Strigocuscus celebensis (Tate, 1945)

CROSBY, KIRSTEN & NORRIS, CHRISTOPHER A., 2003, Periotic Morphology in the Trichosurin Possums Strigocuscus celebensis and Wyulda squamicaudata (Diprotodontia, Phalangeridae) and a Revised Diagnosis of the Tribe Trichosurini, American Museum Novitates 3414, pp. 1-16 : 4-6

Strigocuscus celebensis

The overall morphology of the periotic in S. celebensis is remarkably similar to that of Trichosurus vulpecula ( Norris, 1994) , particularly in its elongate promontorium, which accounts for almost half the total length of the pars cochlearis, and small internal auditory meatus ( fig. 2). However there are a number of distinctive features not shared with Trichosurus (table 2). The caudal margin of the subarcuate fossa is greatly thickened, reducing the overall size of the opening. At the anterior margin of the fossa, an overhang in the crista petrosa produces a small, pocketed area. As in Trichosurus , the crista petrosa is expanded to form part of the septum separating the middle and posterior cranial fossae and is notched anterodorsally for the passage of the prootic sinus. However, unlike in Trichosurus , this expansion does not result in the development of a large, platelike lamella or lead to the crista petrosa becoming contiguous with the tegmen tympani. In Strigocuscus , the crista petrosa is a comparatively small, robust structure, which is clearly separated from the tegmen tympani. It is comparatively low at the point where it meets the margin of the subarcuate fossa, and it is grooved along its lateral margin by the sulcus of the prootic sinus. In Trichosurus this sulcus is deflected laterally by the expansion of the crista petrosa, but in Strigocuscus it follows a more or less straight dorsal­ventral path.

Dorsally, the lamella that is often seen around the posterodorsal margin of the peri­

TABLE 2 Comparison of the Periotics of Trichosurins

otic in phalangerids ( Norris, 1994) is absent, with the exception of a small flap just dorsal to the crus commune. On the left periotic of AMNH 195602, which remained in situ, this flap carried a small circular foramen, which is assumed to be the opening of the prootic sinus. There is a faint ridge between the crus commune and the prefacial commissure, which is sharply depressed. A small aqueductus vestibule is present just ventral to the crus commune. The internal auditory meatus is dominated by the large foramen of the vestibulocochlear nerve, with the small foramen of the facial nerve lying just dorsolateral to it.

From a cerebellar aspect, the promontorium is separated from the remainder of the pars cochlearis by a faint discontinuity, caused by the SIPS crossing the surface of the periotic just rostral to the internal auditory meatus. On the tympanic face, a small, pustular process arises from the ventral surface of the promontorium just caudal to the sulcus of the trigeminal nerve; this structure was not seen in any other phalangerid taxa studied. As in Trichosurus , the promontorium itself is long and slender when viewed from both the tympanic and cerebellar aspects. Unlike Trichosurus , however, it retains this slender appearance in its squamosal aspect. This is the result of differences in the morphology of the tympanic surface of the pars cochlearis in the two taxa. In Strigocuscus , the apex of the cupula cochleae is very small and indistinct (unlike that of Trichosurus , which is prominent and well defined), and the rostral tympanic process is small. The squamosal surface of the promontorium is depressed by a narrow trigeminal fossa. The large opening of the hiatus Fallopii lies just posterodorsal to the fossa; just anterior to the fossa, the medial margin of the promontorium is depressed by the SIPS.

Ventrally, the pars cochlearis is bordered by a long CPM. This structure is not nearly as large and ventrally extensive as that seen in Trichosurus , where the CPM completely fills the gap between the basioccipital and the tympanic wing of the alisphenoid ( Norris, 1994). This is linked, in part, to the development of the ventral portion of the rostral tympanic process in the two taxa. In Trichosurus this is a large, well­defined structure, which is greatly expanded ventrally. In Strigocuscus , by contrast, the process, though large, is only poorly developed; it is rounded and shows little or no ventral expansion. The lambdoidal face of the pars cochlearis is relatively smooth and lacks the deep sulci seen in a number of extinct and living phalangerid taxa. The acqueductus cochleae is small and has only a very shallow sulcus leading to it.

Further differences between the periotics of Strigocuscus and Trichosurus can be seen in the roof of the tympanic sinus. The fenestra cochleae and fenestra vestibulae are much smaller in Strigocuscus . The recessus mesotympanicus is curved rather than straight and is bordered by a caudal tympanic process that is only poorly defined. The epitympanic recess is shallow and seems to lack the prominent bordering septa seen in Trichosurus : this is reflected in the lack of continuity between the crista petrosa and tegmen tympani in Strigocuscus .