Lagosuchus

*Lagosuchus talampayensis*

***"Lagosuchus"*** is a dubious genus of small archosaur resembling a crocodilian. It was originally thought to have been a dinosaur from the middle Triassic period. It is generally thought to be closely related to dinosaurs, as a member of the Dinosauromorpha. Its fossils were found in the Chañares Formation of Argentina, dating to the Middle Triassic (Ladinian age), about 230 million years ago.

*Lagosuchus* was a lightly-built archosaur, and is notable for its long slender legs and well-developed feet - features it shares with certain dinosaurs. These features, along with the shape of its hips, suggest that it could run on its hind legs for short periods, although it probably moved on all fours most of the time. *Lagosuchus* was likely an agile predator that could use speed to chase its prey and to escape larger predators.

*Lagosuchus* is regarded by some to be nomen dubium and *L. lilloensis* has been referred to *Marasuchus* by Paul Sereno in 1994.

## History

The type species *Lagosuchus talampayensis* was first described by Alfred S. Romer in 1971, who considered it a "pseudosuchian" (then a collection of various non-dinosaurian "thecodonts"). In 1972 he named a second species, *Lagosuchus lilloensis*, known from larger and more well-preserved specimens. A later review by Jose Bonaparte in 1975 synonymized the two species and considered *Lagosuchus* intermediate between "pseudosuchians" and saurischian dinosaurs.

Modern authors now consider at least *L. lilloensis* to be firmly on the lineage of archosaurs leading to dinosaurs. However, the genus Lagosuchus is regarded by some to be dubious. Paul Sereno and Andrea Arcucci considered *L. talampayensis* to be undiagnosable in a 1994 study, and reclassified *L. lilloensis* as a new genus, Marasuchus. In 2019, PULR 09, the holotype skeleton of *L. talampayensis*, was redescribed by Federico Agnolin and Martin Ezcurra. They argued that the skeleton was not only diagnostic, but indistinguishable from specimens of *Marasuchus lilloensis*. As a result, they supported the synonymy proposed by Bonaparte, referring specimens of *Marasuchus lilloensis* back to *Lagosuchus talampayensis*.

The Chañares Formation's age has been through much debate. It has traditionally been considered to belong to the Ladinian stage, the last stage of the Middle Triassic. Radiometric dating in 2016 has dated the main fossiliferous section of the formation to the early Carnian stage, near the start of the Late Triassic.

## Description

*Lagosuchus* is known from very incomplete remains (only a hind leg plus a shoulder blade and vertebrae can be definitely assigned to it). However, features of the leg show that it was a lightly built archosaur, and is notable for its long slender legs and well-developed feet - features it shares with certain dinosaurs. These features, as well as comparisons to close relatives, suggest that it could run on its hind legs for short periods, although it probably moved on all fours most of the time. *Lagosuchus* was likely an agile predator that could use speed to chase its prey and to escape larger predators. *Lagosuchus* was about the size of a ferret.

*Lagosuchus* is regarded by some to be dubious and the second species assigned to the genus, *L. lilloensis*, was reclassified as *Marasuchus* by Paul Sereno in 1994.

## Paleobiology

### Metabolism

It is believed that *Lagosuchus* and *Marasuchus* were transitional between cold blooded reptiles and warm blooded dinosaurs.

## Dinosaur Field Guide Description

Lagosuchus ("rabbit crocodile) is a little archosaur the size of a rabbit. It had a small. pointed snout, long hind legs, and a long tail. There were many larger and more interesting creatures living in the Middle Triassic, but Lagosuchus is important to paleontologists for a good reason: It helps us understand where dinosaurs came from. Lagosuchus had long shins and feet. Its legs were not sprawled out at the side (as in most reptiles) but were held directly underneath the body. This would have allowed it to run fast for a long time. Dinosaurs also have these features. In fact, Lagosuchus seems either to be the ancestor of the dinosaurs or a close relative of that ancestor. Although we often think of dinosaurs as giant creatures, they didn't start that way. The earliest dinosaurs were small. Having legs directly underneath the body helped early dinosaurs run after insects and small reptiles, and run away from bigger meat-eaters. The same trait helped the later dinosaurs become giants because legs like that could better support their weight.

## Fun Facts

This possible ancestor of the giant dinosaurs was small enough to have been threatened by a cat!

## Trivia

Some paleontologist call this archosaur Marasuchus, after the mara-~a rodent from South America that looks and acts like a rabbit

## References

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