Are Birds Dinosaurs?

[CDarwin]

Until yesterday I had always thought that the answer from the palaeontological community was a resounding "yes." I had no idea that there was any decent. I just started reading Ernst Mayr's What Evolution Is, however, and lo and behold, he quite persuasively presented a quite different view.

 

Mayr seems to feel that birds descended from thecodont reptiles of the late triassic, and evolved in parallel to the dinosaurs. He cites the similarity of the pelvises of birds and the bipedal dinosaurs as a remarkable instance of parallel evolution; similar environments acting on similar genetic potentials in related lineages similaraly.

 

I'll summarize his argument by copying Box 3.3 on page 68, "Refutation of the Dinosaurian Origin of Birds" verbatim.

 

"1. Age- The dinosaurs structurally most similar to birds are very recent (80-110 million years ago), whereas Arcaeopteryx is a great deal older (145 million year old) and no birdlike dinosaurs are known from lower Jurassic or Triassic that could qualify as ancestors of birds.

 

2. The three digits of the hand of the dinosaurs are 1,2,3, those of a bird are 2,3,4. It is quite impossible to derive the avian digits from those of dinosaurs.

 

3. Teeth- Theropods have recurved, flattened, serrated teeth, quite different from the simple peglike teeth of Arcaeopteryx and other early birds.

 

4. The pectoral girdle and anterior extremities of the late theropod dinosaurs are much too small and weak to lift and incipient bird from the ground. No factors are known that could have caused a sudden drastic growth of the anterior extremities.

 

5. The leading aerodynamic experts of bird flight claim that an origin of flight from the ground up is a near impossibility."

 

I believe his book predated the discovery of the "feathered dinosaurs", and this would have to be seen as another case of parallel evolution, but the argument is still persuasive. I don't really know a great deal about our avian friends, so I'm not entirely sure what to make of the whole thing. Any responses?

[lucaspa]

Until yesterday I had always thought that the answer from the palaeontological community was a resounding "yes." I had no idea that there was any decent. I just started reading Ernst Mayr's What Evolution Is, however, and lo and behold, he quite persuasively presented a quite different view.

 

Mayr seems to feel that birds descended from thecodont reptiles of the late triassic, and evolved in parallel to the dinosaurs. He cites the similarity of the pelvises of birds and the bipedal dinosaurs as a remarkable instance of parallel evolution; similar environments acting on similar genetic potentials in related lineages similaraly.

 

I'll summarize his argument by copying Box 3.3 on page 68, "Refutation of the Dinosaurian Origin of Birds" verbatim.

 

"1. Age- The dinosaurs structurally most similar to birds are very recent (80-110 million years ago), whereas Arcaeopteryx is a great deal older (145 million year old) and no birdlike dinosaurs are known from lower Jurassic or Triassic that could qualify as ancestors of birds.

 

2. The three digits of the hand of the dinosaurs are 1,2,3, those of a bird are 2,3,4. It is quite impossible to derive the avian digits from those of dinosaurs.

 

3. Teeth- Theropods have recurved, flattened, serrated teeth, quite different from the simple peglike teeth of Arcaeopteryx and other early birds.

 

4. The pectoral girdle and anterior extremities of the late theropod dinosaurs are much too small and weak to lift and incipient bird from the ground. No factors are known that could have caused a sudden drastic growth of the anterior extremities.

 

5. The leading aerodynamic experts of bird flight claim that an origin of flight from the ground up is a near impossibility."

 

The book was published in 2001. You need some history. Huxley introduced the idea that birds were descended from dinos. Then in the 1920-1980 the idea went out of favor and most paleontologists argued that dinos and birds had a common archosaurian ancestor and the two lineages independently evolved the pelvis and legs that are so similar. The general idea is parallelophyly.

 

In 1982 Ostrom revived the dino-bird lineage. Cladistically, this is the lineage that works. On page 227 Mayr states "the dinosaur origin [of birds] has been proclaimed by the cladists with such vigor that at present it seems to be the most widely accepted explanation of the origin of birds."

 

The alternative archosaurian common ancestor -- and the arguments -- has been most promoted vigorously by Alan Fedducia and colleagues. Mayr is just putting this out as an example of controversy within evolution and the concept of parallelophyly. I'll go down the list.

 

1. Compsagnathus is very similar to Archie. So similar, in fact, that many fossils of Archie without feather impressions were mistakenly identified as Comsagnathus -- which was a contemporary of Archie. The similarities are not just in the pelvis and legs. http://www.dinosauria.com/jdp/archie/dinoarch.htm

 

New fossils have pushed the theropods with feathers back in time: http://research.amnh.org/vertpaleo/dinobird.html 130 mya theropod with feathers. and this gives a skeleton similar to Archie. Microraptor is also about that age: http://www.dinohunters.com/History/Microraptor.htm

 

Then there are the feathered dinos. Parallelophyly might be able to account for bones, but bones AND feathers? http://www.dinosauria.com/jdp/archie/protocaud.html

What's worse, there are later birds with the theropod sickle claw. http://www.dinosauria.com/jdp/archie/sickle.htm So now parallelophyly has to produce 3 common features with theropod dinos.

 

2. This is from Feduccia. It was countered here: Science, Volume 280: 355, 17 April 1998. Counting the Fingers of Birds and Dinosaurs

 

3. Confusciornis -- a contemporary of Archie -- has no teeth at all! Microraptor has the teeth of a theropod: http://www.dinosauria.com/jdp/archie/sickle.htm

 

4. This one seems to just be untrue.

 

5. A new article by Dial shows just how ground up flight becomes possible. It's really a cool study because it uses contemporary birds -- really chicks. Before the flight feathers come in, flapping the forelimbs lets the birds run up inclined surfaces (like Microraptor chasing insect prey?) up to even over the vertical! Just as the feathers get really good for running up inclined surfaces, that is just the point where the motion starts getting the animal off the ground! Kenneth P.Dial, Wing-Assisted Incline Running and the Evolution of Flight. Science, 299: 402-405, Jan 17, 2003.

 

Now, before I leave this, I want you to notice the general trend of these arguments you found "persuasive". ALL of them are NOT based on evidence supporting the alternative hypothesis: archosaurians with feathers and other bird features.

 

Instead, ALL the arguments are supposed "problems" with the dino-bird theory. At the most, they could be used to question/refute that theory. They do not support an archosaurian ancestry of birds.

 

Arguments 1, 4, and 5 rely on "gaps" in data. Argument 2 and 5 rely on the supposed "impossibility" of something.

 

When your arguments supporting a theory are really on the supposed weakness of a competing theory, then you are in serious trouble.

 

In the last couple of years, the controversy has vanished and the dino-bird theory is accepted. Feduccia seems to have retired and is now Professor Emeritus at UNC. Sometimes "controversies" in science are "settled" simply because the major protagonists for one side retire or die. Since they were the only ones keeping the "controversy" alive, once they leave the scene there is no one to keep up the argument.

 

Or Feduccia may simply have accepted overwhelming data. I can hope it is this one.