Pterosaurs and the evolution of melanin-based colours in feathers: pterosaurs had feathers and they were coloured!

Pterosaurs and the evolution of melanin-based colours in feathers: pterosaurs had feathers and they were coloured!

With their highly diverse sizes and ecologies, pterosaurs are fascinating extinct animals. Pterosaurs are not to be confused with their close relatives, the dinosaurs, from which they were anatomically very different. Pterosaurs indeed developed completely original morphologies; they were the first vertebrates to acquire active flight, well before birds did. Pterosaur membranous wings were supported by their elongated fourth manual digit. The earliest known pterosaurs (collected from late Triassic deposits, some 210 million years ago) had already wings and had therefore the ability to sustain powered flight. Besides having wings, some pterosaurs – such as the tapejarid pterosaur Tupandactylus imperator from the Early Cretaceous (115 million years ago) of Brazil – also had a head crest, made of non-mineralized – also called “soft” – tissues. The exact functions of those head crests are not fully resolved, in part because pterosaur head crests preserving soft tissues are rare. Only a few skulls belonging to Tupandactylus have been found and not all of them preserve details of the large soft-tissue crest.

A few years ago, I was shown a fossil skull of a new specimen of Tupandactylus, which comprises a large head crest and retains exquisite soft tissue preservation. I had in front of my eyes an exceptional pterosaur specimen. The soft tissue crest of Tupandactylus is made of a series of parallel fibres that curve towards the posterior part of the skull. Details of those brown elongate fibres are visible. When looking at the preservation of the crest, I noticed some curious integumentary structures along the occipital process (the posterior extension of the skull). Those small, individual structures were really looking like down feathers. In another area of the skull, still along the occipital process, there was another type of integumentary structure, elongate filaments. The pterosaur skull, in the region located along the occipital process, was associated with two distinct types of feathers; this was such an exciting finding! To add to that excitement, the down-like feathers had a typical branching structure, with short filaments branching at regular intervals from a central shaft. This branching structure is characteristic of open-vane feathers in theropod dinosaurs, including modern birds. This discovery is important as it is the first time that clearly branched feathers are reported from a pterosaur; it brings new information about the evolution of feathers within Avemetatarsalia, the lineage that also comprises the dinosaurs and among them, birds.

Reconstruction of Tupandactylus imperator (MCT.R.1884) showing the distribution of two types of feathers along the occipital process (colours are not reconstructed here). Image credit, Julio Lacerda.

The two types of feathers were examined using a powerful microscope (scanning electron microscope), which allowed to look at their internal structure. What we observed is that the feathers contain fossilized pigment organelles called melanosomes. The colour of feathers in modern birds is strongly linked to melanosome shape. The analysis of the melanosome shape in the feathers, and the skin of the head crest, showed some interesting results: the geometry of the melanosomes is ovoid in the branched feathers and elongated in the monofilaments (it was ovoid to spherical in the skin). Such a diversity of melanosome geometries in feather was previously only known from some non-avian dinosaurs and from extant birds. Our study has therefore important implications for understanding the evolution of melanin-based colouration. Melanosome diversity in other pterosaur fossils is low and resembles that in the skin of extant reptiles, where colours are produced by the interaction of melanosomes with other pigment cells. A low melanosome diversity in pterosaurs was therefore considered as retention of the ancestral, reptilian, state. Our discovery of diverse melanosome geometries in Tupandactylus feathers shows that this character was shared both by pterosaurs and theropod dinosaurs, including birds. This suggests that early feathers, in the common ancestor of pterosaurs and birds, probably already had melanosomes with different geometries. The soft tissues from the head crest also show different melanosome geometries.

Simplified time-tree phylogeny for melanosome geometry. The different categories (or ‘states’) of melanosome geometry are: one geometry (in black), two geometries (in red) and three geometries (in green). Only taxa for which melanosome length and aspect ratio was known have been included in our dataset (n = 20). Tupandactylus shows two melanosome geometries in its feathers.

The fact that both types of feathers (and also the skin) contain melanosomes with distinct shapes also suggests that they had different colours. The presence of similar melanosome geometries in the feathers from pterosaurs and theropod dinosaurs shows that the most common ancestor to both dinosaurs and pterosaurs already had already the genes necessary to modify melanosome geometry, and therefore the colour of feathers. This feature was therefore in place very early in the evolution of Avemetatarsalia, in the Middle to Late Triassic (about 240 million years ago). It is also interesting to mention that melanosomes in mammal hair also show diverse geometries (although to a much lesser extent than what we see in bird’s feathers). The ability to change melanosome geometry was therefore probably present in the genetic material of the ancestor of pterosaurs, birds and mammals, a basal amniote, some time during the Carboniferous period.  

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