A new large 'silesaur' specimen from the ?Late Triassic of Zambia; taxonomic, ecological and evolutionary...

'Silesaurs' are bird-line archosaurs that either represent the sister clade to dinosaurs or a paraphyletic grade of early ornithischians. Here, we describe a partial silesaur femur, NHMUK PV R37051, from the Ladinian/Carnian Ntawere Formation of Zambia. This femur is notable for being one of the largest silesaur specimens yet known, both globally and from the Ntawere Formation. The description of this new specimen means that two out of the three largest silesaur individuals are now known from the latter stratigraphic unit. A detailed morphological and osteohistological description of the specimen is presented alongside comparisons with other silesaur femora. Although NHMUK PV R37051 cannot be confidently referred to the only named Ntawere silesaur, Lutungutali sitwensis, it is also not morphologically distinct enough to justify erecting a new taxon. Furthermore, osteohistological data rule out a simple ontogenetic explanation for these large Ntawere silesaurs. This taxonomic uncertainty suggests that a more conservative approach is needed when scoring L. sitwensis into phylogenetic datasets. The existence of these large silesaurs challenges our understanding of dinosaur size evolution and its implications for the very early stages of their rise to ecological dominance.