Caenolestes, , AUSTRALIDELPHIA

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 : 106

( CAENOLESTES, AUSTRALIDELPHIA )))

This clade is supported by seven unambiguous and one ambiguous synapomorphies.

The mastoid tympanic process is weakly developed and forms a distinct posterior wall of the stylomastoid notch (24 1> 2, RI = 0.846).

The lateral wall of the epitympanic recess is raised and triangular (31 0> 1, RI = 0714).

The alisphenoid develops a ventral wing (43 0> 1, RI = 1.000; 42 0> 1, RI = 1.000), which floors a hypotympanic sinus (46°> 2, RI = 0.714). An alisphenoid tympanic process is present in the Late Cretaceous metatherians Asiatherium reshetovi and Gurlin Tsav skull ( Trofimov & Szalay, 1994; Szalay & Trofimov, 1996; Rougier et al., 1998), and thus was thought to be a synapomorphy of metatherians. However, the Early Palaeocene Pucadelphy s, Mayuleste s, and Andinodelphys show the plesiomorphic state ( de Muizon et al., 1997; Wroe, 1997; de Muizon, 1998). The auditory bulla is formed by the alisphenoid (48 0> 1, RI = 1.000). The late Palaeocene Pucadelphys and Andinodelphys show neither an ossified bulla nor any alisphenoid tympanic process ( Marshall & de Muizon, 1995; S. Ladevèze, pers. observ.), and that condition may represent the plesiomorphic state in metatherians. However, an alisphenoid contribution to the formation of the bulla is present in some basal metatherian taxa from the Late Cretaceous of Asia (i.e. Gurlin Tsav skull and A. reshetov i; Trofimov & Szalay, 1994; Szalay & Trofimov, 1996; Rougier et al., 1998).

The transverse canal is present in most extant metatherians (50 0> 1, RI = 0.750), but its occurrence and location appear variable among extant didelphids ( Sánchez-Villagra & Wible, 2002).

The development of a tympanic wing of the petrosal, which extends anterolaterally, but not over the whole length of the promontorium (18 0> 1, RI = 1.000), is a synapomorphy of extant metatherians plus the Type-I petrosal. The Type-III, -IV, and -V petrosals do not develop any rostral tympanic process of petrosal, and were optimized erroneously in the analysis as possessing a tympanic wing.