Osteohistological and taphonomic life-history assessment of Edmontosaurus annectens (Ornithischia: Hadrosauridae) from the Late Cretaceous (Maastrichtian) Ruth Mason dinosaur quarry, South Dakota, United States, with implication for ontogenetic segregation between juvenile and adult hadrosaurids

The Late Cretaceous (Maastrichtian) Ruth Mason Dinosaur Quarry (RMDQ) represents a monodominant Edmontosaurus annectens bonebed from the Hell Creek Formation of South Dakota and has been determined as a catastrophic death assemblage likely belonging to a single population, providing an ideal sample to investigate hadrosaurid growth and population dynamics. For this study, size-frequency distributions were constructed from linear measurements of long bones (humeri, femora, tibiae) from RMDQ that revealed five relatively distinct size classes along a generally right-skewed distribution, which is consistent with a catastrophic assemblage. To test the relationship between morphological size ranges and ontogenetic age classes, subsets from each size-frequency peak were transversely thin-sectioned at mid-diaphysis to conduct an ontogenetic age assessment based on growth marks and observations of the bone microstructure. When combining these independent datasets, growth marks aligned with size-frequency peaks, with the exclusion of the overlapping subadult-adult size range, indicating a strong size-age relationship in early ontogeny. A growth curve analysis of tibiae indicated that E. annectens exhibited a similar growth trajectory to the Campanian hadrosaurid Maiasaura, although attaining a much larger asymptotic body size by about 9 years of age, further suggesting that the clade as a whole may have inherited a similar growth strategy. This rich new dataset for E. annectens provides new perspectives on other hypotheses of hadrosaurid life history. When the RMDQ population was compared with size distributions from other hadrosaurid bonebed assemblages, juveniles (categorized as ages one and two) were either completely absent from or heavily underrepresented in the samples, providing support for the hypothesized segregation between juvenile and adult hadrosaurids. Osteohistological comparison with material from polar and temperate populations of Edmontosaurus revealed that previous conclusions correlating osteohistological growth patterns with the strength of environmental stressors were a result of sampling non-overlapping ontogenetic growth stages.

Histology of Caenagnathid (Theropoda, Oviraptorosauria) Dentaries and Implications for Development, Ontogenetic Edentulism, and Taxonomy

The fossil record of caenagnathid oviraptorosaurs consists mainly of their fused, complexly sculptured dentaries, but little is known about the growth and development of this diagnostic structure. Previous work has suggested that the ridges and grooves on the occlusal surface are either the vestiges of teeth and their alveoli or were adaptations to increase shearing action during mastication. In addition, the distinctiveness of the dentaries has led to their use for species-level taxonomy, without a complete understanding of their variation through ontogeny. Here, we describe additional caenagnathid mandibles from the Dinosaur Park Formation of Alberta, Canada, and perform histological analyses to assess relative ontogenetic stage and the nature of the occlusal elaborations. The results show that the mandibular symphysis is synostosed early in ontogeny and does not accurately reflect ontogenetic stage in caenagnathids. In contrast, the presence of cyclical growth marks in a large specimen shows that mandibles can be used for relative histological maturity estimation. Histological features of the ridges of bone surrounding the lingual groove indicate that they are not the vestiges of tooth-bearing tissues and that caenagnathids did not lose their teeth through ontogeny as suggested in previous work. Instead, increased secondary remodeling in these structures is consistent with their use for food processing. Unexpectedly advanced maturity in a small specimen suggests that at least three caenagnathid species of varying body sizes coexisted in the Dinosaur Park Formation. These results stress the necessity of histological analysis when assessing maturity or ontogenetic trends in fossil material. Anat Rec, 303:918-934, 2020. © 2019 Wiley Periodicals, Inc.

Histological evidence for a dynamic dental battery in hadrosaurid dinosaurs

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.