The femoral ontogeny and long bone histology of the Middle Triassic (?late Anisian) dinosauriformAsilisaurus kongweand implications for the growth of early dinosaurs

Anomalously high variation in postnatal development is ancestral for dinosaurs but lost in birds

“…The large number of conflicting developmental pathways consisting of semaphoronts with suites of character states that preclude their belonging in other developmental pathways strongly suggests that variation in the relative order of these developmental characters is widespread throughout the populations of these early dinosaurs. Future histological analyses may help to determine whether this lack of correspondence between size and morphological maturity is an expression of how body size is related to ontogenetic age, and although these analyses are outside the scope of the current study, histological data were unhelpful in resolving this problem in silesaurids because of a lack of histological indicators of ontogenetic age in these taxa (21).…”

“…Therefore, developmental plasticity undoubtedly played at least some role in the ontogeny of early dinosaurs. However, the taxa that possess this variation occur across a wide temporal spread [from the Middle Triassic, A. kongwe (21), to the Late Cretaceous, M. knopfleri (27)], suggesting that this variation was not primarily influenced by external features typical of a certain period or stage (e.g., a more arid or humid environment). The large paleolatitudinal range [from tropical paleolatitudes (e.g., Asilisaurus, Coelophysis) to high paleolatitudes (e.g., Megapnosaurus, Plateosaurus)], and varied ecological niches of these taxa (ranging from small-to large-bodied herbivores and carnivores alike) also suggests that this high level of variation is not solely driven by a common environment or ecology shared by all taxa that possess this level of variation.…”

“…OSA has normally been used to understand variation and development in extant organisms (32,(56)(57)(58)(59)(60), with only a few previously published studies using this method in extinct taxa (21,61). We followed standard OSA procedure (32) to conduct this analysis.…”

“…Most studies of the ontogeny of more-derived theropods (15)(16)(17) suggest that the low levels of variation that characterize avian ontogeny were present in close nonavian relatives as well. Closer to the origin of Dinosauria, morphological variation within species is widespread (18)(19)(20)(21)(22)(23)(24)(25)(26), but whether this variation is the result of taxonomic diversity, ontogeny, sexual dimorphism, or simple individual variation is not clear (22,23,27). Furthermore, close dinosaurian relatives or "dinosaur precursors" (e.g., Silesaurus, Asilisaurus) possess high intraspecific variation in growth sequences [i.e., sequence polymorphism (28)], suggesting that this condition could be ancestral for Dinosauria (21).…”

Developmental patterns and variation among early theropods

“…Coelophysis bauri and M. rhodesiensis provide excellent study taxa to study morphological changes in ontogeny in early theropods and other dinosaurs because: (i) they have been reported to possess a high amount of variation in the presence of bone scars and co‐ossifications (= bone ‘fusions’; Raath, , ; Colbert, , ; Genin, ; Griffin & Nesbitt, ; Barta et al. ), similar to non‐dinosaurian dinosauriforms (Griffin & Nesbitt, ,b); (ii) they are both early‐diverging neotheropods, and therefore in a close phylogenetic position to the common dinosaurian ancestor, possessing many character states in common with this ancestor (Nesbitt et al. ; Nesbitt, ; Sues et al.…”