Evolution of Dinosaur Skin and Feathers: A Complex Story
Today, Paul Barrett from the Natural History Museum, Nicolas Campione, a lab alumnus and researcher at Uppsala University, and I published a detailed study of the evolution of dinosaurian integument. Despite extensive recent speculation, our analyses suggest that the majority of non-avian dinosaurs were more likely to have scales than to exhibit ‘feather-like’ structures that are the direct precursors to the feathers we see in modern birds.
Over the past two decades, a number of spectacularly preserved dinosaur fossils with feathers have revolutionized the field of palaeontology. The presence of feathers in birds and their immediate dinosaurian ancestors – theropods like Velociraptor – is uncontroversial, but the presence of true feather homologues, or protofeathers, in other major groups, such as ornithischian dinosaurs, has been highly debated. Several recent discoveries have suggested that, along with scales, filament-like ‘protofeathers’ might have been present in the common ancestor of all dinosaurs and ubiquitous in the entire group.
In order to test the idea that dinosaurs were primitively feathered, we compiled a comprehensive database of dinosaur skin fossils- the most complete to date- and attempted to reconstruct the evolutionary history of dinosaur scales and feathers using a maximum likelihood approach. Most of our analyses provide no support for the appearance of feathers in the majority of non-avian dinosaurs, and although many meat-eating dinosaurs were feathered, the ancestor of all dinosaurs was probably scaly. Interestingly, the quills and filaments in some major plant-eating ornithischian dinosaur groups were evolutionary experiments that were independent of true feather origins.
Untangling when particular integumentary features first evolved will help us understand the origins of feathers and why they first arose, but our analyses are limited to the data we have at hand and questions still remain. Importantly, our research also quantifies taphonomic biases and identifies gaps in the fossil record of dinosaur skin. The biggest and most significant gap is in the Triassic and Early Jurassic, when dinosaurs first originated and diversified, where very few integument fossils are known. Rocks of this age also lack significant lake/lagoonal fossil sites where delicate feather-like structures are preferentially preserved. This makes the origin of the direct filamentous precursors to feathers difficult to pinpoint. Whether or not the first dinosaurs had true ‘protofeathers’ may only be finally resolved with the discovery of more fossils, particularly from early in dinosaur evolutionary history.
P. M. Barrett, D. C. Evans, and N. E. Campione. 2015. Evolution of dinosaur epidermal structures. Biology Letters. 20150229. Doi: http://dx.doi.org/10.1098/rsbl.2015.0229