Keys to the Origin of the Dinosauria
Upon its discovery, little bipedal Lagosuchus talampayensis (Romer 1971. 1972, UPLR 09, fig. 1, Middle Triassic, 230 mya) was hailed as a key taxon in the story of the origin of dinosaurs. Unfortunately, later workers were not so happy with that species. Sereno and Arcucci 1994 wrote, “Lagosuchus talampayensis does not exhibit any autapomorphies to distinguish it from contemporary dinosauromorphs. In contrast, the holotype of Lagosuchus lilloensis is a well-preserved skeleton that exhibits several distinctive features… we therefore designate a new genus, Marasuchus for the species ‘Lagosuchus’ lilloensis.” The two species had been recognized, by Romer, but there was more to be said.
Sereno and Arcucci (1994) determined that the lilloensis specimen was not congeneric with Lagosuchus, so they renamed this more complete and more distinct specimen ‘Marasuchus‘ (Fig. 1). And it has become one of the keys to the origin of the Dinosauria. Lagosuchus, meanwhile, has languished in relative obscurity.
For some unknown reason
Like Clark Kent and Superman, Lagosuchus and Marasuchus have never been seen together in the same room or in the same illustration (Fig. 1). We’ll fix that today and review the gross similarities and differences.
Both were small, lightly built bipeds.
Lagosuchus had shorter forearms and a smaller (more gracile) pectoral girdle. The tibia/fibula was longer and more robust. That made Lagosuchus slightly taller at the hips.
For some unknown reason, artists like to paint and mount both of these taxa in the quadrupedal configuration. It always looks forced and awkward. Why not let these guys be bipeds? Because they’re not considered dinosaurs by the traditional community. We’ll reexamine that in a future post.
Apparently Lagosuchus and Marasuchus were closely related. The splitters (Sereno and Arcucci 1994) beat out the lumpers in this case and for good reason (see above).
The large reptile tree nested Marasuchus as a basal theropod, not as a basal or pre-dinosaur. The reduction of metatarsal 1 and the lack of phalanges on pedal digit 5 are two key traits to that nesting.
Agnosphitys cromhallensis (Fraser et al. 2002) is known from a selection of uncrushed bones, all of which resemble those from Marasuchus, but slightly larger.
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 .
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