Lagerpetidae
Lagerpetidae (/ˌlædʒərˈpɛtɪdiː/; originally Lagerpetonidae) is a family of basal avemetatarsalians. Though traditionally considered the earliest-diverging dinosauromorphs (reptiles closer to dinosaurs than to pterosaurs), fossils described in 2020 suggest that lagerpetids may instead be pterosauromorphs (closer to pterosaurs).[1][2] Lagerpetid fossils are known from the Late Triassic of Argentina, Arizona, Brazil, Madagascar, New Mexico, and Texas.[3][4][5][6][1] They were typically small, although some lagerpetids, like Dromomeron gigas and a specimen from the Santa Rosa Formation attributed to Dromomeron sp., were able to get quite large (femoral length 150–220 mm (5.9–8.7 in)).[7][8] Lagerpetid fossils are rare; the most common finds are bones of the hindlimbs, which possessed a number of unique features.[9]
As with most early avemetatarsalian, the most characteristic adaptations of lagerpetids occurred in their hip, leg and ankle bones, likely as a result of these being the bones most commonly preserved. Hip material is only known in Lagerpeton and Ixalerpeton, which share three adaptations of the ilium (upper blade of the hip). The supraacetabular crest, a ridge of bone which lies above the acetabulum (hip socket), is thickest above the middle portion of the acetabulum, rather than the front of it. However, it also extends further forwards than in most dinosauromorphs, snaking along the length of the pubic peduncle (the area of the ilium which connects to the pubis). The ilium's facet for the pubis opens downwards, a trait also acquired by ornithischian dinosaurs.[5] The hip in general was wide, had a closed acetabulum (i.e. one with a bony inner wall), and had two sacral vertebrae, lacking many specializations of later dinosauromorphs, like dinosaurs.[3]
Like other early archosaurs (and archosaur relatives such as Euparkeria), the femur (thigh bone) was slender and S-shaped. The femoral headwas thin when seen from above, and its apex projected about 45 degrees between medially (inwards) and anteriorly (forwards). Most archosaurs had three tubera (bumps) on their flattened femoral head, one at the middle of the anterolateral (forwards/outwards) surface, another at the middle of the posteromedial (backwards/inwards) surface, and a small third one which was near the apex of the femoral head. However, lagerpetids lack the anterolateral tuber, instead having an emargination in the head just below where the tuber would normally be expected. The femoral head itself was notably hook-shaped when seen from the side. The distal portion of the femur (i.e. the portion near the knee) had a pair of condyles (knobs) on either side of the rear surface, as well as a third knob-like structure known as a crista tibiofibularis,which was present just above thelateral condyle. The crista tibiofibularis was uniquely enlarged in lagerpetids, and undergoes further evolution in Ixalerpeton and particularly Dromomeron.[9]
