Namacalathus
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Namacalathus Temporal range: Terminal Ediacaran550–542 Ma |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | incertae sedis |
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†Namacalathus Grotzinger et al. 2000 |
Namacalathus is a problematic metazoan fossil occurring in the latest Ediacaran. The first, and only described species, N. hermanastes, was first described in 2000 from Nama Group of central and southern Namibia.
A U–Pb zircon age from the fossiliferous rock in Namibia and Oman provides an age for the Namacalathus zone in the range from 549 to 542 Ma, which corresponds to the Late Ediacaran. These organism and Cloudina are the oldest known evidence in the fossil record of the emergence of calcified skeletal formation in metazoans, a prominent feature in animals appearing later in the Early Cambrian. There are only four occurrences of Namacalathus (Namibia, Canada, Oman, Siberia) known to date, all of which are found in association with Cloudina fossils.
Among the late Precambrian fossil assemblage in the Nama group, Namibia, 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 Proterozoic stromatolitic reefs. Namacalathus lived a benthic existence with its stalk attached to the sea floor by means of a holdfast, or possibly to algal mats growing on the reef surface.
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. 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.
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