More On the Early Jurassic Resting Trace Paper

Andrew R. C. Milner, Jerald D. Harris, Martin G. Lockley, James I. Kirkland, Neffra A. Matthews (2009). Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace PLoS ONE, 4 (3) DOI: 10.1371/journal.pone.0004591

This recent article has been getting a bit of attention in the blogosphere (e.g., here, here, and even here), although mainly on the functional implications of theropod manus and arm orientation. However, there are a few other tidbits in this article that I find of interest (and which are relevent to the Chinle Formation and the Late Triassic.

First up is this little comment regarding the rank of the Chinle:

"The Moenave Formation overlies the Chinle Formation (Chinle Group of Lucas [24], but in Utah, group status is not recognized for these same strata)."

By the way, I really dislike numerical citations in journal articles as you have to continuously flip to the reference section to follow along. The same goes for putting all of the figure abbreviations for an entire paper into a single list or appendix rather than under each figure. Hate, it, hate it, hate it.

Anyhow, as many of my readers are possible aware, the rank of the Chinle is heavily debated. As I have stated before I have no problem with the Chinle being raised to Group status, but I do have a problem with it subsuming the Dockum Group (which is an older established name and thus has priority) as advocated by Lucas (1993) (Lucas [1993] states that the Dockum should be lost because of inconsistent usage, but this would also be true for the name Chinle). Thus we are kind of stuck with not using Chinle as a Group because of the confusion any future work advocating this (and leaving the Dockum separate) would cause with the necessity to then state continuously who's version of "Chinle Group" you are using.

In their paper Milner et al. (2009) have come right out (in a Jurassic-themed paper) and voiced their disapproval with using Chinle "Group" in Utah. We feel the same way in Arizona and I know my Texas colleagues feel just a strongly. This leaves only some workers in New Mexico using "Group" and two different versions at that. Of course, the North American Stratigraphic Code allows for units to be members, formations, or groups in different areas, however, it can cause some confusion.

Another important aspect of this paper is the useful taxonomic review of quite a few Late Triassic and Early Jurassic footprint taxa, which have been assigned to theropods, especially those that may be synonymous with Eubrontes and those that show manus impressions. Interesting is the hypothesis that based on the associated manus impressions the tracks of Atreipus are not theropodan, but rather those of a non-dinosaurian dinosauriform or possibly an ornithischian. Based on the body fossil record for the Late Triassic the former is most likely as was previously attributed to such by Olsen and Baird (1986) and Nesbitt et al. (2007).

In summary, although this new paper mainly focuses on proposed posture and behavior in theropods it also contains some key information on Triassic and Jurassic ichnofossil taxonomy and stratigraphy. For those interested this paper is available for free here.

REFERENCES

Lucas, S.G. 1993. The Chinle Group: revised stratigraphy and biochronology of Upper Triassic nonmarine strata in the western United States. Museum of Northern Arizona Bulletin 59: 27–50.

Andrew R. C. Milner, Jerald D. Harris, Martin G. Lockley, James I. Kirkland, Neffra A. Matthews (2009). Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace PLoS ONE, 4 (3) DOI: 10.1371/journal.pone.0004591

Nesbitt, S. J., Irmis, R. B., and W.G. Parker. 2007. A critical reevaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5:209-243.

Olsen, P. E. and D. Baird. 1986. The ichnogenus Atreipus and its importance for Triassic biostratigraphy. Pp. 61–87 in K. Padian (ed.) The beginning of the age of dinosaurs: faunal change across the Triassic–Jurassic boundary. Cambridge University Press, Cambridge.