Chapter 11: A Biography of Earth
Feature Articles
1: The Moon: A Source of Insight into the Hadean Earth?
2: The Cradle of Life
3: Clues in the Fossil Record
Clues in the Fossil Record
by Stephen Marshak
Early Cambrian fossil
Credit: Science
Paleontologists have the unenviable task of trying to piece together the history of evolution using tiny bits of information separated by huge gaps of time. This effort is particularly difficult in the Precambrian where the fossil record is exceptionally sparse. Mysteriously following this scarcity, almost all the main types of animals (or phyla) that exist today, suddenly appear in the fossil record at the beginning of the Cambrian, around 545 million years ago.
Many paleontologists concluded that an explosion of evolutionary activity during the early Cambrian must be responsible. But others aren't convinced that evolution that rapid is possible. They have suggested a longer period of evolution that left no fossil record must have preceded the Cambrian.
German and British researchers recently made a discovery that lends support to the idea that the Cambrian explosion might not have been that explosive after all, and may have been preceded by a long evolutionary fuse in the Precambrian. They found some of the oldest known crustaceans in Lower Cambrian limestone deposits in Shropshire, England. The 511 year-old fossils are very well preserved with some of the soft body parts cast in calcium phosphate, allowing them to be identified with confidence unlike other rare fossils of this age.
Crustaceans such as these, as well as modern animals like crabs, lobsters and shrimp, are members of the arthropod phylum. Previously, the oldest undoubted crustaceans were from the late Cambrian, which left 40 million years for them to evolve from a primitive arthropod at the beginning of the Cambrian. But the new discovery of early Cambrian crustaceans suggests the process must have started sooner to allow enough time for all the necessary evolutionary steps from arthropod to crustacean.
Though the newly discovered fossils are good evidence for Precambrian evolution, paleontologists would like to find an early arthropod fossil that can be identified with confidence. The search is on, but uncovering such a fossil is akin to finding the proverbial needle in a haystack. The ancestral arthropods would most certainly have been very small and lacking a shell or skeleton that could be preserved for discovery by scientists millions of years later. Still, soft animal parts may be phosphatized as in the case of the recently discovered crustaceans. In spite of the odds mounted against them, fossil hunters continue their search in hopes of finding a Precambrian piece to the evolutionary puzzle.
REFERENCES:
- Siveter, D.J., Williams, M., Waloszek, D. 2001, A phosphatocopid crustacean with appendages from the Lower Cambrian: Science, v.293, p. 479-481.
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