Description and Classification

Revueltosaurus callenderi, the type species, was a relatively small reptile, estimated to be about 1 to 1.5 meters (3 to 5 feet) in length. It likely walked on four legs (quadrupedal) and possessed a body covered in protective bony plates called osteoderms, similar to modern crocodiles and other pseudosuchians like Aetosaurus. The most distinctive feature of Revueltosaurus initially known was its teeth. These teeth are triangular or leaf-shaped (lanceolate) with coarse serrations along the edges, perfectly suited for shredding tough plant material.

The classification of Revueltosaurus underwent a significant revision. Initially described in 1989 by paleontologist Adrian P. Hunt based solely on teeth, it was placed within the Ornithischia, the group of “bird-hipped” dinosaurs that includes animals like Stegosaurus and Triceratops. This was because its teeth strongly resembled those of early herbivorous dinosaurs. However, the discovery of associated skeletal material, including skulls and bones from the rest of the body, by paleontologist William Parker and colleagues in 2005, definitively showed that Revueltosaurus was not a dinosaur. Instead, it belongs to Pseudosuchia (also known as Crurotarsi), the group of archosaurs that includes modern crocodiles and their extinct relatives. Within Pseudosuchia, its exact placement is still studied, but it is often considered to be a basal member or related to groups like the Aetosauria. Other species, such as Revueltosaurus olseni and Revueltosaurus hunti, have also been named, primarily based on tooth differences from various locations.

Distinguishing Features

Revueltosaurus can be identified by several key characteristics:

• Leaf-shaped (lanceolate) teeth with noticeable serrations on both edges, adapted for eating plants. These were once mistaken for ornithischian dinosaur teeth.
• Presence of osteoderms (bony scutes or plates) arranged in rows along its back, providing armor and protection.
• A body plan suited for walking on four legs, unlike many early dinosaurs which were bipedal (walked on two legs).
• Skull features that are distinctly pseudosuchian (crocodile-line), differing from those of dinosaurs. For example, the structure of its ankle is typical of pseudosuchians.

Paleoenvironment and Diet

Revueltosaurus lived during the Late Triassic period in a region that had a warm, seasonally dry climate with river systems and floodplains. Its fossils are commonly found in the Chinle Formation of the southwestern United States, a rock unit famous for preserving a diverse Triassic ecosystem. This environment was home to a variety of other archosaurs, including early theropod dinosaurs like Coelophysis, large crocodile-like phytosaurs, heavily armored aetosaurs, and rauisuchians, which were top predators. Amphibians and early mammal relatives also shared this habitat.

The teeth of Revueltosaurus clearly indicate a herbivorous diet. The way its serrated, leaf-shaped teeth could slice and dice would have been effective for processing tough vegetation. Common plants during the Triassic period included ferns, cycads, and conifers, which Revueltosaurus likely ate. This makes Revueltosaurus one of several plant-eating pseudosuchian lines that evolved during the Triassic, showing that herbivory was not exclusive to dinosaurs in these ancient ecosystems.

Significance and Ongoing Research

The story of Revueltosaurus is very important in paleontology because it is a classic example of convergent evolution. This is when unrelated organisms independently evolve similar features. Its dinosaur-like teeth led to its misidentification for over a decade, which affected how scientists understood early dinosaur diversity and where they lived. The reclassification of Revueltosaurus as a pseudosuchian archosaur made scientists re-evaluate what was thought to be evidence of early ornithischian dinosaurs in the Late Triassic of North America. It highlighted how important it is to find more complete skeletons for accurate identification, especially for animals from times of major evolutionary change like the Triassic.

Ongoing research continues to improve our understanding of Revueltosaurus and its relatives. This includes studying the differences in tooth shapes among various populations and locations, which may help clarify how many valid Revueltosaurus species there were. More fossil discoveries could provide more details about its anatomy, how it moved, and its lifestyle. Paleontologists are also still working to determine its exact place within the pseudosuchian family tree and to understand the broader patterns of how plant-eating evolved among crocodile-line archosaurs during the Triassic Period. Its reclassification has also been important for biostratigraphy (the science of using fossils to determine the age of rock layers).