Wednesday, December 6, 2017

Halszkaraptor and what it means for Dromaeosaurids

Halszkaraptor escuilliei by Joschua Knuppe

Today saw the announcement of one of if not the biggest dinosaur discovery of 2017: the description of Halszkaraptor escuilliei, a small, semi-aquatic fishing dromaeosaurid from Mongolia. Yes, seriously, swimming raptors. Not only is this inferred to be a fish-eating animal (With ISOs to boot!) but the specimen preserves a flattened forelimb that forms a paddle-like hand, and its proportions cluster with wing-propelled diving birds and marine reptiles in morphometric analyses. This thing is a true aquatic swimmer, and basically what happens when a raptor dinosaur copies a penguin or auk.

But while I could go on and on about the anatomy and what that means for the biology of this interesting taxon, I'll refrain from that here because, in all honesty, it's gonna be talked about everywhere else by people better-equipped to talk about it than me. Instead what I'll be talking about here is the new subfamily, halszkaraptorines, and their placement among other dromaeosaurids. The newly-founded members of this subfamily include the aforementioned Halszkaraptor escuilliei, the similar but incomplete Mahakala omnogovae, and the fragmentary Hulsanpes perlei, known only from a foot and partial braincase. All of these taxa seem to be from inner Mongolia (though note that Halszkaraptor's exact location of origin is unknown), but there's fragmentary material of what might be more specimens from Canada. The incomplete nature of Mahakala and Hulsanpes make it difficult to figure out if they have all the same aquatic adaptations as Halszkaraptor, but what we do know of them is that they're similarly small-bodied and long-legged dinosaurs with short forelimbs. Mahakala might even preserve a similar paddle-like morphology of the forelimb, but it's difficult to tell without more material.

Traditional dromaeosauridae is made up of three groups: the thin-snouted and long-legged unenlagiinae of the southern continents; the mostly small-bodied, winged microraptorines; and the extremely popular macropredatory eudromeosaurines (which are split up further into velociraptorines and dromaeosaurines). Halszkaraptorines are right at the base of all other groups as the earliest-branching dromaeosaurids, and thus some parts of their anatomy might be a decent example of what the ancestral dromaeosaurid looked like. But here's the thing: as you move from them up the tree closer to eudromaeosaurs, an interesting pattern emerges.

Phylogeny from Cau et. al. 2017 with added taxa notes.
According to this phylogeny, dromaeosaurids start out as short-armed, long-legged taxa, something akin to terrestrial storks, and presumably feeding on small prey items like fish and small vertebrates before developing longer forelimbs, shorter legs, and boxier skulls. The two basalmost groups, halszkaraptorines and unenlagiines, both show adaptations for aquatic feeding, and even further up the tree Microraptor shows direct evidence of fish-eating. So three out of the four major divisions of dromaeosaurids show evidence of piscivory.

Would this suggest that all dromaeosaurids came from a piscivorous or stork-like, long-legged ancestor? I wasn't the only person to suggest this, as some of my friends came to a similar conclusion after reading through the paper. If this is the case then what does that mean for evolutionary models, particularly involving the origin of birds and flight, like the neoflightlessness hypothesis? Is a long-legged, short-armed body form ancestral to paravians? Troodontids also have this kind of bodyplan and they're even closer to birds in most recent phylogenies. If non-eudromaeosaurian dromaeosaurids are engaging in extensive fish-eating, then would the shift to eudromaeosaurs be a shift from fish-eating to macropredation, similar what's seen in some birds and marine mammals? Certainly there are a lot of interesting perspectives to think about going forward.

This was a short post but I hope it'll help everyone realize just how bizarre and fascinating this new little theropod is. I hope it and its relatives get a lot of love and attention from theropod workers, and hopefully new members of its subfamily will start popping up now that people know what to be on the lookout for.

Until next time, stay sharp!

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