Anomalocaris is an icon of the enigmatic marine realms of the Cambrian. Anomalocaridids are considered the top predators of Cambrian and Ordovician ecosystems, as suggested by their large size (~1m) and large frontal appendages appropriate for grasping prey, among other circumstantial evidence. However, the eyes of Anomalocaris were poorly understood, even though the fossil record of this group is fairly good . It was known previously that the eyes were large for given body size and placed on stalks, yet detailed information on eye structure was unavailable. On the basis of size, overall shape, and phylogenetic position as an arthropod stem group it was speculated that Anomalocaris had compound eyes.
Paterson et al. describe a new find from an apparently very fossil-rich Cambrian locality in South Australia (see my previous post). This new fossil features a well-preserved set of paired, large (~2cm) eyes, belonging to Anomalocaris. These eyes clearly are compound eyes, which suggests that compound eyes evolved outside Euarthropoda (Schinderhannes* + other crown-group arthropods). There are at least 16,000 lenses per eye, arranged in a dense hexagonal packing, with each lens having a diameter of about 100 μm.
The small size of the lenses indicates a very fine ‘sampling grid’, resulting in excellent resolving power. The inferred acuity of this eye actually may have exceeded the acuity of living arthropods. Paterson et al. think that Anomalocaris was a “large, macrophagous nektonic predator possessing sharp vision”, which may have influenced the tempo of the evolutionary arms race between predators and prey.
*as a footnote, the etymology of Schinderhannes is totally awesome. The legendary Schinderhannes is Germany’s most famous outlaw. Schinderhannes’ gang operated in the region where the fossil was found until Schinderhannes was guillotined in 1803.