Walking With Triceratops

26 06 2007

It’s funny how certain things come into style, only to be swept away by a new trend. Clothes, music, and television all are mediums in which the bang-and-bust cycle of trends is apparent, but things can come “back into style” in paleontology as well. Not very long ago I read Robert Bakker’s The Dinosaur Heresies, a wonderful book that certainly shook things up. Not everything within its pages is valid, but Bakker certainly did a lot of heavy-lifting in order to show that dinosaurs were not glorified lizards. One of the points he makes involves the posture of one of the most beloved dinosaurs, Triceratops, but as I have learned the problem of Triceratops posture is a bit more complicated than I had originally thought.

Triceratops Sprawling
From “Mounted Skeleton of Triceratops elatus” by Henry Fairfield Osborn, American Museum Novitiates, Sept. 6, 1933

On the fourth floor of the American Museum of Natural History, the mounted skeleton of Triceratops horridus (the same mount referred to as T. elatus by Osborn, above) stands today in the same posture as it was mounted in the 1930’s; back legs directly under the hips like a good dinosaur, but with the front limbs sprawled out to the sides, almost resembling a body-builder who bulked up his arms but neglected his legs. In fact, this analogy is not very far off from what Osborn himself envisioned, writing;

The pose of the fore limb, set out widely apart from the body, is also designed to withstand attack, like the widely spreading feet of the pugilist or wrestler.

This is Osborn’s idea of the adaptive value for the stance, but it does not explain why his Triceratops is sticking out its upper arms nearly horizontal to the body. The problem in determining a proper mount for the composite skeleton (fossil material coming from 4 skeletons) was that the front limbs would not articulate in the way a mammals forelimbs do. After observing various reptiles and mammals, as well as trying different poses, it became apparent that Triceratops could not carry its forelimbs directly under the body without disarticulating its own skeleton; the sprawling pose was the only one that seemed to work. I’m sure the appearance of Triceratops as being front-heavy played into the pose as well, such a massive head requiring plenty of support from the arms, although in the position chosen a huge amount of strain would be put on the front limbs. It’s arms would be spread out away from the body in a push-up position, with the legs directly under the body; if this dinosaur walked in such a position it would be a very amusing sight.

Then I read Bakker, who argued that fossil trackways (in addition to his own research and position that dinosaurs were active, dynamic animals) showed that Triceratops did, in fact, carry its front legs underneath its body and was even capable of galloping. How fast this dinosaur may have been is something for another day (I’m not even aware of any studies that have attempted to answer that question, in fact), but Bakker seemed very confident in his idea of posture, and I had my own misgivings about the super-sprawl of the AMNH mount. The trick is, however, remebering that this is not an either or question where the super-sprawl or “good dinosaur” pose must be right to the exclusion of all others. In truth, the real answer seems to lie somewhere in between.

In the 1933 paper, Osborn mentions the analysis of W.D. Matthew, tells us that other scientists were mounting their ceratopsians in different ways;

The heavy strain of supporting the great body on these widely-spread fore limbs is very apparent but there seems to be no other way to pose the skeleton. A compromise pose, such as that of the National Museum mounted skeleton (or of Marsh’s restorations so far as they can be interpreted) serves to reduce, not to banish the anatomically impossible disjointing.

Triceratops Profile
From “Mounted Skeleton of Triceratops elatus” by Henry Fairfield Osborn, American Museum Novitiates, Sept. 6, 1933

With such problematic limbs, different mounts were bound to take different poses, and the Smithsonian dinosaur is of further interest to us because it recent has helped to answer the very questions we’re asking. In May of 2001, the Smithsonian unveiled its new mount for Triceratops (dubbed “Hatcher” after the man who discovered it), featuring limbs that were closer to being under the body than Osborn’s dinosaur, but still bend slightly outward from the body. The reanalysis that led to this new pose was undertaken by Ralph Chapman, who used computer modeling to help properly restore the dinosaur (i.e. it, too was a composite, and its back feet were actually from a hadrosaur) and figure out how it might have moved. Not everyone is behind the semi-sprawl, however, and just prior to the Smithsonian skeleton being unveiled Gregory S. Paul and Per Christiansen issued the paper “Forelimb posture in neoceratopsian dinosaurs: implications for gait and locomotion” in the journal Paleobiology, arguing that the problem with Triceratops posture often stems from problems articulating other parts of the skeleton; if you get the vertebrae and ribs wrong, you’ll probably get the arms wrong, too.

