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Ornithomimid bone study shows individual variations in growth marks (free pdf)



Ben Creisler
bcreisler@gmail.com

A recent paper in open access, not yet mentioned:



Thomas M Cullen, David C Evans, Michael J Ryan, Philip J Currie and
Yoshitsugu Kobayashi (2014)
Osteohistological variation in growth marks and osteocyte lacunar
density in a theropod dinosaur (Coelurosauria: Ornithomimidae).
BMC Evolutionary Biology 2014, 14:231
doi:10.1186/s12862-014-0231-y
http://www.biomedcentral.com/1471-2148/14/231/abstract



Abstract (provisional)


Background.
Osteohistological examinations of fossil vertebrates have utilized a
number of proxies, such as counts and spacing of lines of arrested
growth (LAGs) and osteocyte lacunar densities (OLD), in order to make
inferences related to skeletochronology and mass-specific growth
rates. However, many of these studies rely on samplings of isolated
bones from single individuals. These analyses do not take individual
variation into account, and as a result may lead to misleading
inferences of the physiology of extinct organisms. This study uses a
multi-element, multi-individual sampling of ornithomimid dinosaurs to
test the amount of individual variation in the aforementioned
osteohistological indicators. Based on these results we also assess
the conclusions of previous studies that tested paleohistological
hypotheses using isolated elements.


Results.
LAG number was found to be consistent within the hind limb bones of
each individual, with the exception of the fibula, which preserves one
additional LAG. Considerable differences in LAG spacing were found
between elements of the sampled individuals, with larger variation
found in elements of the foot compared with the femur, fibula, and
tibia. Osteocyte lacunar density ranged between 29000 and 42000
osteocyte lacunae per mm3, and was found to vary more between hind
limb bones of an individual and within bones, than between the average
values of individuals.


Conclusions.
The variation between hind limb elements in LAG number and LAG spacing
suggests that direct comparisons of these elements may be misleading,
and that LAG spacing is not a reliable proxy for mass-specific growth
rates of an individual. Sampling of multiple bones should be performed
as an internal check of model-based LAG retro-calculation and growth
equations. The observation that osteocyte lacunar density varies more
between individual bone elements than between average individual
values suggests that the choice of sampled element can greatly
influence the result, and care should be taken to not bias
interpretations of the physiology of fossil tetrapods.