Petrosals of Late Cretaceous marsupials from North America, and a cladistic analysis of the petrosal in therian mammals

ABSTRACT Ten isolated petrosals of Late Cretaceous marsupials belonging to three types—A, B, and C—are described from the Lance Formation of Wyoming and the Bug Creek Anthills of Montana. These are compared with ear regions of other Mesozoic and Recent mammals in a cladistic analysis of 24 petrosal characters among ten taxa. A clade comprising marsupials, eutherians, and Vincelestes from the Early Cretaceous of Argentina is supported by five synapomorphies of the ear region—coiled cochlea, perilymphatic duct enclosed in an osseous canal, post-promontorial tympanic sinus present, caudal tympanic process of the petrosal present behind stapedius fossa, and lateral wall of fossa incudis formed by squamosal. Marsupialia is diagnosed by four synapomorphies of the ear region—stapedial artery absent in adult, prootic canal reduced and with intramural opening, prootic sinus within deep sulcus between petrosal and squamosal, and sphenoparietal emissary vein present. Eutheria is also diagnosed by four synapomorphies of the ear region—fenestra vestibuli elliptical, prootic canal absent, prootic sinus absent in adult, and capsuloparietal emissary vein present. Following the pattern in Recent didelphids, vessels and nerves are reconstructed on the most complete Late Cretaceous marsupial petrosal, Type A, which probably belongs to the pediomyid Pediomys hatcheri or P. florencae. A prootic canal, which transmits the lateral head vein, is reported for all three Late Cretaceous petrosal types and for Recent adult didelphids, caenolestids, and some dasyurids. Comparison with other mammals reveals that a major dichotomy in the basicranial vasculature occurs between adult marsupials and eutherians. Both the lateral head vein and stapedial artery run through the middle ear in the common ancestor of marsupials and eutherians, but in the former the artery is lost and in the latter the vein disappears. A reevaluation of the ontogeny of the cranial venous system reveals that the vein passing through the postglenoid foramen is not homologous in marsupials and eutherians.