Metatherian petrosals from the late Paleocene of Itaboraí, Brazil, and their phylogenetic implications

ABSTRACT

Four Paleocene petrosal bones from the Itaboraí fissure fillings of Brazil belong to two metatherian morphotypes. They are compared with ear regions of other fossil and recent marsupials. The results of a parsimony analysis using thirty-eight characters from the petrosal and ear region are partly congruent with recent studies and confirm that characters from the ear region are highly informative for phylogenetic reconstruction. Metatheria are diagnosed by five petrosal synapomorphies: stapedial artery absent in adults; reduced, intramural prootic canal; extrabullar internal carotid artery; inferior petrosal sinus between petrosal, basisphenoid, and basioccipital; and cava supracochleare and epiptericum completely separated. The stem group of South American and Australian metatherians is composed of the Paleocene taxa Pucadelphys, one of the oldest known metatherians from South America, and Petrosal Type II from Itaboraí. Among the South American-Australian metatherians two clades are defined here. The crown clade is composed of Australian taxa plus Dromiciops (here referred to Australidelphia sensu stricto), while they plus Caenolestes form the Australidelphia sensu lato. Petrosal Type I from Itaboraí is the most basal taxon of Australidelphia sensu lato, the latter diagnosed by three significant synapomorphies: ventral part of the mastoid rounded and bulbous owing to enlargement of the fossa subarcuata; posttemporal foramen absent; and foramen for a branch of the transverse sinus present on the posterior edge of the ventral pars canalicularis.