Namacalathus
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Namacalathus Fossil range: Ediacaran — 550–543 Ma |
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A sketch of Namacalathus cup on stalk with 7 windows
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Scientific classification | ||||
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Namacalathus[1] is a problematic metazoan fossil occurring in the latest Proterozoic Nama Group of central and southern Namibia. The single species N. hermanastes[2] was first described in 2000.[3] The animal is more likely to be a Cnidarian than a sponge or primitive Bilaterian.
The Ediacaran Nama fossils dating from about 550 to 543 million years ago comprise a distinctly Proterozoic assemblage of limited diversity with no evidence of arthropods, bivalves,or echinoderms that characterize early Cambrian life. Importantly, however, these fossils are the oldest known evidence in the fossil record of the emergence of calcified skeletal formation, a prominent feature in phyla appearing in the Cambrian Explosion. Among the Nama fossils, Namacalathus far outnumber Cloudina and other poorly preserved taxa and ichnofossils found in the formation.
The Nama Group fossils occur within thrombolitic facies of immense Proterozic stromatolitic reefs. Namacalathus lived a benthic existence with its stalk attached to the sea floor or possibly to algal mats growing on the reef surface.
[edit] Morphology
It has a unique shape with a cup on a stalk. The stalk is hollow all the way through and tapered from the bottom, ranging from 1 to 2 mm in diameter, and reaching 30 mm in length. The narrower top of the stalk connects to the cup. The cup is hollow and has a large hole in the top with the shell curving over forming a cup lip. Around the side of the globe are six or seven symmetrically arranged holes, called "windows". The wall curves inwards around each window in a formation called window lips. Each hole is slightly elongated vertically and expanded on the higher side. The size of the cup varies from two to about 25 mm, but averages 6.1 mm. The ratio of the height of the cup to the diameter is from 0.7 to 1.3. The fossil is lightly calcified, preserved as calcite crystals; its original morphology is unknown.[4] The walls in Namacalathus are only 0.1 mm thick, and often deformed by the weight of the sediment. The windows were probably originally filled with organic matter during life, but the cup was likely to be open.
Because the three dimensional shape of Namacalathus is complex, and the wall is so thin, the fossils appear as a two dimensional sections in a wide variety shapes, including closed and open circles, irregular hexagons or heptagons, as well as heart and moon shapes. [5]
[edit] Notes and references
- ^ Derivation: Nama, from the geological group where it was described, + Greek "kalathos", meaning "basket in the shape of a lily", or "wine goblet"
- ^ Derivation: Greek 'herma', reef, + 'nastes', inhabitant
- ^ Grotzinger, J.P.; Watters, W.A., Knoll, A.H. (2000). "Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia". Paleobiology 26 (3): 334–359. doi: .
- ^ Susannah M. Porter (1 June 2007). "Seawater Chemistry and Early Carbonate Biomineralization". Science 316 (5829): 1302. doi: . PMID 17540895.
- ^ Watters, W.A.; Grotzinger, J.P. (2001). "Digital reconstruction of calcified early metazoans, terminal Proterozoic Nama Group, Namibia". Paleobiology 27 (1): 159–171. doi: .