Field of Science

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.

4 comments:

  1. Can't wait til Tschopp et al. becomes available. Potentially a lot more interesting.

    I'm not sure if I'd be very convinced by enamel microstructure characters alone either, there's a thesis out there where the author was unable to find phylogenetic signal in a set of them for even closely related crocodilians, IIRC. I'm sure there are plenty of cases where they are relevant, but they are probably subject to the same variation that everything else is.

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  2. It's true that the squamosal and armor found at St Johns quarry was found associated with teeth of Krzyzanowskisaurus. But the lack of overlap with the teeth is an obstacle to referring the osteoderms to K. hunti.

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  3. Again that would suggest the presence of more than one Revueltosaurus-like animal. The teeth of R. hunti possess characters that are shared with R. callenderi exclusive of other taxa. This is why Heckert referred them to Revueltosaurus in 2002. After it was determined that R. callenderi was not a dinosaur, R. hunti was assigned to a new genus in hopes that it still would prove to be dinosaurian. However, the current evidence is against this (i.e. presence of Revueltosaurus bones in same quarry, tooth microstructure). The lack of overlap is always a problem but we can use autapomorphies to make referrals.

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  4. This enamel microstructure data is very cool and informative. It would be interesting to know what patterns might be found with other Triassic teeth such as Tecosaurus, Pretecovasaurus, Crosbysaurus, etc.

    The comments by Heckert & Miller-Camp on the Blue Hills material and whether or not Krzyzanowskisaurus hunti should be referred to Revueltosaurus are dissappointing, because they unfortunately do not accurately summarize what we said in Irmis et al. 2007 (Historical Biology). We actually *never* officially referred the osteoderms and squamosal to R. hunti, all we said is that they're diagnostic of the genus Revueltosaurus, and this would be *consistent* with R. hunti being assignable to Revueltosaurus. Here's the full passage:

    "We tentatively retain R. hunti in Revueltosaurus (as a probable pseudosuchian) based on the characters it shares with R. callenderi that Heckert (2002) outlined and that the teeth were found at the Blue Hills locality in Arizona (UCMP loc. 7308) in association with a squamosal (UCMP 165205), quadrate (UCMP 165206), and osteoderms that are identical to those of R. callenderi. These osteoderms were described by Heckert and Lucas (2002) as juvenile specimens of the aetosaur Stagonolepis wellesi, but they are indistinguishable from the osteoderms described by Parker et al. (2005) for R. callenderi. This evidence, combined with the fact that R. hunti lacks a true asymmetric basal swelling, prevents these teeth from being unambiguously assigned to the Ornithischia."

    Yes, the lack of unambiguous association and overlapping elements within the Blue Hills specimens makes it difficult to say the dental material belongs to the same taxon as the non-dental skeletal material. But, as Bill points out, each individual element can be referred to Revueltosaurus based on unambiguous apomorphies, and this includes the teeth of R. hunti. The important point is that this is *consistent* with the hypothesis that the cranial and osteoderm material belongs to the R. hunti material. An alternative hypothesis would be that there are two or more species of Revueltosaurus in the Blue Hills assemblage, but this seems less parsimonious without any evidence of multiple taxa (e.g., seeing two morphotypes of the same element).

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