Israeli-Chinese Team Identifies Earliest Feather Molt in Dinosaurs

Four-winged Microraptor could fly and fly well, says team, negating the theory that the chicken-sized early Cretaceous creature could only glide

Ruth Schuster
Ruth Schuster

Beautifully preserved fossil of Microraptor complete with feathers
Beautifully preserved fossil of Microraptor complete with feathersCredit: Yosef Kiat
Ruth Schuster
Ruth Schuster

It’s become quite the consensus that being feathered was the basal dinosaurial condition. It is also becoming clear that the very earliest ancestral dinosaurs were tiny rat-sized things and the weather could get very cold so they may have evolved downy feather coats to keep warm.

Now the earliest known incidence of feather molting in an aviating dinosaur has been reported by doctoral student Yosef Kiat of the Animal Flight Laboratory at the University of Haifa and a team from the Chinese Academy of Sciences in Beijing. Their paper was published this week in Current Biology.

The winged dinosaur in which molting was detected was the remarkable four-winged dromaeosaurid Microraptor, which isn’t new to science. It had long feathers not only on its wings but on its legs too. We have known about the dromaeosaurid Microraptor from the early Cretaceous formations of Liaoning, China for some time. Now the binational team reveals the first fossilized evidence of sequential wing feather molt, which, they point out, is how today’s more vulnerable birds molt their feathers – i.e., sequentially.

The researchers also suggest that in contrast to theories to the contrary, the thing could fly and well at that.

The volant dromaeosaurid Microraptor, which lived 130 to 120 million years ago, was all of 2.5 feet (about 77 centimeters) long, and its sequential feather molting could have been a survival strategy. The upshot is that they could continue to fly while replacing their wing and leg feathers, which would reduce their risk of being eaten, the team observes.

Reproduction of Microraptor, which could fly with its four wings (two of which were legs)Credit: Fred Wierum

Microraptor, the four-winged flying dinosaur

Feathers need replacing periodically in order to remain functional, or they’d become tatty and the bird wouldn’t be able to deflect rain or fly. Think what a feathered earring or hat looks like after a week, a month or a year – and that’s without flying.

In fact, once a feather reaches its full size, it dies and gets shed and replaced. That, explains the University of Haifa scientists, is what molting is.

Clearly if the feathers all grew together and dropped off together, the dinosaur or bird would periodically be left naked, which is not an evolutionarily sound strategy. Hence the evolutionary logic of sequential molting, a strategy known among today’s birds. Sequential molting is characterized bygradation: Especially vulnerable species molt very gradually whereas among birds that do not fly much, or that can effectively hide from predators without resorting to aviation, may molt within just two to three weeks.

The team – Kiat, his supervisor Nir Sapir, head of the Animal Flight Laboratory at the University of Haifa and Amir Balaban, a leading Israeli ornithologist, collaborating with paleontologists Xing Xu, Jingmai O’Connor and Min Wang from the Chinese Academy of Sciences – began with the modern ornithology and applied modern ornitholgical and other scientific theories to an exquisitely preserved fossil of Microraptor in China.

A: Highlighted colorized bit shows feathers in sequential molt; B:Illustrated Microraptor wing: fully grown primary feathers (left) and during wing moltingCredit: Yosef Kiat

This specific Microraptor was preserved with its four wings retaining most of their feathers, and the researchers realized they were seeing six feathers of differing sizes. Were they evidence of molting or were some broken or otherwise mangled? Could the team tell whether said dinosaur had sequential molt, simultaneous molt (all at once, more or less) or haphazard irregular molt, as some birds have?

They first of all admit that they couldn’t identify irregular molt today or 130 million years ago, and thus focused on sequential and non-sequential and compared their observations of the long dead feathered dinosaur’s plumage with 302 extant birds. And lo, they concluded that the basal ancestral strategy of bird-dom is apparently sequential molt, and non-sequential molt is a derived strategy of birds that don’t need to get about.

“There is not a single flightless bird species that molts its primary feathers sequentially,” the team writes. Good to know.

Now they had to check whether the different feather sizes on the dead Microraptor were really the result of molting or mutilation. That involved identifying the edges of the feathers and presto, they concluded – going into exquisite detail of each feather – they really were looking at a wing with feathers of different lengths in a sequence.

Ostriches: being flightless it doesn't mater how or when they moltCredit: Nir Kafri

So, Microraptor employed sequential molting. QED. It could thus escape from the predators in its environment by taking to the skies with its four wings, molting or not. It makes sense since the animal, which is quite richly represented in the fossil record, was a titch – weighing about as much as a small chicken, about a kilogram.

“We are used to examining and studying molting in our regular bird-ringing work. Coming face to face with a flying dinosaur that sunk into a primeval swamp millions of years ago is a very rare and exciting event,” stated Balaban, adding that the team are slavering to go back and continue the work in China when the coronavirus wanes.

Extrapolating from today’s birds, the researchers suggest that Microraptor was airborne a great deal and probably hunted for its food, which likely included small mammals, spotted from the air.

So much for the argument that the four-winged midget that thronged the early Cretaceous environs of today’s China couldn’t fly, but only glide when it had to. “The new finding supports those the claim that the Microraptor could fly – and could fly well,” the University of Haifa researchers observe.

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