Age Determination in Dinosaurs - Growth Lines

Growth Lines

Two types of growth lines exist: annuli, and lines of arrested growth (LAGs). Histological examinations have revealed that annuli are composed of thin layers of avascular bone with parallel-aligned bone fibers. The growth line annuli are found compressed between broad vascularized regions of bone with randomly oriented fibrillar patterns, known as zones.

Lines of arrested growth, similar to annuli, are found between zones are avascular. They are, however, much thinner, and have relatively fewer bone fibers by volume.

Studies on extant vertebrates indicate that the vascularized zones form during moderate to rapid skeletogenesis, and that abrupt metabolic disruptions of bone formation can trigger growth line deposition.

Both types of growth lines may be deposited in synchrony with endogenous biorhythms. For example, captive crocodilians exposed to constant temperature, diet, and photoperiod, still exhibit the periodic and cyclical skeletal growth banding of their wild counterparts. Consequently, it is assumed that many paleontologists that the growth lines of dinosaurs reflect annual rhythms, and that they may be used to determine individual ages. However, in the large and long bones of many dinosaurian taxa, resorption of internal and external bone proceeds even as new cortical bone continues to be deposited, so that growth lines deposited early in development may need to be inferred.