Five minutes with Ornithoscelida

If you are the kind of person who reads a blog like this, you've probably already heard the news about a new analysis (Baron et al. 2017) finding that sauropodomorphs and theropods may not be the closest of evolutionary siblings, as we've long believed. Instead, theropods were paired with ornithischians, and Sauropodomorpha got to pair with the herrerasaurids in the settlement. For the new alliance of bitey (theropod) and beaky (ornithischian) dinosaurs, the authors went back into the mists of time and pulled out Ornithoscelida, a name first proposed by Thomas Henry Huxley for a roughly similar grouping of dinosaurs. There has already been a lot of discussion about the publication, and apart from some criticisms of the exclusion of certain forms and how names were redefined, the tone at this moment is open-minded.

The idea that the relationships of the three major lineages of (non-avian, or "classic") dinosaurs are not what we thought them to be is not far-fetched. The earliest theropods, sauropodomorphs, and ornithischians all had the same grandparents, so to speak. Unsurprisingly these early forms all look kind of similar, had similar lifestyles, and thus are liable to be mixed up by later observers, especially when the observation is happening 230 million years later and the subjects have been reduced to skeletons. (It's a common problem for species near the base of branching lineages to be difficult to place.) Going from (S+T)+O to S+(T+O) does require some rethinking on how and when certain features appeared. It also throws a bit of a kink into the branch-swapping basal saurischians: Eoraptor, herrerasaurids, and friends. Eoraptor had seemed reasonably comfy among the sauropodomorphs and herrerasaurids with the theropods, and here they are switching places.

There is also an interesting question concerning the origin of ornithischians. Historically, Triassic ornithischians have been a problem due to their frustrating insistence on not being there. After a few decades of redating various formations and reassessing a bunch of teeth, we're down to Pisanosaurus (which is itself attracting questions) and Eocursor. Woo-wee. Under the traditional Saurischia–Ornithischia split, there should be more Triassic ornithischians, because of the record of Triassic saurischians. Pairing ornithischians with theropods has the potential to resolve this if it should turn out that ornithischians and theropods actually branched later in the Triassic (which also results in some "theropods" becoming basal ornithoscelidans, but it's not as if current basal theropods haven't been living under taxonomic instability). The results of the current study don't support this, but you never know. (In fact, the authors find that ghost lineages may go back all the way to nearly the beginning of the Triassic, because Nyasasaurus has suddenly started acting like a massospondylid, but then again the rocks it came from may be younger than currently thought.)

At any rate, an injection of controversy is good for the field. Either other studies support the authors, in which case we learned something new, or they don't, in which case the authors got people looking at dinosaur relationships and evolution from new angles, which is also good. In fact, now that I'm thinking about it, it's kind of odd that we have as much consensus as we do. Off the top of my head, for persistent trouble spots we've got Triassic groups and species that can't seem to make up their minds about where they belong, megaraptorans as carnosaurs or coelurosaurs, the knot of undecideds where Dromaeosauridae, Troodontidae, and birds meet, and what to do with various "hypsilophodonts". This doesn't count a few other areas that are questionable because of lack of attention, like the Box of Mystery that is Titanosauria.

All that said, I *am* going to let the issue sit before, say, updating The Compact Thescelosaurus; publications find things all the time that are not supported by later analysis (hey, Phytodinosauria, the hip alternative of the '80s and '90s). Also, redefining Saurischia to hold just sauropodomorphs and herrerasaurs was a mis-step. Just let herrerasaurs into Club Sauropodomorpha, let Saurischia go into dignified retirement, and call it a day.

References:

Baron, M. G., D. B. Norman, and P. M. Barrett. 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543(7646):501–506.