Chinle Silesaurid and the Importance of Field Notes

Adam Yates nailed it. The specimen is the proximal end of the right femur of a silesaurid (PEFO 34347) from the Upper Triassic Blue Mesa Member (Chinle Formation) of Arizona. This specimen is significant because it represents the only known unambiguous silesaurid element from the lower portion of the Chinle Formation and from Arizona. It demonstrates that silesaurids were a portion of the fauna at Petrified Forest National Park.

This specimen was discussed in more detail by Parker et al. (2006) and Nesbitt et al. (2007). It is identical to the proximal ends of the femora of Silesaurus opolensis (Carnian of Poland) and Eucoelophysis baldwini (Norian of New Mexico) and thus cannot be assigned to a specific genus although the age (Norian) and stratigraphic position (Chinle Formation) of the specimen would suggests that it is probably could represent Eucoelophysis rather than Silesaurus. However, until more material is found this cannot be considered. A key characteristic of the proximal end of the femur in Silesaurus, Eucoelophysis, and PEFO 34347 is that the element is triangular in proximal view and has a mediolaterally trending sulcus. Whereas this sulcus is present in other taxa, most notably the pseudosuchian Shuvosaurus, the femur of silesaurids differs in having a subrectangular femoral head in lateral view with a slightly offset head as in dinosaurs.

Now for the promised “interesting” (and frustrating) story regarding this specimen. This specimen was collected sometime in the late 1990s by an unknown individual who was part of a larger research project. It went unrecognized and was included in a large amount of material deemed unworthy of study and potentially to be disposed of. When going through this material to see if anything was salvageable I came across this specimen. Unfortunately, the exact spot where the specimen was collected was not recorded. There are no known field notes for the project and field tags contain minimal information, in this case just a vague geographical reference. This reference is enough to pinpoint the specimen to a small geographical area and limited stratigraphic level; however, it will be nearly impossible to find the rest of the specimen if it exists (and the break is clean, suggesting that more of the specimen was preserved and awaits discovery).

I cannot emphasize enough (and I stress this to, and require it from, all of my employees and interns) the importance of collecting and recording accurate field data on every specimen collected whether the specimen seems important or not. I don’t know how I would function without my past field notes when it comes to identifying and interpreting specific specimens. I have found when I get lax (because of time, weather, arrogance, etc..) I usually end of regretting not having a key piece of information regarding a specimen at some point. At the absolute minimum for EVERY specimen collected there should be GPS coordinates, a photograph of the site showing the surrounding landscape (most important), and a brief description of the sediments and stratigraphic position. Taphonomic notes are also extremely important and often forgotten. Bottom line, you cannot collect too much information. Anyone who has worked with older specimens when simply a stratigraphic unit and state were deemed sufficient information will understand. PEFO 34347 currently represents to earliest known silesaurid from North America, yet its provenance cannot be precisely determined and clarification regarding this specimen depends exclusively on luck. Was more of the specimen preserved? Is it still present and will we be able to stumble across it? I certainly hope so.

REFERENCES

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

Parker, W.G., Irmis, R.B., and S.J. Nesbitt. 2006. Review of the Late Triassic dinosaur record from Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62:160-161.