Updated March 13 with text and figure corrections and overlooked references.
Very few skull bones are known for Marasuchus, the tiny theropod-like dinosaur or proto-dinosaur. Here’s a shot at a restoration of the skull (Fig. 1).
Marasuchus would make a pretty good little basal theropod/basal dinosaur EXCEPT,
- each cervical is too short
- no cervicals have epipophyses
- the pubis is too short
- and it has no boot
- the ischium is too deep and V-shaped along its entire length.
- the femur lacks some grooves and bumps found in sister taxa
- the proximal tibia has a lateral bump does not reach the posterior rim
- distal tarsal 4 is not flat
- the astragalus has a larger facet for a larger fibula
Marasuchus is also smaller than basal dinosaur/theropod sisters (Fig. 2) and, considering this list, one wonders if some of these traits are due to neotony, the juvenilization of traits when a taxon experiences generational miniaturization.
If not as a basal theropod close to the odd theropods, Procompsognathus and Segisaurus, then where else could Marasuchus more parsimoniously nest? Most of the above traits can be found individually far from bipedal dino-types, but the suite cannot be found elsewhere. I think we have to rely on maximum parsimony here.
Bonaparte JF 1975. Nuevos materiales de Lagosuchus talampayensis Romer (Thecodontia – Pseudosuchia) y su significado en el origen de los Saurischia, Chañarense Inferior, Triasico Medio de Argentina [New materials of Lagosuchus talampayensis Romer (Thecodontia – Pseudosuchia) and its significance on the origin of the Saurischia, Lower Chañares, Middle Triassic of Argentina]. Acta Geológica Lilloana 13:5-90.
Romer AS 1971. The Chanares (Argentina) Triassic reptile fauna X. Two new but incompletely known long-limbed pseudosuchians: Brevoria 378: 1-10.
Romer AS 1972. The Chanares (Argentina) Triassic reptile fauna. XV. Further remains of the thecodonts Lagerpeton and Lagosuchus: Breviora 394: 1-7.
Sereno PC and Arcucci AB 1994. Dinosaurian precursors from the Middle Triassic of Argentina: Marasuchus lilloensis gen. nov. Journal of Vertebrate Paleontology, 14: 53-73