Dromiciops, Thomas, 1894

Ladevèze, Sandrine, 2007, Petrosal bones of metatherian mammals from the Late Palaeocene of Itaboraí (Brazil), and a cladistic analysis of petrosal features in metatherians, Zoological Journal of the Linnean Society 150 (1), pp. 85-115 : 108

DROMICIOPS PLUS DASYURIDS

The dasyurid Phascogale and Dasycercus , and the microbiotheriid Dromiciops , share five unambiguous but controversial synapomorphies and four ambiguous synapomorphies. The formation of a hypotympanic sinus of the petrosal (19 0> 1, RI = 1.000) represents probably a synapomorphy of the Dasyuromorphia ( Wroe, 1997, 1999; Wroe et al., 2000). The formation of the petrosal plate (27 0> 1, RI = 1.000) occurs in some specialized didelphids (e.g. Caluromysiop s), microbiotheriids, some derived peramelomorphians, and all recent dasyurids ( Wroe, 1999). The presence of a tubular foramen for the Eustachian tube (47 0> 1, RI = 1.000) is probably a synapomorphy of dasyurids ( Wroe, 1997, 1999; Wroe et al., 2000), even if it is absent in Thylacinus and found in Dromiciop s. A secondary facial nerve canal enclosed within the petrosal (21 0> 1, RI = 1.000) is an apomorphy shared by Dromiciops and dasyurids, which was thought to be unique among extant dasyurids ( Wroe, 1999).

The crista petrosa extends as a thin lamina covering the anterior part of fossa subarcuata (5 0> 1, RI = 1.000).

In those taxa, the complexity of the auditory bulla results from the expansion and fusion of the alisphenoid and petrosal processes (49 0> 1, RI = 1.000).

The bulla of Dromiciops is peculiar (49 1> 2). It is a ventrally closed, globular structure formed by four components: the ectotympanic (lateral), the tympanic process of the alisphenoid (anterior), the caudal tympanic process of the petrosal (posterolateral), and the rostral tympanic process of the petrosal (mesial and ventral) ( Sánchez-Villagra & Wible, 2002). The bulla is completed by two small processes, one from the basioccipital and another from the exoccipital ( Giannini, Abdala & Flores, 2004). According to Hershkovitz (1999), the evolution of the bulla is characterized by the expansion of its components and its progressive enclosure by the processes developed by the alisphenoid and petrosal (this enclosure is complete in Dromiciops and partial in Caluromys and some other taxa).