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Long bone histology of Middle Triassic dinosauriform Asilisaurus



Ben Creisler
bcreisler@gmail.com

A new paper:

C. T. Griffin & Sterling J. Nesbitt (2016)
The femoral ontogeny and long bone histology of the Middle Triassic
(?late Anisian) dinosauriform Asilisaurus kongwe and implications for
the growth of early dinosaurs.
Journal of Vertebrate Paleontology (advance online publication)
DOI: 10.1080/02724634.2016.1111224.
http: // www.tandfonline.com/doi/full/10.1080/02724634.2016.1111224

The ontogeny of early-diverging dinosauromorphs is poorly understood
because few ontogenetic series from the same species-level taxon are
known and what is available has not been extensively documented. The
large numbers of skeletal elements of the silesaurid Asilisaurus
kongwe recently recovered from Tanzania provide an opportunity to
examine the ontogenetic trajectory of the earliest known member of
Ornithodira and one of the closest relatives to Dinosauria. We
examined the ontogeny of the femur and the histology of a series of
long bone elements. We observed bone scar variation in a series of
femora (n = 27) of different lengths (73.8–177.2 mm). We hypothesize
that most femora follow a similar developmental trajectory; however,
we observed sequence polymorphism in the order of appearance and shape
of bone scars, and we quantified this polymorphism using ontogenetic
sequence analysis (OSA). Additionally, five femora, three tibiae, a
fibula, and a humerus were thin-sectioned to examine osteological
tissues. No lines of arrested growth (LAGs) are present in any
specimen, and there is little histological information about the
ontogenetic stage of femora, although none have slowed or ceased
growth. The woven-fibered bone present in the cortex of elements
sectioned is similar to that of the earliest dinosaurs. This sequence
polymorphism provides an alternate hypothesis for the robust/gracile
dichotomy found in early dinosaurs often interpreted as sexual
dimorphism. The shared femoral features found in Asilisaurus and early
dinosaurs suggest that this ontogenetic pattern is plesiomorphic for
Dinosauria, and that size is a poor predictor of maturity in early
dinosauriforms.