Abstract
BackgroundOsteohistological 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.ResultsLAG 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.ConclusionsThe 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-014-0231-y) contains supplementary material, which is available to authorized users.
Highlights
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
Few studies to date have focused on variation in LAG spacing between different limb bones of a single individual; individual variation in osteocyte lacunar morphology [30,31] and density [27,28] in the long bones of tetrapods suggests that element choice and sampling location may influence studies using these histological parameters to infer life history traits in fossil taxa
We describe bone microstructure and assess intra- and inter-skeletal variation in skeletochronological indicators (LAGs) and osteocyte lacunar density (OLD) in a sample from the Horseshoe Canyon Formation (Maastrichtian, Alberta) of ornithomimid theropod skeletons, three of which were found closely associated in the same stratigraphic horizon and likely represent individuals dervied from the same population [32]
Summary
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. We use the intra-specific, multi-element osteohistological data to test two separate hypotheses: 1) that patterns of lines of arrested growth (LAG) spacing in different elements of a single individual provides a consistent signal for evaluating its relative maturity, and, 2) osteocyte lacunar densities (OLDs) are generally consistent between limb elements of a single individual, and between individuals of a given taxon due to their shared physiology and loading regimes Growth marks, such as LAGs, are well documented in theropod dinosaurs [5,7,18,25,33,34,35,36], and this detailed case study of individual variation in a single theropod taxon may provide insights that can be extended to other theropod dinosaurs, as well as tetrapods more generally
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