Are Spondylosoma and Staurikosaurus (Santa Maria Formation, Middle-Upper Triassic, Brazil) the oldest saurischian dinosaurs?

Abstract

Spondylosoma absconditum (Ladinian, Middle Triassic) lacks certain Dinosauriformes-Dinosauria characters (sigmoid curve and epipophyses in neck, distally placed deltopectoral crest on humerus) and those present (accessary hyposphene-hypantra vertebral articulations, three sacral vertebrae, elongate pubis) also occur in the Rauisuchia (Crurotarsi, Suchia, Rauisuchiformes).Spondylosoma is tentatively referred to the Rauisuchidae because, as indicated by the ventrolaterally directed sacral ribs, the ilium was held obliquely (and not vertically as in the Poposauridae).Staurikosaurus pricei (Carnian, Upper Triassic), one of the oldest dinosaurs, possesses autapomorphies of Dinosauriformes, Dinosauria, Saurischia, and Herrerasauridae, but it retains certain plesiomorphic characters (subcircular distal end to tibia, ?proximally placed deltopectoral crest, axially unshortened neural spine to sacral 1, ?scapular spine unreduced distally) not present inHerrerasaurus (Carnian, Ischigualasto Formation, Argentina). However, the femur is more derived in the loss of the trochanteric shelf and reduction of the lesser trochanter (as also occurs in some prosauropods). The two vertebrae sacrum ofStaurikosaurus is a plesiomorphic character for Dinosauria (also in other herrerasaurids, German Upper Triassic prosauropodSellosaurus gracilis — ?males), to which was added either a caudosacral (e.g. Santa Maria prosauropodSaturnalia, Sellosaurus gracilis — ?females) or a dorsosacral (e.g. Ischigualasto theropodEoraptor and prosauropodRiojasaurus). Agrosaurus (Upper Triassic, Durdham Down, England — not Australia) is not a herrerasaurid; it is referred to the prosauropod dinosaurThecodontosaurus antiquus (Upper Triassic, Durdham Down).