Fossils reveal how ancient birds molted their feathers -- which could help explain why ancestors of modern birds survived when all the other dinosaurs died

Every bird you've ever seen -- every robin, every pigeon, every penguin at the zoo -- is a living dinosaur. Birds are the only group of dinosaurs that survived the asteroid-induced mass extinction 66 million years ago. But not all the birds alive at the time made it. Why the ancestors of modern birds lived while so many of their relatives died has been a mystery that paleontologists have been trying to solve for decades. Two new studies point to one possible factor: the differences between how modern birds and their ancient cousins molt their feathers.

Feathers are one of the key traits that all birds share. They're made of a protein called keratin, the same material as our fingernails and hair, and birds rely on them to fly, swim, camouflage, attract mates, stay warm, and protect against the sun's rays. But feathers are complex structures that can't be repaired, so as a means of keeping them in good shape, birds shed their feathers and grow replacements in a process called molting. Baby birds molt in order to lose their baby feathers and grow adult ones; mature birds continue to molt about once a year.

"Molt is something that I don't think a lot of people think about, but it is fundamentally such an important process to birds, because feathers are involved in so many different functions," says Jingmai O'Connor, associate curator of fossil reptiles at Chicago's Field Museum. "We want to know, how did this process evolve? How did it differ across groups of birds? And how has that shaped bird evolution, shaped the survivability of all these different clades?" Two of O'Connor's recent papers examine the molting process in prehistoric birds.

A paper in the journal Cretaceous Research published in May 2023 detailed the discovery of a cluster of feathers preserved in amber from a baby bird that lived 99 million years ago.

Today, baby birds are on a spectrum in terms of how developed they are when they're born and how much help they need from their parents. Altricial birds hatch naked and helpless; their lack of feathers means that their parents can more efficiently transmit body heat directly to the babies' skin. Precocial species, on the other hand, are born with feathers and are fairly self-sufficient.

All baby birds go through successive molts -- periods when they lose the feathers they have and grow in a new set of feathers, before eventually reaching their adult plumage. Molting takes a lot of energy, and losing a lot of feathers at once can make it hard for a bird to keep itself warm. As a result, precocial chicks tend to molt slowly, so that they keep a steady supply of feathers, while altricial chicks that can rely on their parents for food and warmth undergo a "simultaneous molt," losing all their feathers at roughly the same time.

The amber-preserved feathers in this study are the first definitive fossil evidence of juvenile molting, and they reveal a baby bird whose life history doesn't match any birds alive today. "This specimen shows a totally bizarre combination of precocial and altricial characteristics," says O'Connor, who was the first author of the paper alongside senior author Shundong Bi of the Indiana University of Pennsylvania. "All the body feathers are basically at the exact same stage in development, so this means that all the feathers started growing simultaneously, or near simultaneously." However, this bird was almost certainly part of a now-extinct group called the Enantiornithines, which O'Connor's previous work has shown were highly precocial.

O'Connor hypothesizes that the pressures of being a precocial baby bird that had to keep itself warm, while undergoing a rapid molt, might have been a factor in the ultimate doom of the Enantiornithines. "Enantiornithines were the most diverse group of birds in the Cretaceous, but they went extinct along with all the other non-avian dinosaurs," says O'Connor. "When the asteroid hit, global temperatures would have plummeted and resources would have become scarce, so not only would these birds have even higher energy demands to stay warm, but they didn't have the resources to meet them."

Meanwhile, an additional study published July 3 in Communications Biology by O'Connor and Field Museum postdoctoral researcher Yosef Kiat examines molting patterns in modern birds to better understand how the process first evolved.

In modern adult birds, molting usually happens once a year in a sequential process, in which they replace just a few of their feathers at a time over the course of a few weeks. That way, they're still able to fly throughout the molting process. Simultaneous molts in adult birds, in which all the flight feathers fall out at the same time and regrow within a couple weeks, are rarer and tend to show up in aquatic birds like ducks that don't absolutely need to fly in order to find food and avoid predators.

It's very rare to find evidence of molting in fossil birds and other feathered dinosaurs, and O'Connor and Kiat wanted to know why. "We had this hypothesis that birds with simultaneous molts, which occur in a shorter duration of time, will be less represented in the fossil record," says O'Connor -- less time spent molting means fewer opportunities to die during your molt and become a fossil showing signs of molting. To test their hypothesis, the researchers delved into the Field Museum's collection of modern birds.

