Many dinosaurs, particularly ornithischians (bird-hipped dinosaurs) have elaborate bony projections like the horns and frills of ceratopsians, the crests of hadrosaurs, and the plates and spines of stegosaurs and ankylosaurs. The evolution and function of these features has been a subject of significant scientific debate. Current research proposes that these structures evolved in the context of socio-sexual selection to communicate with members of the same species, but our understanding of the structure of these features has been limited. A new scientific article by Dr. Caleb Brown published today in the journal PeerJ analyzes the armour of the nodosaur Borealopelta markmitchelli. Borealopelta is unique in that it preserves both the body core of the armour (the osteoderm) and the keratinous coverings.
This research marks the first analysis of the growth patterns of ornithischian soft tissues and allows for analysis of regional sheath changes in the armour and scales covering the animal. The majority of scientific research on the evolution and function of dinosaur bony projections has been only on the bony core, due to the lack of preservation of the keratinous sheath during fossilization. Borealopelta is the first specimen with keratinous sheaths preserved in good enough condition and high enough quantities for analysis. Dr. Brown measured 172 osteoderms to analyze the size, shape, and relative proportion of sheath changes across the body. This research shows that the height of the osteoderms increases at a higher rate than the base.
As the osteoderms grow in size, their shape changes to become taller. This pattern is consistent across the body with the exception of the large spine above the shoulder. Due to the unique preservation of soft tissue, Dr. Brown was also able to analyze the relation between the horn core and the keratinous sheath. In the smaller, flatter osteoderms on the back of the body, the sheath contributes a very small percentage to the overall size of the armour. This growth pattern does not remain constant across the body and the spines on the neck and shoulder region have a much higher rate of growth between the keratinous sheath and the bony core.
Although this is the first specimen with enough soft tissue preserved to facilitate this analysis, we are able to compare Borealopelta to living animals with horns. Dr. Brown’s research compared the osteoderms to the horns of a number of living lizards and mammals. The smaller osteoderms do not follow a similar growth rate to the horns of living animals, but the large shoulder spine does, converging on a horn-like morphology. By comparing Borealopelta to living animals that use their horns as display structures and jousting, Dr. Brown inferred that although the role of the small osteoderms may have been mainly for defence from predators, the role of the larger osteoderms, like the large shoulder spine, may have evolved as a socio-sexual structure to communicate and interact with members of the same species.