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 ver- tebrae, elongate pubis) also occur in the Rauisuchia (Crurotarsi, Suchia, Rauisuchiformes). Spondylosoma is tentatively re- ferred to the Rauisuchidae because, as indicated by the ventro- laterally 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, pos- sesses autapomorphies of Dinosauriformes, Dinosauria, Sauri- schia, 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 in Herrera- saurus (Catalan, Ischigualasto Formation, Argentina). How- ever, 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 of Stauriko- saurus is a plesiomorphic character for Dinosauria (also in other herrerasaurids, German Upper Triassic prosauropod Sellosaurus gracilis - ?males), to which was added either a caudosacral (e.g. Santa Maria prosauropod Saturnalia, Sello- saurus gracilis - ?females) or a dorsosacral (e.g. Ischigualasto theropod Eoraptor and prosauropod Riojasaurus). Agrosaurus (Upper Triassic, Durdham Down, England - not Australia) is not a herrerasaurid; it is referred to the prosauropod dinosaur Thecodontosaurus antiquus (Upper Triassic, Durdham Down).