Seven fossil specimens have so far been attributed to L. chanarensis, five of which were found in the Chañares Formation on the Northern branch of the Chañares River, La Rioja, Argentina:
- UPLR 06 (holotype) – articulated right hindlimb
- PVL 4619 – articulated pelvis with sacrum, partial right and complete left hindlimbs
- PVL 4625 – left pelvis with left femur and articulated vertebral column (dorsal, sacral and anterior caudal vertebrae
- PVL 5000 – proximal end of left femur
- MCZ 4121 – complete left, and partial right, femur.
The two remaining specimens were collected from the Chinle Formation, Colorado, USA and the Ischigualasto Formation; they consist of femora and a femur respectively.
Lagerpeton is estimated to have been 70 cm (28 in) in length based on the length of the hindlimb; the most complete hindlimb specimen, from PVL 4619, measures 257.9mm from proximal femur to distal ungual. Body mass has been estimated as no more than 4 kg based on the slender cross section of limb bones and estimates between more derived dinosauromorphs such as Silesaurus, and basal saurischians like Eoraptor. 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. L. chanarensis lacks many dinosaurian characters, such as the anterior trochanter, placing it basal within Dinosauromorpha.