Episode 183 – Feathers

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They are among the most diverse and complex forms of integument, occurring in a huge variety of forms and functions on living and extinct dinosaurs. This episode, we discuss the diversity and exciting unfolding evolutionary story of Feathers.

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Feathers

Feathers are a type of integument found exclusively (today at least) on birds. Feathers come in a wide variety of forms – broad plumes, fluffy down, short bristles, and more – and they serve a long list of functions – insulation, display, camouflage, tactile sense, flight, and plenty more.

Left: The main features of a “standard” body feather of a bird. This basic structure (and other structures such as down and bristles) has been modified over the course of bird evolution into an incredibly versatile and complex variety of shapes. Image in the Public Domain.
Right: A small sampling of the diversity of modern bird feathers. Image in the Public Domain.

Feathers are similar to other integument structures like scales and hair; they develop in a similar way, their growth is regulated by similar genes, and they’re composed of similar proteins. But the details of feather origins are unclear. It might be that they derived from ancestral scales, but it’s also possible they evolved separately.

This image shows several possible stages in feather evolution, starting as simple filaments and gradually developing more complex branching structures. There have been many hypotheses about what forms early feathers might have taken and in what order.
Image by Matt Martyniuk, CC BY-SA 3.0
Like most integument, feathers don’t often make great fossils, but there are exceptions. Most fossil feathers are preserved as compressions or in amber.
Left: Jurassic feather compression fossil. Image by H. Raab, CC BY-SA 3.0
Right: Cretaceous feathers in amber. Image by Tiouraren, CC BY-SA 4.0

Fossil feathers are often found on fossil birds, especially in sites of exceptional preservation like the Messel Pit or Jehol. But Mesozoic fossils also reveal an extraordinary variety of feathers on non-bird dinosaurs.

Top left: Fossil of Archaeopteryx, one of the oldest members of the bird lineage, fossilized with abundant impressions of feathers. Image by H. Raab, CC BY-SA 3.0
Top right: Fossil of the dromaeosaur Microraptor with associated feathers much like modern birds, even including aerodynamic wing feathers. Image from Hone et al 2010, CC BY 2.5
Bottom left: Fossil of the small theropod Sciurimimus with evidence of fuzz-like feathery covering. Image by Ghedoghedo, CC BY-SA 3.0
Bottom right: Fossil of Sinosauropteryx, the first non-bird dinosaur discovered with feathers, surrounded by feathery filaments. Image by Sam, CC BY-SA 2.0
The discovery of such a diversity of feathers on such a variety of dinosaurs has changed the way we think about the appearance and lifestyles of these animals.
Top left: Velociraptor. Image by Fried Wierum, CC BY-SA 4.0
Top right: Oviraptor. Image by Conty, CC BY-SA 4.0
Bottom left: Yutyrannus. Image by Tomopteryx, CC BY-SA 4.0
Bottom right: Therizinosaurus. Image by PaleoNeolithic, CC BY 4.0

Several major lineages of non-bird dinosaurs are now known to be ancestrally feathered, including dromaeosaurs (the “raptor” dinosaurs), oviraptorosaurs, ornithomimosaurs, and tyrannosaurs, among others.

These dinosaurs have also been found with feather-like integument, but they aren’t closely related to other feathered dinosaurs. Whether these are true feathers or different, similar structures is unclear.
Top: Psittacosaurus. Image by Robert Nicholls, CC BY 4.0
Middle: Kulindadromeus. Image by Tomopteryx, CC BY-SA 4.0
Bottom: Tianyulong. Image by Nobu Tamura, CC BY 3.0
This is a dinosaur family tree from Benton et al 2019 showing integuments known from major dinosaur groups. The fact that feather-like structures are known from several distantly-related groups suggests that either feather-like structures arose multiple times or that simple feathers were a feature of the earliest dinosaurs. The exact nature of these feather-like filaments and the exact position of certain dinosaurs on this tree aren’t fully clear, so other studies find slightly different results.

True feathers clearly evolved among theropod dinosaurs and were then inherited by birds and several related lineages. A number of more distantly-related dinosaur groups also have feather-like integument, which means either that feather-like structures convergently evolved multiple times in different dinosaur groups, or that the earliest dinosaurs already had some sort of fuzzy covering. Both options suggest feathers might be much more widespread among dinosaurs than we currently know.

The question of feather evolution is complicated even further by the fuzzy pycnofibers of pterosaurs. It’s unclear whether these are related to true feathers, but they do indicate that fuzz might have been common – or even ancestral – to early archosaurs even before dinosaurs arose.

Learn More

A Guide to Bird Feathers, Avian Report
Feathers Through Time, Cornell Lab

Dinosaurs Take Flight (including an overview of dinosaur feathers)

Feather Function and the Origin of Birds, 2022 paper (technical, open access)
The complex relationship between scales and feathers, 2018 paper (technical, open access)
The Early Origin of Feathers, 2019 paper (technical, paywall)

Pterosaur feathers (2022 news article)

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