"We tested more than 600 skins of modern birds stored in the ornithology collection of the Field Museum to look for evidence of active molting," says Kiat, the first author of the study. "Among the sequentially molting birds, we found dozens of specimens in an active molt, but among the simultaneous molters, we found hardly any."

While these are modern birds, not fossils, they provide a useful proxy. "In paleontology, we have to get creative, since we don't have complete data sets. Here, we used statistical analysis of a random sample to infer what the absence of something is actually telling us," says O'Connor. In this case, the absence of molting fossil birds, despite active molting being so prevalent in the sample of modern bird specimens, suggests that fossil birds simply weren't molting as often as most modern birds. They may have undergone a simultaneous molt, or they may not have molted on a yearly basis the way most birds today do.

Both the amber specimen and the study of molting in modern birds point to a common theme: prehistoric birds and feathered dinosaurs, especially ones from groups that didn't survive the mass extinction, molted differently from today's birds.

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Scales or feathers? It all comes down to a few genes

Scales, spines, feathers and hair are examples of vertebrate skin appendages, which constitute a remarkably diverse group of micro-organs. Despite their natural multitude of forms, these appendages share early developmental processes at the embryonic stage. Two researchers from the University of Geneva (UNIGE) have discovered how to permanently transform the scales that normally cover the feet of chickens into feathers, by specifically modifying the expression of certain genes. These results, published in the journal Science Advances, open new perspectives for studying mechanisms that have enabled radical evolutionary transitions in form among species.

The skin of terrestrial vertebrates is adorned with diverse keratinized appendages, such as hair, feathers, and scales. Despite the diversity of forms within and among species, the embryonic development of skin appendages typically begins in a very similar way. Indeed, all of these structures develop from cells that produce a localized thickening on the skin surface and express particular genes. One of these genes, called Sonic hedgehog (Shh), controls a signaling pathway -- a communication system that allows the transmission of messages within and between cells. Shh signalling is involved in the development of diverse structures, including the neural tube, limb buds and skin appendages.

A common ancestor

The laboratory of Michel Milinkovitch, professor in the Department of Genetics and Evolution at the Faculty of Science of the UNIGE, is interested in the physical and biological processes that generate the diversity of skin appendages in vertebrates. In particular, his group has previously demonstrated that hair, feathers and scales are homologous structures inherited from a reptilian common ancestor.

Feathers of the chicken embryo are used by scientists as a model system to understand skin appendage development. While it is known that certain breeds of chickens, such as the 'Brahma' and 'Sablepoot' varieties, exhibit feathered legs and dorsal foot surfaces, the genetic determinism of this trait is not fully understood.

A transient modification for a permanent change

As the signaling pathways responsible for this transformation have not been fully determined, Michel Milinkovitch's group investigated the potential role of the Shh pathway. "We used the classic technique of 'egg candling', in which a powerful torch illuminates blood vessels on the inside of the eggshell. This allowed us to precisely treat chicken embryos with a molecule that specifically activates the Shh pathway, injected directly into the bloodstream,'' explains Rory Cooper, a post-doctoral researcher in Michel Milinkovitch's laboratory and co-author of the study.

The two scientists observed that this single stage-specific treatment is sufficient to trigger the formation of abundant juvenile down-type feathers, in areas that would normally be covered with scales. Remarkably, these experimentally-induced feathers are comparable to those covering the rest of the body, as they are regenerative and are subsequently and autonomously replaced by adult feathers.

After comparison with embryos injected with a 'control' solution (without the active molecule), RNA sequencing analysis showed that the Shh pathway is both immediately and persistently activated following injection of the molecule. This confirms that activation of the Shh pathway underlies the conversion of scales into feathers.

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Fossils reveal the long-term relationship between feathered dinosaurs and feather-feeding beetles

New fossils in amber have revealed that beetles fed on the feathers of dinosaurs about 105 million years ago, showing a symbiotic relationship of one-sided or mutual benefit, according to an article published in Proceedings of the National Academy of Sciences today.

The main amber fragments studied, from the Spanish locality of San Just (Teruel), contain larval moults of small beetle larvae tightly surrounded by portions of downy feathers. The feathers belonged to an unknown theropod dinosaur, either avian (a term referring to "birds" in wide sense) or non-avian, as both types of theropods lived during the Early Cretaceous and shared often indistinguishable feather types. However, the studied feathers did not belong to modern birds since the group appeared about 30 million years later in the fossil record, during the Late Cretaceous.

