Deltatheridium, Gregory & Simpson, 1926

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 : 104-105

METATHERIANS EXCEPT DELTATHERIDIUM

This clade is supported by three unambiguous and two ambiguous synapomorphies.

In metatherians and eutherians, the cavum epiptericum has a complete floor mainly formed by the alisphenoid (2 1> 2, RI = 0.750) ( Wible, 1990: fig. 5.2D). In some metatherians, this floor may be completed by the petrosal (i.e. Deltatheridium and Gurlin Tsav Skull, see Rougier et al., 1998; Mayulestes , Pucadelphys , Andinodelphys ).

The internal acoustic meatus is subequal to the fossa subarcuata and separated from the latter by a sharp wall (8 1> 0, RI = 0.286), but this feature is highly convergent among the metatherians studied here.

The lateral wall of the epitympanic recess formed by the petrosal (either adjoined or not by the squamosal) is generally present in most therians (30 0> 1, RI = 0.667). In the outgroup taxa, the epitympanic recess, when present, is only bordered by the squamosal.

A hypotympanic sinus is formed by the squamosal, petrosal, and alisphenoid (46 0> 1, RI = 0.714) in Didelphodon , Mayulestes , and borhyaenids, whereas it is formed by the petrosal and alisphenoid in didelphids, Dromiciops , and dasyurids. Pediomys have an alisphenoid hypotympanic sinus, but the contributing elements are uncertain ( Wible et al., 2001). This character may represent a synapomorphy of the clade formed by metatherians except Deltatheridium (reversal in Didelphodon and Mayulestes ).