Temporal range: Ladinian, 237–228Ma
|Species:||† L. chanarensis|
The genus Lagerpeton, first described by A. S. Romer in 1971, sits basalmost within Dinosauromorpha and includes only the species L. chanarensis. This species is incompletely known with fossil specimens accounting for the pelvic girdle, hindlimbs and posterior presacral, sacral and anterior caudal vertebrae.
- 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.
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.
Palaeogeography and phylogeny
The oldest fossils of L. chanarensis were found in the Chañares Formation and originate from the Upper Middle Triassic (Ladinian) of Southern Pangaea. All Lagerpeton specimens share this geographic location, including other fossils from the Lower Late Triassic (Carnian). Recently Lower (early to late Olenekian) Triassic trackways from Poland (Northern Pangaea) have yielded footprints of a Lagerpeton-like quadrupedal dinosauromorph. This ichnogenus, named Prorotodactylus shares multiple synapomorphic characters with Lagerpeton including approximately parallel digits II, III and IV, fused metatarsus, digitigrade posture and reduced digits I and V. Prorotodactylus also shares the, previously autapomorphic, pes morphology of Lagerpeton. If this ichnogenus represents a close relative of Lagerpeton it would push back the origin of this taxon to the Early Triassic; as a quadrupedal basal dinosauromorph, it also raises questions debating the theory that bipedalism is ancestral to dinosaurs.
The genus Lagerpeton is currently accepted as the basalmost clade within Dinosauromorpha and the sister taxon to Dinosauriformes. Presently, Lagerpeton sits within the family “Lagerpetidae”, also occupied by the more derived genus Dromomeron, known from Late Triassic rocks of the southwestern USA.
It has been suggested that the extant analogues most similar to L. chanarensis are small bipedal mammals, which are often saltators. Three morphological characteristics in L. chanarensis fossils have been putatively cited as evidence of saltation in this taxon.
The neural spines of the posterior dorsal vertebrae are inclined anteriorly, a character not observed in any other archosaur, but common in saltatory mammals. This feature is suggested to allow for greater vertebral flexibility, correlated with leaping and bounding locomotor styles.
Relative to the hindlimb length, the pelvic girdle is remarkably small. The distance from the pelvic girdle to the femur is therefore also small, more so than most other archosaurs apart from closely related taxa. This reduction in distance may increase the force production during hip extension in extant small mammals.
The narrow and functionally didactyl pes are a further similarity to modern saltators. By condensing into a single unit, the metatarsus gains strength without the drawback of increased weight. It also appears likely that consequently to the reduction of digit II, digit IV may have elongated to balance the pes.
The hypothesis of saltatorial locomotion is debated, however. Vertebral adaptations of extant organisms exceed those seen in Lagerpeton; the sacral vertebrae of modern saltators are fused and the neural spines reduced. Furthermore, the size of the pelvic girdle and lateral digital reduction may be equally used as evidence for cursorial locomotion.
- Romer, A. S. (1971). "The Chanares (Argentina) Triassic reptile fauna. X. Two new but incompletely known long-limbed pseudosuchians.". Breviora 378: 1–10.
- Sereno, P. C. and Arcucci, A. B. (1994). "Dinosaurian precursors from the Middle Triassic of Argentina: Lagerpeton chanarensis.". Journal of Vertebrate Paleontology 13: 385–399. doi:10.1080/02724634.1994.10011522.
- Langer, M. C.; Nesbitt, S. J.; Bittencourt, J. S. and Irmis, R. B. (2013). "Non-dinosaurian Dinosauromorpha.". Geological Society, London, Special Publications 379.
- Nesbitt, S. J.; Irmis, R. B.; Parker, W. G.; Smith, N. D.; Turner, A. H. and Rowe, T. (2009). "Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America.". Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218.
- Fechner, R. (2009). "Morphofunctional evolution of the pelvic girdle and hindlimb of Dinosauromorpha on the lineage to Sauropoda.". lmu.
- Brusatte, S. L.; Niedźwiedzki, G. and Butler, R. J. (2011). "Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic.". Proceedings of the Royal Society B: Biological Sciences 278 (1708): 1107–1113. doi:10.1098/rspb.2010.1746.