Namacalathus

Namacalathus Temporal range: Ediacaran — 550–542 Ma
Scientific classification
Kingdom:	Animalia
Phylum:	incertae sedis
Genus:	Namacalathus Grotzinger et al. 2000

Namacalathus^{[derivation 1]} is a problematic metazoan fossil occurring in the latest Ediacaran. The single species N. hermanastes^{[derivation 2]} was first described in 2000 from Nama Group of central and southern Namibia.^[1] The animal is more likely to be a Cnidarian than a sponge or primitive Bilaterian.

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.^{[1][2][3][4][5]}

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 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.

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.^[6] 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.

Siberian specimens from the borehole Vostok 3 were designated as new species, because they have, unlike the type species N. hermanastes, a significantly smaller size. Most specimens show sections of the perforated cup, ranging from 110 to 230 μm in diameter; one specimen (with a cup 120 μm across) has a stalk (30 μm in diameter). The walls of the cup are 10 μm thick.^[4]

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.^[7]

See also

List of Ediacaran genera

Notes and references

1. ^ Nama, from the geological group where it was described, + Greek κάλαθος, kalathos, meaning "basket in the shape of a lily", or "wine goblet"
2. ^ Greek ἕρμα, herma, "sunken rock, reef", + νάστης, nastes, "inhabitant"

1. ^ ^{a b} 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:10.1666/0094-8373(2000)026<0334:CMITSR>2.0.CO;2. http://paleobiol.geoscienceworld.org/cgi/content/abstract/26/3/334. 
2. ^ Amthor, J.E.; Grotzinger, J.P., Schroder, S., Bowring, S.A., Ramezani, J., Martin, M.W., Matter, A (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman". Geology 31 (5): 431–434. Bibcode 2003Geo....31..431A. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2. http://geology.geoscienceworld.org/cgi/content/abstract/31/5/431. 
3. ^ Hofmann, H.J.; Mountjoy, E.W. (2001). "Namacalathus-Cloudina assemblage in Neoproterozoic Miette Group (Byng Formation), British Columbia: Canada's oldest shelly fossils". Geology 29 (12): 1091–1094. Bibcode 2001Geo....29.1091H. doi:10.1130/0091-7613(2001)029<1091:NCAINM>2.0.CO;2. http://geology.geoscienceworld.org/cgi/content/abstract/29/12/1091. 
4. ^ ^{a b} A.E. Kontorovich; A.I. Varlamov, D.V. Grazhdankin, G.A. Karlova, A.G. Klets, V.A. Kontorovich, S.V. Saraev, A.A. Terleev, S.Yu. Belyaev, I.V. Varaksina, A.S. Efimov, B.B. Kochnev, K.E. Nagovitsin, A.A. Postnikov and Yu.F. Filippov (2008). "A section of Vendian in the east of West Siberian Plate (based on data from the Borehole Vostok 3)". Russian Geology and Geophysics 49 (12): 932–939. Bibcode 2008RuGG...49..932K. doi:10.1016/j.rgg.2008.06.012. http://linkinghub.elsevier.com/retrieve/pii/S106879710800206X. 
5. ^ A.E. Kontorovich; B. S. Sokolov, V. A. Kontorovich, A. I. Varlamov, D. V. Grazhdankin, A. S. Efimov, A. G. Klets, S. V. Saraev, A. A. Terleev, S. Yu. Belyaev, I. V. Varaksina, G. A. Karlova, B. B. Kochnev, K. E. Nagavitsin, A. A. Postnikov and Yu. F. Filippov (2009). "The first section of vendian deposits in the basement complex of the West Siberian petroleum megabasin (resulting from the drilling of the Vostok-3 parametric borehole in the Eastern Tomsk region)". Doklady Earth Sciences 425 (1): 219–222. Bibcode 2009DokES.425..219K. doi:10.1134/S1028334X09020093. http://www.springerlink.com/content/h16441754732xr54. 
6. ^ Susannah M. Porter (1 June 2007). "Seawater Chemistry and Early Carbonate Biomineralization". Science 316 (5829): 1302. Bibcode 2007Sci...316.1302P. doi:10.1126/science.1137284. PMID 17540895. 
7. ^ Watters, W.A.; Grotzinger, J.P. (2001). "Digital reconstruction of calcified early metazoans, terminal Proterozoic Nama Group, Namibia". Paleobiology 27 (1): 159–171. doi:10.1666/0094-8373(2001)027<0159:DROCEM>2.0.CO;2. http://www.mit.edu/~watters/watters_etal_paleobio_2001.pdf. ^{[dead link]}