The evolution of dinosaur tooth enamel microstructure

Abstract

The evolution of tooth enamel microstructure in both extinct and extant mammalian groups has been extensively documented, but is poorly known in reptiles, including dinosaurs. Previous intensive sampling of dinosaur tooth enamel microstructure revealed that: (1) the three-dimensional arrangement of enamel types and features within a tooth-the schmelzmuster-is most useful in diagnosing dinosaur clades at or around the family level; (2) enamel microstructure complexity is correlated with tooth morphology complexity and not necessarily with phylogenetic position; and (3) there is a large amount of homoplasy within Theropoda but much less within Ornithischia. In this study, the examination of the enamel microstructure of 28 additional dinosaur taxa fills in taxonomic gaps of previous studies and reinforces the aforementioned conclusions. Additionally, these new specimens reveal that within clades such as Sauropodomorpha, Neotheropoda, and Euornithopoda, the more basal taxa have simpler enamel that is a precursor to the more complex enamel of more derived taxa and that schmelzmusters evolve in a stepwise fashion. In the particularly well-sampled clade of Euornithopoda, correlations between the evolution of dental and enamel characters could be drawn. The ancestral schmelzmuster for Genasauria remains ambiguous due to the dearth of basal ornithischian teeth available for study. These new specimens provide new insights into the evolution of tooth enamel microstructure in dinosaurs, emphasizing the importance of thorough sampling within broadly inclusive clades, especially among their more basal members.

Tooth enamel microstructure of Revueltosaurus and Krzyzanowskisaurus (Reptilia:Archosauria) from the Upper Triassic Chinle Group, USA: Implications for function, growth, and phylogeny

“…We recognize the difficulty of replicating measurements, especially if the section images are not perfectly perpendicular to the electron gun, and so made 10 measurements (close together) on one image to assess the precision of the tpsDig measurement protocol. After accepting the mean of these measurements as the "true value" (and the Sander (1999) and Hwang (2005Hwang ( , 2009Hwang ( , 2011) ***= Taxon sampled by Sander (1999), Hwang (2005, 2009), and Stokosa (2005 Crurotarsi Ornithodira…”

“…We note that this means that we use mesial/distal, labial/lingual, and basal/apical (or occlusal) to describe and orient isolated teeth and features of the tooth crown. Descriptions of microstructural features follow the standardized format of Hwang (2005Hwang ( , 2011, with enamel described from the enamel-dentine junction (EDJ) outward to the outer enamel surface (OES). Enamel thickness and distribution is described first, followed by type and schmelzmuster, followed by discussion of any special features.…”

“…His work, especially the 1999 monograph, remains the standard reference for microstructural features in non-synapsid amniotes. More recently Stokosa (2001Stokosa ( , 2005 examined the microstructure of a variety of tyrannosaurid and dromaeosaurid taxa from the Upper Cretaceous of western North America, and Hwang (2005Hwang ( , 2006Hwang ( , 2007Hwang ( , 2009Hwang ( , 2011 sampled an array of dinosaurian teeth from the Mesozoic of North America as well as the Cretaceous of Asia en route to completing a dissertation (Hwang, 2007) and related abstracts and papers (Hwang, 2005(Hwang, , 2006(Hwang, , 2009(Hwang, , 2011. Unlike Sander (1999), who saw little phylogenetic signal in enamel microstructure, especially at lower (family-level) taxonomic rank, Hwang (2005Hwang ( , 2009) identified some similarity in tooth enamel microstructures related to phylogenetic position, although she concurs with Sander (1999) that relatively few unambiguous synapomorphies are evident in the enamel microstructure at lower taxonomic levels.…”

A troodontid dinosaur from the latest Cretaceous of India

“…Enamel microstructure has recently been suggested as a phylogenetically informative character for some dinosaurian clades 50 . Unfortunately, theropods display a large amount of homoplasy in these characters, and troodontids in particular are highly variable.…”

Revisão sistemática e ontogenética dos materiais cranianos atribuídos ao gênero Mariliasuchus (Crocodyliformes, Notosuchia) e suas implicações taxonômicas e paleobiológicas

“…Suas carenas mesial e distal apresentam dentículos robustos e arredondados, que visualmente (uma vez que dentes mandibulares não foram sacrificados para a análise de microestrutura dentária) parecem apresentar o mesmo padrão observado nos molariformes maxilares: dentículos formados apenas por espessamento da camada de esmalte nas regiões próximas ao ápice do dente e dentículos verdadeiros, formados com a participação da dentina, nas regiões próximas à base da coroa. O segundo e o quarto molariformes são frequentemente os molariformes mais largos e robustos dentre os cinco últimos dentes da mandíbula, mas parece haver alguma anos, algumas contribuições importantes foram realizadas no estudo da microestrutura do esmalte dentário em répteis (Creech, 2004;Hwang, 2005;Hwang, 2011), mas o amplo e diverso clado dos Notosuchia nunca teve nenhum representante estudado neste sentido. A presente contribuição representa, então, a primeira descrição da microestrutura do esmalte dentário para o grupo dos notossúquios.…”