Taxonomic utility of isolated ankylosaurian dinosaur teeth using traditional and geometric morphometrics with implications for ankylosaur paleoecology

Abstract

The presence of a basal cingulum, fluting, and overall size have been used to differentiate nodosaurid and ankylosaurid teeth for decades. The taxonomic utility of tooth morphology in ankylosaurs, however, has not been quantitatively tested. In addition, new phylogenetic hypotheses recognize four ankylosaur families (Panoplosauridae, Polacanthidae, Struthiosauridae, and Ankylosauridae), rather than the traditional nodosaurid–ankylosaurid dichotomy. Understanding ankylosaur tooth variation could better help identify taxa with ambiguous phylogenetic affinities or allow isolated teeth to test paleoecological questions such as a potential extirpation of mid-Cretaceous ankylosaurids from Laramidia. We analyzed a large sample of ankylosaur teeth using traditional and geometric morphometrics and investigated the utility of size and the presence of a cingulum and fluting for differentiating ankylosaur teeth. Morphometric analyses show that “nodosaurids” had the greatest variation in tooth shape and size. Panoplosauridae and Struthiosauridae account for a large amount of “nodosaurid” variation, whereas basal ankylosaurs, Polacanthidae, and Ankylosauridae share a similar restricted morphospace. Teeth with a crown base length or height over 10 mm are found only in panoplosaurids, struthiosaurids, and Peloroplites, but smaller sizes are found in all clades. A basal cingulum and fluting are associated with Ankylosauridae and Panoplosauridae. Linear discriminant analyses could accurately identify only between 50% and 75% of the teeth in our sample; thus, they should be used in conjunction with size and discrete traits when identifying isolated teeth. With these findings, caution should be used when attempting to use isolated ankylosaur teeth in broader paleoecological questions, and reclassification of museum collections should be undertaken. Non-technical Summary Ankylosaur isolated teeth have traditionally been identified to the family level (ankylosaurid or nodosaurid) using overall size and the presence or absence of features such as grooves on the tooth face (fluting). Recent hypotheses recognize four ankylosaur families (Panoplosauridae, Polacanthidae, Struthiosauridae, and Ankylosauridae). Understanding ankylosaur tooth variation could better help identify taxa where the family is unclear or allow isolated teeth to test paleoecological questions such as a potential extinction of mid-Cretaceous ankylosaurids from North America. We analyzed the shapes of a large sample of ankylosaur teeth using traditional and geometric morphometrics and investigated size and the presence of features such as a cingulum and fluting for differentiating ankylosaur teeth. Morphometric analyses show that “nodosaurids” had the greatest variation in tooth shape and size. Panoplosauridae and Struthiosauridae account for a large amount of “nodosaurid” variation, whereas basal ankylosaurs, Polacanthidae, and Ankylosauridae share a similar restricted morphospace. Teeth with a length or height over 10 mm are found only in panoplosaurids, struthiosaurids, and Peloroplites, but smaller sizes are found in all clades. Our analyses could accurately identify only between 50% and 75% of the teeth in our sample; they should be used in conjunction with size and traits when identifying isolated teeth. With these findings, caution should be used when attempting to use isolated ankylosaur teeth in broader paleoecological questions, and reclassification of museum collections should be undertaken.