Monday, August 19, 2013

Follow-Up: Judith River Formation = Oldman Formation

In a previous post, I hung my tentative re-identification of the holotype teeth of Deinodon horridus on a rough correlation between the Judith River and Oldman formations, the latter of which is more precisely dated and, more importantly, contains Daspletosaurus torosus, which is a candidate for the owner of Deinodon teeth.

While researching a different topic, I stumbled across a more definitive published correlation of these two formations I wasn't previously aware of. In their 2001 paper on the stratigraphy of the Two Medicine Formation, Horner et al. discuss the correlation of parts of that formation with the Judith River. Horner et al. note that the Judith River can be separated into two basic units divided by a disconformity, corresponding with a marine transgression (when the terrestrial ecosystem was swamped by the rising of the Western Interior Seaway, the sediments deposited by which appear to have been lost in this instance).

Helpfully, Horner et al. note that it is from the lower unit that Hay collected numerous dinosaur teeth which were later described by Leidy as the infamous tooth taxa such as Deinodon, AublysodonTrachodon, and Troodon. More helpful still, the paper provides a handy chart showing the arrangement of the strata and including points at which radiometric dates have been taken. The base of the lower Deinodon-bearing unit is dated at about 78 million years old. The next available date is from just above the disconformity (i.e. after the seaway had retreated again) and shows an age of 75.4 million years ago. That's narrowing it down, but there's no date from within the formation from just below the disconformity, which would give us an upper boundary for the Deinodon strata.

But, there's hope. Horner et al. note that Rogers (1998) suggested the disconformity itself probably correlates to around the Willow Creek Anticline in the middle Two Medicine Formation (which contains the famous Egg Mountain Maiasaura nesting site). This segment of the TMF has been dated to 76.7 Ma ago, which may give us a rough upper boundary for the age of Hay's fossil tooth collection.

So, based on this paper at least, it looks like Deinodon and friends were collected from rocks aged somewhere between 78 and 76.7 million years old. Which is about the same age range as the Oldman Formation to the north. So, Deinodon horridus and Daspletosaurus torosus did indeed live at about the same time and in the same region (there were no checkpoints at the US-Canadian border back then!), making it more likely that they represent the same species, and the possibility that Deinodon actually represents Gorgosaurus less likely.

Looks like I'm going to have to create a new tag for Arcane Biostratigraphy and Geology Stuff...

Oh, and somebody in the comments last time asked me to get into Trachodon. This is definitely a subject for a longer blog post, though I'm a bit less excited about it because I'm more pessimistic that it's identity is knowable. But maybe this new info can help us get started. I already mentioned that the Trachodon teeth appear to come from the same strata as Maiasaura (and it's well known that Trachodon's contemporary Troodon formosus is reported from Egg Mountain as well, though T. formosus is also reported from pretty much everywhere and everywhen else...). Could Maiasaura be Trachodon? Perhaps! But it could also be Brachylophosaurus, or maybe even Gryposaurus. And... there's been a rumor going around for a while now that Trachodon teeth are referable to Lambeosaurinae. Which lambeosauines are known from this time and place that could fit the bill? Both Parasaurolophus and Corythosaurus have been reported from the uppermost Oldman, though these may be too young. Hypacrosaurus sp. seems to have been contemporary with Maiasaura, so that could be it...

Yeah, you can see why I'm pessimistic. Tyrannosaurids are rare, and there tend to be only one or two species of tyrannosaurids present in any given ecosystem. Hadrosaurids are... the opposite.

References

Horner, J. R., Schmitt, J. G., Jackson, F., & Hanna, R. (2001). Bones and rocks of the Upper Cretaceous Two Medicine-Judith River clastic wedge complex, Montana. In Field trip guidebook, Society of Vertebrate Paleontology 61st Annual Meeting: Mesozoic and Cenozoic Paleontology in the Western Plains and Rocky Mountains. Museum of the Rockies Occasional Paper (Vol. 3, pp. 3-14).

