When is a Triassic fauna not Triassic?

In recent years among paleontologists who work on the Triassic/Jurassic boundary there has been some serious excitement about a new locality in northeastern Utah that hosts a wide variety of cool fossils. It has been named the Saint's and Sinners Quarry and has been actively worked by crews from Brigham Young University in Provo, Utah since 2009. Based on abstracts and news articles it is clear that the fauna is diverse and well represented by multiple specimens. Having been at SVP in recent years I have been able to see images of the fossils coming out of the quarry first hand. Over 11,500 fossils have been removed from the quarry which Brooks Britt (from BYU) and others estimate is only 33% excavated. Virtually all of the fossils are preserved in 3D, allowing us to have spectacular insights into animals we do not have much data from, due to crushing and other concerns. Most of the specimens are even articulated! My hat is off to all of the BYU and Dinosaur National Monument crews who have been literally working on the edge of a cliff to extract these remains.

But. You knew there was a "but" coming, didn't you? But while the fossils themselves are spectacular there has been a trend in the last couple years to refer to this bone bed as being Late Triassic in age.  Admittedly aeolian deposits are hard to date; they tend to lack any significant ash deposits and detritial zircons (which can be used to constrain ages in other sedimentary rocks) are not really useful in sand dunes. That is what the Nugget Sandstone is - a deposit of windblown sand in western North America that began during the latest Triassic Period and persisted well into the Early Jurassic (see Sprinkel et al., 2011 for more details). This sand sea expanded as paleolatitude changed and western North America drifted further away from the equator and into the "dry belt" where warm, arid climatic conditions exist. This pattern can even be seen in the Late Triassic Chinle Formation at Dinosaur National Monument, as presented on at SVP this year (Irmis et al., 2015).

The first reports of the quarry (Chambers et al., 2011) suggested that Britt and colleagues at first assigned an Early Jurassic age to the deposit. This date was keeping with the general consensus that the Triassic/Jurassic boundary was somewhere within the Nugget. By 2012, however, it appeared that the teams views changed. That year Engelmann and others (note -the actual abstract doesn't appear to be available any longer) presented an abstract at the GSA conference in Charlotte, NC. In the title they state that a new drepanosaur has been found in the Nugget Sandstone and state that it has biostratigraphic importance. They also explicitly question the Jurassic age of the Nugget (they literally put a question mark in front of the word Jurassic) based on this new find. This new drepanosaur is pretty dang cool! The team expanded on it in recent SVP meetings (Chure et al., 2013; Chure et al., 2015). This critter seems to show highly derived characters shared only with Drepanosaurus (a European form) that indicate it was a specialized fossorial (digging) animal. The kicker here is that all other known drepanosaurs come from definitive Triassic strata. The Nugget drepanosaur comes from a quarry 55 meters above the last reliably dated strata (the Bell Canyon Formation, which sits between the Chinle and Nugget in northeastern Utah).

So what's the problem? Well this year the team again presented on some more spectacular fossils from the Saints and Sinners Quarry, including a large toothed pterosaur that is very closely related to the Early Jurassic European pterosaur Dimorphodon (Britt et al., 2015). This story has been picked up by the national media who have been reporting this site as being Late Triassic in age. Let's do a quick review of the evidence for a Late Triassic age.

Evidence of a Triassic Age of the Saints and Sinners Quarry

• Presence of a drepanosaur
• Presence of several small sphenosuchians
• In a formation that is traditionally considered to span the Triassic/Jurassic Boundary

Okay...that's not really a convincing list. This is especially true if you are claiming that this extraordinary interdunal wetland deposit represents a Triassic assemblage unlike any other in western North America. In fact two of the "pros" can actually be taken as a "con" and the third I think is ambiguous.
Allow me to present a list of why I have concerns about a Triassic age for this quarry.

Why the Saints and Sinners Quarry may be Jurassic in age

• In a formation that is traditionally considered to span the Triassic/Jurassic Boundary
• Quarry located 55 meters above the last Triassic-dated rocks (~1/2 the thickness of the Nugget)
• Presence of the most-derived drepanosaur yet discovered
• Presence of a pterosaur that is most similar to a Jurassic pterosaur
• Presence of a medium-large bodied theropod in the quarry in addition to a coelophysoid
• Presence of several small sphenosuchians
• No phytosaurs
• No aetosaurs
• No metoposaurs
• Upper Nugget lacks a Triassic ichnofauna

Well, does this mean case closed? No. While my list may be longer it isn't the final word on anything. Several of these points rely on the absence of taxa like phytosaurs and we all know that the absence of evidence is not the evidence of absence. Still, taken as a suite of things, I am not convinced that this quarry is Triassic. There are a few ways that perhaps we could do to see if I'm wrong.

