Analysis of Molar Structure and Phylogeny of Docodont Genera

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Docodonts are a Mesozoic mammal group (Synapsida: Mammaliaformes) with a fossil record from the Middle Jurassic to the late Early Cretaceous. With highly distinctive molars for both shearing and grinding functions, docodonts are inferred to have diverse dietary adaptations, including insectivory, omnivory, and even carnivory. This group also offers the earliest-known case of mammalian swimming adaptation; at least two docodonts are inferred to have occupied a semi-aquatic niche. Here we present a phylogenetic analysis of 31 dental characters of docodonts (12 genera), plus six outgroups, including the taxa considered by some to be close relatives to docodonts. Our analysis recognizes a clade of Itatodon and Krusatodon and a clade of Tashkumyrodon and Borealestes, both of the Middle Jurassic. There is also a well-supported Late Jurassic Euroamerican clade (Dsungarodon, Docodon, and Haldanodon) and an Asiatic clade (Sibirotherium and Tegotherium). The Late Jurassic and Early Cretaceous clades are nested in a paraphyletic series of plesiomorphic taxa from the Middle Jurassic of Eurasia. Our re-analysis of Gondtherium from the Kota Formation of India confirms its docodont affinity and supports a prior hypothesis that this group dispersed to Gondwana during the Middle Jurassic. We hypothesize that docodonts and Late Triassic Tikitherium are sister taxa, and that the Tikitherium-docodont clade, in turn, is related to the mammaliaforms Woutersia and Delsatia. Contingent on the current scheme of cusp homology, docodonts are related to some Late Triassic mammaliaforms with triangular molar cusps, a paraphyletic group commonly known as “symmetrodonts.”