramina pierce the antotic wall ( Fig. 7H ). These have the same configuration, and transmit the same structures as those seen in the dermophiid species (referred to here as Pattern 1; Maddin, 2011). An additional foramen just ventral to that for the dorsal vein is present in E. bicolor . Histological data reveal that this additional foramen transmits a vein. This foramen is absent on the right side of the head in BM 1946.9.5.66 ( Fig. 7H ).

The dorsal surface of the otic-occipital complex is nearly horizontal, and forms the entire occipital surface (the parietal does not contribute; Fig. 7A ). It tapers dramatically in width towards the midline ( Fig. 7A ). A sutural surface that receives the parietal is present along the anterior margin of the surface, but narrows substantially at the midline and is absent at the midline in R. bivittatum (Fig. S15A). A process extending laterally, just anterior and dorsal to the fenestra vestibuli, is present but poorly developed in E. bicolor ( Fig. 7B ). The region occupied by the inner ear is large and fills almost the entire extent of the otic-occipital complex in lateral view. The fenestra vestibuli is large and slightly ovoid ( Fig. 7A ). In posterior view the lateral edges of the otic-occipital complex are incised by the fenestra vestibuli ( Fig. 7G ). The occipital condyle is small, and in lateral view it is almost level with the outline of the posterior margin of the otic-occipital complex ( Fig. 7A ). The jugular foramen in the base of the condyle is only partially visible in lateral view. The foramen magnum is large and subcircular in outline ( Fig. 7G ).

The medial wall of the otic capsule is perforated by eight foramina ( Fig. 7H ). The four foramina common to all species are present in their usual locations (endo- and perilymphatic foramina, anterior and posterior vestibulocochlear nerve foramina). Four small foramina conducting components of the medial branch of the vestibulocochlear nerve occur along the ventral margin ( Maddin, 2011).

The anterior portion of the floor of the os basale is triangular in outline. It is relatively very long, extending to the region of the anterior half of the nasal septum. A thin medial ridge that fits into a groove in the ventral surface of the sphenethmoid is present. The floor of the os basale is nearly flat and the median posterior depression is only weakly defined ( Fig. 7B ). A well-developed basicranial articulation is absent from rhinatrematids ( Fig. 7A, C ), and only a narrow shelf lateral to the antotic wall is all that represents the articulation. In ventral view there is no constriction of the lateral margin of the floor posterior to the basicranial articulation (arrowhead; Fig. 7C ). A very weak ridge is present instead of a wing-like ventral projection on the ventral surface of the otic capsule ( Fig. 7A ). Posteriorly the ventral surface of the floor of the os basale terminates in a rounded point that closely approaches the ventral margin of the foramen magnum ( Fig. 7C ).

The foramen that leads to the canal carrying the carotid artery enters the ventral surface of the floor of the os basale, ventral to the anterior limit of the otic capsule ( Fig. 7C ). The canal terminates at lateral and medial foramina located just ventral to the trigeminal foramina ( Fig. 7H ).

The footplate of the stapes is large and ovoid in rhinatrematids ( Fig. 7I, J ), and its anterior margin is closely applied to the anterior margin of the fenestra vestibuli. The remaining margins nearly fill the fenestra. The medial surface of the footplate is strongly concave. The columellar process is short, rod-like, and extends anteriorly in roughly the horizontal plane. The base of the columellar process is pierced by a foramen ( Fig. 7L ). This foramen is variably present in one specimen ( FMNH 152310 ), being present on the right and absent from the left .

Figures

Figure 7. The morphology of the braincase (sphenethmoid and os basale) and stapes, as revealed through microcomputed tomography, of Epicrionops bicolor (BM 1946.9.5.66). 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.