COMPARISON OF BONE TISSUES AROUND HEALING FRACTURES IN HADROSAUR AND ALLIGATOR

CT (computerized tomographic) imaging, histology, and geochemistry were used to compare bone tissues in partially healed shaft fractures from a late Cretaceous hadrosaur and a modern alligator. In both adults, a traumatic fracture resulted in separation of the shaft ends, and subsequent callus formation reunited and immobilized the fragments with some reorientation. In the dinosaur, an extensive callus sheaths the fracture in low-density non-laminar bone with features suggesting rapid repair; in the alligator, a separate dense callus grew around each end of the fracture, probably as a result of continuous aggravation of the injury, until the calluses interlocked. Damaged cortical bone in the dinosaur was partially resorbed, and remodeling began before the fracture was immobile; in the alligator, edges of original dense bone protrude from the callus, and the boundary between cortex and new mineral is sharp throughout the repair. Cancelli inside the dinosaur bone were nearly filled with lamellar bone and/or callus tissue, whereas trabecular bone of the alligator appears unaffected by the injury. Stable oxygen isotopes of callus mineral phosphate indicate elevation of temperature around the fracture in the dinosaur, probably reflecting aggressive cellular activity, but these effects are not evident in the alligator. These healed fractures suggest that although bone repair in dinosaurs followed a similar general pathway as modern alligators, it produced very different structures at significantly faster rates.