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.


  1. Bill, Thanks for your comment on Dracovenator, very much appreciated. Anyway - your mystery bone. I Can't help but think of the scapula of Heterodontosaurus, which has the same sinuous blade. A triassic ornithischian in North America afterall? Well not with just this bone but it is intriguing. As my phd supervisor would say "Where's the rest of it?"

  2. Adam,

    Yes, dinosaur scapula was my first guess in 2002. I will compare to some ornithischian scapulae. What a shock it if it is. Of course, typical for the Chinle, that is all there was.


  3. I'd like to suggest a third option, Bill. An ischium.

    While the idea that this element is a rather large occipital element (implying a skull about a foot wide, I'd say) is intriguing, the curvature seems to rule it out, while the "ventral" portion appears to be the only portion that is thickened in any way, and there is no apparent endocranial aspect to the structure. As a scapula, it's also intriguing, especially in similarity to Heterodontosaurus, although the apparent glenoid region of such a bone appears strongly angled forward implying a very erect shaft if in life position, and without a thickening of the coracoid margin that accompanies such bones.

    My suggestion of an ischium can relate to the thin proximal margin, though I guess it fails as does the argument against the scapula idea because the acetabular margin is very thin save for a peduncular thicken, and how the proximal margin (either as a scapula or ischium) is broken.

    Just an idea, but I'd back up Adam's idea, anyway. Very cool for an EJ or LT heterodontosaur to back up the apparent Fruita specimen.

  4. Having seen the specimen in person, I vote for archosauriform scapula of some sort...

  5. Jaime,

    Ischium is also a good guess and one that I had considered as well, although scapula does seem the favored ID at this point for readers (including yourself).

    BTW...what is the Fruita specimen?


  6. The Fruita taxon is an undescribed heterodontosaurid from the Morrison Fm of the Fruita Paleo Area. Its supposed to be similar to Echinodon from the lower Cretaceous of England. I believe some folks are working on it, but I don't want to spill beans prior to publication.

  7. I'm a few years late to the party, but could it be part of the temporal arch of a synapsid?


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