Anomalocaris

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A Cambrian predatory arthropod relative reaching perhaps half a metre or more, with a pair of large grasping frontal appendages, a circular plated mouth, stalked compound eyes and lateral swimming flaps. Its interest here is less zoological than historical, because it is the clearest demonstration in palaeontology of how easily disarticulated evidence misleads. Its parts entered the literature separately over decades and were each described as a different animal: the frontal appendage as a shrimp-like crustacean, from which the name Anomalocaris — "odd shrimp" — derives, the circular mouth as a jellyfish, and the body as a sponge. Only in the Cambridge re-study of the Burgess material were the pieces recognised as one organism, and the reconstruction has continued to change since. Anomalocaris also settles a question about Cambrian ecology: it is a large, mobile, visually guided predator, which means that by roughly 508 million years ago marine food webs already had several trophic levels rather than a flat mat-grazing structure.

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