Updated March 12, 2015 with a new skull for Marasuchus.
Agnosphitys cromhallensis (Fraser et al. 2002, Late Triassic, 25 cm snout/vent length estimated) is a small dinosaur enigma known from a half dozen perfectly preserved bones, none of which would articulate with each other in vivo.
Fraser et al. considered Agnosphitys a dinosauriform, closer to true dinosaurs than either Eoraptor (here considered a basal phytodinosaur) and Herrerasaurus. The ilium indicates that only two sacrals were present. Unfortunately Fraser et al. considered three sacrals to be a dinosaur minimum, which is not true.
One solution is to pretend
the rest of the bones are present and create a reasonable restoration (Fig. 1). The pelvis has been compared to that of Marasuchus, only larger. So with that start, if we add the bones to a Marasuchus blueprint we find that the maxilla of Agnosphitys is relatively longer, the humerus is relatively shorter and the astragalus is similar in size.
Basal dinosaurs with a shorter rostrum are generally phytodinosaurs. Those with a longer rostrum and sharp teeth are generally theropods. Those with a short and gracile humerus generally have short forelimbs, another theropod clue, also found in ornithischians. Those with a rostrum as long as the humerus generally remove basal ornithischian candidates, but retain basal theropods like Tawa, which has a typical ilium shape, unlike that of Agnosphitys.
Former dinosauriform candidates, like Lagerpeton, no longer nest near basal dinosaurs. New pro to-dinosaurs like Lewisuchus, Saltoposuchus, Pseudhesperosuchus and Trialestes are better candidates to compare to Agnosphitys, but none share a similar ilium shape.
The lean of the anterior maxilla appears to favor a long low rostrum, like that of Tawa, rather than the more robust rostrum of Herrerasaurus.
Most workers do not nest Marasuchus with theropod dinosaurs.
Here it nests with a branch of theropods that are not usually included in phylogenetic analysis. That branch also includes Procompsognathus and Segisaurus. These three are not in the main line of theropods that led to T-rex and birds.
Fraser NC, Padian K, Walkden GM and Davis LM 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology. 45(1), 79-95.