33.3: To Feather or Not To Feather - Dinosaur Body

Early Feathers

Our interpretations about the coverings of dinosaurs have gone through many iterations over the years. The first dinosaur reconstructions were entirely scaly, in keeping with reptilian ancestry, even with finds like Archaeopteryx pointing to a feathered linage. Early fossil hunters reportedly only cared for the bones, and potentially hundreds of skin and feather impressions were lost to their carelessness. After the dinosaur renaissance and new finds in the later half of the 20th Century, feathers became more accepted. If anything, the feather debate has swung too far, suggesting all dinosaurs were covered in fuzz and fluff. Most dinosaurs have no remains that indicate feathers or not, so plenty is still unknown about dinosaur skin covering. But let us review what is known.

Figure \(\PageIndex{1}\): Diagram showing the stages of feather evolution as outlined by Xu & Gou 2009. 1. Single filament 2. Multiple filaments joined at their base 3. Multiple filaments joined at their base to a central filament 4. Multiple filaments along the length of a central filament 5. Multiple filaments arising from the edge of a membranous structure 6. Pennaceous feather with vane of barbs and barbules and central rachis 7. Pennaceous feather with an asymmetrical rachis 8. Undifferentiated vane with central rachis. (Drawing by Matt Martyniuk via Wikimedia.)

How and why did feathers evolve in the first place? Feathers are not fundamentally different from other vertebrate animal body coverings. The proteins found in feathers, mammal hair, and nails are keratin, similar to what is found in scales of reptiles. This means that feathers can be considered a homologous trait via divergent evolution from a common ancestor with modern reptiles. According to a recent overview of feather evolution (Xu and Guo, 2009), feathers first evolved as a single hair-like object and eventually became more complex. Most scientists infer that feathers would have started out for insulation, and eventually been adapted for service in new functions, such as display and flight. This is corroborated by the fact that smaller dinosaurs have been the most often found with feathers and other coverings. The smaller the animal, the higher the animal’s body surface area to volume ratio, meaning heat loss can occur more quickly. Feathers could reduce this heat loss.

Figure \(\PageIndex{2}\): Sketch of Longisquama (by Nobu Tamura via Wikimedia.)

The fossil record does not give us many early examples of feathers, so the initial origin of feathers as a biological structure is hard to know. What is known is that almost all examples of feathers are found on dinosaurs, both avian and non-avian. One possible exception to that rule is found on the enigmatic fossil reptile Longisquama, known from the Triassic of Kazakhstan. Its name, meaning “long scales,” refers to the large feather-like protrusions sticking from the animal’s back. There is much debate over the purpose of these features, or if they are truly feathers in the first place. One animal that Logisquama may be related to is Coelurosauravus, an early reptile which has a membrane protruding from its sides, and presumably used for gliding. Could Logisquama’s long scales be used for gliding, too? Or were they for display to attract mates or identify members of its kind? More specimens and more discoveries of related animals may need to be found before we can know for sure.

Which dinosaurs had feathers?

Figure \(\PageIndex{3}\): hylogeny and integument description based on Benton et al. (2019). Images of scales and feather types taken from File:Distribution of feathers in Dinosauria.jpg. Silhouettes from works by Slate Weasel, Matt Martyniuk, Audrey.m.horn, Tom Parker, Michael B. H. and Gareth Monger. (Cladogram by Kiwi Rex via Wikimedia.)

Dinosaurs with feathers or feather-like structures have been found on several branches of the dinosaur family tree. Interestingly, even some ornithischian dinosaurs had feather-like filaments. The pterosaurs had pycnofibres, which were dense bristle-like structures. While these were not exactly feathers, recent studies have shown more variety suggesting that some of these might correspond to dinosaur feathers. Another example, Psittacosaurus, a small early member of the ceratopsian lineage, has been found with a large number of filaments sticking out of its back, from the hip to the mid-tail. Full pennaceous (flying style) feathers, like those seen in modern birds, are found only in maniraptorans, with the most basal example from the ornithomimids. This means that the famous raptors of the Jurassic Park movies would almost certainly have been feathered. In fact, Velociraptor itself has been found with quill knobs, small impressions in the bone of the forearm where a feather would originate. Of course, complex feathers and significant coverings are only found on the coelurosaur branch of the theropods, with some paleontologists thinking all coelurosaurs had feathers.

Figure \(\PageIndex{4}\): Photo of Yutyrannus tail feathers (by Kumiko via Wikipedia.)

How about the Jurassic Park co-star, Tyrannosaurus rex? In 2012, a dinosaur named Yutyrannus was described. Meaning “feathered tyrant,” it is the largest dinosaur yet found with evidence of feathers. It has also been phylogenetically placed within the superfamily Tyrannosauridea, and thus could properly be called a tyrannosaur. Tyrannosaurus is considered further down the evolutionary tree with respect to Yutyrannus. Does that mean T. rex had feathers?

The most up-to-date information, based on both T. rex and closely-related large animals, has concluded that the feathers they had were scarce at best. The video game Saurian compiled this information based on published studies to make possibly the most accurate reconstruction yet. Despite this, a 2019 American Museum of Natural History exhibit shows Tyrannosaurus with a feathery body, making the argument that related animals indicate feathers were possible. The exhibit also shows a juvenile with feathers. This is another possibility: even if the adult version was bald, hatchlings may have needed the ‘down’ and could have lost it as they aged.

Dinosaurs to Birds

Figure \(\PageIndex{5}\): Photograph of Confuciusornis specimen (a) and close up of wing (b) IVPP V13156 (by Amanda R. Falk, Thomas G. Kaye, Zhonghe Zhou, David A. Burnham via Wikimedia.)

As evolution progressed, several non-avian dinosaur groups became more and more bird-like, with feathers being only part of the story. This group of animals belongs to the clade Parves, which means “next to birds.” The ornithomimids, for example, derive their name from Ornithomimus, whose name means “bird mimic.” This is due to their toothless beaks and overall body plan that resembled an ostrich. Other dinosaurs, like Microraptor, had an even more bird-like appearance, with long flight-like feathers on its arms… and also its legs! Microraptor was a dromaeosaur, in the same group as other famous animals like Velociraptor and Deinonychus. The dinosaur group that is most closely related to birds but not technically birds are the troodontids. The first non-pterosaur that could have flown was Archaeopteryx. It is part of a group called Avialae, “bird wings.” This is the group where the line between non-avian dinosaurs and birds becomes blurry. Euornithies is the clade that contains true birds, and a subgroup of that is called Aves, the crown group of modern birds.

Figure \(\PageIndex{6}\): Reconstruction of Archaeopteryx in a tree (by Nobu Tamura via Wikimedia.)

One of the biggest ongoing debates in bird evolution is how flight evolved itself. There are two main camps on this question. One idea is that feathers enabled these theropods to be more agile on the ground. As their agility increased, feather complexity also increased, eventually aiding their flight. The other idea proposes these animals already lived in trees, and were gliding from branch to branch. Their feathers assisted in gliding, and eventually led to flight. There is also an in-between, in which the wings helped with climbing, as some modern birds have been experimentally shown to do today. There are compelling arguments on all sides, so no one explanation has yet achieved widespread acceptance.