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 : 107

( CAENOLESTES, AUSTRALIDELPHIA ))

As pointed out by Ladevèze (2004), Petrosal Type I is the sister taxa of Australidephia plus Caenolestes , and thus is likely to belong to either Paucituberculata or Microbiotheriidae , which are both present at Itaboraí ( de Muizon & Brito, 1993). The other petrosals from Itaboraí studied here (Types III, IV, and V) are also closely related to these taxa. The consensus tree resulting from a successive character weighting reveals the monophyly of Petrosal Type I and Petrosal Type III, which are probably from the same taxon, and which form the sister group of the Type IV, Type V, Caenolestes and Australidelphia.

These relationships are supported by three unambiguous synapomorphies. The ventral part of the mastoid is rounded and bulbous owing to the fossa subarcuata enlargement (4 0> 1, RI = 0.857, reversal in Peramele s and Echymipera ). According to Sánchez- Villagra (2002), the volume of the fossa subarcuata grows allometrically.

These taxa show a modification of the stapedial system, with the loss of the post-temporal canal (37 0> 1, RI = 0.750; 38 0> 1, RI = 0.900), a condition that occurs independently in extant nondidelphid marsupials ( Wible, 1990) and in the Bolivian Palaeocene taxa Pucadelphys and Andinodelphys .

The second topology, resulting from successive weighting ( Figure 6 ), points to a sister-group relationship of the Petrosal Type I and Petrosal Type III. They share two unambiguous synapomorphies. They exhibit a deep groove for the internal carotid artery on the anterior pole of the promontorium (10 0> 1, RI = 0.800). This feature is also present in the South American metatherians Pucadelphy s, Andinodelphy s, Mayulestes ( de Muizon et al. 1997) , borhyaenoids ( Rougier et al., 1998), and Petrosal Type II ( Ladevèze, 2004).

The presence of a vascular groove adjacent to the prootic sinus sulcus may conduct a vein from the prootic sinus (36 0> 1, RI = 0.500), and was also identified in Pucadelphys , Andinodelphys , the extant dasyurid Phascogale , and the extant didelphid Monodelphis ( Wible, 2003) .

In the second topology ( Figure 6 ), the Petrosal Type V is the sister taxon of the clade comprising Caenolestes plus australidelphians. They share three unambiguous synapomorphies. The tympanic aperture of the hiatus Fallopii is dorsal (20 2> 1, RI = 0.700). The mastoid tympanic process is indistinct to absent (24 2> 3, RI = 0.923). The presence of foramina on the sigmoid sinus and/or prootic sinus, which apparently connects both vessels (35 0> 1, RI = 0.500) is a feature that appeared independently in Petrosal Type I, and is lost in Perameles . Pucadelphys is polymorphic for this character.

Figures

Figure 6. Consensus tree of the two parsimonious trees resulting from a second analysis using reweighted characters. Each node is named and described in the text. The Bremer index is given on the branches, followed by the number of non-ambiguous synapomorphies in parentheses. The Bremer index mean is 2 (36/18).