• Phylogenetic analysis of the sphenosuchians - closely related to Chinle or Kayenta taxa?
• Phylogenetic analysis of new drepanosaur compared to the still-unnamed Ghost Ranch form
• Phylogenetic analysis of the theropods - are they closer to Coelophysis or later taxa?
• Additional fieldwork to look for unambiguous biostratigraphic markers

To me this fauna looks like a typical Early Jurassic fauna from western North America with a drepanosaur thrown in. Could it be an impoverished Late Triassic fauna that also has several highly derived taxa in it? I suppose and I will happily eat my hat if that is the case. What a great collection of Triassic taxa it would be! With the data that have been presented thus far I just can't see it though.

Why does this matter? Timing is everything in evolution. One of the big ways we as paleontologists talk about paleobiogeography is in terms of dispersal and vicariance. Are animals (and plants, and fungi, etc.) slowly moving into new areas or are populations split up by new barriers, isolating groups that then adapt in their own directions? To put it in the context of the Nugget fossils, are we seeing evidence that many disparate clades were widespread in the Late Triassic, or are we seeing similar taxa from elsewhere in North America in the Early Jurassic adapting to new environments? These questions have serious implications for our understanding of the rate of evolution among all these groups. By tying down the date of the Saints and Sinners Quarry we will be better able to answer some of these questions.

Final caveat: this is all based off of abstracts, talks, and posters and conferences, some of which I was unable to attend or access (this is why people should archive their conference presentations on FigShare - but I digress). I am extremely excited to see the peer reviewed publications that should result from these finds. And it may very well be that their method for dating the quarry is more nuanced than they have already presented. As always, I suppose, "Wait for the paper."

Works Cited
Britt, B. B., Chure, D., Engelmann, G., Dalla Vecchia, F., Scheetz, R. D., Meek, S., Thelin, C., Chambers, M. A NEW, LARGE, NON-PTERODACTYLOID PTEROSAUR FROM A LATE TRIASSIC INTERDUNAL DESERT ENVIRONMENT WITHIN THE EOLIAN NUGGET SANDSTONE OF NORTHEASTERN UTAH, USA INDICATES EARLY PTEROSAURS WERE ECOLOGICALLY DIVERSE AND GEOGRAPHICALLY WIDESPREAD. Journal of Vertebrate Paleontology, Program and Abstracts, 2015 p. 97

Chure, D. J., Andrus, A. S., Britt, B. B., Engelmann, G. F., Pritchard, A. C., Scheetz, R., Chambers, M. MICRO CT IMAGERY REVEALS A UNIQUE MANUS MORPHOLOGY WITH DIGGING/SCRATCHING ADAPTATIONS IN THE SAINTS AND SINNERS QUARRY (SSQ) DREPANOSAUR, NUGGET SANDSTONE (LATE TRIASSIC), NORTHEASTERN UT Journal of Vertebrate Paleontology, Program and Abstracts, 2015 p. 107

Chure, D., Britt, B., Engelmann, G., Andrus, A., Scheetz, R. DREPANOSAURS IN THE DESERT: MULTIPLE SKELETONS OF A NEW DREPANOSAURID FROM THE EOLIAN NUGGET SANDSTONE (?LATE TRIASSIC - EARLY JURASSIC), SAINTS AND SINNERS QUARRY, UTAH: MORPHOLOGY, RELATIONSHIPS, AND BIOSTRATIGRAPHIC IMPLICATIONS Journal of Vertebrate Paleontology, Program and Abstracts, 2013 p. 106

Chambers, Mariah, Hales Kimberly, Brooks B. Britt, Daniel J. Chure, George F. Engelmann, and Rod Scheetz. "Preliminary taphonomic analysis of a Ceolophysoid theropod dinosaur bonebed in the Early Jurassic Nugget Sandstone of Utah." In Geological Society of America Abstracts with Programs, vol. 42, no. 4, p. 16. 2011.

Engelmann, G., Britt, B., Chure, D., Andrus, A., Scheetz, R. MICROVERTEBRATES FROM THE SAINTS AND SINNERS QUARRY (NUGGET SANDSTONE: ?LATE TRIASSIC–EARLY JURASSIC): A REMARKABLE WINDOW ONTO THE DIVERSITY AND PALEOECOLOGY OF SMALL VERTEBRATES IN AN ANCIENT EOLIAN ENVIRONMENT  Journal of Vertebrate Paleontology, Program and Abstracts, 2013 p. 122

Engelmann, George F., Daniel J. Chure, Brooks B. Britt, and Austin Andrus. "The biostratigraphic and paleoecological significance of a new drepanosaur from the Triassic-? Jurassic Nugget Sandstone of northeastern Utah." In 2012 GSA Annual Meeting in Charlotte. 2012.

Irmis, R. B., Chure, D. J., Wiersma, J. P. LATITUDINAL GRADIENTS IN LATE TRIASSIC NONMARINE ECOSYSTEMS: NEW INSIGHTS FROM THE UPPER CHINLE FORMATION OF

NORTHEASTERN UTAH, USA Journal of Vertebrate Paleontology, Program and Abstracts, 2015 p. 149

Sprinkel, Douglas A., Bart J. Kowallis, and Paul H. Jensen. "Correlation and age of the Nugget Sandstone and Glen Canyon Group, Utah." Utah Geological Association Publication 40 (2011): 131-149.