Lewisuchus: an early dinosauriform from Argentina

Lewisuchus admixtus was a small, long-legged archosaur from the Chañares Formation of north-western Argentina, close to the early radiation of dinosaurs. Alfred Romer named it in 1972 from an incomplete skeleton. New articulated finds and revisions of the original fossil have since changed which bones are included and how the animal is classified. It is securely an early dinosauriform; whether its broader lineage sits outside dinosaurs or within an early ornithischian branch remains part of a wider phylogenetic debate.

Quick facts

The type skeleton and its revisions

The holotype, PULR 01, preserves parts of the skull, vertebral column, shoulder girdle and limbs. It includes incomplete upper jaws and a braincase, neck vertebrae from the atlas through the seventh cervical, eleven dorsal vertebrae, nine tail vertebrae, both scapulocoracoids and humeri, plus parts of the lower legs. Romer's original reconstruction also included a lower jaw and foot bones. A later re-examination found that some elements in the same concretion belonged to other reptiles, likely proterochampsids. Museum study therefore corrected the specimen without requiring a new field discovery.

Two partial articulated skeletons, described in 2022, added information about the skull, front of the spine and limbs. Their overlapping bones share a distinctive combination of features with the holotype, supporting their referral to Lewisuchus. These finds improve knowledge of the animal but still do not produce a complete skeleton. Other bones reported from the formation should not automatically be folded into its anatomy.

Age and setting of the Chañares Formation

The type locality is Los Chañares in La Rioja Province. The formation contains fine-grained deposits and fossil-bearing concretions associated with a floodplain. Rapid burial helped preserve delicate bones, while fossils of different animals can occur together in a concretion and require careful separation.

Older accounts often called the Chañares Formation Middle Triassic. Radiometric dating has shown that much of it is Late Triassic, with the Ladinian–Carnian boundary likely falling near its lower part. The type material is generally placed near that transition, around 236 million years ago, but the exact age depends on which bed and dating framework are used. The fauna included cynodonts, dicynodonts, rhynchosaurs, pseudosuchians and several early dinosauriforms.

Predator, but with limits to the evidence

The skull was low and elongated, and its teeth were sharp and recurved. Those features support a carnivorous or animal-eating role, perhaps involving small vertebrates, but no stomach contents identify prey. Its relatively light skeleton and long limbs are consistent with an active terrestrial animal. They do not by themselves tell us how fast it ran or whether it hunted in a particular way.

Lewisuchus has also been central to discussion of Pseudolagosuchus major, another dinosauriform named from the same formation. A 2010 analysis proposed that the two names represented the same animal, and a 2020 study formally treated Pseudolagosuchus and its species as junior subjective synonyms of Lewisuchus admixtus. The synonymy is an evidence-based taxonomic proposal, not a direct physical connection between every specimen assigned to the two names. The new 2022 skeletons strengthen the anatomical definition of Lewisuchus, while comparisons of separated material still require caution.

Where it sits on the tree

Many analyses place Lewisuchus among silesaurids, close relatives of dinosaurs. Some recent models distribute traditional silesaurids among the earliest ornithischians. The change is consequential for the origin of plant-eating dinosaurs, but it does not make every silesaurid classification settled. In the dinosaur catalogue, Lewisuchus is treated as an early dinosauriform profile, with the uncertainty stated rather than hidden. Its relationship to Kwanasaurus is useful for comparison, not evidence that the two lived together or occupied identical ecological roles.