Bone microstructure of the Upper Cretaceous theropod dinosaurTroodon formosus

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Abstract
The bone microstructure of three third metatarsals and two tibiae of differing sizes from Troodon formosus was examined. Bone in T. formosus passed through three ontogenetic stages: rapid fibro-lamellar, moderate lamellar-zonal, and slow avascular lamellar growth. Highly-vascularized fibro-lamellar bone accounted for the majority of growth, with adult size possibly being reached in 3 to 5 years. No significant growth occurred after T. formosus reached a body weight of roughly 50 kg (femur: tibia length 310:425 mm). Abundant vasculature together with dense Haversian bone suggests T. formosus had a relatively high metabolism. Other gross morphological features (large brain, large eyes, cursorial adaptations including high tibia: femur ratio) would indicate T. formosus was very active and possibly endothermic.