An Eye-Opening Discovery: The Remarkable Vision of Anomalocaris
Readers may recall the Illustra Media production Darwin's Dilemma: The Mystery of the Cambrian Fossil Record. The documentary opens with an animation of the hideous Anomalocaris, undisputed terror of the Cambrian seas. The creature is in the news today thanks to new revelations about its fantastic vision. Sadly, it's proven to be another tough day to be a Darwinian.
Like something out of science fiction, Anomalocaris (whose name means "strange shrimp"), with lengths ranging up to two meters, propelled itself with ease through the Cambrian waters by moving the lobes on the side of its body in a smooth, wavelike motion. Its mouth was shaped like a disk. Made up of 32 overlapping plates, the mouth could close somewhat to devour and crush its prey. The mouth, however, never fully closed. In front of the creature's mouth were two formidable grasping claws, armored with barb-like spikes, which it likely used to capture and devour its prey.
This beast poses mysteries both small and large. The diet of Anomalocaris, for example, is not clear. Some have argued that the creature fed on trilobites. The evidence cited for this includes the fact that trilobite fossils from the period commonly exhibit injuries such as bite marks. But of late some experts have challenged this notion . For one thing, no fossilized Anomalocaris feces (or, for that matter, gut contents) has been shown to contain signs of indigestible crustacean exoskeleton. The mouth of Anomalocaris also appears too weak to crush trilobites shells. In view of the evidence of bite marks on trilobite fossils, it has been suggested that the creature may have chewed or ingested trilobites but didn't swallow them and spat them out. The creature's primary diet was probably soft-bodied organisms.
Bizarrely, the eyes of Anomalocaris were positioned on stalks on the side of the animal's head. The popular science media are abuzz about a recent discovery -- published in Nature and based on fossils found on Kangaroo Island, South Australia -- that Anomalocaris possessed compound eyes similar to those that modern insects and arthropods have today. The only difference is that Anomalocaris had even more lenses per eye (according to the Paterson et al. Nature article, as many as 16,000 hexagonal lenses per eye) than most of those modern groups do. These remarkable eyes (which presumably would have required a reasonably advanced brain) would have afforded the creature very sharp vision. In fact, according to the Nature News report,
It is possible that the eyes of Anomalocaris had even more than 16,000 lenses -- the fossils are detailed, but they are not perfect. In fossil form, the stalked eyes are flattened, like pancakes. But Paterson speculates that the eyes of a living anomalocaridid would have been bulbous, and that if non-flattened eyes were to be found, many more lenses would be discovered on the other side.New Scientist reports:
"Very few modern animals, particularly arthropods, have eyes as sophisticated as this," says Paterson. Houseflies, for instance, have a mere 3000 lenses. The only comparable species are some predatory dragonflies that have up to 28,000 lenses in each eye.Anomalocaris is one of the many extraordinary creatures that emerge during the period in the history of life known as the Cambrian explosion (~530 million years ago), seemingly without clear precursors. What's more, there is no evidence to suggest that the Cambrian morphologies were fundamentally simpler in their composition than representatives of those phyla living today -- for example, in having fewer cell types or more rudimentary eye structures. Rather, the Cambrian animals are highly complex and sophisticated. Stephen Meyer also has a great paper from 2004, published in the journal Proceedings of the Biological Society of Washington (the paper that got the journal's editor Richard Sternberg into trouble).
For responses to objections to the challenge of the Cambrian to Darwinian evolution, see this article. It seems that with each new discovery, neo-Darwinism becomes less and less plausible as a viable candidate explanation for the complexity and organization we find in living systems.