Letting our Science show: Torosaurus and the Temporally Transforming Triceratops

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At long last, we're finally going to talk about Torosaurus! I'll start off by saying that one of the things I've really been looking forward to doing with this blog is sharing just how much research goes into making Saurian. We go through all the same development steps as any other video game does, but on top of that, Tom and I in particular spend a LOT of time keeping up with the latest dinosaur news and discoveries because of how important accurate and up to date information is to Saurian's premise. So with that in mind, its time to let a little bit of our science show.

One of the most common questions we get from fans and followers is "Will (insert dinosaur name here) be in the game?" Normally this is a question we don't answer because you can find out what's in game by checking out the website, the animal obviously isn't from Hell Creek or other nearby equivalent formation, or we just enjoy the thought of surprising you all later on. I'm going to make an exception here for Torosaurus, both because it's one of the animals asked about most frequently, and because whether or not Torosaurus is actually a unique dinosaur is a matter of hot debate in paleontology right now.

(If you're not familiar with the "Toroceratops" debate, good god (wo)man! Climb out from under your rock and check out Torosaurus' Wikipedia page and if you want more, check out this, this, Jack Horner's TED Talk and these two papers for some "light" reading.)

In a nutshell, the debate boils down to this question: Are Torosaurus and Triceratops either each other's closest relative, or are they different growth stages of the same animal? We can't answer that question because 1.) We're here to make a video game and 2.) Data is still being collected, analyzed, published and debated. We CAN say though that Torosaurus will not be appearing in Saurian because of this paper:

Evolutionary trends in Triceratops from the Hell Creek Formation, Montana and it's supporting information

(There's a LOT of cool science in this paper regarding the relationship between Triceratops and Torosaurus, we highly recommend you check it out!)

One of the recent discoveries that has come out of the Museum of the Rockies 10-year Hell Creek Project is that the two currently recognized species of Triceratops (T. horridus and T. prorsus) are not found evenly throughout the formation. T. horridus is found in the lower half of the formation, while T. prorsus is found in the upper third of the formation, just before the layer marking the end of the Cretaceous. We know this because unlike most other fossils of Triceratops collected in the past 100+ years, the Museum of the Rockies staff cataloged the exact location of each of the Triceratops specimens they collected as they related to the top and bottom of the formation, as well as significant sandstone layers within. Thanks to this precision, we've made several other discoveries about Triceratops. Tracking skulls from top to bottom, we can see that length of nasal horn, snout shape, brow horn length and other features changed over the course of Hell Creek time. Triceratops skulls from the middle of the formation have features found in both T. horridus and T. prorsus. This change over time in an entire population, rather than showing one species gradually displace another is called anagenesis, and it's pretty freaking cool that we can even know this about any Mesozoic dinosaur because it takes a large sample size to detect.

Comparison of Triceratops skulls from the Hell Creek and other latest Cretaceous formations of the western interior. Using the skulls discovered in the different layers of Hell Creek, it is possible to estimate the stratigraphic position of skulls in other formations as well based on their similarity. As an example, Triceratops from the Frenchman Formation are practically identical to those from the upper third (U3) of the Hell Creek formation, indicating they are probably the same geologic age.

A growth series of skulls from Triceratops prorsus found in the upper third of the Hell Creek Formation. Even without Torosaurus-morph skulls, Triceratops goes through significant changes to its skull, snout and frill during the course of an individual's lifetime. MOR 004 was the basis of Saurian's Triceratops model.

'But what about Torosaurus?' you say? Here's where it gets interesting.

Torosaurus and Triceratops are largely differentiated from each other by their frill: Torosaurus has large holes called 'fenestrae' in it's frill while Triceratops has a solid frill. (To date, Triceratops is unique among all ceratopsians for its lack of fenestrae) Without finding enough of the frill to determine if there are fenestrae however, there are no major described differences in the skeletons of Torosaurus and Triceratops, so most fossils are by default assigned to Triceratops or even indeterminate ceratopsians. As a result, fossils that can be confidently assign to Torosaurus are rare. Additionally, most specimens of Torosaurus lack provenance (data relating as to where they were found in the formation, what type of rock they were found in, and in some cases exact location) so before the Hell Creek Project, it was possible to assume Torosaurus was found throughout the Hell Creek Formation. This study demonstrates that assumption is not accurate.

Stratigraphic placement of Hell Creek Formation (HCF) Triceratops reveals trends in cranial morphology including elongation of the epinasal and change in morphology of the rostrum. (A) HCF stratigraphic units. (B) Magnetostratigraphic correlation. (C) Stratigraphic positions of Triceratops specimens within a generalized section. Specimens plotted by position, not by facies [Type of sediment fossils were found in] Relative position of specimens from different areas are approximate at the meter scale. Scale in meters. (D) MOR 2702 nasal horn from U3. (E) UCMP 113697 nasal horn from upper part of M3; black arrow indicates epinasal-nasal protuberance. (F) MOR 2982 nasal horn from lower part of M3 (image mirrored). (G) MOR 1120 nasal horn from L3 (H) MOR 004 young adult Triceratops skull from U3 (cast; image mirrored).  Postorbital horn cores reconstructed to approximate average length of young adult specimens from this unit. (I) UCMP 113697 late-stage subadult/young adult Triceratops skull from the upper part of M3 (J) MOR 1120 (cast) subadult triceratops from L3. (Scale bars = 10cm)

Plotting out the stratographic location of Torosaurus fossils found during the course of the Hell Creek Project reveals that they cluster in the bottom half of the formation. In the chart above, MOR 1122 and MOR 981 are the only confirmed specimens of Torosaurus discovered during the course of the Hell Creek Project. Other than their fenestrae, the skull features of Torosaurus are also shared with Triceratops skulls found at similar layers. Even more interesting is that skulls of large Triceratops from the upper half of the formation appear to have thin spots in their frills, while those from the lower half of the formation show a more gradual thinning of the frill across several growth stages. Taken together, these findings have been suggested to indicate that Torosaurus fossils are those of skeletally mature Triceratops, and that over the course of Hell Creek time, the development of fenestrae happened later and later in the life cycle of the animal, until by the upper third of the formation (the time represented in Saurian) the development of fenestrae was very rare or ceased to happen in the course of the animal's life at all. Even if it is the case that Torosaurus and Triceratops are different animals and not growth stages of one dinosaur, no identifiable Torosaurus remains have ever been found in the upper third of the formation. This means that regardless of how the 'Toroceratops' hypothesis settles out, the 'Torosaurus-morph' of ceratopsian was extinct before the final third of Hell Creek's timespan. As these are the layers where our game is set, we have no plans to include Torosaurus in Saurian.

Triceratops by Doc Lewko

Juvenile Triceratops by Doc Lewko

(We'd like to take a moment to thank Get Away Trike! for creating the handy skull chart in our post, you can read the original post here)