Comments on: A critical evalution of Tianyulong confiusci – part 3: Plucking at the idea of feathered dinosaurs

By: Jura

Jura — Thu, 01 Oct 2009 14:15:06 +0000

Dr. Lawrence Witmer of Ohio University, has a nice little write up in Nature, on Anchiornis huxleyi, and what it means to bird origins. It would appear that Sawyer's initial hypothesis - that four wings were the basal stage to flight- would be correct. Though if I remember correctly, the concept of the "four winged bird" is even older than that.

By: Maija Karala

Maija Karala — Sat, 26 Sep 2009 18:15:57 +0000

“… and that the ancestral feathered dinosaur was feathered all the way down to the feet.”
This hypothesis just got quite a bit of new points: The troodontid Anchiornis seems to be feathered all over, from the snout down to the toes.

By: Maija Karala

Maija Karala — Thu, 17 Sep 2009 13:14:55 +0000

“Sawyer and Knapp�s 2003 work on avian integument (cited above) suggests that scales in bird legs are a secondary characteristic, and that the ancestral feathered dinosaur was feathered all the way down to the feet.”
I understood this was based on the fact that bird scales are formed by actively suppression of feather growth? It might mean that there have been some completely feathered animals, but doesn’t in any sense mean a dinosaur has to be either completely feathered or completely scaly. Since the birth of feathers, their growth may have been suppressed in different ranges and different parts of animals.

And the genetic mechanisms of feather and scale growth regulation haven’t necessarily been identical for the last 200 million years, but there’s no way to find out about that.

“As for the owl, I would not be surprised if a molecular study of those filaments found them to be scale derivatives rather than feathers.”
This might well be true, although I’m skeptical about the owl fuzz being scales since they smoothly blend into leg feathers without any difference to be seen. There might well have been scale derivative “hairs” or “feathers” in dinosaurs, and this, of course, would have an impact on our knowledge of feather evolution. But for drawings of fuzzy dinosaurs, it’s essentially the same if they are scales or feathers in their molecular basis.
Maybe molecular test from the fossils will give more light in this problem.

“I�d have to wonder how much bigger we would need to go in order to see this change take place.”
Actually, the largest birds known to have lived relatively recently (the elephant birds and Dromornis) have weighed around 450-500 kg, but since no living human has seen them and there’s no skins or photographs, they don’t actually help. When comparing to tropical African mammal fauna, we see that hairs start to disappear from animals weighing more than 1000 kg. In cooler climates, even larger animals need to be hairy (or feathered).

By: Jura

Jura — Tue, 15 Sep 2009 03:10:22 +0000

As mentioned in the above post, the presence of scales and feathers in birds is a bit of a red herring. Sawyer and Knapp’s 2003 work on avian integument (cited above) suggests that scales in bird legs are a secondary characteristic, and that the ancestral feathered dinosaur was feathered all the way down to the feet. As for Scansoriopteryx the scale impression described by Czerkas and Yuan, is not actually preserved on the base of the tail, but rather is preserved near it; suggesting the scales had moved out of place.

As for the owl, I would not be surprised if a molecular study of those filaments found them to be scale derivatives rather than feathers. Sawyer and Knapp’s work on avian integument showed that many structures that one would not normally associate with feathers, are actually feathers molecularly. I imagine a similar study on scales would prove the same.

I�m not sure if it�s possible for an animal to change from feathers to scales during growth, since we don�t have big enough birds alive to observe.

Extant ostriches can reach 130kg in size, while the largest birds known to have lived (moas) are estimated to have hit 230kg. I’d have to wonder how much bigger we would need to go in order to see this change take place.

By: Maija Karala

Maija Karala — Sat, 12 Sep 2009 22:58:01 +0000

“Taken together all of this suggests an antagonistic relationship between scales and feathers.”
Actually, nothing suggests a relationship like that: the only living dinosaurs are birds, which DO have both scales and feathers at the same time. This means that every ancestor of modern birds since the invention of feathers has had them both, if we don’t assume re-evolution of either one. So, we only need one birth of feathers and different emphasis on either integument in the later species. The fact that, for example, Tyrannosaurus, was scaly, doesn’t mean that it didn’t have any feathers anywhere in it’s body in any phase of it’s life. And the other way around: the fact that maniraptorans had feathers doesn’t mean that they didn’t have any scales. We might not need any more than _one_ evolutionary step (and maybe a few complete losses of feathers in large-bodied taxa).
Actually, the type specimen of Scansoriopteryx has both feathers and scales preserved. And, of course, all extant birds have them both.

And of course, you can’t always simply count the steps in a phylogeny. New traits (like filamentous integument) are much less probable to evolve independently than losses of said traits.

“In fact, the presence of the Psittacosaurus �quills� alongside scales, suggest that they are more likely to be a scaly derivative, than a feathery one.”
So, do you suggest that the presence of simple filamentous structures alongside scales in the feet of Barn owl suggest that they are scaly derivatives rather than feathery ones?
Photo here: http://www.flickr.com/photos/25918339@N00/1874087885

It’s possible that the quills of Psittacosaurus or pterosaur pycnofibers have evolved independently. We just need more evidence to be sure, one way or another. I’m not sure if it’s possible for an animal to change from feathers to scales during growth, since we don’t have big enough birds alive to observe.

By: Matt Wedel

Matt Wedel — Tue, 07 Jul 2009 03:07:07 +0000

Damn good post. Thanks for pulling it all together–and for the refs.