164 Br J Ophthalmol 2004;88:164 Br J Ophthalmol: first published as 10.1136/bjo.2003.037259 on 21 January 2004. Downloaded from

COVER ILLUSTRATION ...... To see what you eat

ith the imagination and crea- Anomalocaris, too, must have had a wide tivity of an artist’s commune, field of view. Frontal views of the Wthe Cambrian explosion is creature suggest stereopsis was possible, believed to have produced almost all and stereopsis almost certainly must be the body plans on earth today within a present for the most efficient predators. geologically short period of perhaps Unlike that found in another early 30 million years (540–510 million years creature from this period, the trilobite ago) or less. All but one of the extant (BJO, April 2002), a calcite lens system phyla, and the basic body plans they was not present, or at least not pre- represent, had their beginnings in this Anomalocaris. served, so little can be said about the period. Furthermore, in a spectacular eyes, although they were not found to display of metazoan bloom, evolution have facets as would be seen with Nevertheless, the visual diversity of the forged several other phyla in this same compound eyes. But while Cambrian is worth considering. Opabinia period that are now extinct, although may have been agile, Anomalocaris prob- Opabinia (upper left on this month’s perhaps not as many as some enthu- ably wasn’t. It is believed that cover) must have been a predator, and siasts believe. It was the big bang of Anomalocaris, a protoarthropod, probably evolution. Curiously, the first known an odd one at that, with five eyes, a segmented body, and a miniature ele- more resembled a manta ray in its eyes appeared then, too, and there may swimming. With its anterior grasping be a relation. phant’s trunk for a nose. It was thor- oughly described by Harry Whittington forelimbs for feeding, and its circular Predation is a terrible and swift sword mouth with massive teeth, it probably for prey species and there is good (Cambridge palaeontologist in a mono- graph in 1975) as one of the most was accustomed to less well detended or evidence that predation drives evolu- slower, organisms, perhaps resembling tion, at least to some extent. Sensory remarkable creatures in the history of sea cucumbers, or their equivalent in the modalities drive predation. After all, the science, and is now considered a close Cambrian. It would not have needed predator must find the prey. Vision is cousin of the arthropods. Whittington stereopsis. the most far ranging and comprehensive found five eyes with two stalked pairs The Cambrian had organisms that sense and must be a principal facilitator and one smaller central eye in an represented extant phyla too. Waptia for predation. Auditory sense must be extraordinary creature approximately (lower left) was an arthropod, and entrained, hence cannot be as efficient 50–70 mm in length. The eyes are probably had compound eyes as do or as global as vision, and although it is almost spider-like in distribution, and an excellent sensory modality, we recog- probably provided stereopsis. arthropods today. This benthic species Whittington dissected beneath the car- walked along the bottom, feeding on nise auditory predators as exceptions— http://bjo.bmj.com/ for example, bats, dolphins. apace (the stuff of genius since these are organic debris. Its eyes were stalked and So, did vision drive the Cambrian fossils compacted into slate!) and found probably more sensitive to movement explosion? Dr Andrew Parker of the the animal to be bilaterally symmetrical than possessive of good acuity, if mod- zoology department at Oxford (like us, but in contrast with radially ern correlates are an example. University thinks so and describes his symmetrical, like a starfish). Opabinia Other more evolutionarily pedestrian thesis in his book In the Blink of an Eye. had this peculiar and flexible ‘‘elephant animals were present and probably repre- But, it cannot be this simple an expla- trunk’’ proboscis with what were most sent direct predecessors of contemporary nation, as evolution is neither directed likely grasping spines at the end of the phyla. Perspicaris (lower right) was a on September 24, 2021 by guest. Protected copyright. nor compelled, especially by so single a trunk. This trunk with its prehensile common species in the Cambrian, and is force. There are many other factors at spiny lips was probably designed to thought to be a crustacean predecessor. work including climate, geography, deliver food to the mouth on the ventral Many other creatures have been found water currents, oxygen levels, competi- surface. Although these creatures were from the Cambrian, and many of these tion, and even chance. Nevertheless, the rare within the Burgess shale, where are distant predecessors of contemporary formation of eyes, and especially the they were first found, it is likely that the species. And, at least one, Pikaia,a neurological processing necessary to large visual field the eyes would have protochordate, represents the lineage process the image must have had a produced, the streamlined segmented that is distantly related to vertebrates, significant role in the ‘‘Cambrian explo- body, and presumed aquatic speed made although perhaps a direct ancestor. But sion.’’ for an agile, and successful predator. that is a story for another day. Even the Cambrian explosion may not If Opabinia was the resourceful, bar- I R Schwab have been as dramatic and sudden as racuda-like predator of the Cambrian University of California, Davis, Sacramento, once assumed by some. The soft bodied seas, then Anomalocaris was the looming CA, USA; [email protected] fauna that must have presaged the lord and master. Anomalocaris (upper Cambrian and betokened evolutionary right of cover and this page) was the ThankstoDouglasErwin,PhD,forhis creativity—the Ediacaran fauna—were largest predator of the Cambrian seas comments and the National Museum of Natural History, Washington, DC for the probably more complicated than we with some species known to be up to generous use of these cover images. Thanks realise. The pre-Cambrian fauna, 2 metres in length. This widely distrib- to Raymond Rye for his assistance with these whether Ediacaran or not, probably laid uted animal was much more common images. The image of Anomalocaris was extensive molecular and phylogenetic than Opabinia, and certainly less subtle. provided by the Royal Ontario Museum and groundwork for the explosion to come. With large, laterally placed eyes, Desmond Collins, PhD.

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