A major portion of my doctoral thesis was centred around untangling a taxonomic mess: how many species of caenagnathids are present in the Dinosaur Park Formation, and which specimens belong to which species?
I’m very pleased to say that my review on the caenagnathids of the Dinosaur Park Formation has now been published at VAMP: Vertebrate Anatomy Morphology Palaeontology. It’s accessible to all here: https://journals.library.ualberta.ca/vamp/index.php/VAMP/article/view/29362
Short note: if you haven’t heard of VAMP before, you should definitely consider it for any papers for which you’re struggling to find a home. They’re free, open access, they don’t filter by perceived impact, and they’re willing to handle longer-format papers. I found the reviews incredibly quick and thorough, and I would highly recommend them to anyone looking to publish open impact.
If, like most readers, you just don’t have time to pore through 27,000 words on caenagnathid taxonomy, I’ve broken down my main findings in a more digestible form below!
How many species are valid?
In short: three. The long story is that this has been distilled from all of my work and all of the specimens I’ve seen over the many years. Every time we have enough diagnostic elements, there are three morphotypes: mandibles, ilia, and metatarsals are the most common. We never have more than three morphotypes, and where there are only two morphotypes (e.g. claws), we simply don’t have very many or they’re mostly incomplete.
Now, three morphotypes have been proposed since the 1940s, so this is by no means a new result. However, these three morphotypes were first based on a set of hands, a foot, and a jaw, none of which overlapped. So there were arguments that two of these morphotypes actually represented different parts of the same animal. Others argued that they were different in size, so they must be different, which was countered by arguing that some were juveniles.
Until we have more complete skeletons that overlap and show which bones belong together, we will never be 100% certain of whether any two of these are the same animal. But, there are some ways we can start to solve this issue. The approach I take in this paper was to thin-section the bones to determine their age and maturity (e.g., baby, juvenile, subadult, adult).
The evidence from the thin sections supports the idea of three species in two ways: first, bones from each of the three morphotypes are from adult individuals. Second, adult individuals are of significantly different size. So, we can say that the differences between the morphotypes aren’t based solely on juveniles vs. adults, and we can also say that these three species were different sizes at adulthood. This is useful for sorting out which specimens belong to which species.
Which specimens are from which species?
Once the validity of the three species was sorted out, I could start categorizing the new specimens to fill in the blanks in these animals. Based on comparisons between the morphotypes and the sizes of the elements where I could tell maturity, I could fit in some specimens and improve our representation of these animals.
One of the big differences my paper proposes from other previous papers is that we can’t really tell if some of the little jaws are different because of their age or because they’re from a different species. So I took the conservative approach and limited that species to only specimens we can be certain are from adults. This species is the problematic “Ornithomimus/Elmisaurus/Leptorhynchos” elegans that some specialist readers will be familiar with. It has always been referred to another genus, never its own, but because I restrict the specimens included in it now, it doesn’t fit with any of those other genera. So, I finally gave it it’s very own genus: Citipes (from the Latin for ‘fleet-footed’). Citipes elegans will now always have a home in its own genus, unless future work shows that it’s better off in one of the existing genera.
Caenagnathid evolution and ecology
I use my new taxonomy to update the phylogeny of caenagnathids and learn a bit more about their evolution. This phylogeny has some new results: strong support for Nomingia as a caenagnathid, not an oviraptorid, and Chirostenotes as a somewhat early-diverging caenagnathid. Like most other studies, the ‘elmisaurs’ don’t make their own clade.
The differences in body size in the Dinosaur Park caenagnathids got me thinking about body size variation in oviraptorosaurs, and analyzing this in an evolutionary context, I find that caenagnathids expanded their body size ranges earlier and to a much greater degree than oviraptorids. This is probably the result of their dispersal to North America at about the same time, and it’s likely they were able to expand into new niches quite successfully.
We’re not quite sure yet what niche caenagnathids filled (probably wading/running omnivores), but it seems like they could coexist in the Dinosaur Park by avoiding competition. They probably ate different things and moved in different ways, suggesting they may have lived in different sub-habitats or, at the very least, weren’t competing with each other for resources. They’re among the rarest parts of the Dinosaur Park fauna, but they’re found all over Dinosaur Provincial Park, and they lived together pretty much throughout the 1.7 million or so years that the Dinosaur Park Formation was deposited.
For me, probably a bit of a break from caenagnathids, at least after I finish my active projects. I’m focusing right now on the physiology of mammals after the extinction of the dinosaurs, and there will be some really interesting results on that sometime soon. As for caenagnathids, there’s still a lot to be learned. More thin-sections will be important for checking my work and making sure that the three species don’t overlap in body size, and that there isn’t evidence for a fourth species. CT scans can probably help in this aspect too. Last but not least, there’s a lot of work to be done in understanding the diets and lifestyles of caenagnathids, and approaches like biomechanics can probably help with that.