Jul 28 2011
Archaeopteryx lithographic is one of my favorite fossils. The Berlin specimen is practically a work of art. It is also a classic example of a transitional species, with a compelling blend of avian and dinosaur features. As much as it’s possible for a single fossil to be so, Archaeopteryx is a smoking gun of the evolution of birds from theropod dinosaurs.
It is also a much maligned fossil. Creationists have attacked it in every way imaginable, calling it a fraud, and ironically at times saying it’s just a dinosaur, and at others saying it’s just a bird.
But perhaps the most common misconception about the fossil, and about transitional fossils in general, is that their value as evidence for an evolutionary connection is dependent on their being a literal ancestor of the descendant group (in this case, birds). In other words, Archaeopteryx’s value as a transitional fossil is dependent on it being on the direct line that led to birds. This, however, is almost certainly not the case, and is also mostly irrelevant.
A new computer analysis of feathered dinosaur species suggests that Archaeopteryx should be placed with the feathered theropods and not in the proto-avian line. This is not surprising, and does nothing to diminish the value of Archaeopteryx as a fossil. But this fact is being reported as if it somehow destroys Archaeopteryx as a transitional fossil. One headline declares: “Charles Darwin may have just lost Evolution Exhibit A, otherwise known as Archaeopteryx.” The article states:
It turns out Archaeopteryx, the iconic 150 million year old “original bird”, is probably just another dino with feathers, of which there are many, the researchers said.
It is hard to imagine a tougher fall from evolutionary grace.
The implication is as I stated above – unless Archaeopteryx is the “original bird” it has fallen as a transitional fossil. Rubbish. The article does a great disservice to the understanding of evolution and fossil evidence.
The problem with this reasoning is that we almost never find specimens that are literal direct ancestors to extant species – a representative of the small population that directly gave rise to the new species. The chances of finding a fossil of such a transitional population is exceedingly small. Evolution is also not a linear process but a messy branching bushy process. Now that we understand that, we expect to finds lots of cousins and uncles, and only rarely actual parents and grandparents. We are finding random samples from somewhere in the bush of variation, but unlikely to be directly on the tiny branch that led to the descendant group.
It is helpful to know how close to that direct line a fossil species is. Most specimens are “side branches,” but can still be very close to the branch we are interested in.
When it comes to the evolution of birds from dinosaurs, prior to the discovery of Archaeopteryx we had dinosaurs fossils and living birds – with a pretty huge gulf in between. Archaeopteryx filled that gulf with a species that is just about half dinosaur and half bird. It is a transitional species – it is morphologically transitional and it does represent a group of animals that exist evolutionarily between dinosaurs and birds.
We now have evidence from multiple fossils that there was a large group of feathered dinosaurs branching out in many directions during this time (and apparently, mostly in what is now China). Only one path meandering through this forest of feathered dinosaurs actually led to modern birds. It would be amazing if Archaeopteryx, the first such specimen found, lay on this path rather than some side path. But it still occupies this evolutionary space between dinosaurs and birds.
The article also quotes professor Lawrence Witmer as saying:
“Perhaps the time has come to finally accept that Archaeopteryx was just another small, feathered, bird-like theropod fluttering around in the Jurassic.”
I disagree with this characterization. Archaeopteryx could fly – that makes it very bird-like; much more so than velociraptor, which had feathers but not flight feathers, and could not fly. Archaepteryx may, cladistically, be in the theropod clade rather than the bird clade, but it branched off pretty far down the line to birds.
I guess it’s possible that flight evolved independently in Archaeopteryx from the line that led to birds. It’s possible that small feathered theropods independently evolved flight multiple times. I don’t think that’s what this new computer analysis is saying, however. Even still – that would not diminish Archaeopteryx as a representative (the first discovered) of the larger group of theropods that was evolving feathers and flight.
I have followed the discovery of the many feathered dinosaurs over the last 20 or so years with great interest. This is a fascinating and beautiful group of animals, and is a stunning vindication of evolutionary theory. This is just what we would expect must exist if dinosaurs evolved into birds – a large branching group of transitional species with some dino and some bird characteristics.
Creationists, however, are likely to latch onto the terrible reporting about this new analysis as if it destroys one of the “icons of evolution.” The fact that Darwin himself referred to Archaeopteryx as a transitional species is all the better, because then they get to say that “Darwin was wrong” about something. That is how creationists twist new fossil evidence for evolution into an argument against evolution. This is in line with the broader denialist strategy of focusing on changes or disagreements with the small details (where exactly to place Archaeopteryx in the feathered-dino to avian bush) as if it calls into question the large picture of evolution.
The article relies on a couple of cheap shots in order to sensationalize this story. Good science reporting, however, uses new studies or information in order to inform the reader about the broader relevant concepts – how to put the information into context.
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