The Cambrian Explosion (the abrupt appearance of animal phyla in the earliest fossil layers bearing multicellular body plans) remains unmuffled. Known by Darwin as a problem for his theory, it has become more problematic to his followers over time. There are now many more Cambrian fossils than Darwin knew of, and they continue the pattern: sudden appearance of complex animals, complete with legs, digestive systems, eyes, and nervous systems. Discoveries of Precambrian fossils have not helped: the ones that are more than microbial appear to be mere colonies of cells with no relationship to animals. Here are more discoveries that fit this pattern.
Burgess shale tulip animals: A new species of filter-feeding marine animal that resembles a tulip has been found on Mt. Stephen in British Columbia, Canada, site of the internationally famous Burgess Shale fossil bed. Pictures of the fossils and an artist reconstruction can be found on PhysOrg. Named Siphusauctum gregarium, the animal is about 8” high and lived in colonies. The Burgess Shale is dated Middle Cambrian, but the existence of this fully-formed animal, complete with gut, foot anchor and pump to drive water through its “unusual filter-feeding system” implies a very short fuse between the Precambrian and its abrupt appearance.
Crustaceans with modern aspect: Like lobster? Think of the complexity of this animal with eyes, antennae, claws, legs, mouth parts equipped with mandibles, and internal organ systems, including a digestive tract and sexual organs. Crustaceans are a highly diverse group of arthropods that include lobsters, crayfish, and crabs. Among the crustacean subphylum are the branchiopods (which includes the fairy shrimp and water flea), ostracods (small shelled crustaceans; see diagram of complex internal organs on the Lake Biwa Museum site), and copepods (“oar-foot” swimmers; see Smithsonian for description).
At another site in western Canada called the Deadwood Formation, a trio of scientists from Cambridge, Hawaii and Canada found exquisitely-preserved crustaceans “of surprisingly modern aspect” in mudstone, a type of rock that was thought to form very slowly (12/14/2007). The abstract of their paper in PNAS says it best:1
The early history of crustaceans is obscured by strong biases in fossil preservation, but a previously overlooked taphonomic mode yields important complementary insights….
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