The evolution of the feather: scales on the tail of Sinosauropteryx and an interpretation of the dinosaur’s opisthotonic posture

Abstract

The epidermis and dermis are exposed in the tail region of the theropod dinosaur Sinosauropteryx. The specimen under study, like many others of the genus and other air-breathing vertebrates discovered in the Jehol biota, shows strong opisthotonus (i.e., recurvature of the spine) that includes the neck and tail. Here, recurvature of the tail upwards is considered to have aided the separation of the dermal and epidermal elements of the skin. Addressing a somewhat controversial question, the sequence of events in which this apparently occurred also suggests that the development of opisthotonus may have occurred post mortem rather than perimortem in this specimen. Crucially, epidermal structures considered to be scales are preserved overlying the posterior part of the tail and alongside it. They are approximately 2.0–2.5 mm in diameter and have distinctive papillae radiating around a central point, comparable to scales in some modern day lizards. Some of these scales overlie thick structural fibres external to the body outline, extending posteriorly at steep angles to the body's long axis, considered by many workers to be protofeathers. Intervening between the epidermal scales and the deeper structural fibres are preserved traces of a dermal fibre meshwork with two layers of oppositely oriented fibres.