When looking at modern ecosystems, we see how ticks infest cattle, frogs capture insects with acrobatic tongues, or some barnacles grow on the skin of whales. These are just a few of the diverse and complex ecological relationships between vertebrates and arthropods, which have coexisted for more than 500 million years. The way that these two groups have interacted throughout deep time is thought to have critically shaped their evolutionary history, leading to coevolution. Nevertheless, evidence of arthropod-vertebrate relationships is extremely rare in the fossil record.

The larval moults preserved in the amber were identified as related to modern skin beetles, or dermestids. Dermestid beetles are infamous pests of stored products or dried museum collections, feeding on organic materials that are hard for other organisms to decay such as natural fibres. However, dermestids also play a key role in the recycling of organic matter in the natural environment, commonly inhabiting nests of birds and mammals, where feathers, hair, or skin accumulate.

"In our samples, some of the feather portions and other remains -- including minute fossil faeces, or coprolites -- are in intimate contact with the moults attributed to dermestid beetles and show occasional damage and/or signs of decay. This is hard evidence that the fossil beetles almost certainly fed on the feathers and that these were detached from its host," explains Dr Enrique Peñalver, from the Geological and Mining Institute of Spain of the Spanish National Research Council (CN IGME-CSIC) and lead author of the study.

"The beetle larvae lived -- feeding, defecating, moulting -- in accumulated feathers on or close to a resin-producing tree, probably in a nest setting. A flow of resin serendipitously captured that association and preserved it for millions of years."

"Three additional amber pieces each containing an isolated beetle moult of a different maturity stage but assigned to the same species were also studied, allowing a better understanding of these minute insects than what is usually possible in palaeontology," says Dr David Peris, from the Botanical Institute of Barcelona (CSIC-Barcelona City Council) and co-author of the study. The most impressive, complete specimen was found in the amber deposit of Rábago/El Soplao in the northern Spain, roughly of the same age as San Just.

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A stone age child buried with bird feathers, plant fibers and fur

The exceptional excavation of a Stone Age burial site was carried out in Majoonsuo, situated in the municipality of Outokumpu in Eastern Finland. The excavation produced microscopically small fragments of bird feathers, canine and small mammalian hairs, and plant fibres. The findings gained through soil analysis are unique, as organic matter is poorly preserved in Finland's acidic soil. The study, led by Archaeologist Tuija Kirkinen, was aimed at investigating how these highly degraded plant- and animal-based materials could be traced through soil analysis.

During the Stone Age in Finland, the deceased were interred mainly in pits in the ground. Little of the organic matter from human-made objects have been preserved in Stone Age graves in Finland, but it is known, on the basis of burial sites in the surrounding regions, that objects made of bones, teeth and horns as well as furs and feathers were placed in the graves.

Teeth and arrowheads found in the red ochre grave

The Trial Excavation Team of the Finnish Heritage Agency examined the site in 2018, as it was considered to be at risk of destruction. The burial place was located under a gravelly sand road in a forest, with the top of the grave partially exposed. The site was originally given away by the intense colour of its red ochre. Red ochre, or iron-rich clay soil, has been used not only in burials but also in rock art around the world.

In the archaeological dig at the burial site, only a few teeth were found of the deceased, on the basis of which they are known to have been a child between 3 and 10 years of age. In addition, two transverse arrowheads made of quartz and two other possible quartz objects were found in the grave. Based on the shape of the arrowheads and shore-level dating, the burial can be estimated to have taken place in the Mesolithic period of the Stone Age, roughly 6,000 years before the Common Era.

What made the excavation exceptional was the near-complete preservation of the soil originating in the grave. A total of 65 soil sample bags weighing between 0.6 and 3.4 kilograms were collected, also comparison samples were taken from outside the grave. The soil was analyzed in the archaeology laboratory of the University of Helsinki. Organic matter was separated from the samples using water. This way, the exposed fibres and hairs were identified with the help of transmitted-light and electron microscopy.

Oldest feather fragments found in Finland

From the soil samples, a total of 24 microscopic (0.2-1.4 mm) fragments of bird feathers were identified, most of which originated in down. Seven feather fragments were identified as coming from the down of a waterfowl (Anseriformes). These are the oldest feather fragments ever found in Finland. Although the origin of the down is impossible to state with certainty, it may come from clothing made of waterfowl skins, such as a parka or an anorak. It is also possible that the child was laid on a down bed.

In addition to the waterfowl down, one falcon (Falconidae) feather fragment was identified. It may have originally been part of the fletching of the arrows attached to the arrowheads, or, for example, from feathers used to decorate the garment.

Dog or wolf hairs?

