Cladistic analysis of therapsid relationships

Abstract

A series of cladistic analyses of 47 taxa of synapsids, including Dimetrodon and Haptodus as consecutive outgroups to Therapsida and Morganucodon representing Mammalia, was performed using PAUP 3.0 to determine most parsimonious trees. The analyses were based on 210 characters: 106 from skull and lower jaw; 37 from dentition; and 67 from postcranial skeleton. Analyses including all characters yield the following increasing-derived therapsid clades: Biarmosuchia, Dinocephalia, Anomodontia, and Theriodontia, the latter including Gorgonopsia as sister group to Therocephalia and Cynodontia. Advanced cynodonts (Eucynodontia) fall into two clades: Cynognathia, consistently containing Cynognathidae and Gomphodontia, and Probainognathia, consistently containing Probainognathidae and Chiniquodontidae. Although Tritylodontidae, Tritheledontidae, and Morganucodon (Mammalia) form a single clade, this clade varies in relationship to other cynodonts, joining either with gomphodonts or with probainognathians. Analyses using only craniodental characters yield the same major therapsid groups, but britylodontids now link with gomphodonts whereas tritheledonts/Morganucodon link with probainognathians. Analyses using only postcranial characters give paraphyletic groups representing therapsid locomotor grades, with intermixing of gomphodont and probainognathian taxa. I accept the results of the craniodental analysis and conclude that homoplasy in the therapsid postcranium is greater than in the skull and dentition.