Novas and Agnolin (2015) have an abstract on Lagerpeton that proposes it is actually a proterochampsian sister to Tropidosuchus. If true, this would be a rare time David Peters got something right, as he had this idea back in 2011- https://pterosaurheresies.wordpress.com/2011/10/15/what-is-lagerpeton-the-heretical-view/ . I hope Novas and Agnolin credit him if they write a paper on this.
However, something not addressed by Novas and Agnolin is how Dromomeron relates. According to Nesbitt (2011), Tropidosuchus lacks all lagerpetonid characters shared by Lagerpeton and Dromomeron- Anterolateral tuber of the proximal portion of the femur absent, the anterolateral face is flat (302-1), Femoral head hook shaped in medial and lateral views (306-1), Ventral emargination present on anterolateral side of the femoral head (310-1), Crista tibiofibularis larger than the medial condyle (326-1), Dorsally expanded process on the posterolateral portion of the tibial facet of the astragalus expanded into a distinct, raised process (5 posterior ascending process of Sereno and Arcucci, 1994a) (355-1), Concave articular surface for the fibula on the calcaneum (378-2). Of the proterochampsian characters noted by Novas and Agnolin as present in Lagerpeton, Dromomeron lacks two- femoral 4th trochanter proximodistally expanded; caudal surface of distal tibia with a middle tubercle surrounded by two shallow concavities; but has one- astragalus with anteromedial corner acute. Others are unknown since Dromomeron is so fragmentary, but at least "elongate and compact metatarsus with metatarsal V reduced and devoid of phalanges" would be expected in a taxon close to Marasuchus (which has the conditions), so that works for either placement of Lagerpeton. Then there are the dinosauriform-like characters of Lagerpeton scored by Nesbitt as absent in Tropidosuchus- Straight cnemial crest (328-1); Longest metatarsal longer than 50% of tibial length (383-1); Metatarsal V "hooked" proximal end absent, and articular face for distal tarsal 4 subparallel to shaft axis (398- 1). And the archosaurian characters of Lagerpeton scored as absent in Tropidosuchus- Articular surfaces for fibula and distal tarsal IV on the calcaneum continuous (380-1), Anteromedial tuber of the proximal portion of the femur present (300-1), Tibial facet of the astragalus divided into posteromedial and anterolateral basins (366-1).
|Phylogeny of eucrocopodans recovered by Ezcurra (2016), with Novas and Agnolin's (2015) proposed reidentification of Lagerpeton indicated in red (modified from Ezcurra, 2016).|
Taking all of this into account (with the caveats that I haven't checked the accuracy of any characters except the proterochampsian states of Dromomeron, and that this only uses unambiguous characters listed by Nesbitt), Novas and Agnolin have ten characters that might need to converge if Lagerpeton is a dinosauriform relative, but there are nine characters that might need to converge if Lagerpeton is a proterochampsian (the seven from Nesbitt listed here plus two listed by Novas and Agnolin). So that's one step in favor of a proterochampsian identity. But if Dromomeron is a proterochampsian too, that adds three more steps (the two listed above plus dinosauromorph character 313 of Nesbitt that is present in Dromomeron but unknown in Lagerpeton). Thus dinosauriform-relative Lagerpeton is two steps shorter. And if Dromomeron is a dinosauriform relative but Lagerpeton is not, that adds five steps (the six lagerpetonid characters that would need to be convergent minus the proterochampsian-like acute astragalar corner).
So basically, ignoring Dromomeron might make proterochampsian Lagerpeton a slightly better option but a proterochampsian Dromomeron turns those odds around, and a proterochampsian Lagerpeton but a dinosauriform-relative Dromomeron is even worse. So pending further analysis, I favor the traditional view (note Ezcurra's huge 2016 archosauromorph analysis which sampled proterochampsian diversity also recovered this topology). If Lagerpeton is a proterochampsian, then Lagerpetonidae is a junior synonym of Proterochampsia definition-wise (and Proterochampsidae ICZN-wise). However, Novas and Agnolin are wrong in claiming "The exclusion of Lagerpeton from the dinosaur lineage results in the removal of the clade Dinosauromorpha, which was originally conceived to encompass Lagerpeton plus Dinosauriformes." The only proposed definition of Dinosauromorpha that mentions Lagerpeton is one of Sereno's (1991) which is "Lagerpeton chanarensis, Lagosuchus talampayensis, Pseudolagosuchus major, Dinosauria (inc. Aves), and all descendants of their common ancestor." Following this definition, Dinosauromorpha merely moves stem-ward to be the Proterochampsia+Archosauria clade which is AFAIK otherwise unnamed. The other proposed definitions are all basically "taxa closer to dinosaurs than pterosaurs", so Dinosauromorpha would stay where it is even if Lagerpeton isn't a member.
References- Sereno, 1991. Basal archosaurs: Phylogenetic relationships and functional implications. Society of Vertebrate Paleontology Memoir. 2, 1 53 pp
Nesbitt, 2011. The early evolution of archosaurs: Relationships and the origin of major clades. Bulletin of the American Museum of Natural History. 352, 292 pp.
Novas and Agnolin, 2015. Lagerpeton chanarensis Romer (Archosauriformes): A derived proterochampsian from the Middle Triassic of NW Argentina. Libro de resúmenes del V Congreso Latinoamericano de Paleontología de Vertebrados. 48.
Ezcurra, 2016. The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms. PeerJ. 4:e1778.