HOW TO GROW A DINOSAUR: THE HISTOLOGY AND FEMORAL ONTOGENY OF THE MIDDLE TRIASSIC (?LATE ANISIAN) DINOSAURIFORM ASILISAURUS KONGWE AND IMPLICATIONS FOR THE GROWTH OF EARLY DINOSAURS

The ontogeny of dinosaurs and their closest relatives is poorly understood due to the lack of ontogenetic series from the same species-level taxon. The large numbers of skeletal elements of the silesaurid Asilisaurus kongwe recently recovered from the Anisian of Tanzania provides an opportunity to closely examine the ontogenetic trajectory of the earliest known member of Ornithodira and one of the closest relatives to Dinosauria. We examined the histological tissues and the appearance of muscle scars over a series of different lengths of long bone elements. Five femora, as well as three tibiae, a fibula, and a humerus were thin sectioned to examine osteological tissues. No annual lines of arrested growth (LAG) are present in any of the specimens, and it is likely that A. kongwe did not lay down LAGs, although all specimens thin sectioned may be <1 year old. The woven bone present in the cortex is similar to that of the earliest dinosaurs in all elements sectioned. We also observed muscle scar appearance and shape change throughout an ontogentic series of femora (n = 26) of different lengths (73.8 to 177.2 mm). Femoral muscle scars develop at different ontogenetic stages, and we hypothesize that the majority of femora follow a similar developmental trajectory, e.g. the anterior trochanter and trochanteric shelf develop separately and roughly simultaneously, but fuse later in ontogeny in the most common developmental path. However, we did observe developmental polymorphism in the order of appearance and shape of muscle scars, e.g. there is high morphological variability in the fourth trochanter throughout most of the series, and although fusion of the trochanteric shelf and the anterior trochanter is only common in larger specimens, it is present in the second smallest specimen and conspicuously lacking in one of the largest specimens.

The ontogenetic pattern of Asilisaurus femora provides a baseline for understanding growth in early dinosaurs. This developmental trajectory provides an alternate explanation for the robust/gracile dichotomy found in early dinosaurs (e.g. Coelophysis, Syntarsus) that commonly has been interpreted as sexual dimorphism. The shared femoral scar features found in Asilisaurus and early dinosaurs suggest this ontogenetic pathway may be pleisiomorphic for Dinosauria.