Basicranial Evidence for Early Mammal Phylogeny

Abstract

OverviewThe distribution of thirty-eight basicranial characters is considered among monotremes, marsupials, placentals, and the following extinct taxa—Tritheledontidae, Tritylodontidae, Sinoconodon, Morganucodontidae, Haldanodon, Triconodontidae, Multituberculata, and Vincelestes. PAUP analysis of the ensuing data matrix supports the following conclusions: 1.Marsupialia and Placentalia form a clade supported by an anterior lamina of the petrosal that is greatly reduced or absent, a cavum epiptericum floored primarily by the alisphenoid, major basicranial drainage via the postglenoid foramen, and a squamosal contributing broadly to the cranial wall.2.A clade with Vincelestes from the Early Cretaceous of Argentina as the sister taxon to Marsupialia plus Placentalia is supported by a caudal tympanic process of the petrosal, a post-promontorial tympanic sinus, a true cochlear aqueduct, and a cochlear duct coiled through at least 270 degrees.3.A clade comprising the taxa in (2) along with Multituberculata plus Monotremata is supported by loss of support for the ventromedial part of incus on the cranium and a greatly reduced quadrate ramus of the alisphenoid. A clade with Multituberculata and Monotremata is supported by a common tympanic aperture for the prootic canal and pterygoparoccipital foramen.4.A clade including Triconodontidae and the taxa in (3) is supported by loss of the vascular foramen in the periotic lateral flange and the suspension of the postdentary bones from the cranium.5.A clade with Haldanodon and the taxa in (4) is based on the loss of the quadratojugal notch in the squamosal.6.A clade including the taxa in (5) plus Sinoconodon and Morganucodontidae is supported by an anterior lamina of the petrosal expanded forward dorsal to the exit of the maxillary and mandibular nerves, a cavum epiptericum partially floored by the petrosal, facial ganglion floored by petrosal, stapes length less than 5.5% of skull length, ossified base of the pila antotica absent, major basicranial drainage via a large prootic canal that opens endocranially, a paroccipital process with a distinct ventrally directed projection for muscle attachment, a petrosal promontorium, and a well-developed dentary-squamosal contact.7.Tritylodontidae shares support of the ventromedial part of the quadrate on the cranium via a convex surface on the crista parotica of the petrosal and a fossa for the stapedius muscle on the petrosal with the taxa in (6). Only the groupings in (1) and (2) are congruent with those produced from analyses of other anatomical systems; the others are controversial.