EXAMINATION OF ENAMEL GROWTH RATES IN THE HADROSAURIAN DINOSAUR EDMONTOSAURUS USING OXYGEN ISOTOPE VARIABILITY

Stable isotope analysis can be a powerful tool for determining physiological and developmental information about extinct organisms. Oxygen isotopes are derived from ingested food and water and are incorporated into biological tissues (including tooth enamel and bone) during growth; the process is temperature dependent and causes d¹⁸O to vary seasonally, creating a temporal record of mineralization. Using this relationship, we investigated enamel growth rates and duration of mineralization in a conspecific growth series of the Late Cretaceous (Hell Creek Formation) hadrosaurian dinosaur Edmontosaurus.

Oxygen isotopes were obtained by sampling numerous consecutively-erupted teeth from a growth row from one juvenile, two sub-adults, and an adult Edmontosaurus. Individual teeth were sectioned and enamel drilled at very fine-scale increments from the base of the enamel (younger) to the outer surface (older). Combining multiple isotope values from each tooth with multiple teeth from each individual in the growth series gave a composite ontogenetic record of isotope incorporation for Edmontosaurus.

Tests for diagenesis (to show isotopic integrity) included: 1) microprobe analyses of sectioned teeth (prior to sampling) to locate those areas least likely to have been altered; 2) comparison of oxygen from the carbonate component of enamel to samples from the phosphate component; and 3) comparison of oxygen from enamel carbonates to oxygen from carbonate cements in sediment samples collected with the fossils. Isotope values from fossils were also compared to those of extant bird bone and alligator enamel as a means of phylogenetically bracketing values for the Edmontosaurus samples.

Preliminary results suggest that the enamel does (in this instance) retain an original isotopic signature - suggesting that even very old biological materials can be reliable recorders of isotopic data - and that hadrosaurs displayed variable growth rates through ontogeny.