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, Aublysodon, Trachodon, 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.
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).
Monday, August 19, 2013
Tuesday, August 13, 2013
|T. rex illustration by Matt Martyniuk, licensed.|