What are “opossum-like” fossils? The phylogeny of herpetotheriid and peradectid metatherians, based on new features from the petrosal anatomy

The phylogenetic relationships of “opossum-like” metatherians, Herpetotheriidae and Peradectidae, continue to be debated, and yet they are key taxa in the evolutionary history of the crown-group Marsupialia. With state-of-the-art technologies such as X-ray computed tomography (CT scanning), it is now possible to access the inner anatomy of the skull and particularly the ear region, which is more and more frequently used in phylogenetic analyses. Here we describe for the first time the petrosal and inner ear anatomy of the type species <i>Peratherium elegans</i> from the early Oligocene of Ronzon, together with <i>Amphiperatherium elegans</i> from the Eocene of the Montmartre Gypsum. A parsimony analysis of cranial (including petrosal), dental and postcranial characters in 25 metatherians (extant and fossil taxa including the four herpetotheriids studied herein) does not support inclusion of herpetotheriids or peradectids within Didelphimorphia. The results indicate the monophyly of <i>Peratherium cuvieri</i>, <i>P. elegans</i> and <i>A. minutum</i>, but the other herpetotheriid in the analysis, <i>Herpetotherium</i> cf. <i>fugax</i>, is more closely related to the crown-group clade Marsupialia. Peradectids and herpetotheriids, traditionally considered as “opossum-like”, are here found to be stem metatherians and stem marsupials, respectively. The hypothesis in favour of an ancient origin of opossums (Didelphidae) is therefore questioned.