Xiaotingia

The subject of feathered dinosaurs and the evolution of birds is something that fascinates me and captures my imagination, as I'm sure it does a lot of people. Not only because it changes the way we look at the world around us, specifically birds, but also because there's a lot of cool evolutionary science involved in the study of the early evolution of birds from... well, yes that's the question, isn't it? From what exactly? We know that in any evolutionary sense that matters, birds are dinosaurs, but the questions remain. How long ago did the evolution of birds start? What did the closest bird-related dinosaurs look like? What did the first birds look like, and what did they do?



A highly publicized article appeared in Nature yesterday morning, describing the lovely creature so artistically reconstructed in the image above: Xiaotingia zhengi, an early bird-like dinosaur that might help answer these questions. The findings were very quickly picked up by The Guardian, Wired Science, Not Exactly Rocket Science, Pharyngula, Live Science and Nature News, of course, among other media outlets and blogs. Partly because the archetypical "original bird" Archaeopteryx is involved. Do head over to any of those pages first for pithy summaries of the findings.

Both major Swedish newspapers were content with copying the news telegram: "Original bird was not a bird". Also here.

The description of the Xiaotingia fossil and phylogenetic analyses comparing it to other bird-like dinosaurs and early birds seems to place it together with Archaeopteryx among a dinosaur group, the Deinonychosaura, and not at the base of bird evolution. So the Xiaotingia findings call into question where the line should be drawn between birds and dinosaurs, more so than having something specific to say about Archaeopteryx itself. As with most phylogenetic trees, many of the branches are missing in the evolutionary history of the dinosaur-bird transition. The really interesting thing to consider in the light of Xiaotingia and other bird-like dinosaur findings is how beautifully gradual the transition must have been, and how remarkably early bird-like traits appeared in dinosaur evolution, in what now appears to have been a diverse and successful group of organisms. As PZ Myers put it in his Pharyngula post:

There was a whole assortment of delicate-boned, feathered, bipedal dinosaurs that were flourishing and diversifying in that window of time, and we've now got enough data that we can distinguish details in the family tree, which is absolutely fabulous.

Xu, X., You, H., Du, K., & Han, F. (2011). An Archaeopteryx-like theropod from China and the origin of Avialae Nature, 475 (7357), 465-470 DOI: 10.1038/nature10288

"Warm-blooded" dinosaurs and "warm blooded" fish*

At 80beats (@Discover blogs) there is a post about a method of inferring the body temperature of large dinosaurs by looking at the temperature that would be needed for the enamel of Brachiosaurus and Camarasaurus teeth to form. The post references a recent study published in Science. The conclusion is that big dinosaurs were as warm as mammals, but that's not to say that they had the same temperature regulation as mammals.

Here, we used clumped isotope thermometry to determine body temperatures from the fossilized teeth of large Jurassic sauropods. Our data indicate body temperatures of 36 to 38°C, which are similar to most modern mammals. This temperature range is 4 to 7°C lower than predicted by a model that showed scaling of dinosaur body temperature with mass, which could indicate that sauropods had mechanisms to prevent excessively high body temperatures being reached due to their gigantic size.

It's always been a pet peeve of mine to note when the terms "warm-blooded" and "cold-blooded" are used indiscriminately - in popular use they are incredibly widespread - or even when the more scientific terms endothermy and ectothermy are put against each other. This doesn't actually give the right view of the diverse temperature-regulation strategies that different animals can have. At Deep Sea News, one of my favourite blogs, there is a great post about those strategies that are somewhere in between the notions of warm- and cold-bloodedness, highlighting the really interesting strategies in pelagic fish*, such as lamnid sharks, tunas, billfishes and several others.

One important pattern that emerges from these observations is that body-warming is not a taxonomic thing: it has evolved several times in several different lineages, at least twice for sharks and once each for rays, tuna/billfish and opah. Rather, body warming is an ecological thing because it occurs in many species that are not related but all share pelagic migratory habits. Doubtless a closer look at other pelagic species will show that it has evolved in quite a few other species of the open ocean too.

Considering this, it's certainly not far-fetched to assume that "warm-bloodedness" has evolved several times in land-living tetrapod vertebrates as well. To me there is little doubt that the small theropod dinosaurs that birds arose from were able to regulate their body temperature internally, but perhaps sauropods, or some other dinosaur groups, were able to do it as well to some extent.

* I use the term fish very loosely here. Lamnid sharks are as related to us humans as they are to tunas and billfishes.

Anchiornis

Who wouldn't be completely smitten with this lovely little critter!


The latest issue of Nature has published the description of what is billed as the oldest feathered dinosaur, Anchiornis huxleyi. It's adorable! Read more here and here.

It's not surprising anymore to find dinosaurs with feathers, as a matter of fact there are several small toy models of feathered dinosaurs on my desk right now looking up at me so they've started to find their way into popular culture. What makes Anchiornis so striking is that both the sediments in which it was found and its characteristics place it at the very base of the origin of birds from small feathered dinosaurs in the late Jurassic, before the oldest known bird, Archaeopteryx. It provides evidence of bird-like features in a time before there were birds. All other feathered dinosaur fossils that have been found have been dated to a later time.

There are of course many more interesting details surrounding this finding, but I'll just refer you to the far more knowledgeable articles I linked to above. I just wanted to get something down because this is exactly the kind of thing that gets my imagination going and really takes me back to when I was first "bitten" by dinosaurs when I was seven or eight years old. I can only imagine what I must have thought or felt back then if someone had told that not only had there been small, feathered, four-winged (!) dinosaurs hopping and flapping around on earth, but that all birds were actually living breathing dinosaurs; great, great, great, grand-cousins of Velociraptor or T. rex. I still can't walk past the thrushes and blackbirds that skittishly and vigilantly peck for earth worms out on the yard without smiling to myself imagining a much, much older, but maybe not too dissimilar scenario in the Mesozoic.

Hu, D., Hou, L., Zhang, L., & Xu, X. (2009). A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus Nature, 461 (7264), 640-643 DOI: 10.1038/nature08322

Swedish blog tags: Dinosaurier, Paleontologi, Vetenskap
Technorati tags: Dinosaurs, Paleontology, Anchiornis, Science