Although isolated mammalian petrosals often are encountered in the fossil record, few detailed descriptions of these bones exist for extant taxa. As a contribution to that void, isolated petrosals are described in detail for two nine-banded armadillos, Dasypus novemcinctus Linnaeus, 1758, and are placed in the context of the basicranium based on an additional 18 specimens, all from Carnegie Museum of Natural History. Neurovascular structures are reconstructed based on study of serially sectioned fetal specimens from the Anatomisches Instituts in Frankfurt and Heidelberg, Germany. Preliminary comparisons are made with other extinct and extant eutherians that the author has described in recent years. Unexpectedly, quite a few similarities are found between the petrosals of D. novemcinctus and the chiropteran Pteropus livingstonii Gray, 1866, which in light of the divergent phyletic affinities and biologies of these animals are remarkable convergences.