Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian

Abstract

Di¡raction gratings are reported from external surfaces of the hard, protective parts of Wiwaxia corrugata, Canadia spinosa and Marrella splendens from the Burgess Shale (Middle Cambrian (515 million years), British Columbia). As a consequence, the above animals would have displayed iridescence in their natural environment; Cambrian animals have previously been accurately reconstructed in black and white only. A diversity of extant marine animals inhabiting a similar depth to the Burgess Shale fauna possess functional di¡raction gratings. The Cambrian is a unique period in the history of animal life where predatory lifestyles and eyes capable of producing visual images were evolving rapidly. The discovery of colour in Cambrian animals prompts a new hypothesis on the initiation of the`Big Bang' in animal evolution which occurred during the Cambrian: light was introduced into the behavioural systems of metazoan animals for the ¢rst time. This introduction, of what was to become generally the most powerful stimulus in metazoan behavioural systems, would have consequently triggered turbulence in metazoan evolution.

Early crest animals and the insight they provide into the evolutionary origin of craniates

“…In contrast to this prediction, the image-forming paired eyes are, however, not present in most animal groups but arose independently only among few but distantly related animal groups including molluscs, arthropods, chordates, and annelids. The sudden emergency of image-forming eyes among these distantly related animal groups is intriguing, and it recently has been interpreted as a response to increased predation and visual hunting (Fortey et al, 1997;Parker, 1998). The first arthropods as exemplified by Lower Cambrian Fuxianhuia and Shankouia were, however, definitely not active predators, but inactive suspension and microphagous feeders although these animals had paired, large, image-forming eyes.…”

Environmental factors which may have led to the appearance of colour vision

“…Motley rocks and di¡erent algae existed 500 Myr ago in the same way as now and were coloured regardless of the eyes which examined them. In addition, colour vision in invertebrates seems to have arisen even earlier than in vertebrates and, judging from their remains, some extinct marine invertebrates already possessed bright colours which may have been of use for intra-and interspeci¢c communications (Parker 1998).…”

Natural photonics for industrial inspiration

“…The earliest known examples of diffraction gratings, or indeed any form of structural colour, are from the Cambrian period and belong to the famous Burgess Shale animals of the Canadian Rockies, 508 Ma (Parker 1998). These linear, two-dimensional diffraction gratings have not survived in their entirety, rather as mosaics (e.g.…”

“…Soon after the first eyes, at least by 508 Ma (Parker 1998; new date for the Burgess Shale fossils), animals began to evolve sophisticated optical reflectorsnanostructures that interact with (reflect, refract and diffract) light rays, such as surface corrugations or internal stacks of thin layers. Today we find an array of optical reflectors in animals that have resulted from millions of years of evolutionary 'fine-tuning'.…”

“…Trilobites are the first animals known to host image-forming eyes, of ca 521 Ma (figure 1); eyes that contain efficient optics in their own right (Parker 1998(Parker , 2003. In the absence of vision, any incidental iridescence appearing before the Cambrian period would have been neutrally selective.…”