Namacalathus

Namacalathus is the first animal mineralised fossil. It is from the Ediacaran Period from later deposits dated from 550 to 543mya. 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 3 cm 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 holes symmetrically arranged. 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 2.5 cm but averages 0.61 cm. 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 proabably 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 fossil is always seen in a two dimensional secion in a variety of appearances. It can appear as a circle, a broken circle, a knobbly haxagon or heptagon, a Y or club shape, or even a broken heart or moon shape. Namacalathus lived on a reef with stromatolites and thrombolites. The stalk was attached to the sea floor or possibly algae growing on the reef surface. The environment was quite differnt to other Ediacaran assemblenges having a rough substrate. It is found in the Nama formation in Namibia. Namacalathus is found along with Cloudina, but it is far more frequent where it occurs by about ten to one.

Namacalathus is likely to be a cnidarian. The one named species is Namacalathus hermanastes. The name Namacalathus comes from firstly the Nama group in which it occurs, and secondly the Greek word kalathos, which means a basket in the shape of a lily, or wine goblet. The species name hermanastes comes from herma a Greek word for reef, and nastes which menas inhabitant. The organism was fisrt described by Grotzinger, Watters, and Knoll in 2000.

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 descruption 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