A new paper out in Paleobiology by Verracchio et al. describes the porosity of Troodon formosus eggs and uses the data as evidence to support the hypothesis that troodontids brooded their eggs, like modern birds and other known maniraptorans, rather than burying them, like crocodilians and some other modern birds. In and of itself, this conclusion is interesting in that it pretty much solidifies nest brooding (as opposed to burial) as the ancestral trait for modern birds, and for maniraptorans (or at least chuniaoans*) in general.
|Study coauthor Darla Zelenitsky with Troodon formosus nest. Photo by Jay Im, University of Calgary.|
(Of course, this is not to imply that all chuniaoans must have brooded their eggs. It's entirely possible that reversals to burial nesting occurred, as with modern megapodes, and this seems especially likely for very large species like some dromaeosaurines. But the odds that any given chuniaoan would not be a brooder are low.)
|Arctic troodontids, anatomy based on Troodon formosus. |
Matt Martyniuk, all rights reserved.
Back in the 1990s, the small group of scientists who contend that birds are not dinosaurs (BANDits) spent most of their time arguing that deinonychosaurs like Troodon formosus were in no way, shape, or form related to birds, and that any similarities were due to convergent evolution.
And then we found Sinornithosaurus.
Suddenly, the BAND was faced with a dilemma. Here was an irrefutable deinonychosaur covered in irrefutable feathers, including irrefutable vaned wing and tail feathers. There's no way this this was not related to birds. Rather than acknowledge their error (to my knowledge, no BANDit has ever tried to explain why they were so wrong on this based on the skeletal evidence), the BAND engaged in the classic logical fallacy of moving the goalposts. Rather than admit defeat on this particular point, they said essentially that yes, deinonychosaurs were clearly related to closely related to birds, but were not themselves dinosaurs. They went from BANDits to MANIACs - Maniraptorans Are Not In Actuality Coelurosaurs!
So, in a way, continuing to use evidence from maniraptorans as evidence for the dinosaur/bird link is completely in vain. No scientist working today believes that these animals are anything but birds or bird relatives. If BADists (birds are dinosaurs hypothesis supporters) ignore the shifted goalposts, arguments like this are being made to nobody, and we may as well stop making them unless a serious counter-hypothesis is presented. Except that would be ignoring the fact that vocal opponents still exist, just not in the same form they did in the 1990s, when this sort of evidence would have been compelling.
So what to do? Continue trumpeting the ever-mounting evidence that deinonychosaurs are close evolutionary relatives of birds despite the fact that literally everybody is now on board with that idea? Or do we play the BANDits' game, addressing only actual holdouts to the hypothesis, but kicking for their new goalposts rather than their old ones?
Let's say we do go for the "new" goal as defined by BAND. What would constitute support for the dino/bird link today? A nest-brooding compsognathid or ornithomimid might do (and would be exponentially more exciting than a broodong troodontid, since it would push back the origins of brooding itself). But it's hard to tell what they would accept as evidence for BAD, since the BAND position seems to consist of little more than "BAD is wrong no matter what." I would assume that, according to BAND, it's now up to BAD to demonstrate that maniraptorans are related to compsognathids, coelurids, and other coelurosaurs. The caveat is that BAND has made it clear they are not convinced by cladistic, integumentary (other than vaned feathers), or osteological evidence. So maybe evidence based on eggs or brooding would do the trick after all, since that's really all there is left.
And if we found avian-style brooding in, say, a carnosaur or coelophysoid? Surely at least some in the BAND will simply move the goalposts again, declare all theropods to be birds, and dare BAD to demonstrate that they are related to sauropods and ornithischians.
So... should we keep playing this game of shifting hypotheses and moving goalposts? Or should we simply ignore these blatant uses of logical fallacy and acknowledge that no reasonable counter-hypothesis currently exists? It may be best to treat challenges to BAD like challenges to evolution itself--as not worthy of attention, or of headlines.
At this point, isn't the title "Egg Study Supports Evolutionary Link Between Birds and Troodontids" just as redundant as "Egg Study Supports Evolution"?
*Chuniaoae is the clade comprising oviraptorids and birds, usually called Aviremigia nowadays. However, Aviremigia is defined by the apomorphy of vaned feathers on the arms... and on the tail, for some reason. We now know that vaned arm feathers occur further down the tree in ornithomimosaurs. We don't yet know if they also had vaned tail feathers, so we can't yet sink Aviremigia as a synonym of Maniraptoriformes, and it technically remains a senior synonym of Chuniaoae. But since it is now a much shakier application of this particular apo-clade, I'm going to hedge my bets and start referring to Chuniaoae more often for the ovi+bird clade.
- Varricchio, D.J., Jackson, F.D., Jackson, R.A., and Zelenitsky, D.K. (2013). "Porosity and water vapor conductance of two Troodon formosus eggs: an assessment of incubation strategy in a maniraptoran dinosaur." Paleobiology, 39(2): 278-296.