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Anisodontosaurus greeri  is an enigmatic small-bodied tetrapod with a heterodont dentition from the Holbrook Member of the Moenkopi Formation (Middle Triassic) of Arizona (U.S.A.). The evolutionary relationships of this taxon have long been debated and remain uncertain. Using micro-computed tomography (µCT) scans we redescribe the holotype of  Anisodontosaurus greeri  (UCMP 37804), as well as an additional specimen (MNA.V.1478) that had been informally referred to this taxon. Our new data reveal hidden details of the dentition (i.e., ankylothecodonty and absence of replacement teeth) that, in combination with the tricuspid and mediolaterally expanded crowns, support a referral of both specimens to the archosauromorph clade Trilophosauridae. The referral of MNA.V.1478 to  Anisodontosaurus greeri  is supported by the unique anatomy of the highly differentiated dentition (i.e., ‘figure of 8’-shaped premolariforms in occlusal views; mesiodistally short tooth row; presence of a distal ‘molariform’ crown). Comparison of  Anisodontosaurus greeri  with other trilophosaurids highlights marked similarities with  Variodens inopinatus  from the Upper Triassic deposits of the U.K. Our cladistic analyses confirm these observations, and recover, for the first time, two distinct lineages within Trilophosauridae: one geologically long-lived and comprising  Anisodontosaurus  and  Variodens ; and one comprising  Tricuspisaurus ,  Trilophosaurus  spp., and  Spinosuchus . These results imply that: (i)  Anisodontosaurus  is one of the oldest known trilophosaurids worldwide and the oldest in North America; (ii) trilophosaurids achieved a broad distribution at low latitudes within western Pangea by the Middle Triassic; (iii) small body sizes were more common than previously thought among trilophosaurids.

New specimen and redescription of Anisodontosaurus greeri (Moenkopi Formation: Middle Triassic) and the spatiotemporal origins of Trilophosauridae