The story began in the summer of 2015 when I was invited by Baoyu Jiang from the University of Nanjing to go in the field with his team. The site to be investigated was Daohugou in Inner Mongolia, which had produced an amazing assemblage of fossils, including mammals, pterosaurs and dinosaurs.
We were interested in the pterosaurs, however. Our aim was to investigate the evolution of feathers, and Daohugou is famous for the exquisite preservation of feathers. Since 1990, hundreds of dinosaur specimens from China have shown that bird-like feathers originated many millions of years before the origin of birds. In fact, isolated fossils had shown feather-like structures in some ornithischian dinosaurs, very distant from birds in the evolutionary tree. Needless to say, many palaeontologists disputed the homology of the ornithischian fluff with avian feathers and preferred to call it something else.
So, what about pterosaurs? Pterosaur fluff had been identified in the nineteenth century, but palaeontologists had always been cautious and named it ‘pycnofibres’, suggesting by the special name that the fluff of pterosaurs might have evolved independently from the feathers of birds. We eventually found the specimen we were looking for, a Daohugou pterosaur with thick fluff all over its body, and which had not been painted over with lacquer. We need a clean surface for microscopic and geological study.
We intended to challenge the orthodoxy and make a case that pterosaurs and ornithischian dinosaurs all possessed feathers. The driver came from recent work in genomic controls on development which told an exactly opposite story to the cautious scepticism of many palaeontologists. The Wnt, BMP and Shh developmental pathways in vertebrates are shared by the denticles of sharks, the scales of bony fishes and of reptiles, and the hair of mammals and feathers of birds. Further, genomic work shows that lizard scales, bird feathers and mammal hairs are the default, and they have to be suppressed by additional genomic regulators to stop them developing.
Zixiao Yang, lead author of our paper, discovered four pycnofibre types in our pterosaur specimen, and in another specimen – filaments, filament bundles, tufted whiskers and down – all of which occur in specific areas over the body and wings. We recognised all of them as present in dinosaurs and could see no reason to continue calling them pycnofibres. In all details of their morphology these three occur widely among dinosaurs, including close relatives of birds, so we say they are all feathers. The absence of feathers in large sauropod dinosaurs and armoured dinosaurs is explained by suppression (as hair is suppressed in elephants and whales).
Feathers are not unique to birds. They occur in birds, dinosaurs, and pterosaurs, and this brings their origin down to the Early Triassic, some 250 Mya, a time when the two main tetrapod lineages, the diapsids and synapsids, were speeding up and heating up, as life recovered from the devastating end-Permian mass extinction 252 Mya. Independent evidence shows that these animals shifted from sprawling to upright posture (footprints) and speeded up their thermoregulation (bone histology, bird-like respiration, oxygen isotopic data). This radical new vision clarifies and simplifies our understanding, and definitively strips birds of the uniqueness of their feathers.