Earlier this summer Lee et al. published a very nice finding in Nature: spectacular fossils of compound eyes of a marine arthropod from the Early Cambrian (~515ma) of South Australia. A summary of this paper, including photographs of the actual fossils, has already been posted by Matthew Cobb, but I still wanted to point out some really interesting aspects. The Early Cambrian is well-known for showing a remarkably fast evolution of diversity, both in terms of the numbers of species as well as morphology. Compound eyes have been previously described for Early Cambrian but the new findings add a lot of new information about this interesting time period. Not only are these specimens the oldest non-biomineralized, non-trilobite eyes in the fossil record with such great preservation, these compound eyes also have a very large number of lens ommatitidia per eye (~3000), far more than the eyes of contemporaneous trilobites (<50). If one measures complexity by means of ommatidia count, the fossil record showed an almost gradual increase in complexity over more than 40 million years. This is a very long time span, especially given that complex eyes can evolve in just a few hundred thousand years (Nilsson, D.-E., and S. Pelger. 1994. A pessimistic estimate of the time required for an eye to evolve. Proc. Roy. Soc. Lond. B 256:53-58.). Furthermore, all parts of the compound eye were already in place by the Early Cambrian, hence a simple evolutionary increase of the number of ommatitida, hand in hand with possibly advanced vision by having specialized receptor fields, should happen quickly. So, the new findings make perfect sense from an evolutionary perspective. Interesting from an ecomorphological perspective is the presumed predatory lifestyle of this marine arthropod, potentially capable of seeing in dim light. I’m sure this paper will re-fuel the discussion whether advanced vision drove the Cambrian increase in diversity.
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