Marrella
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| Marrella Fossil range: Mid Cambrian |
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| Marrella splendens Walcott, 1912 |
Marrella splendens is an unusual arthropod known from fossils found in only a single stratum of the Middle Cambrian Burgess Shale of British Columbia. It is, however, the most common fossil in the Burgess Shale, with over 15,000 specimens catalogued.[citation needed] Marrella was the first fossil collected by Charles Walcott from the Burgess Shale. Walcott described Marrella informally as a "lace crab" and described it more formally as an odd trilobite. It was later reassigned to the now defunct class Trilobitoidea in the Treatise on Invertebrate Paleontology. In 1971, Whittington did a thorough redescription of the animal and, on the basis of its legs, gills and head appendages, concluded that it was neither a trilobite, nor a chelicerate, nor a crustacean.[1]
Marrella itself is a small animal, 2 cm or less in length. The head shield has two pairs of long rearward directed spikes. On the underside of the head are two pairs of antennae, one long and sweeping, the second shorter and stouter. Marrella has a body composed of 24–26 body segments, each with a pair of branched appendages. The lower branch of each appendage is a leg for walking, while the upper branch is a long, feathery gill. There is a tiny, button-like telson at the end of the thorax. It also contains eyes under its head. It is unclear how the unmineralized head and spines were stiffened. Marrella has too many antennae , too few cephalic legs, and too few segments per leg to be a trilobite. It lacks the three pairs of legs behind the mouth that are characteristic of crustacea. The legs are also quite different from those of crustaceans. The identification of a diffraction grating pattern on well-preserved Marrella specimens proves that it would have harboured an iridescent sheen — and thus would have appeared colourful.[2]
It is currently accepted that Marrella is a stem group arthropod — that is, it's descended from a common ancestor common to it and most or all of the major later arthropod groups. It is thought to have been a benthic (bottom-dwelling) marine scavenger living on detrital and particulate material. Marrella had a hard shell-like head that was not only hollow but like the shell of a crab.[3] Taken with two other unexpectedly unique arthropods, Opabinia and Yohoia, Marrella has demonstrated that the soft bodied Burgess fauna were much more complex and diverse than anyone had previously suspected.[4]
[edit] References
- ^ Whittington, H. B. (1971). Redescription of Marrella splendens (Trilobitoidea) from the Burgess Shale, Middle Cambrian, British Columbia. Geological Survey of Canada Bulletin 209: 1-24.
- ^ Parker, A.R. (1998). "Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian". Proceedings of the Royal Society B: Biological Sciences 265 (1400): 967–972. doi:.
- ^ Whittington, H. B. (1971). Redescription of Marrella splendens (Trilobitoidea) from the Burgess Shale, Middle Cambrian, British Columbia. Geological Survey of Canada Bulletin 209: 1-24.
- ^ Gould, Stephen Jay (2000). Wonderful Life: Burgess Shale and the Nature of History. Vintage.
