Lagerpeton chanarensis (Romer 1971) Middle Triassic, ~ 240 mya, ~0.7 meters long (Fig. 1) was originally and traditionally considered a dinosauromorph, like Marasuchus. For several decades it has played a pivotal role in traditional dinosaur evolution studies, nesting at the base of the Dinosauria along with pterosaurs, which were considered and dismissed here earlier.
Only the hindlimb and a portion of the lumbar and caudal regions of the vertebral series in Lagerpeton have been found. That makes it something of an enigma wondering what the rest of it looked like. The pedal proportions don’t match those of pterosaurs or Marasuchus. Neither does the pelvis. The rise in the astragalocalcaneum occurs behind the tibia/fibula, not in front, as in Marasuchus and dinosaurs.
Recently a femur described by Irmis et al. (2007) and Nesbitt et al. (2009) as a sister to Lagerpeton was named Dromomeron. It’s a pretty good match. However, in those studies, pterosaurs and both of these lagerpetids were found to be derived from parasuchians, like Parasuchus, which are almost polar opposites in terms of morphology. Such mismatches should raise a red flag that a problem is present in the matrix of data.
Boy It Would Be Great If We Only Had a Skull!
So what is Lagerpeton, if not a sister to dinosaurs, phytosaurs and pterosaurs? Given all the reptiles now know, what would be its closest match? Ideally it would be great to find a specimen with more of the skeleton preserved. Well that has already happened with little to no fanfare.
A Specimen Attributed to Tropidosuchus
In his book, Dinosaurios de America del Sur, Bonaparte (1994) included a photograph of a nearly complete and articulated specimen attributed to Tropidosuchus. That specimen has not been described yet. Interestingly the pes (Fig. 2) shares more traits with Lagerpeton, which also has an elongated metatarsal 4.
Chanaresuchus (Figure 4). Cerritosaurus was a sister to their common ancestor. The BPI 2871 specimen attributed to Youngina by Gow (1974) also nested here. It is currently under study by an unknown worker, so we should be hearing some news about this specimen soon. It was not illustrated with an antorbital fenestra (AOF) and fossa, but I suspect it had one considering the small size of the AOF in its sisters.
In the chanaresuchid clade the pedal digits did not share many traits with dinosaurs. The pelvis was distinct as well. The reason why parasuchians and phytosaurs nested with pterosaurs and dinosaurs in prior studies seems to be largely due to the inclusion of Lagerpeton and the exclusion of Cerritosaurus, Doswellia and the Choristodera. In a larger study including all these taxa and many more (see below), these problems become fully resolved and all nested sisters more closely resemble each other in whole and in detail.
The large reptile tree not only split the reptiles into two distinct clades. It also split the archosauriformes. On one branch are the Euarchosauriformes from Proterosuchus to birds and crocs. On the other branch are the Pararchosauriformes from the RC 91 specimen of Youngoides to Lagerpeton. This clade also includes choristoderes, Proterochampsa, phytosaurs and chanaresuchids, taxa largely united by a long snout with dorsal nares. An antorbital fenestra appeared in derived members of this clade by convergence with the Euarchosauriformes.
Lagerpeton nests with Tropidosuchus, Chanaresuchus and Cerritosaurus in order of increasing distance. Lagerpeton does share a common ancestor with dinosaurs, but it goes all the way back to basal Youngina. Lagerpeton does share a common ancestor with pterosaurs, but it goes all the way back to Cephalerpeton, the most primitive known reptile.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Bonaparte JF 1994. Dinosaurios de America del Sur. Impreso en Artes Gráficas Sagitario. Buenes Aires. 174pp. ISBN: 9504368581
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH 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.
Romer AS 1971 The Chanares (Argentina) Triassic reptile fauna X. Two new but incompletely known long-limbed pseudosuchians: Brevoria, n. 378, p. 1-10.
Sereno PC and Arcucci AB 1993. Dinosaurian precursors from the Middle Triassic of Argentina: Lagerpeton chanarensis. Journal of Vertebrate Paleontology, 13, 385–399.