Falcarius, Clarified

I'm not even going to try to make excuses. This paper has been sitting on my desktop since January because I forgot about it. I found it this morning when I executed a sorely needed declutterization operation. So, six months after it was published by the Linnaean Society, I'm going to write about paleontologist Lindsay Zanno's detailed description of the therizinosaur Falcarius. Like Jell-O desserts, there's always room for therizinosaurs.


Falcarius, by Michael Skrepnick. From the University of Utah via NatGeo.

I've written about the therizinosaurs before, because they're some of my favorite dinosaurs: fiercely clawed feathered theropods who've gone vegetarian, some of whom grew to huge sizes. They're primarily known from Asia; North America joined the club in the early 2000's with the discovery of late Cretaceous Nothronychus in New Mexico. Soon afterwards, Falcarius was discovered in Utah, and was found to be contemporaneous with the then-earliest known therizinosaur, China's early Cretaceous Beipiaosaurus. As Zanno notes in her analysis, Falcarius differs from Beipiaosaurus in lacking many of its more derived features - those that would be further refined and emphasized by their ancestors. It's an evolutionary puzzle: why is Falcarius so primitive compared to its Chinese contemporary? Zanno puts forward three hypotheses that will require new discoveries or research to clear up:

1. Better dating of the sediments in which the two therizinosaurs were found could reveal that Falcarius is actually a bit older.
   
2. Environmental factors in Asia may have favored therizinosaurs there to evolve at a faster rate than their North American relatives.
3. Falcarius shared North America with other therizinosaurs more similar to Beipiaosaurus, which haven't been found yet.

One of Falcarius' distinguishing features is its teeth. Like primitive relatives of the oviraptorosaurs, which also evolved toothless beaks, Falcarius had relatively large teeth in the front of its jaws. It's likely due to the therizinosaurs' shift from carnivory to omnivory and then herbivory (that last step isn't necessarily where the oviraptorosaurs settled). What's interesting is that a herbivorous diet required the suite of anatomical adjustments that make the later therizinosaurs so unique - large guts to breakdown nutrient-stingy plant material, wider pelvises, stouter legs. Falcarius shows some initial steps toward those features, probably necessary to compete in North American ecosystems with the dromaeosaurs and tyrannosaurs who dominated predatory niches. As Zanno says in her conclusion, we're going to need some serious functional studies of the skeletons of derived therizinosaurs to really determine how they lived and moved, and this in-depth look at Falcarius provides a "baseline" for that work.

Of course, maybe an apparently advanced therizinosaur from the Jurassic will be discovered in Australia and turn the world upside down. That could happen, too.