Lagerpeton is a genus of lagerpetid avemetatarsalian, comprising a single species, L. chanarensis.[1] First described from the Chañares Formation of Argentina by A. S. Romer in 1971, Lagerpeton's anatomy is somewhat incompletely known, with fossil specimens accounting for the pelvic girdle, hindlimbs, posterior presacral, sacral and anterior caudal vertebrae.[2] Skull and shoulder material has also been described.[3]
Lagerpeton fossils have only been collected from the Chañares Formation in La Rioja Province, Argentina. The first of these fossils were discovered in a 1964-1965 expedition by the Museum of Comparative Zoology (MCZ) and Museo de la Plata (MLP), although some were also discovered in 1966 by paleontologists from the Miguel Lillo Institute (PVL) of the University of Tucuman.[2][4][5]
Alfred Romer named Lagerpeton chanarensis in 1971, based on a complete right hindlimb discovered during the MCZ-MLP expedition. The specimen was initially stored at the Museo de la Plata with catalogue number MLP 64-XI-14-10, but by 1986 it had been transferred to the Paleontology Museum at the National University of La Rioja (PULR) and given the designation PULR 06,[5][6] though some studies alternatively call it UPLR 06[1] or UNLR 06.[7] Some of the foot bones from this specimen have gone missing.[1] Romer also mentioned PVL material collected by Jose Bonaparte.[2]
In 1972, Romer described MCZ 4121, which was a specimen smaller than the holotype. It was preserved in a nodule alongside the holotype of Lewisuchus admixtus and a few Lagosuchus bones. MCZ 4121 represents a few vertebrae, a pair of scapulocoracoids (mislabeled as belonging to Lagosuchus) and portions of the hip and hindlimbs, including two complete femurs. He also suggested that Lagerpeton was the probable identity of several incomplete tibiae and fibulae preserved along with several gomphodont skeletons in slab MCZ 3691.[4] However, later authors have doubted the referral of most MCZ material to Lagerpeton, with only the MCZ 4121 femurs being confidently referred to the genus.[1][7]
Andrea Arcucci described two PVL specimens, PVL 4619 and 4625, in 1986. PVL 4619, the PVL specimen mentioned by Romer, was a partial skeleton including a complete pelvis and left hindlimb, as well as a partial right hindlimb. PVL 4625 was another skeleton discovered later and originally described as including portions of the left hip, left hindlimb, and vertebral column.[5] Paul Sereno and Arcucci redescribed the known material in 1994 and mentioned that an isolated partial femur of this species was also present at the PVL,[1] although Martin Ezcurra (2016) noted that the provided catalogue number, PVL 5000, actually referred to a notoungulate mammal.[6] Further preparation of PVL 4625 has revealed the presence of a scapula, dentary, and cranial fragments.[3]
Lagerpeton is estimated to have been 70 cm (28 in) in length based on the length of the hindlimb;[8] the most complete hindlimb specimen, from PVL 4619, measures 257.9mm from proximal femur to distal ungual.[1] Body mass has been estimated as no more than 4 kilograms (8.8 lb), based on the slender cross section of limb bones and estimates between more derived dinosauromorphs, such as Silesaurus, and basal saurischians like Eoraptor.[9] Twenty one autapomorphic characters have been identified in L. chanarensis, these include: the anterior inclination of the posterior dorsal neural spines, the hook-shaped femoral head and the length of digit IV and metatarsal IV being greater than digit III and metatarsal III.[1] L. chanarensis lacks many dinosaurian characters, such as the anterior trochanter, placing it basal within Dinosauromorpha[8] or even outside the group altogether.[3]