OSTEOHISTOLOGY OF CRYOLOPHOSAURUS ELLIOTI: TEMPO AND MODE OF GROWTH IN A LARGE-BODIED POLAR DINOSAUR

The Early Jurassic theropod, Cryolophosaurus ellioti (Dinosauria: Theropoda), was discovered in 1991 in the Hanson Formation of Antarctica. To date, it is the most complete dinosaur skeleton found in Antarctica. Here we describe bone microstructure in some axial and appendicular bones of C. ellioti and discuss histological variance at the individual level. Thin sections were produced and digitized in the histology lab at The Field Museum of Natural History in Chicago. Examination of histological slides under a light microscope with polarizing lens reveals mild to strong presence of lines of arrested growth (LAGs) in three to five elements, indicating slowing growth. The femur, fibula, and radius show signs of moderate to severe remodeling, diminishing the number of annual markers, whereas axial elements preserve more of the growth record. The number of LAGs missing due to remodeling were estimated by fitting sigmoidal growth curves to LAG circumference in PAST®. Based on these results, the estimate age of the C. ellioti holotype is roughly 16 years old. Of those 16, 11-13 growth marks were directly observed in thin section while up to 5 were inferred missing by PAST® calculations. Patterns of diminishing growth match other skeletal indicators of subadult status (e.g., closed, but visible neurocentral sutures in the vertebrae), but contrast with rapid bone deposition observed in the femur. Other histological observations reveal great variability between elements (e.g., bone matrix classification, fiber organization, vascular canal types, cyclical bone marks, degree of remodeling, and behavior under polarized light). Prevalence of woven bone between widely spaced LAGs in the femur, indicating ongoing rapid growth, differs from the lamellar bone between narrowing LAGs found in the axial elements indicating slowing growth. Remodeling in the sections varied from minor in the rib to extreme in the fibula. These variances may have implications for proper histological study to understand growth in theropods. Regardless, these observations add to the evidence that C. ellioti represents the largest known Early Jurassic theropod. Combining our age estimates with estimated body mass and femoral circumference measurements allows comparisons to growth patterns of other theropods such as Allosaurus or Tyrannosaurus.