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Masiakasaurus (noasaurid theropod) bone histology and growth

From:  Ben Creisler

A paper in the new JVP:

Andrew H. Lee & Patrick M. O’Connor (2013)
Bone histology confirms determinate growth and small body size in the
noasaurid theropod Masiakasaurus knopfleri.
Journal of Vertebrate Paleontology 33(4):  865-876

Noasauridae is a clade of ceratosaurian theropods that evolved small
body size independently of other non-avian theropods. The
best-preserved and most complete noasaurid is Masiakasaurus knopfleri
from the Maastrichtian-aged Maevarano Formation in Madagascar. An
abundance of skeletal material from several individuals spanning a
wide range of ontogeny makes Masiakasaurus an ideal candidate for the
analysis of growth. We histologically sampled a growth series of
elements consisting of four femora and three tibiae. Bright-field and
circularly polarized light microscopy were used to distinguish between
slowly and rapidly growing forms of bone. To simultaneously estimate
age at death and reconstruct growth trajectories, we measured the
perimeters of growth lines in each specimen and fitted models to these
data using a novel application of mixed-effects regression. Our
histological results show an external fundamental system in the
largest tibial specimen and confirm that Masiakasaurus grew
determinately, matured at small body size, and is not the juvenile
form of a larger-bodied theropod. Parallel-fibered bone is unusually
prominent and suggests relatively slow growth. Moreover, our
quantitative analysis shows that the average individual took about
8–10 years to get to the size of a large dog. Although Masiakasaurus
grew 40% faster than crocodylians, it grew about 40% slower than
comparably sized non-avian theropods. Slowed growth may have evolved
as a means to minimize structural and maintenance costs while living
in a semiarid and seasonally stressful environment. Dimorphism does
not appear related to asymptotic size or growth rate but seems to
reflect the degree of skeletal maturity.