Jun 26, 2016

Feathers, Hair, Scales Evolved from One Ancestor

(left) Human hair, (center) Peacock feather, (right) Crocodile skin.
Hairs, feathers and reptile scales share a common ancestry, according to a new study that shows how interconnected many members of the animal kingdom are.

The coverings all develop from the same primordial structure that might have first emerged in the common reptilian ancestor of birds, mammals and reptiles, the new research -- published in the journal Science Advances -- strongly suggests.

"What we found is that they (hair, feathers, scales) all start from the same micro-anatomical structure, called a placode, made of a local thickening of the epidermis due to epidermal cells taking a columnar shape," Michel Milinkovitch, who co-authored the paper with Nicolas Di-Poï, told Discovery News.

Placodes (spots stained in dark blue by the expression of an early developmental gene) are visible before the development of hair, scales and feathers in (from left to right) the mouse, the snake, the chicken and the crocodile.
Prior research, published last year and authored by a team at Yale, found that scales, hairs and feathers share molecular signatures during their development. But data on reptile scales did not clearly align them with hairs and feathers.

Milinkovitch and Di-Poï, from the University of Geneva's Department of Genetics & Evolution, were curious to learn more. They analyzed the molecular characteristics of skin during embryonic development in crocodiles, snakes and lizards. In doing so, they found the same anatomical placode and molecular signatures seen in mammals and birds.

Reptiles, birds and egg-laying mammals were already connected because all are known as amniotes, referring to their type of egg (amniotic) in which their embryos are embedded. Now it's known that the three animal groups share basic skin features as well, so it is likely that their common ancestor had placodes, too.

"I would say that the patterning of the skin with placodes is an evolutionary developmental innovation that allowed the ancestor of amniotes to develop some sort of bumps distributed across the skin and providing some additional mechanical protection," Milinkovitch said.

He said modern amphibians could even sport this ultra-ancient look.

"These animals are supposed to be 'naked,' but quite a few species exhibit bumps and scaly appendages that might have a link with the placodes of amniotes," he said. "After all, this could be an older innovation that occurred in the ancestor of all terrestrial vertebrates (animals with a backbone)."

Marcelo Sánchez-Villagra, a professor at the University of Zurich's Paleontological Institute and Museum, told Discovery News that the common ancestor of birds, mammals and reptiles "would not be a reptile by definition, but an animal that lived around 330 million years ago."

Sánchez-Villagra's colleague, Torsten Scheyer, added that the common ancestor was likely a "reptiliomorph," named as such "because they combine amphibian features with reptilian features."

Illustration of the reptiliomorph Limnoscelis paludis.
Scheyer says reptiliomorphs were mostly aquatic predatory animals that inhabited a variety of different habitats ranging from coal swamps and forests to warm Savanna-like regions. Their fossils have been found in Germany, Texas and at other locations.

Although it appears that birds, mammals and reptiles inherited their placodes from this reptiliomorph common ancestor, clearly a lot of evolution has taken place since that early time.

To better determine how placodes can result in so many different looks, Milinkovitch and Di-Poï investigated bearded dragons. These lizards vary in appearance, with one type of bearded dragon having a lot of scales, another featuring reduced-size scales, and a third with no visible scales whatsoever.

Read more at Discovery News

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