Variation in dinosaur skeletochronology indicators: implications for age assessment and physiology

Twelve different bones from the skeleton of the holotype specimen of the ha drosaurian dinosaur Hypacrosaurus stebingeri were thin-sectioned to evaluat e the significance of lines of arrested growth (LAGs)in age assessments. Th e presence of an external fundamental system (EFS) at the external surface of the cortex and mature epiphyses indicate that the Hypacrosaurus specimen had reached adulthood and growth had slowed considerably from earlier stag es. The number of LAGs varied from none in the pedal phalanx to as many as eight in the tibia and femur. Most elements had experienced considerable Ha versian reconstruction that had most likely obliterated many LAGs. The tibi a was found to have experienced the least amount of reconstruction, but was still not optimal for skeletochronology because the LAGs were difficult to count near the periosteal surface. Additionally, the numbers of LAGs withi n the EFS vary considerably around the circumference of a single element an d among elements. Counting LAGs from a single bone to assess skeletochronol ogy appears to be unreliable, particularly when a fundamental system exists . Because LAGs are plesiomorphic for tetrapods, and because they are present in over a dozen orders of mammals, they have no particular physiological me aning that can be generalized to particular amniote groups without independ ent physiological evidence. Descriptions of dinosaur physiology as "interme diate" between the physiology of living reptiles and that of living birds a nd mammals may or may not be valid, but cannot be based reliably on the pre sence of LAGs.