Date Published: November 7, 2018
Publisher: Public Library of Science
Author(s): Jun Chen, Aaron R. H. LeBlanc, Liyong Jin, Timothy Huang, Robert R. Reisz, Cyril Charles.
The great diversity of dinosaurian tooth shapes and sizes, and in particular, the amazing dental complexity in derived ornithischians has attracted a lot of attention. However, the evolution of dental batteries in hadrosaurids and ceratopsids is difficult to understand without a broader comparative framework. Here we describe tooth histology and development in the “middle” Cretaceous ornithischian dinosaur Changchunsaurus parvus, a small herbivore that has been characterized as an early ornithopod, or even as a more basal ornithischian. We use this taxon to show how a “typical” ornithischian dentition develops, copes with wear, and undergoes tooth replacement. Although in most respects the histological properties of their teeth are similar to those of other dinosaurs, we show that, as in other more derived ornithischians, in C. parvus the pulp chamber is not invaded fully by the newly developing replacement tooth until eruption is nearly complete. This allowed C. parvus to maintain an uninterrupted shearing surface along a single tooth row, while undergoing continuous tooth replacement. Our histological sections also show that the replacement foramina on the lingual surfaces of the jaws are likely the entry points for an externally placed dental lamina, a feature found in many other ornithischian dinosaurs. Surprisingly, our histological analysis also revealed the presence of wavy enamel, the phylogenetically earliest occurrence of this type of tissue. This contradicts previous interpretations that this peculiar type of enamel arose in association with more complex hadrosauroid dentitions. In view of its early appearance, we suggest that wavy enamel may have evolved in association with a shearing-type dentition in a roughly symmetrically-enameled crown, although its precise function still remains somewhat of a mystery.
Comparative dental histology is an emerging area of study in dinosaur palaeontology [1–18] and the breadth and depth of this research is increasing. Several studies have focused on the hard tissues -enamel and dentine—and their phylogenetic and functional significance [4,19–22], but the field has steadily broadened to include more holistic descriptions and comparisons of teeth, their constituent tissues, and their interactions with the jaws. The result has been a broader picture of how teeth and jaws have evolved in association with dietary shifts and what developmental processes could have given rise to novel dentitions [2,5,7–9,13,14,17,23–26]. Some of the most dramatic and most studied evolutionary innovations are found in derived ornithischian dinosaurs.
We examined five partial dentaries from different individuals of Changchunsaurus parvus, from Jilin University Museum collections (JLUM; Table 1), and carried out the histological section experiments at the ROM vertebrate palaeohistology laboratory.
Given the previous focus on dental histology in derived, Late Cretaceous carnivores and herbivores in the literature , the present histological analysis of the teeth and jaws of Changchunsaurus parvus from the Cretaceous Quantou formation in Jilin Province, China, provides the first in-depth look at tooth histology and development in a dinosaur that may lie close to the base of the ornithopod radiation . This has important bearing on our understanding of the evolution of the more complex dental batteries seen in hadrosaurids from the more modest, single-rowed dentitions of most herbivorous dinosaurs. These histological sections have revealed several surprising features that may have evolved in association with herbivory in this early ornithischian, including the presence of wavy enamel and an unusual tooth replacement mode. In contrast, the presence of root cementum and a periodontal ligament connection of the tooth to the alveolus are probably symplesiomorphic features in ornithischians and in dinosaurs more generally. The presence of replacement foramina close to the apex of the jaw and a short contribution of interdental bone contrasts strongly with the condition in hadrosaurids. We use this comparison to hypothesize that hadrosaurids have evolved a reduced lingual wall of the jaws and, correspondingly, greater contributions of interdental bone via a shift in the position of the dental lamina towards the base of the jaws. Using these data as an example, we advocate that similar studies of early members of major dinosaurian clades are needed in order to incorporate histological characters into phylogenetic analyses and to understand how dinosaur dentitions have evolved over millions of years to cope with their diverse diets.