11 comments:

  1. Keep in mind that there has to be an albertosaurine SOMEWHERE in the world at the time the lower Judith River is deposited. So we can't dismiss the possibility that Deinodon is that animal.

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    1. Agreed! Though I'd still argue that it's more parsimonious to assume it belongs to the coeval local species we happen to also have skeletal remains for in the absence of anything but a hypothetical alternative, pending further evidence (as always).

      I haven't really looked into it, but do we have good evidence for albertosaurines in the southwestern US, i.e. Bistaheversor and Teratophoneus country? I'm just now realizing our info about the split between albertosaurines and tyrannosaurines (i.e. were, when, etc.) is basically nil?

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  2. At present, no definite presence of albertosaurines in the American SW (I have a little note about this in the Tyrannosauroidea chapter of Dinosauria II; basically, albertosaurines have the same distribution of the small-browhorned centrosaurines (or, as we called them back in 2004, centrosaurines).

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    1. Hi Tom,

      There's no way that albertosaurines could not have been present in the American SW because Diabloceratops and Nasutoceratops represent the southernmost occurrences of centrosaurines, while unnamed representatives of the two subfamilies of Late Cretaceous ankylosaurs and hadrosaurids are known from the Aguja Formation in Texas and the Kirtland Formation of New Mexico. Therefore, if we haven't yet described any albertosaurines or centrosaurines from New Mexico, it's merely a sampling artifact (not to forget that the dinos of the Mesaverde Group have only recently come into light and remain to be fully described, including a new horned dinosaur from the Almond Formation).

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  3. Hi Matt,

    Sternberg (1936) was actually the first author to consider Trachodon a possible lambeosaurine, based on comparisons of ANSP 9260 with those of North American lambeosaurines. If Trachodon is lambeosaurine, then we might consider the possibility that an undiscovered lambeosaurine distinct from Corythosaurus and Parasaurolophus might be the source of the Trachodon teeth because Hypacrosaurus stebingeri is found in the upper part of the Two Medicine and no lambeosaurine species has been named from the lower member yet.

    Sternberg, C.M. (1936). The systematic position of Trachodon. Journal of Paleontology 10(7):652-655.

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    1. Thanks David, I've never been able to track down any actual sources to back up the Trachodon = lambeosaurine thing before! Previously I'd just seen it mentioned online, occasionally attributed to Horner (I think).

      One thing I didn't mention in the article is that if Trachodon is indeed a lambeosaurine, Trachodontinae has priority over Lambeosaurinae under the ICZN.

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    2. The paleontological community would be extremely averse to see Lambeosaurinae abandoned in favor of Trachodontinae even if Trachodon is a lambeosaurine because Lambeosaurinae has gained universal currency in the annals of nomenclature, meaning that Trachodontinae would be a nomen oblitum.

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    3. "Universal currency" seems to be pretty ephemeral. Trachodontidae had gained universal currency by the time it was abandoned in favor of hadrosauridae in the mid-20th century. I'm not sure why everybody thinks our current set of universal names are so sacred when those of the last generation were unceremoniously dumped in a matter of a few decades. I'm sure the nomenclature will turn over once again in 50 years or so.

      Anyway, it would take an ICZN petition to sink Trachodontinae. Contrary to popular belief, "nomen oblitum" does not mean "name abandoned on a whim for an arbitrary set of reasons." See http://dinogoss.blogspot.com/2010/09/what-is-nomen-oblitum-not-what-you.html

      Basically, according to the current edition of the ICZN, if a name was used in the literature as valid any time after 1899, it can never be a nomen oblitum, no matter what.

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    4. To be clear, I know there's no compulsory law saying anybody has to follow the ICZN. But it annoys me that many professional paleontologists have such a flippant attitude towards it, and then in the next breath correct a layperson for using the name Brontosaurus, which is a popular name sunk on an ICZN technicality.

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