Comparing the Tooth Enamel Microstructure of the Pseudosuchian Revueltosaurus and the Proposed Triassic Ornithischian Krzyzanowskisaurus

This is a new paper testing the relationships between the pseudosuchian Revueltosaurus callenderi and the hypothesized ornithischian Krzyzanowskisaurus hunti (originally Revueltosaurus hunti) utilizing tooth enamel microstructure. The study finds that the tooth enamel structure of these two taxa share many characters not found in other taxa and thus they are probably closely related.  The authors advocate that generic distinction should be maintained until the skeletal remains of K. hunti are discovered. However, the teeth of K. hunti were recovered from a microvertebrate deposit in the Blue Mesa Member of the Chinle Formation near St. Johns Arizona.  As mentioned by Parker et al (2005) and Irmis et al. (2007) the same deposit included an autapomorphic squamosal of Revueltosaurus as well as numerous osteoderms also referable to the taxon.  Thus the assemblage possibly contains Revueltosaurus hunti or Revueltosaurus as well as a second taxon with Revueltosaurus-like teeth called Krzyzanowskisaurus. Heckert and Miller-Camp argue that this could be circumstantial given the purported lack of element association; however, no evidence exists either that they weren't originally found in association. There is simply just drawers of microvertebrate material from the same quarry. I and my colleagues have just maintained that the former is more parsimonious (Revueltosaurus hunti) and the shared characteristics of the teeth revealed by this study seem to support that hypothesis. In any case there still is no strong evidence for an ornithischian dinosaur affinity for K. hunti.

Heckert, A. B., and J. A. Miller-Camp. 2013. Tooth enamel microstructure of Revueltosaurus and Krzyzanowskisaurus (Reptilia:Archosauria) from the Upper Triassic Chinle Group, USA: Implications for function, growth, and phylogeny. Palaeontologia Electronica Vol. 16, Issue 1; 1A,23p;  palaeo-electronica.org/content/2013/344-revueltosaurus-tooth-enamel
 
Abstract - Tooth enamel microstructure can carry significant phylogenetic, ontogenetic, and functional information within amniotes. Here we provide the first descriptions of the tooth enamel microstructure of two Late Triassic taxa, the crurotarsan Revueltosaurus callenderi Hunt and the putative ornithischian Krzyzanowskisaurus hunti (Heckert), which some consider closely related. To test the hypotheses that enamel thickness corresponds to function and/or phylogeny we analyzed the enamel of each at various scales, measuring enamel thickness and examining microstructural features throughout both longitudinal and cross-sectional thickness using previously established techniques to facilitate comparisons. Both taxa possess thick (up to ~150 µm) enamel for their size (< 20 mm crown height). Enamel in R. callenderi ranged from ~5-152 µm across a premaxillary tooth in longitudinal section, and ~42-92 µm in a maxillary/dentary tooth transverse section. K. hunti enamel thickness was ~18-155 µm longitudinally and ~29-75 µm transversely. Both also had well-developed basal unit layers (BUL) and weakly developed columnar microstructure. Well-developed lines of incremental growth (LIG) are present in both taxa, through which the columnar enamel grades into parallel crystallite enamel. Their enamel microstructure is therefore grossly similar to that of several ornithischian taxa, especially ankylosaurs, with which they are strongly convergent, and also compares well to rauisuchids and tyrannosaurids. The relatively unique combination of microstructural characteristics in the schmelzmuster of R. callenderi and K. hunti supports the hypothesis that they are closely related, but does not conclusively preclude a different taxonomic placement for K. hunti so we retain its separate generic designation.Image may be NSFW.
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