Uninterrupted growth in a non-polar hadrosaur explains the gigantism among duck-billed dinosaurs

Duck-billed dinosaurs (Hadrosauridae) were the most common ornithopods of the Late Cretaceous. Second only to sauropods and in many cases exceeding the sizes of the largest land mammals (such as indricotheres or proboscideans), they represent the largest terrestrial herbivores that walked the Earth. Despite their gigantic sizes, diversity, and abundance, their growth strategies remain poorly understood. Herein, we examine the bone microstructure of several Mongolian hadrosauroids of varied adult sizes. The small and middle-sized species have lines of arrested growth (LAGs). On the other hand, one of the largest duck-billed dinosaurs, Saurolophus angustirostris, shows an uninterrupted growth, comparable with other big hadrosaurs for which the lack of cyclical growth arrests was interpreted as a result of living in the polar region. Since both of the studied taxa inhabited warmer, continental, monsoon-influenced environments of the Late Cretaceous Mongolia, we propose that the absence of LAGs is not a climatic-driven condition but rather connected with the animal’s size (i.e., ontogeny). Our results show that hadrosaurs, similar to sauropods changed their growth dynamics from cyclical to continuous during their evolution, which made it possible for them to achieve comparable body sizes.

Methods
The 3D images of the specimens were prepared using the Shining 3D EinScan Pro 2X 3D scanner fixed on a tripod with EinScan Pro 2X Color Pack (texture scans), EinTurntable (alignment based on features), and EXScan Pro 3.2.0.2 software. The number of turntable steps was varied, chosen depending on the specimen. The models were meshed using the Watertight Model and High Detail presets.