Wyulda squamicaudata, Alexander, 1918

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 : 8

Wyulda squamicaudata

As in Trichosurus , the crista petrosa of Wyulda is expanded to form a flattened, anteriorly directed, platelike lamella ( fig. 3 ). The prefacial commissure is not especially depressed and instead forms more of a right angle between the crista petrosa and the promontorium. The subarcuate fossa is large and lacks the thickening of the caudal margin seen in Strigocuscus , and the internal auditory meatus is relatively large. The most striking feature, however, is the overall morphology of the pars cochlearis, which although almost as long as those of the other two trichosurin taxa, is much broader, especially in its tympanic and squamosal aspects. The ventral edge of the cerebellar face extends downward as a ventrally directed lamella, bordering the inferior petrosal sinus medially and giving the cerebellar surface of the periotic a flattened appearance. This lamella ends rostrally at the point where the inferior petrosal sinus emerges onto, and crosses, the promontorium. The depth of the lamella exaggerates the degree of discontinuity between the promontorium and the remainder of the pars cochlearis. The shallow sulcus of the inferior petrosal sinus is relatively close to the tip of the promontorium and stops part way across the cerebellar face; this is in contrast to the situation seen in the Riversleigh specimens, where the sulcus is deep and runs the entire width of the cere­ bellar surface of the promontorium (K.C., in prep.).

The breadth of the pars cochlearis is particularly striking when the periotic of Wyulda is viewed from its tympanic aspect. The rostral tympanic process is robust, well defined, and ventrally oriented, as in Trichosurus . The apex of the cupula cochleae is better defined than in Strigocuscus , and the rostral tympanic process is larger. The fenestra cochleae is more ventrally oriented than in either Trichosurus or Strigocuscus . The epitympanic recess is well defined, with a distinct lamina connecting the crista petrosa to the tegmen tympani. The recessus mesotympanicus is curved, and the caudal tympanic process is small and poorly defined.

From the squamosal aspect, the much greater depth and bulk of the pars cochlearis in Wyulda is clearly evident, and it can be seen that this is a function of the size of the rostral tympanic process. The discontinuity between the promontorium and the cerebellar face of the periotic is also very apparent, together with the sulcus of the inferior petrosal sinus that is responsible for the discontinuity. As in Strigocuscus , the acqueductus cochleae is small and indistinct. The platelike lamella of the crista petrosa dominates the lambdoidal aspect of the specimen. Expansion of this structure leads to a marked lateral displacement of the sulcus of the prootic sinus, similar to that which is seen in Trichosurus ( Norris, 1994) .

Figures

Fig. 3. Periotic of AMNH 196920, Wyulda squamicaudata. A. Tympanic face. B. Cerebellar face. Abbreviations: DL, dorsal lamella; MCF, exposure of periotic in middle cranial fossa. Other abbreviations as for figure 2. Approximately 10× life size.