Histological evidence for a dynamic dental battery in hadrosaurid dinosaurs

Abstract

The first histological study of an entire hadrosaurid dental battery provides a comprehensive look at tooth movement within this complex structure. Previous studies have focused on isolated teeth, or in-situ batteries, but this is the first study to examine an entire dental battery of any dinosaur. The absence of direct tooth-to-tooth contact across the entire battery and a unique arrangement of the dental tissues in hadrosaurids led us to compare their teeth with the ever-growing incisors of mammals. The similarity in the distributions of tissues along the incisor, coupled with continuous eruption, make for helpful comparisons to hadrosaurid teeth. The mammalian ever-growing incisor can be used as a model to extrapolate the soft tissue connections and eruptive mechanisms within the hadrosaurid dental battery. Serial sections across the adult dental battery reveal signs of gradual ontogenetic tooth migration. Extensive remodeling of the alveolar septa and the anteroposterior displacement of successive generations of teeth highlight the gradual migration of tooth generations within the battery. These eruptive and ontogenetic tooth movements would not be possible without a ligamentous connection between successive teeth and the jaws, underscoring the dynamic nature of one of the most unique and complex dental systems in vertebrate history.

Tooth development, histology, and enamel microstructure in Changchunsaurus parvus: Implications for dental evolution in ornithopod dinosaurs

“…The enamel in Chengchunsaurus teeth is between 30 and 55 μm thick in erupting, unworn replacement teeth (and thus the closest approximation for maximum enamel thickness). In thin section, the enamel cap is the same thickness and roughly symmetrical on the labial and lingual sides of the tooth, unlike the condition in hadrosaurid and ceratopsid dinosaurs [ 2 , 6 , 8 , 9 , 13 , 29 ]. One thin section revealed that the labial face of the developing tooth crown may have a slightly shorter enamel-covered portion compared to that along the lingual surface, however the difference is subtle ( Fig 3F ).…”

“…In section, every tooth root is separated from the surrounding socket bone by a mineral-filled space. In modern crocodilians, this space houses the periodontal ligament in life [ 6 , 45 – 47 ]: a network of collagen fiber bundles that anchor into the walls of the tooth socket and the surface of the tooth root. The surfaces of the tooth roots in Changchunsaurus parvus are coated in a thin band of acellular cementum, and a thicker, outer layer of cellular cementum ( Fig 6 ), which would have been the anchoring points for the periodontal ligament [ 8 ].…”

“…Dental innovations have been a focal point in ornithischian research, particularly because of the evolution of complex dental batteries in hadrosaurid and ceratopsid dinosaurs [ 2 , 6 , 9 , 13 , 31 , 51 ]. Hadrosaurid teeth in particular are touted as the most histologically complex of reptilian teeth [ 2 ], because of the presence of novel dental tissues (e.g.…”

“…coronal cementum, longitudinally and transversely oriented “giant tubules”). This view was challenged more recently [ 6 , 8 , 9 ], because dinosaur dental tissue homology, development, and evolution have only begun to be studied in other groups and compared to hadrosaurids. What remains is a need for histological analyses of early members of ornithischian clades in order to reconstruct the evolutionary origins of such complex structures.…”

“…The teeth of most early ornithischian dinosaurs were relatively simple triangular or leaf-shaped teeth for shearing plant material [ 32 ], but hadrosaurid and ceratopsid dinosaurs displayed extensive modifications to their dentitions, forming complex dental batteries for more efficient oral processing. The evolution of these complex structures, particularly the dental batteries of hadrosaurids, has been studied extensively and figures prominently in many histological analyses of dinosaur teeth [ 1 – 4 , 6 , 9 , 16 , 22 , 33 ]. The appeal of understanding how such complex structures originated and functioned is obvious, however without a broader comparative framework, it is difficult to characterize the evolutionary history of dental complexity in derived dinosaurian clades [ 5 ].…”