Besides the feathers, 24 fragments of mammalian hair were identified, ranging from 0.5 to 9.5 mm in length. Most of the hairs were badly degraded, making identification no longer possible. The finest discoveries were the three hairs of a canine, possibly a predator, found at the bottom of the grave. The hairs may also originate, for example, in footwear made of wolf or dog skin. It is also possible a dog was laid at the child's feet.

"Dogs buried with the deceased have been found in, for example, Skateholm, a famous burial site in southern Sweden dating back some 7,000 years," says Professor Kristiina Mannermaa, University of Helsinki.

"The discovery in Majoonsuo is sensational, even though there is nothing but hairs left of the animal or animals -- not even teeth. We don't even know whether it's a dog or a wolf," she says, adding: "The method used, demonstrates that traces of fur and feathers can be found even in graves several thousands of years old, including in Finland."

"This all gives us a very valuable insight about burial habits in the Stone Age, indicating how people had prepared the child for the journey after death," says Kirkinen.

The soil is full of information

Also found were three fragments of plant fibres, which are preserved particularly poorly in the acidic Finnish soil. The fibres were what are known as bast fibres, meaning that they come from, for example, willows or nettles. At the time, the object they were part of may have been a net used for fishing, a cord used to attach clothes, or a bundle of strings. For the time being, only one other bast fibre discovery dating back to the Mesolithic Stone Age is known in Finland: the famed Antrea Net on display in the National Museum of Finland, laced with willow bast fibres.

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Pterosaur discovery solves ancient feather mystery

An international team of palaeontologists has discovered remarkable new evidence that pterosaurs, the flying relatives of dinosaurs, were able to control the colour of their feathers using melanin pigments.

The study, published in the journal Nature, was led by University College Cork (UCC) palaeontologists Dr Aude Cincotta and Prof. Maria McNamara and Dr Pascal Godefroit from the Royal Belgian Institute of Natural Sciences, with an international team of scientists from Brazil and Belgium.

The new study is based on analyses of a new 115 million year old fossilized headcrest of the pterosaur Tupandactylus imperator from north-eastern Brazil. Pterosaurs lived side by side with dinosaurs, 230 to 66 million years ago.

This species of pterosaur is famous for its bizarre huge headcrest. The team discovered that the bottom of the crest had a fuzzy rim of feathers, with short wiry hair-like feathers and fluffy branched feathers.

"We didn't expect to see this at all," said Dr Cincotta. "For decades palaeontologists have argued about whether pterosaurs had feathers. The feathers in our specimen close off that debate for good as they are very clearly branched all the way along their length, just like birds today."

The team then studied the feathers with high-powered electron microscopes and found preserved melanosomes -- granules of the pigment melanin. Unexpectedly, the new study shows that the melanosomes in different feather types have different shapes.

"In birds today, feather colour is strongly linked to melanosome shape." said Prof. McNamara. "Since the pterosaur feather types had different melanosome shapes, these animals must have had the genetic machinery to control the colours of their feathers. This feature is essential for colour patterning and shows that coloration was a critical feature of even the very earliest feathers."

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Migratory birds have lighter-colored feathers

Migratory birds are specially adapted to find their way over extreme distances that represent remarkable tests of endurance. Now, researchers reporting December 6 in the journal Current Biology have discovered an unexpected way that migratory birds keep their cool during such arduous journeys: lighter-colored feathers.

"We found across nearly all species of birds, migratory species tend to be lighter colored than non-migratory species," said Kaspar Delhey of the Max Planck Institute for Ornithology, Seewiesen, Germany. "We think that lighter plumage coloration is selected in migratory species because it reduces the risk of overheating when exposed to sunshine. Lighter surfaces absorb less heat than darker ones, as anybody wearing dark clothes on a sunny day can attest! This would be particularly important for long-distance migrants that undertake extensive flights during which they cannot stop to rest in the shade."

Delhey and colleagues had been studying the effects of climate on bird coloration. Their earlier studies showed that, in general, lighter colored birds are found where temperatures are high and there is little shade. Presumably that's at least in part because the birds' lighter plumage helps to keep them cooler in the hot sun. Around that same time, the researchers came across studies by others showing that some birds fly at much higher altitudes during the day compared to at night.

"Because flying at high altitude is likely costly, these changes required an explanation," Delhey says. "One possibility was that flying higher, where it is colder, would offset the heat absorbed by the plumage when the sun was shining."

If so, they realized, another way to reduce the risk of overheating would be to absorb less solar radiation in the first place. It raised a question: have migratory species evolved lighter feathers?

To find out, they quantified overall plumage lightness (from 0 = black to 100 = white) for all bird species, using bird images from the Handbook of the Birds of the World. Next, they compared the data on coloration with the species' migratory behavior, while controlling for other factors known to effect plumage color.

