Intraskeletal histovariability and skeletochronology in an ornithopod dinosaur from the Maestrazgo Basin (Teruel, Spain).

Ornithopods are an extinct group of dinosaurs that were particularly abundant and diverse in the Cretaceous of the Iberian Peninsula, and whose abundance in the Maestrazgo Basin has allowed numerous taxa to be identified over the last decade. Many of these fossil remains are still taxonomically indeterminate and require a more detailed study on both a macroscopic and microscopic scale. In this contribution, an osteohistological analysis is carried out on a partial skeleton-composed of five incomplete vertebrae, two dorsal ribs, an ischium, a fibula, and a tibia-found in the province of Aliaga (Teruel, NE Spain). We identified a progressive slowdown in tissue apposition and a variation in the type of growth marks generated in every bone, allowing a more precise identification of the ontogenetic stage of the specimen as a subadult individual. The skeletochronological correlation between the different elements also suggests that the specimen reached sexual maturity at around seven years of age and died between nine and twelve years of age. Likewise, the usefulness of the three-front model is proven for the first time in an ornithopod dinosaur, as a tool for analysing the histology expressed by the different bone elements of a single specimen and inferring their skeletochronological potential. Comparison with other ornithopod taxa reveals the great variability that each bone element shows depending on the taxon analysed, which prevents us from determining a single element suitable for studying the skeletochronology of any ornithopod taxon.