Namacalathus
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Namacalathus (name derives from the Nama group where it was described, and the Greek word kalathos, meaning basket in the shape of a lily, or wine goblet) is a problematic metazoan fossil occurring in the latest Proterozoic Nama Group of central and southern Namibia. The single species hermanastes (from herma a Greek word for reef, and nastes for inhabitant) was first described by Grotzinger, Watters, and Knoll in 2000. The authors propose that the animal is more likely a Cnidarian than a Poriferan (sponge) or primitive Bilaterian.
The Ediacaran Nama fossils dating from about 550 to 543 Mya 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 two species of Cloudina, and other taxa and ichnofossils found in the formation that are poorly preserved.
The Nama Group fossils occur within thrombolitic facies of immence 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. The stalk is from 1 to 2 mm in diameter, and can be up to 30 mm long. 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. The holes are 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 mm 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 with calcite crystals precipitated in an organic matrix. 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.
[edit] References
Watters article on digital reconstruction Wesley A. Watters and John P. Grotzinger: Digital reconstruction of calcified early metazoans, terminal Proterozoic Nama Group, Namibia: in Paleobiology, 27(1), 2001, pp. 159–171
Original description John P. Grotzinger, Wesley A. Watters, and Andrew H. Knoll: Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia: in Paleobiology, 26(3), 2000, pp. 334–359
