The Evolution of Speed: an empirical and comparative analysis of drag-reducing scales in early fishes

From their earliest origins fishes have developed a suite of adaptations for locomotion in water. Even without data from behaviour, soft tissue, and extant relatives, it is possible to infer a wealth of palaeobiological and palaeoecological information. As in extant species, aspects of gross morphology such as streamlining, fin position and tail type are optimised even in the earliest fishes, indicating similar life strategies have been present throughout their evolutionary history. Drag-reducing riblets ornamenting the scales of fast-moving sharks have been subject to particularly intense research, but this has not been extended to extinct forms. The crowns of shark scales are exposed directly to the aquatic environment offering a unique opportunity to elucdate scale function in some of their long-extinct Palaeozoic relatives.
This thesis is the first comprehensive study of scale function in fossil fishes, and demonstrates that sophisticated adaptations for drag-reduction existed at a remarkably early stage in fish evolution. It is shown that riblet spacing reflects swimming speed in modern sharks, and that drag-reduction morphology evolved in the denticles of the earliest vertebrates, including the oldest examples known (~460 million years ago). Comparative analysis between modern sharks and fossil taxa reveals distinct and diverse regionalisation of scale features across the body, as well as ontogenetic changes. In addition to riblet spacing, the variation of other scale features is investigated quantitatively in modern sharks, revealing the importance of riblet angle in overall drag reduction. Furthermore, experimental skin friction measurements of fossil fish scales demonstrate the potential drag reduction of a diverse range of morphologies, even without riblets.