The dentary of hadrosauroid dinosaurs: evolution through heterochrony

The near-global success reached by hadrosaurid dinosaurs during the Cretaceous has been attributed to mastication, a behaviour commonly recognized as a mammalian adaptation. Its occurrence in a non-mammalian lineage should be accompanied by the evolution of several morphological modifications associated with food acquisition and processing. This study investigated morphological variation in the dentary, a major element of the hadrosauroid lower jaw. Eighty-four hadrosauroid dentaries were subjected to geometric morphometric and statistical analyses to investigate their taxonomic, ontogenetic, and individual variation. Results suggest increased food acquisition and processing efficiency in saurolophids through a complex pattern of evolutionary and growth-related changes. The edentulous region grew longer relative to dentary length, allowing for food acquisition specialization anteriorly and processing posteriorly, and became ventrally directed, possibly associated with foraging low-growing vegetation, especially in younger individuals. The saurolophid coronoid process became anteriorly directed and relatively more elongate, with an expanded apex, increasing moment arm length, with muscles pulling the jaw more posteriorly, increasing mechanical advantage. During growth, all hadrosauroids underwent anteroposterior dental battery elongation by the addition of teeth, and edentulous region ventralization decreased. The dental battery became deeper in saurolophids by increasing the number of teeth per tooth family. The increased coronoid process anterior inclination and relative edentulous region elongation in saurolophids are hypothesized to have evolved through hypermorphosis and/or acceleration, peramorphic heterochronic processes, and the anteroposteriorly shorter but dorsoventrally taller saurolophid dentary, likely emerged due to post-displacement in dental battery elongation and edentulous region decreased ventral orientation, a paedomorphic heterochronic process.

Methods
The dataset was collected from the literature (see the publication's online Supporting Information for a dataset reference list) and digital libraries of Dr. Albert Prieto-Márquez. Information pertaining to hadrosauroid dinosaur dentary specimens was written into the dataset using Excel. Images of specimens were made into a .TPS file in tpsUtil, and then digitized with equidistant semilandmark curves in tpsDIG2. The sliders file was created in tpsUtil. The .TPS file and sliders file were then transferred into the program R, and used together with the dataset (see the file "SOEDERBLOM_et_al_Appendix_S1.txt" in Zenodo or the publication's online Supporting Information), to analyze the digitized data, using the R script.