LADINIAN–CARNIAN (MIDDLE–LATE TRIASSIC) EVOLUTION OF DINOSAURIFORMES: INDICATIONS FROM: History Edit
Subjects: Paleontology

The rise and early evolution of Dinosauromorpha is documented by skeletal remains, beginning during the Middle Triassic (Anisian). Small lacertoid footprints from Olenekian- Anisian deposits of North America and Europe (Rotodactylus, Prorotodactylus) have also been attributed to this group, but this relationship is still debated. Mesaxonic tridactyl footprints (Atreipus/Grallator), widely considered as those of the dinosaur stem (Dinosauriformes), are documented from the Anisian-Ladinian, together with tetradactylpentadactyl, functionally tridactyl footprints (Parachirotherium). They were described with trackways demonstrating facultative bipedality of their producers from the late Ladinian of the Germanic Basin. In recent years, similar ichnofaunas with abundant Atreipus/Grallator and Parachirotherium footprints were discovered in Carnian deposits of the Argana Basin, Morocco. Other records are from the early late Carnian of eastern North America and Argentina. In particular, their dominance on surfaces of the Central European and North African localities suggests a larger diversification and dispersal of Dinosauriformes near the Ladinian–Carnian boundary, about 3–5 million years before the so-called Carnian pluvial event and climate change in the mid-Carnian, which has recently been proposed as a crucial factor favoring the diversification of dinosaurs. This was partly based on data from the track record of Northern Italy. However, Triassic tridactyl footprints mostly lack diagnostic dinosaurian features and therefore scarcely permit a distinction between non-dinosaurian dinosauriforms and "true" dinosaurs. Although skeletal remains provide no evidence of Dinosauria before the late Carnian (~ 231 Ma), the footprint recordsuggests an earlier appearance, and assemblages with abundant footprints that can be attributed to Dinosauriformes/Dinosauria are observed beginning from the late Ladinian (238 Ma). Therefore, a more continuous evolution and diversification of dinosaurs and some basal sister groups (e.g., silesaurids) from the Middle Triassic, rather than the proposed "explosion" in the mid-Carnian, is the likely scenario. This may have been unrecognized thus far due to a collecting bias in late Ladinian–early Carnian deposits outside of the Germanic Basin and Morocco.