So what about trackways? If we have the ichnofossils, then we should be able to tell what the limb posture of this animal was, right? This seems to be a lot to ask; both in Bakker’s book and the Louie Psihoyos’ book Hunting Dinosaurs (the only two books with depictions of Triceratops tracks I’ve come across) the trackways are illustrated. In Bakker’s book, the tracks align just the way he says they should; the tracks suggesting the feet were under the body. Psihoyos’ book, however, depicts tracks made by the front limbs being just slightly outside those made by the rear limbs, suggesting a semi-sprawl. If that was not enough, fossil trackways are notoriously hard to match up with known fossils, tracks of the same type receiving their own name (like Grallator here in New Jersey) when we can’t figure out what dinosaur made them. This isn’t to say that we can’t identify any tracks or even any tracks made by ceratopsian dinosaurs, but given how massive the heads of ceratopsian dinosaurs could be, it’s important information to know when determining posture that trackways don’t tell us much about.

And so the arguments will likely continue; while we now know that Triceratops and its relatives like Torosaurus did not hold their front limbs out to the point where their chests would be scraping the ground while their butts were in the air, the way their skeletons are constructed seems to suggest a small amount of sprawl in the front limbs. Carrying such a massive skull would have certainly affected the front limbs, and if the Triceratops carried its limbs directly under its body it may have been a little more unstable/more likely to tip over. I don’t know if it has been done, but I for one would love to see what happens to the animal’s center of gravity when switched between the various postures, and I think that the semi-sprawl would have offered Triceratops more support without putting undue strain on its limbs.

It’s funny how old ideas come back; in such a short amount of time, Triceratops was thought to have its arms out to the side, then under its body, and now we’re at a point of compromise. While the logic of uniformitarianism may be “the present is the key to the past,” the history of the science of paleontology has taught me that the past is the key to the present, and old “fossils” can still tell us a lot about debates that continue today.


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5 responses

26 06 2007
Zach Miller

Interesting you should bring this topic up. The most recent issue of Palaeontologica Electronica has a very good paper about reconstructing the walk cycle of Chasmosaurus using a physical model of the limb bones. I highly recommend it. If correct, the paper shows that the majority of movement in the forelimbs occurred at the shoulder, and that their forelimbs had a distinct “high sprawl” reminicent of running crocodilians.

And then in Sereno’s recent description of Psittacosaurus major (in the new Acta Palaeontologica Polonica), he emphasizes that quadrapedality is directly related to head size, so Osborn’s original idea is still valid after all these years!

19 06 2015
Ben Patton

Thanks for the write up. It’s fascinating the more I research Dinosaurs the more it becomes evident how little we really know. We have a lot of fragmented bones, shards of skulls, or disarticulated toe bones that we somehow build into a skeleton. Do you have any information on how the T-Rex came to be? The more I look at the T-Rex the more I become unsure of it’s existence.

26 06 2007
laelaps

Thanks Zach! I’ll certainly check that paper out (and append my entry if necessary, too). Like I mentioned in the article, I find it funny how the old becomes new again in paleontology; I certainly have found much enjoyment in dusty old books, although professors in other disciplines wonder why I “waste my time” with so many studies that must be out of date by now.

26 06 2007
Will Baird

The Horned Dinosaurs by Peter Dodson, while not a scholarly treatise like Zach brings up, does have a nice section on this and within that the trackways of Triceratops. It is probably a bit dated now since it came out in the late 90s.

26 06 2007
laelaps

Thanks Will; I actually met Dodson a number of years ago (I was just a kid) and have gone on digs with some of his students. The book is definitely on my list, I just haven’t had the chance to pick it up yet. Thanks for the reminder!

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