Overall, the findings show that bird species get increasingly lighter as they migrate more. So, resident birds tend to be darker than short-distance migrants. Short-distance migrants are darker than bird species that travel farther. Delhey said that the one of the biggest surprises was how consistent the effect was across different types of birds. They saw the same pattern in birds large and small. The same held true in waterbirds and land-dwelling birds, too.

The findings are another reminder of the important role of temperature and climate factors more broadly in shaping the evolution of animal coloration. They also have clear implications for understanding the impacts of global warming and potential adaptive evolutionary responses, the researchers say.

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Dinosaur feather study debunked

 

Archaeopteryx fossil

A new study provides substantial evidence that the first fossil feather ever to be discovered does belong to the iconic Archaeopteryx, a bird-like dinosaur named in Germany on this day in 1861. This debunks a recent theory that the fossil feather originated from a different species.

The research published in Scientific Reports finds that the Jurassic fossil matches a type of wing feather called a primary covert. Primary coverts overlay the primary feathers and help propel birds through the air. The international team of scientists led by the University of South Florida analyzed nine attributes of the feather, particularly the long quill, along with data from modern birds. They also examined the 13 known skeletal fossils of Archaeopteryx, three of which contain well-preserved primary coverts. The researchers discovered that the top surface of an Archaeopteryx wing has primary coverts that are identical to the isolated feather in size and shape. The isolated feather was also from the same fossil site as four skeletons of Archaeopteryx, confirming their findings.

"There's been debate for the past 159 years as to whether or not this feather belongs to the same species as the Archaeopteryx skeletons, as well as where on the body it came from and its original color," said lead author Ryan Carney, assistant professor of integrative biology at USF. "Through scientific detective work that combined new techniques with old fossils and literature, we were able to finally solve these centuries-old mysteries."

Using a specialized type of electron microscope, the researchers determined that the feather came from the left wing. They also detected melanosomes, which are microscopic pigment structures. After refining their color reconstruction, they found that the feather was entirely matte black, not black and white as another study has claimed.

Carney's expertise on Archaeopteryx and diseases led to the National Geographic Society naming him an "Emerging Explorer," an honor that comes with a $10,000 grant for research and exploration. He also teaches a course at USF, called "Digital Dinosaurs." Students digitize, animate and 3D-print fossils, providing valuable experience in paleontology and STEAM fields.

From Science Daily

Naked prehistoric monsters! Evidence that prehistoric flying reptiles probably had feathers refuted

 The debate about when dinosaurs developed feathers has taken a new turn with a paper refuting earlier claims that feathers were also found on dinosaurs' relatives, the flying reptiles called pterosaurs.

Pterosaur expert Dr David Unwin from the University of Leicester's Centre for Palaeobiology Research, and Professor Dave Martill, of the University of Portsmouth have examined the evidence that these creatures had feathers and believe they were in fact bald

They have responded to a suggestion by a group of his colleagues led by Zixiao Yang that some pterosaur fossils show evidence of feather-like branching filaments, 'protofeathers', on the animal's skin.

Dr Yang, from Nanjing University, and colleagues presented their argument in a 2018 paper in the journal Nature Ecology and Evolution. Now Unwin and Martill, have offered an alternative, non-feather explanation for the fossil evidence in the same journal.

While this may seem like academic minutiae, it actually has huge palaeontological implications. Feathered pterosaurs would mean that the very earliest feathers first appeared on an ancestor shared by both pterosaurs and dinosaurs, since it is unlikely that something so complex developed separately in two different groups of animals.

This would mean that the very first feather-like elements evolved at least 80 million years earlier than currently thought. It would also suggest that all dinosaurs started out with feathers, or protofeathers but some groups, such as sauropods, subsequently lost them again -- the complete opposite of currently accepted theory.

The evidence rests on tiny, hair-like filaments, less than one tenth of a millimetre in diameter, which have been identified in about 30 pterosaur fossils. Among these, Yang and colleagues were only able to find just three specimens on which these filaments seem to exhibit a 'branching structure' typical of protofeathers.

Unwin and Martill propose that these are not protofeathers at all but tough fibres which form part of the internal structure of the pterosaur's wing membrane, and that the 'branching' effect may simply be the result of these fibres decaying and unravelling.

Dr Unwin said: "The idea of feathered pterosaurs goes back to the nineteenth century but the fossil evidence was then, and still is, very weak. Exceptional claims require exceptional evidence -- we have the former, but not the latter."

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