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

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. Additionally, 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 palaeoecological questions like
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 accounts 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 only found 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
only accurately identify between 50-75% 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
palaeoecological questions and reclassification of museum collections
should be undertaken.