In evolution, scale is important

Scales, for most of us, are nothing more than an annoyance while having a fish dinner.

But without them, you might not be able to eat at all. And, if you enjoyed some chicken too, scales might also explain your snack’s former ability to fly. 

According to two new studies, scales are the key to understanding the evolution of teeth in vertebrates, and feathers in birds.

The earliest mineralised skeletons in vertebrates, made from calcium-rich minerals such as dentine, were thick armoured scales. These scales would have provided protection from predators such as Eurypterids, extinct sea scorpions, that could grow to a formidable 2.5 metres long and pack a real pinch.

Now J. Andrew Gillis, Els Alsema and Kathrine Criswell from the Department of Zoology at Cambridge University argue in a paper published in the Proceedings of the National Academy of Sciences that such scales are the evolutionary origin of vertebrate teeth.

“Primitive scales were much more tooth-like in structure,’ says Gillis, “but have been retained in only a few living lineages, including that of cartilaginous fishes such as skates and sharks.” 

The researchers investigated this evolutionary link by studying the embryonic development of skates, which like other cartilaginous fish have ‘dermal denticles’ – spiky scales that are remarkably like teeth.

By labelling and tracking cell-types through development, the researchers revealed that “the denticle scales of sharks and skate develop from neural crest cells, just like teeth.” This common cellular origin lends support to the theory that denticle scales became teeth after the evolution of jaws in vertebrates, a feature of almost all living vertebrates today.

Coincidentally, another recent publication highlights the evolutionary versatility of the humble scale. 

Long after the evolution of the jaw, giant reptiles called Archosaurs put them to good use while dominating the planet 250 million years ago. These “ruling reptiles”, were the common ancestor of the dinosaurs, as well as two still-living groups: birds and crocodilians. 

Birds, the only modern dinosaurs, are feathered, while crocodiles and their kin have scales, and this implies an evolutionary connection.

Finds in the last few decades have provided examples of transitionary dinosaur fossils with “proto-feathers”. However, the exact genetic underpinnings of the development of feathers, and the evolution of flight more generally, has remained elusive.

Cheng-Ming Chuong, of the University of Southern California, US, with his international team, has just shed more light on the genetics of scale and feather development. “We now have a potential molecular explanation for these hypothesised missing links,” says Chuong.

Apart from identifying genes involved in the expression of both scales and feathers, the researchers have gone a step further and demonstrated them in action. 

As detailed in their article in the journal Molecular Biology and Evolution, Chuong and his team took genes specific for feather development from chickens and placed them in alligator eggs. By carefully manipulating the genes’ expression during development, they were able to induce scales to form various feather-like characteristics and appendages.

“Intriguingly,” says Chuong, “some of these phenotypes are similar to the unusual filamentous appendages found in the fossils of feathered dinosaurs.”

The research adds another layer of understanding to the story of the way scales evolved into feathers that eventually allowed birds to take flight.

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