Now that we’ve gotten a reasonable phylogeny hammered out, it’s time to start putting it to use! Just for fun, I used the (open source and free to download) program Mesquite to plot the femur:tibia ratio (as John Dziak had talked about not so long ago) as it changed from basal ornithischians up through Thyreophora, the clade including ankylosaurs and stegosaurs. The cladogram shown here only illustrates taxa for which we know the ratio, and species with multiple individuals had their data averaged.
Femur:Tibia Ratio, With a Focus on Thyreophora. Larger values equal relatively long femora.
Note that everything up through and including Scutellosaurus is presumed to be bipedal; after that, they’re quadrupedal. There’s a rather major change! Rather than the tibia being longer than or approximately equal in length to the femur, the femur ends up much longer than the tibia (and Stegosaurus is just insane in this respect)! Now the interpretation of this isn’t as easy as you might think. Is it due to being quadrupedal? Or is it due to being a big animal? The bipedal guys are all small, and the quadrupedal guys are all relatively large, so it’s tough to separate these two factors. Either way, we can say something interesting about locomotor evolution! And, note that the branches between nodes are reconstructed, with a reasonably wide error bar. So, please don’t consider them absolute truth.
And while I’m thinking of it. . .why didn’t theropods ever go quadrupedal? Some of them are as big as the largest quadrupedal ornithischians! Quadrupedal vs. bipedal locomotion can’t strictly be a size thing, then. Is there something about being a carnivorous dinosaur that discourages quadrupedalism? Perhaps the use of the forearms for prey acquisition? This is something we’ll want to touch on in the paper.
P.S.: The latest version of the combined spreadsheet is available here (Excel spreadsheet).
The Open Dinosaur Project 
That is soooo cool! What a pretty break in the values between the hypothesized bipedal and quadrupedal taxa. This just begs for hypothesis testing! You can look at correlation between body size and tibia/femur ratio change, etc. More importantly, you can really easily test the hypothesis that a shift in the ratio correlates with inferred quadrupedality.
This ratio is definitely complex. Its weird – in bipedal basal dinosaurs up through theropods, you get shifts in the ratio back and forth across 1, but these taxa are all bipedal. One other thing to be wary of: almost without exception, the tibia scales with negative allometry relative to the femur, both within and among dino taxa. So, getting back to one of your points, it could partially have to do with body size.
Neat! Could you do something similar for the ceratopsia? It looks like you should get similar results.
Andy, there seems to be a Sauropelta (AMNH 3032) that’s significantly different in F:T than the other two examples.
I have no idea if this is a novel or obvious observation (which shows my ignorance), but it looks like Ceratopsia is splitting into two distinct lines of a high (~1.5) F:T ratio and a low (~1.0) F:T ratio. Plot is here:
https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxkYXZpZGRyZWlzaWdtZXllcnxneDo1Yjc3MTdjN2RlY2NmYjQz
with age increasing right to left. (I took the age estimates in my from Wikipedia or other online sources — forgive me.)
I had noticed the Sauropelta, but left it in just for this quick-and-dirty analysis, pending additional data. Re: the Ceratopsia plot, I think you are seeing a genuine pattern; the high ratio ones are the big, quadrupedel ceratopsids, and the low ratio ones are the (presumed bipedal or facultatively quadrupedal) small “leptoceratopsids.”
What I think interesting is that even functionally herbivorous theropods maintained their bipedal stance. One of the ideas I’d heard before dealt with the ability to process plant matter, but given the huge gut cavity of ‘Therizinosaurus’ it doesn’t seem that a quadrapedal gate is entirely necessary.
Though totally speculative, is it possible that the transition had more to do with a defense mechanism than something specifically to do with herbivory? Given the numerous injuries found on many predatory dinosaurs, it would seem that, though dangerous, having to keep both legs has a trade off in the end for a carnivore (as you said, in terms of grasping ability for prey acquisition). But if you’re running away from a two ton predator, having to deal with a much greater chance of tripping and falling wouldn’t seem too ideal.
But I guess that takes me back to the therizinosaurs/Limusaurus: were the skeletal changes needed to maintain a quadrapedal gate too numerous, and the herbivorous theropod lineages too few or short lived? Or was it something to do with their metabolism?
Great food for thought, thanks.
Then, you also have the massive (yet semi-habitually bipedal and herbivorous) hadrosaurs… pretty fascinating subject.
Wow, it’s exceedingly cool just getting a hint of the fruits of our labor! (And doubly exciting to consider the possibilities inherent with PLoS ONE, where pretty data may still look pretty even after publication!)
David: your results are very cool. Note that all the younger ceratopsians with a lower ratio are small-bodied forms that are outside of Ceratopsidae, whereas the higher ratios are within Ceratopsidae. Things like Leptoceratops and Montanaceratops are very closely related to taxa like Protoceratops and Cerasinops. Thus, it makes sense that they would have a similar ratio. Given that Leptoceratops and Montanaceratops are thought to be quadrupedal, just like ceratopsids, I suspect the major signal that this ratio is picking up is body size. Apparently ceratopsids are adjusting their hindlimb ratio as they get bigger through time.
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Wow! this is insanely cool! This is actually something I have wanted to do to pterosaur measurement data but never knew a software that could do stuff like this. Could you perhaps do a short tutorial on how to do this stuff in Mesquite?
Hi Ville–
I would love to post a tutorial here. . .unfortunately, it’s dependent on a lot of other factors (primarily time). If another project member wants to volunteer a post on this, that would be fantastic! In the meanwhile, check out the Mesquite Manual for help, especially the sections on character evolution and continuous characters. To briefly outline the process, I created the list of taxa and then the cladogram in Mesquite, created a new character matrix, for a continuous character, and then input my data there. Then, in the tree window I used Analysis>Trace Character History, using parsimony analysis to create the reconstructions.
Andy,
After some time messing with mesquite I did indeed get the hang of it. The whole “trace charachter history” thing is the easy bit, I just had hard time figuring how to get the whole tree thing sorted out and in the end I just built kindof suppertree from most recent analyses, ran it trough paup, saved the trees and then opened the tree in mesquite and that seemed to do the job. I’m sure there’s easier way but that is how I solved it. 🙂
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I wonder about those Stegosaurus(es) whose tibias are so short. Do you suppose they are specialized for extra-low grazing or something?
Or is it not so much that the tibias are too short, but that the femurs are very long? Why are their backs arched so high anyway? Just wondering. I remember when I was a kid, reading in Dr. Bakker’s book about how — if I remember correctly — he conjectured that they could sort of swivel up on their hind legs. But that would be unusual because in the figure above they seem to be the least bipedal-looking of all.
John Dziak, have you heard of the famous SVP abstract “The case for Stegosaurus as an agile, cursorial biped”?
*sob* and my talks/posters were rejected three years running! *sob*
Oh my goodness! I had to Google that. That must have been some poster!