Troodon Again!

So last time I talked about Troodon and how it was Alaska's most common theropod dinosaur as well as one of the largest. Up here, Troodon was around twice the size of a southern Alberta Troodon. A new paper in Paleogeography, Palaeoclimatology, Palaeoecology (heck of a business card) by Fanti & Miyashita add some surprisingly twists to that story. The paper describes a high latitude vertebrate assemblage from a west-central Alberta. A host of dinosaurs are known from scrappy remains, but include such colorful animals as Saurornitholestes, Troodon, Pachyrhinosaurus, Hypacrosaurus, an ankylosaur, a hypsilophodont, and a tyrannosaur. Evidence of duckbill nesting grounds are included here, with lots of juvenile material and, perhaps not surprisingly, lots of Troodon (more on this in a minute).

What's more important is that lots of squamates and a turtle were found at the site, so the temperatures must not have dipped to the lows of Alaska's Prince Creek formation, where ectotherms are totally unknown. However, it's interesting that most of the dinosaurs in this new Wapiti Formation are also in Alaska (Hypacrosaurus is not). Even more interestingly, the two mammals tentatively identified at Wapiti are also in southern Alberta and Alaska (Didelphodon and Cimolodon). To me, this implies a fairly uniform group of fauna across the northwestern coast of North America during the Late Cretaceous.

But I'm here to discuss Troodon. Fiorillo suggested (in 2008) that Troodon may have grown larger the farther north is occurred, possibly because of its dominance in the carnivore guild thanks to its predisposition to surviving in a colder, darker climate. In fact, the Troodon specimens from Wapiti are comparable to those of southern Alberta and Montana, which means that Troodon did not get larger as a consequence of Bergman's Rule (animals at high latitudes get larger than their more southerly counterparts) but instead thanks to its dominance in the carnivore guild, as Fiorillo suspected. What kept Troodon going during those cold winter months? As it turns out, the presence of hadrosaur nesting sites in both Wapiti and Prince Creek may tell us something about what Troodon did in its spare time.

Ryan, et al. (1998) suggested that the association of baby hadrosaurs and Troodon in the Horseshoe Canyon Formation (where other dinosaurs are uncommon) could mean that Troodon actively targeted baby and juvenile duckbills. Given that Hypacrosaurus apparantly nested at Wapiti and Edmontosaurus nested in Prince Creek, and that both sites are covered with shed Troodon teeth lends credence to this idea. And, in fact, it might be telling that Troodon becomes a larger and larger faunal component the farther north you go while other theropods become more scarce.

The description of the Wapiti Formation also has broad implications for paleogeographical and paleoenvironmental studies. It might also be time to revisit the question of whether any of Alaska's dinosaurs migrated south--the Wapiti Formation isn't that far away, and it was temperate enough to support a range of ecothermic vertebrate fauna!

Also, give Lukaz Panzarin a hand on his absolutely wonderful illustration for the paper. I especially like the trio of pachyrhinosaurs on the left!