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.
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).