Sep 25 2009
I have been following the story of bird evolution for some time. My fascination began, appropriately, with Archaeopteryx – a beautifully transitional species from theropod dinosaurs to modern birds living about 150 million years ago. The Berlin specimen is simply a gorgeous fossil.
The story of bird evolution also has everything a paleontology enthusiast could want – impressive fossils, exotic extinct creatures, an unfolding mystery, and a bit of legitimate scientific controversy. There was also a case of fraud thrown in to keep things interesting.
So of course the news item about the oldest fossil bird being discovered – finally overtaking Archaeopteryx for the title, caught my eye. Anchiornis huxleyi is a feathered theropod dinosaur from the Troodontidae family – the group of theropods most closely related to birds. The specimen was found in a formation in China now dated to between 161 and 151 million years old – making it older than Archaeopteryx. This specimen has great significance, but first let me set the stage with some background.
A Brief History of Bird Evolution
The first fossil species to link theropod dinosaurs and birds was Archaeopteryx, originally found in limestone formations in Germany (leading to the species name lithographica, as limestone from this quarry was used for making lithographs). For years this was the only link. Archaeopteryx is nicely morphologically transitional – it has teeth, a long bony tale, clawed hands and other reptilian features. Yet it also had fully formed feathers and could fly – although it lacked the more sophisticated features of flight seen in modern birds, so it was probably a poor flyer. You could not invent a more perfectly morphologically transitional species than Archaeopteryx.
I stress morphologically because this single species is probably not on the actual line that led to modern birds. There has also been a temporal paradox in that Archaeopteryx is older than many of the Troodontidae from which it allegedly evolved. This apparent paradox fueled much of the minority opposition (mainly held by one paleontologist, Alan Feduccia) that doubted the dino-bird connection.
However, the paradox really was never much of a problem. Species branch off from each other and then persist for millions of years. The spotty fossil record makes it likely that in many cases we will simply lack examples of older common ancestors, while having older specimens with later derived characteristic and younger specimens retaining “primitive” characteristics. For example, if a later paleontologist only had hominid fossils from 5 million years ago and ape fossil from today, they might find it paradoxical that humans evolved from ape-like ancestors.
To support the notion that the apparent temporal paradox is an artifact of a spotty fossil record is the fact that the record of theropod dinosaurs from the Jurassic – the time period before Archaeopteryx, is very sparse. So we lacked access to the very time period that would settle the question. But, the dino to bird hypothesis predicted that as we discovered more fossils we would find older Troodontidae, and that is in fact what happened, even before the Anchiornis huxleyi was discovered.
In recent years several Troodontid specimens from the Tiaojishan Formation of China have been discovered that are from the Jurassic era and are older than Archaeopteryx – so as predicted, the ancestral group to feathered dinosaurs are older than feathered dinosaurs – paradox resolved.
In the last 15 or so year China has also offered up a number of new feathered dinosaur finds. It now seems that feathers were a common feature of theropod dinosaurs of the time, especially the Troodontidae but also dromaeosaurids, another family of theropods. And also there was a robust adaptive radiation of feathered dinosaurs at that time, and surviving past the evolution of birds from one line of this bushy radiation. We now have specimens ranging the morphological range from slightly feathered dinosaurs to protobirds to Archaeopteryx to more bird-like specimens but still not fully modern birds. We have also documented more of the evolution of feathers themselves, and demonstrated a wide range of feathery adaptations in this group.
But still Archaeopteryx was more developed along the avian line than many of the later feathered dinosaurs, and we had not yet found a more primitive and older feathered dinosaur specimen – so the apparent temporal paradox continued.
Enter Anchiornis huxleyi
This brings us to the current specimen – Anchiornis huxleyi – which dates from 161-151 million years ago, is in the line that likely led to birds, and is feathered but more primitive than Archaeopteryx. This shatters the remnants of the alleged temporal paradox – as predicted by the dino to bird hypothesis.
Anchiornis huxleyi also shows a very interesting feature – it has long shafted feathers on all four limbs. In other words – it had four wings. This is not the first specimen to show this feature. Microraptor and Pedopenna are two other related species that have the four wing design. It seems that this feature evolved early in the branching point of what is called Paraves – which contains the Troodontids, the dromaeosaurids, and of course birds.
This makes sense – while wings were still small and not fully developed, you could get more lift from having four wings. Once the forelimbs developed more sophisticated wings, the hind wings became superfluous.
This raises the question of whether or not Anchiornis huxleyi could fly. Right now it seems that it probably could not. While the feathers are well developed, they are symmetrical in their design. Flight feathers are asymmetrical – they have an off-center shaft. Its hind legs also are large and seem well-adapted for running. Perhaps Anchiornis huxleyi was a glider, or used the feathers to pounce on prey from on high.
Creationist Cognitive Dissonance
Of course, I cannot help but contemplate how creationists (those who bother to be even minimally informed) resolve the cognitive dissonance that must result from such evidence. Up until the late 1990s, creationists like Duane Gish simply dismissed Archaeopteryx as a lone weirdo, and pointed out the apparent temporal paradox as well as the lack of more transitions to support the dino to bird hypothesis, and by extension evolution. The same was true of scientific dissenters, like Alan Feduccia (not a creationist, but he maintained that birds evolved from other reptiles, not dinosaurs).
But in 2009 all such objections have been dealt with. We now have a robust group of feathered dinosaurs showing a nice transition from early feathered theropods, with a variety of adaptations, including more and more “modern” feathers and bird-like creatures. The gaps have been shrinking, leaving the creationists like flopping fish in a dried up pond of denial. Anchiornis huxleyi is just the icing on the cake – showing that as we find more fossils the story just gets better and better.
The story of bird evolution also shows that new fossil evidence can simultaneously surprise paleontologists and confirm existing theories. This often confuses non-experts and provides fodder for a sensationalistic press and denying creationists. Specifically, as paleontologists discovered more and more feathered dinosaurs this evidence confirmed the theory that birds evolved from dinosaurs, specifically theropods, and probably Troodontidae. At the same time there were some surprising twists and turns in the story – such as the extent of feathered non-avian dinosaurs, and the evolution of four-winged theropods early on in the evolution of feathers.
Essentially, the evidence confirmed the big picture, while providing interesting and surprising details. This is a common point of confusion among creationists who often exploit surprising details as if they call into question the bigger question of evolution itself.
Anchiornis huxleyi is a nice addition to the story of bird evolution and the expanding picture of feathered dinosaurs. It also seems to put the final nail in the coffin of the (always weak) temporal paradox of bird evolution from theropods.
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