Definition, diagnosis, and origin of Mammalia

ABSTRACT Mammalia is defined by its ancestry as the taxon originating with the most recent common ancestor of extant Monotremata and Theria. To diagnose Mammalia as so defined, 176 character transformations in the skull and postcranial skeleton, distributed among Placentalia, Marsupialia, Multituberculata, Monotremata, Morganucodontidae, Tritylodontidae, and Exaeretodon, were polarized, scored, and subjected to PAUP. Only one most parsimonious tree was identified (BL = 190, CI = 0.926): (Exaeretodon (Tritylodontidae (Morganucodontidae (Monotremata (Multituberculata (Marsupialia, Placentalia)))))). Thirty-seven osteological synapomorphies diagnose Mammalia. Triassic and Early Jurassic taxa commonly referred to as mammals, including Morganucodontidae, Kuehneotheriidae, and Haramiyidae, were found to lie outside of Mammalia. These fossils document that the mammalian lineage had diverged from other known synapsid lineages by the Norian (Late Triassic). However, the earliest evidence that Monotremata and Theria had diverged from their most recent common ancestor, and thus the earliest evidence of Mammalia itself, is of Bathonian (Middle Jurassic) age. Many of the diagnostic attributes of Mammalia are associated with either the sensory organs housed in the skull, the masticatory system, or the craniovertebral and atlas-axis articulations. Modification of each of these regions has long been tied to the origin of mammals. However, other synapomorphies are identified which suggest that additional factors must be sought to fully understand the origin of Mammalia.