Triassic Mystery Fossil

Things are a bit slow news wise over the holidays and I have not done this for awhile, so here goes.  Any guesses on what this is?  Should be relatively easy I think.

Another Mystery Fossil from the Chinle

The identification of these two specimens has been bugging me for a few years now. These are the only two specimens like this that I have ever seen and they were collected from pretty much the same locality and horizon (Sonsela Member, Chinle Formation) in the Petrified Forest. They are thick bones, characterized by their triangular shape (one is missing a corner) and by a short but deep notch in what presumably is the dorsal end. It is bilaterally symmetrical, thus represents a mid-line element, and the lateral edges show possible open sutures. The photos show the tops and updersides of the two elements.

I have a working hypothesis on what I think they are (at least what bone) but I'd like to hear your opinions.

Late Triassic Mystery Fossil #5

OK...this really is a mystery fossil because I am not really sure what it is. It was collected in 2002 from Petrified Forest National Park in the Blue Mesa Member of the Chinle Formation. It was found in a bluish mudstone layer that contains lots of in-situ tree stumps but hardly any vertebrate fossils. This layer occurs just below an extremely productive horizon which produces a wealth of typical Chinle vertebrates (phytosaurs, aetosaurs, metoposaurs, etc...).

I'm sorry for the slightly blurry photo and only one angle but that is all I have right now. I first thought that it was a scapula, but I guess that it could also be a paroccipital process of the opisthotic and a portion of the exocipital of a pseudosuchian archosaur. The scale bar = 5 cm.

I'm open to either other suggestions and/or votes on my hypotheses.

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.

Late Triassic Mystery Fossil #4

This is an interesting specimen with an interesting history.....and proof that, to be cliche, one person's junk is another person's treasure (especially if they don't know what they are looking at). Unfortunately that is only half the story, the rest is highly frustrating. Views are lateral and proximal. Scale bar is 1 cm.

Late Triassic Mystery Fossil #3

Man I am busy (but aren't we all?), family, two jobs, research, upcoming holidays, and now studying for the GRE (I last took it 11 years ago!). Not too sure what I am thinking at times. Anyhow, I have not done one of these for awhile so here is Late Triassic mystery fossil #3. It comes from the Chinle Formation and is shown here in dorsal view. Hopefully I will post on this fossil and its relatives soon. I also know that there are a few new Triassic papers coming down the pipe soon, so there will be lot to cover in the next couple of months....but for now it is back to the quantitative section study guide.

SVP Meeting and Mystery Fossil #2

I am off to attend the annual meeting of the Society of Vertebrate Paleontology in Cleveland. There are lots of interesting abstracts (especially Triassic) and I am hoping it will be a very good meeting.


Because I will not be posting for over a week I've included another mystery fossil for anyone still checking these pages. I'll give you a couple of hints, you are looking the element in distal (ventral) view, the specimen is an archosaurian bone from the Chinle Formation, the specimen is part of the holotype of an animal whose generic name roughly translate as "ghost reptile", and another clue can be found by reading over the comments from Triassic Mystery Fossil #1.

Late Triassic Mystery Fossil #1 is...

...the posterior portion of the right squamosal bone of a phytosaur. Because of the robust nature of these elements they are often preserved even when the rest of the skull is not. In fact, they are a very common fossil in the Chinle Formation. This is actually fortunate because the morphology of the posterior process of the squamosal is diagnostic and therefore the entire skull does not need to be recovered to determine the presence of a taxon. The specimen to the upper left belongs to Pseudopalatus pristinus and is characterized by being extremely thickened and knoblike (down in the photo is posterior), with many surfaces for muscle attachment. Phytosaur evolution is characterized by changes in the "post temporal" arcade, including depression and narrowing of the supratemporal fenestrae as well as a posterior elongation and mediolateral widening of the squamosal process. The common preservation and recovery of isolated squamosals in concert with their diagnostic nature make them excellent biostratigraphic index fossils and thus they should be collected whenever found. The picture below shows a complete upper portion of a skull of P. pristinus with the position of the squamosal fragment above (from a different individual) outlined.

Late Triassic Mystery Fossil #1

One of the key skills needed to work on Late Triassic fossils is the ability to readily identify fragmentary specimens (because that is all that you usually get!). I've seen this on a few other blogs and have found it entertaining, so from time to time I will present a mystery fossil specimen from the Late Triassic for reader identification. I'll start with a fairly easy one. I would ask some of my colleagues who specialize in the Late Triassic and read this blog to please refrain from providing an ID immediately ;).