In 1971, Alfred Sherwood Romer named a new genus and species of archosaurian reptile, Lagerpeton chanarensis. It’s known from several partial specimens found in the Upper Triassic Chañares Formation in La Rioja Province, Argentina. The genus name is derived from Greek and translates to “Hare Reptile.” The species name references the Chañares Formation.
Lagerpeton is known from fragments of the skull, vertebral column, shoulder and forelimb; and nearly complete pelvis and hindlimbs. Although incomplete, the remains show that Lagerpeton was bipedal with long, slender hind limbs, and shorter forelimbs similar to many early dinosaurs. In life, Lagerpeton was about 60-70 cm (24-28 inches) long, and stood 20-30 cm (8-12 inches) tall; about the same size as a crow.
Lagerpeton’s most distinctive feature was its feet and toes. Each foot had four toes of distinctly different lengths. The innermost toes, digits I and II, were shortened and did not contact the ground. The outer toes, digits III and IV were longer and bore almost all the weight of the animal while standing. Uniquely, the outermost toe, digit IV, is the longest in the foot.
This differs from what’s seen in birds and early dinosaurs. Their feet also have four toes, with the innermost toe, digit I, being the shortest. But in birds and early dinosaurs digits II-IV reach the ground and bear all the weight of the animal while standing. Additionally, in dinosaurs and birds, the middle weight-bearing toe, digit III, is usually the longest.
When Romer named Lagerpeton, he considered it to be a “pseudosuchian” of uncertain affinities, but possibly close to the ancestry of “coelurosaurs.” Romer’s use of these names conforms to mid-20th-century usage. Today, pseudosuchians are considered a lineage that includes crocodilians and all archosaurs closer to crocodilians than birds. But to Romer and contemporaneous paleontologists, “pseudosuchians” were a diverse group of Triassic archosaurs independently ancestral to crocodilians, birds, pterosaurs, coelurosaurs, carnosaurs, sauropods, and ornithischians.
Beginning in the 1980s, the application of phylogenetic analyses showed that the old concept of “pseudosuchians” didn’t conform to the actual evolutionary history of the group. They found two major lineages within the archosaurs. One lineage includes crocodiles and crocodile-line archosaurs like aetosaurs, poposaurs, and rauisuchians. As previously mentioned, this lineage retains the name Pseudosuchia. The other major lineage includes birds and bird-line archosaurs like pterosaurs, ornithischians, sauropodomorphs, and non-avian theropods. This lineage is called Avemetatarsalia. Some other pseudosuchians-of-old, like phytosaurs and Euparkeria, are now known to be outside the crocodile-bird lineage, and are considered non-archosaurian archosauriformes.
When included in phylogenetic analyses, Lagerpeton was initially found to be close to the ancestry of dinosaurs, somewhat closely related to the contemporaneous Lagosuchus. Additional members of its family, the Lagerpetidae, were discovered beginning with Dromomeron in 2007. Many recent analyses, including a comprehensive 2020 review by Martín Ezcurra and colleagues, have found that lagerpetids are actually most closely related to the pterosaurs, and share several anatomical characteristics. For instance the ends of the lower jaws of lagerpetids and early pterosaurs are toothless and taper to a point. Likewise, lagerpetids and many early pterosaurs share multi-cusped teeth. Additional details of the forearms and femora link lagerpetids and pterosaurs.
Other members of the family preserve details of the brain and inner ear, and show that lagerpetids share many similarities to pterosaurs there too. These features are also seen in birds and primates and are related to navigating in three dimensions. Lagerpetid forelimb material is rare, but no lagerpetids show any early modifications of their arms into wings. It is thought that lagerpetids climbed and spent a lot of time in trees, leaping from branch to branch in search of insects and other small prey.