Melanosomes are organelles within the cells of Vertebrates within which the pigment melanin is stored. The first fossil melanosomes were described in 2008, since when they have been recorded in a range of fossil taxa, with the different shape of different melanosomes being used to make inferences about the colour of a variety of extinct Animals. One notable group to which this methodology has been applied is Mesozoic Birds, enabling palaeontologists to determine that some of the remarkable colouration, and therefore presumably colour-related behaviour, seen in modern Birds was already present in the Mesozoic.
One notable feature found in the plumage of a variety of Bird groups today is iridescence, a visual effect achieved by the coherent scattering of light at certain frequencies as it passes through the barbules of feathers, where layers of specially modified melanosomes are present. This works because melanin and keratin have different refractive indexes to both one-another and to air, enabling the bending of light at certain wavelengths in consistent ways. This is sometimes used to aid the camouflaging effect of the Bird's plumage, as in the case of the iridescent green plumage on the backs of Hummingbirds, it is more commonly used in displays and signalling between Birds.
Iridescence and plumage colour has been determined for a number of ancient Bird species, but feathers are also known in non-Avian Dinosaurs, with the most famous examples being the Jehol Biota from the Early Cretaceous of northeastern China, where a very large number of feathered Dinosaurs have been found. The feathers of these Jehol Dinosaurs are generally considered to be too modified by burial compression and thermal alteration, as well as oxidation after exposure, which has largely removed the kerogen matrix from which the fossils are made, leaving the majority of the fossils as (very detailed) rock impressions. This might seem to be an obstacle to the reconstruction of colour and iridescence in these fossils, but much of the work that has been done with fossil melanosomes has been based upon the shape of these structures, rather than their chemistry, which has allowed for the reconstruction of both non-iridescent structural colouration and iridescence in the plumage of Enatornithine Birds and non-Avian Paravian Dinosaurs (the Paravia is the group that includes Birds, Dromaeosaurs, and Troodontids).
All known iridescent nanostructures within feathers are found within the barbules (filaments projecting from the barbs of feathers), structures which appeared in early Maniraptorans (the group which includes Alvarezsaurs, Therizinosaurs, Oviraptosaurs, and Paravians), and to have been refined within the Paravians, and the presence of iridescence in a variety of Paravians could imply that the appearance of barbules may have been linked to iridescence.
It is highly likely that colouration has played an important role in the evolution of Birds and their ecology, and that the accumulating body of data on melanosomes in early Birds might provide insight into this. However, if non-Avian Paravians also had pigmented and iridescent plumage, then their evolution and ecology is equally likely to have been influenced by this. At the moment we have very little information on the colouration of these Dinosaurs, but potentially, if it can be collected, it may be possible to tell how their colouration changed as they reached maturity, and if their colouration was sexually dimorphic. To do this for any species of Dinosaur, a large dataset would be necessary, with numerous specimens with preserved feathers, reflecting both sexes and a range of ages.
In a paper published in the journal Acta Palaeontologica Polonica on 13 June 2023, Angus Croudace of the School of Geosciences at the University of Edinburgh, Caizhi Shen of the Dalian Natural History Museum, Junchang Lü of the Institute of Geology of the Chinese Academy of Geological Sciences, Stephen Brusatte, also of the School of Geosciences at the University of Edinburgh, and Jakob Vinther of the School of Earth Sciences at the University of Bristol, reconstruct the plumage colouration of a specimen, DNHM D2933, of Wulong bohaiensis, an Early Cretaceous feathered Microraptorine Dromaeosaurid from the Jehol Biota of Liaoning, China.
Previous studies of the skeleton of DNHM D2933 have determined that it was a juvenile, about one year old when it died and still growing, making it the first juvenile non-Avian Dinosaur to have the colour of its plumage determined, as far as Croudace et al. are aware. In order to determine the colouration, and iridescence, of the specimen, Croudace et al. used a statistical approach to the determining of the purpose of preserved melanosomes from different parts of the body. This works because different coloured melanosomes are known to be different in shape. Eumelanin-rich melanosomes are large and oblate and produce black colours, while pheomelanin-rich melanosomes are smaller and more ovoid, producing ginger or brown colours. Iridescent melanosomes are typically hollow or flat.
Dromaeosaurid Dinosaur Wulong bohaiensis (DNHM D2933), from Shangheshou, Chaoyang,
Liaoning, China, Early Cretaceous Jiufotang Formation with a minimum age of 120.3 million years (A₁). Samples 1–16 were labelled by Stephen Brusatte while taking samples
at the museum. Illustration by Jakob Vinther (A₂) to show distinct plumage groupings on Wulong bohaiensis (DNHM D2933). For clarity, in this illustration only the
samples with successful melanosome preservation are labelled. Preservation on each of the excluded samples was not sufficient for study. Croudace et al. (2023).Samples from DNHM D2933 were examined under a scanning electron microscope from several angles in order to establish the measurements of individual melanosomes. Since these 'melanosomes' are impressions rather than the original structures, it is impossible to determine whether-or-not they were hollow in life. However, the majority of the melanosomes observed appear to be to narrow to have been hollow. The density with which the melanosomes were packed, which has an effect on colour intensity, was not assessed.
Different types of melanosomes were observed on different parts of the body of the Dinosaur, enabling Croudace et al. to build up a picture of the distribution of colours its plumage, by comparison to a database of melanosomes from the plumage of modern Birds. Based upon this, Wulong bohaiensis is presumed to have been largely grey in colour, with patches of highly iridescent feathers on its forelimb and hindlimb remiges.
Wulong bohaiensis is the fifth non-avian Paravian Dinosaur in which iridescence has been described (the others being Microraptor, Caihong, Bohaiornis, and Eoconfuciusornis) and the first juvenile. Two previously described species, Microraptor and Caihong, have also had the colour of their plumage determined, with both thought to be predominantly black.
The presence of iridescence in the feathers of non-Avian Dinosaurs lends support to the proposal that feathers evolved before flight, and originally had a quite different purpose, such as signalling, being later co-opted as flying aids. However, it is possible that the common ancestor of the Paravians was at least partially airborne, and that feathers were later co-opted for signalling in different groups.
Feathers in modern Birds play a variety of roles, with signalling to other members of their species being important. In many modern Birds, iridescence plays an important part of such behaviour, often playing a role in courtship rituals. However, in such species, iridescence is typically only present in adults of reproductive age, not juveniles or subadults. DNHM D2933 is assessed to be a juvenile Dromaeosaur, about a year old, but still with some growing to do. In some Animals, reproduction can occur before the maximum size is reached, though in modern Birds of comparable size to Wulong bohaiensis, the best available analogue for reproductive behaviour, reproduction does typically occur until the individual is several years old, despite the fact that modern Birds typically reach their full size during their first year of life. This makes it unlikely that DNHM D2933 was reproductively active, and therefore unlikely that its iridescence was linked to mating behaviour. Some modern Birds, notably Corvids, develop iridescent feathers before reaching reproductive age. In these Birds, iridescence appears to be linked to the ability to recognise members of their own species, often as individuals (Corvids typically have quite complex social lives).
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