Incremental growth of therizinosaurian dental tissues: implications for dietary transitions in Theropoda

Previous investigations document functional and phylogenetic signals in the histology of dinosaur teeth. In particular, incremental lines in dentin have been used to determine tooth growth and replacement rates in several dinosaurian clades. However, to date, few studies have investigated the dental microstructure of theropods in the omnivory/herbivory spectrum. Here we examine dental histology of Therizinosauria, a clade of large-bodied theropods bearing significant morphological evidence for herbivory, by examining the teeth of the early-diverging therizinosaurian Falcarius utahensis, and an isolated tooth referred to Suzhousaurus megatherioides, a highly specialized large-bodied representative. Despite attaining some of the largest body masses among maniraptoran theropod dinosaurs, therizinosaurian teeth are diminutive, measuring no more than 0.90 cm in crown height (CH) and 0.38 cm in crown base length (CBL). Comparisons with other theropods and non-theropodan herbivorous dinosaurs reveals that when controlling for estimated body mass, crown volume in therizinosaurians plots most closely with dinosaurs of similar dietary strategy as opposed to phylogenetic heritage. Analysis of incremental growth lines in dentin, observed in thin sections of therizinosaurian teeth, demonstrates that tooth growth rates fall within the range of other archosaurs, conforming to hypothesized physiological limitations on the production of dental tissues. Despite dietary differences between therizinosaurians and hypercarnivorous theropods, the types of enamel crystallites present and their spatial distribution—i.e., the schmelzmuster of both taxa—is limited to parallel enamel crystallites, the simplest form of enamel and the plesiomorphic condition for Theropoda. This finding supports previous hypotheses that dental microstructure is strongly influenced by phylogeny, yet equally supports suggestions of reduced reliance on oral processing in omnivorous/herbivorous theropods rather than the microstructural specializations to diet exhibited by non-theropodan herbivorous dinosaurs. Finally, although our sample is limited, we document a significant reduction in the rate of enamel apposition contrasted with increased relative enamel thickness between early and later diverging therizinosaurians that coincides with anatomical evidence for increased specializations to herbivory in the clade.

Scanning electron microscope analysis of enamel microstructure in a Polycotylid (Plesiosauria) from the Pierre Shale Group, South Dakota, U.S.A.

The teeth of polycotylid plesiosaurs are generally simple, cone shaped, non-serrated and only slightly recurved without distinct carinae. The surface of crowns are characterized by a series of vertical enamel wrinkles that are more highly developed on the lingual surface of the crown, and decrease in width and number toward the apex. Some of the most promising research related to fossil dentition, involves the analysis of surface and internal dental microstructure. This study, is an attempt to examine and describe polycotylid dental microstructure. It gives an overview of polycotylid plesiosaur enamel and dentine microstructures using a scanning electron microscope. Enamel type and structures vary, based on its position on the surface of the crown, and its perceived strength requirements. The dentition layer is “honeycombed” with tubular structure, possibly to provide nourishment to fast growing crowns. The study of crown microstructures may lead to a better understanding of polycotylid niche preference in the late Cretaceous oceans.

Scanning electron microscope analysis of enamel microstructure in a Polycotylid (Plesiosauria) from the Pierre Shale Group, South Dakota, U.S.A.

The teeth of polycotylid plesiosaurs are generally simple, cone shaped, non-serrated and only slightly recurved without distinct carinae. The surface of crowns are characterized by a series of vertical enamel wrinkles that are more highly developed on the lingual surface of the crown, and decrease in width and number toward the apex. Some of the most promising research related to fossil dentition, involves the analysis of surface and internal dental microstructure. This study, is an attempt to examine and describe polycotylid dental microstructure. It gives an overview of polycotylid plesiosaur enamel and dentine microstructures using a scanning electron microscope. Enamel type and structures vary, based on its position on the surface of the crown, and its perceived strength requirements. The dentition layer is “honeycombed” with tubular structure, possibly to provide nourishment to fast growing crowns. The study of crown microstructures may lead to a better understanding of polycotylid niche preference in the late Cretaceous oceans.

Dental microstructure and geochemistry of Mosasaurus hoffmanni (Squamata: Mosasauridae) from the Late Cretaceous of Turkey

Mosasaurus hoffmanni from Devrekani, Turkey is among the geologically youngest of the ancient aquatic predators. In addition, M. hoffmanni is the only Mesozoic vertebrate reported from Turkey, and has proven useful in the understanding of paleogeographic segregation within Mosasauridae. Here we provide an analysis of the histology and geochemistry of a functional maxillary tooth of this Turkish mosasaur. Dental histology included descriptions of lines of von Ebner and contour lines of Owen in dentine, as well as microstructural details pertaining to the enamel structure. Considering the spacing of the lines of von Ebner, the odontoblast deposition of the dentine (at the level of sectioning of the crown) was estimated to have taken approximately 511 days. A replacement tooth was fortuitously discovered upon sectioning the functional tooth, and given the thickness of the dentine visible, it is estimated that it took 233 days to deposit the centripetal layer of dentine. Energy dispersive spectroscopy, x-ray diffractometry, fusion disc x-ray fluorescence and Sr isotope measurements suggested that the tooth had undergone heterogeneous diagenetic alteration. Despite signs of alteration, the anatomy and chemistry of the M. hoffmanni teeth provided significant paleobiological and paleo-environmental insight.