title: Osteohistological variation in growth marks and osteocyte lacunar density in a theropod dinosaur (Coelurosauria: Ornithomimidae).
abstract: 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 mm(3), 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.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, ON, M5S 3B2, Canada. thomas.cullen@mail.utoronto.ca.
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Cullen
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, ON, M5S 3B2, Canada. d.evans@utoronto.ca.|Department of Natural History, Royal Ontario Museum, 100 Queen's Park, Toronto, ON, M5S 2C6, Canada. d.evans@utoronto.ca.
authors.forename: David C
authors.initials: DC
authors.lastname: Evans
authors.affiliation: Department of Vertebrate Paleontology, Cleveland Museum of Natural History, Cleveland, OH, 44106-1767, U.S.A. mryan@cmnh.org.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Ryan
authors.affiliation: Department of Biological Sciences, University of Alberta, Edmonton, AB, T6G 2E9, Canada. pjcurrie@ualberta.ca.
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Currie
authors.affiliation: Hokkaido University Museum, Hokkaido University, Sapporo, Hokkaido, 060-0810, Japan. ykobayashi@museum.hokudai.ac.jp.
authors.forename: Yoshitsugu
authors.initials: Y
authors.lastname: Kobayashi
mesh_terms: D000416:Alberta; D000818:Animals; D001842:Bone and Bones / Q000033:anatomy & histology* / Q000166:cytology; D025061:Dinosaurs / Q000033:anatomy & histology*; D005260:Female; D005269:Femur / Q000033:anatomy & histology; D005580:Fossils