Evolution surprise: Parrot-beaked dinosaur had both bird and reptile skin (2024)

In the beginning, over half a billion years ago, there were fish. They had scales. Then some fish would leave the seas and early reptiles would emerge. They had scales too. These scaly reptiles begat the archosaurs, from whom crocodilians, then pterosaurs and dinosaurs, and then birds would arise. What body covering these archosaurs had has been the subject of much controversy.

One would expect archosaurs to have scales, and they did. But at least some archosaurs already sported proto-feathers a quarter-billion years ago, fossil evidence indicates. In short, feathers apparently emerged from scales, but the burning question is how. The transition has not been well understood.

Now, analysis of extraordinarily well-preserved skin of a feathered dinosaur named Psittacosaurus ("parrot-beaked lizard") is reported in Nature Communications Tuesday, by Zixiao Yang of University College Cork, Ireland, and colleagues. The dinosaur in question dwelled in the Jehol biota field in China about 130 million years ago, the Early Cretaceous.

Though this Psittacosaurus lived over 100 million years after the earliest feathers erupted, according to the researchers' own estimates, the deceased dinosaur can help us understand the transition from scales to feathers – because it had both.

Now, dinosaurs with scales and feathers have been discovered before. In 2014, paleontologists reported on the lovely Kulindadromeus zabaikalicus, found in Siberia, which was the first one reported to have both scales and feathers. But it wasn't preserved well enough to study its internal skin structure.

But the Psittacosaurus fossil is that good: paleontologists were able to study the structures of the skin where the animal had scales, and where it had feathers. The results were quite a surprise.

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The feathered dinosaur

As said, previous research shows proto-feathers began to appear as much as a quarter-billion years ago. "Our recent discoveries of feathers in pterosaurs (cousins of dinosaurs) strongly suggest that feathers evolved in the common ancestor of pterosaurs and dinosaurs during the Early Triassic, almost 250 million years ago," Yang tells Haaretz by email. Early feathers evolved as a warm body coat, not for flight.

So, if feathers were present in the common ancestor of pterosaurs and dinosaurs – i.e., before the origin of dinosaurs – then feathering is a basal condition to all dinosaurs. That does not mean apatosaurs and the presumably ferocious Tyrannosaurus rex had feathers from head to toe; gargantuan animals don't need body coating to stay warm. Elephants, rhinos and hippos for instance, have lost almost all their fur.

Psittacosaurus (of which there were a dozen species we know of) had a beak. It also had bristles on its tail, paleontologists reported in 2016, and the rest of its body was scaly, Yang confirms. The bristles had been an enigma when first discerned, though the 2016 team did note a "striking resemblance" between Psittacosaurus' tail bristles and the bizarre "horn" sprouting from the head of the horned screamer (Anhima cornuta).

Yang now confirms that the bristles on Psittacosaurus' tail were feathers.

As for its scales, they were many and myriad. A 2022 study reported on scales in Psittacosaurus found in Germany: "Variations in the scaly skin are found to be strongly regionalized in Psittacosaurus. For example, feature scales consist of truncated cone-shaped scales on the shoulder, but form a longitudinal row of quadrangular scales on the tail," that paper said.

What about the scales on extant birds' feet? Like on chicken feet? These scales were once assumed to be a remnant from the birds' distant reptilian past, but they're not.

"These scales are not a reptilian remnant because they evolved secondarily from feathers – after the evolutionary transition from scales to feathers, some feathers evolved to a structure resembling the scales of reptiles," Yang explains. Note also birds like the Silkie chicken that has feathered feet.

So what have we? So far, that the basal dinosaurian condition was to have both feathers and scales, and our Psittacosaurus who lived a long, long time after the first feathered being sported both.

But how did they get from scales to feathers? The new study sheds light on the transition, revealing for the first time that where it was feathered, Psittacosaurus had avian-type skin and where it wasn't, it retained reptilian skin.

To be clear, birds do not retain any reptilian skin at all.

"The chicken only has bird skin (including the scaly leg and feet), while Psittacosaurus had a mix of reptile-type skin, inherited from its reptilian ancestors, and bird-like skin where the feathers grew," Yang confirms, following the team's analysis of the skin fossils by electron microscopy and ultraviolet fluorescence.

How is avian skin distinguished from the ancestral reptilian skin? The outer skin, the epidermis, is thinner and more pliable to enable movement by the feather shafts during flight. Bird skin has follicles for feather generation and renewal. Bird skin also features muscles to support and control the feathers, which reptilian skin lacks. That said, muscle for feather control goes back early too: Separate work on maniraptoran dinosaurs (ancestral to some non-avian dinosaurs and birds) found they had dermal muscle systems with flight feathers.

The team also identified what they believe are melanosomes in the fossil skin cells: particles that give skin (or feathers) their colors.

Psittacosaurus' color scheme seems to have been like that of some pterosaurs – and our friend the croc. It may even have had spots or stripes like today's river reptiles.

Do birds have melanosomes in their skin? "The feather-covered skin of most birds appears pink because there is very little or absence of melanosomes in the skin," Yang explains. "Most of the melanosomes are in the feathers to produce plumage color. On the other hand, the exposed bird skin, such as the skin on the feet, can contain abundant melanosomes. In some birds, such as the silkie chicken, the feathered skin appears black and contains abundant melanosomes in the dermis layer of the skin. This is because of genetic mutations and has occurred multiple times in bird evolution."

The bottom line is that early evolution of avian skin traits was restricted to feathered body regions, the team concluded. Wow. Psittacosaurus retained reptile-type skin in non-feathered areas of its body to preserve the functions of the scaly skin at a time of "experimentation" in feathering, the team suggests.

So what have we? We still can't say when feathers originated, or even if it only evolved once (as Yang says he believes) or multiple times. But we can say that these transitional dinosaurs who had both feathers and scales had bird skin where they had feathers and reptile skin where they had scales – and that is quite the surprise.