Lagosuchus and Marasuchus, a Field Guide

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.

In short,
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.

Figure 1. Marasuchus lilloensis (above) and Lagosuchus talampayensis (below) compared. The radius and ulna are longer in marasuchus. The hind limbs are more robust in Lagosuchus. The length of the torso in Lagosuchus is based on the insitu placement of the pectoral girdle and forelimb, which may have drifted during taphonomy.

Figure 1. Marasuchus lilloensis (above) and Lagosuchus talampayensis (below) compared. The radius and ulna are longer in Marasuchus. The scapulocoracoid is more robust in Marasuchus. The hind limbs are more robust in Lagosuchus. The length of the torso in Lagosuchus is based on the insitu placement of the pectoral girdle and forelimb, which may have drifted during taphonomy. Neither taxon had any phalanges on pedal digit 5.

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).

Not dinosaurs?
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.

References
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

wiki/Marasuchus
wiki/Lagosuchus

1 thought on “Lagosuchus and Marasuchus, a Field Guide

  1. Your Lagosuchus looks unstable. With such a long torso it looks like it might topple forward, especially with such a short tail. The Center of Mass in a biped basically needs to be right at the acetabulum in order for it to be stable.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.