Two Medicine Tyrannosaurs

[Allosaurus]

I'm hoping someone can provide a little information about tyrannosaurs of the Two Medicine. Somewhere I thought I picked up that Daspletosaurus and Gorgosaurus were the only ones found in Montana (with Albertosaurus being absent in MT). But now I'm not finding that, so I'm wondering if I just imagined it. Additionally, I keep seeing conflicting information as to when Daspletosaurus and Gorgosaurus lived (I'm seeing 83 Ma but other places 76 Ma with several other times around there). Can anyone share an article or two that describes the current understanding of when these animals lived? 

[Paleoworld-101]

"To date, A. sarcophagus remains the only unequivocally identified tyrannosaurid species from the Horseshoe Canyon Formation (Carr, 2010; Eberth et al., 2013). Daspletosaurus spp. are otherwise known only from the older Dinosaur Park, Oldman, and Two Medicine formations (Currie, 2005; Carr et al., 2017)."

 

From: Mallon, J. C., J. R. Bura, D. Schumann, and P. J. Currie. 2019. A problematic tyrannosaurid (Dinosauria: Theropoda) skeleton and its implications for tyrannosaurid diversity in the Horseshoe Canyon Formation (Upper Cretaceous) of Alberta. The Anatomical Record 303(4):673–690.

 

Also see: Tanke, D. H., and P. J. Currie. 2010. A history of Albertosaurus discoveries in Alberta, Canada. Canadian Journal of Earth Sciences 47:1197–1211.  

 

You are correct, Albertosaurus is not presently identified outside of Alberta. Daspletosaurus  first appears in Alberta within the Oldman Formation, which Fowler (2017) assigns an age between ~77 and ~79.3 Ma. The Two Medicine Formation covers a broader span of time, about ~81.5 through to ~75 Ma according to Fowler (2017). So whether the Oldman material is the max known age of Daspletosaurus  in general depends on the stratigraphic position of Daspletosaurus  within the Two Medicine Fm, which i am unsure of but you could probably track this down. A quick check of the wiki page for the Two Medicine Fm shows it's stratigraphic position as "upper", so the Oldman Daspletosaurus material may well be older. Late Campanian tyrannosaurids in western North America seem to show a good degree of provincialism, where only one is usually present in a particular area at any one time (co-existence of Gorgosaurus and Daspletosaurus being an exception). The temporal distinction between Daspletosaurus /Gorgosaurus and Albertosaurus in Alberta probably happens somewhere about 73 to 73.5 Ma (pers. comm with Dave Eberth). The exact timing of their appearance/disappearance in Montana i am less sure of as my research focuses on the Canadian material. 

 

[Troodon]

There are two Tyrannosaurids in the TMF.  Below is Tom Carr's 2017 paper on the described TMF Tyrannosaurid Daspletosaurus horneri.  The other Tyrannosaurid in the TMF is an undescribed Gorgosaurus sp.   Albertosaurus  sarcophagus temporal range was equivalent with that of the younger deposits of the Horseshoe Canyon Fm

 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5372470/

From T Carrs blog 

[Paleoworld-101]

2 hours ago, Troodon said:

There are two Tyrannosaurids in the TMF.  Below is Tom Carr's 2017 paper on the described TMF Tyrannosaurid Daspletosaurus horneri.  The other Tyrannosaurid in the TMF is an undescribed Gorgosaurus sp.   Albertosaurus  sarcophagus temporal range was equivalent with that of the younger deposits of the Horseshoe Canyon Fm

 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5372470/

From T Carrs blog 

Looks like Carr's D. horneri paper came out shortly before the revised dates of Fowler (2017), in which case the appearance of D. torosus (within the Oldman Fm) would be a bit further back than that image indicates.

[Troodon]

45 minutes ago, Paleoworld-101 said:

Looks like Carr's D. horneri paper came out shortly before the revised dates of Fowler (2017), in which case the appearance of D. torosus (within the Oldman Fm) would be a bit further back than that image indicates.

Which paper of Fowlers paperz are you referring to?

[Paleoworld-101]

9 minutes ago, Troodon said:

Which paper of Fowlers paperz are you referring to?

Fowler, D. W. 2017. Revised geochronology, correlation, and dinosaur stratigraphic ranges of the Santonian-Maastrichtian (Late Cretaceous) formations of the Western Interior of North America. PLoS ONE 12(11): e0188426.

 

The dates are best summarised visually in the supplementary Table S1 (an excel spreadsheet). This spreadsheet also nicely summarises local dinosaur stratigraphic ranges but annoyingly theropods seem to be excluded. 

[Allosaurus]

Hmmmm, so are there no known specimens of tyrannosaurs found in the lower Two Medicine (80 Ma) in Montana?

 

Also, thanks everyone for the papers. I think today is going to be a fun filled day of reading. 

[Troodon]

3 hours ago, Allosaurus said:

Hmmmm, so are there no known specimens of tyrannosaurs found in the lower Two Medicine (80 Ma) in Montana?

 

Also, thanks everyone for the papers. I think today is going to be a fun filled day of reading. 

The lower TM is a bit of an unknown but a Daspletosaurini sp. and Gorg sp are two likely candidates.

 

[Allosaurus]

1 hour ago, Troodon said:

The lower TM is a bit of an unknown but Daspletosaurini sp. and Gorg sp are two likely candidates.

 

Why is that? It is just understudied, or are there simply fewer outcrops of lower Two Med? 

[Troodon]

Not sure but thats a reasonable assumption 

[CBchiefski]

Howdy Allosaurus,

You bring up an issue near and dear to my research. The papers already posted by @Paleoworld-101 and @Troodon are great and I would add one more item to read: “An unusual multi-individual tyrannosaurid bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA)". Currie, Philip J.; Trexler, David; Koppelhus, Eva B.; Wicks, Kelly; Murphy, Nate (2005). In Carpenter Kenneth (ed.). The Carnivorous Dinosaurs, Bloomington: Indiana University Press. pp. 313–324. ISBN 978-0-253-34539-4.

 

The Gorgosaurs specimen was found and brought to the attention of Jack Horner, and it is his ID that most go with for the specimen. It was privately collected and unlikely any other paleontologist will be able to view it, so take the presence of Gorgosaurs as not technically accurate in a purely scientific sense. I am not saying Jack is wrong but rather, we should not rely on an ID that cannot be verified.

To your question of why the Two Med is so poorly studied, it is quite simply not a focus of much research. As for the Lower and Upper portions, the problem there is a lack of land access, partially resulting from a long history of disagreements between the landowners and paleontologist. Not pointing blame anywhere, as far as I am concerned, those disputes are in the past and one of my goals is to slowly patch relations. An example in the Upper Two Med is a site on land owned by the State of Montana and fully surrounded by private land. Museum of the Rockies is the only institution that can store specimens collected from state land but they are not allowed on the private land surrounding this site. The Two Medicine Dinosaur Center has permission to collect from said private land but not from the state land. The are many localities sharing a similar problem. There are no obvious solutions as most are not feasible for a host of reasons, and so the lack of specimens in research collections persist. Many exposures are on Tribal land, and after everything with Sue, there are legitimate legal concerns with excavating those. Even most of the Middle Two Med is only known from the Willow Creek Anticline and there are many unexplored outcrops. The amount of mystery is part of why the formation is my favorite.

Now, for your initial question, there are likely unknown apex predators within the Two Med, and I suspect those are tyrannosaurid species but we lack the actual finds. I am one of many working to slowly solve this issue but it will take time. Hope that helps, Eric P.

@Allosaurus