''Namacalathus'' is a problematic

metazoan Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

occurring in the latest Ediacaran. The first, and only described species, ''N. hermanastes'', was first described in 2000 from the Nama Group of central and southern

Namibia Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and ea ...

. A U–Pb zircon age from the fossiliferous rock in

and

Oman Oman ( ; ar, عُمَان ' ), officially the Sultanate of Oman ( ar, سلْطنةُ عُمان ), is an Arabian country located in southwestern Asia. It is situated on the southeastern coast of the Arabian Peninsula, and spans the mouth of ...

provides an age for the ''Namacalathus'' zone in the range from 549 to 542 Ma, which corresponds to the Late Ediacaran. Alongside '' Namapoikia'' and '' Cloudina'', these organisms are the oldest known evidence in the

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

of the emergence of calcified skeletal formation in

metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

ns, a prominent feature in animals appearing later in the

Early Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

. Shore ''et al.'' (2021) reported the first three-dimensional, pyritized preservation of soft tissue in ''Namacalathus hermanastes'' from the Nama Group (

), and evaluate the implications of this finding for the knowledge of the phylogenetic relationships of this animal; they suggest it is an ancestor of

Lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, brachi ...

n animals such as

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

s and worms. There are only five occurrences of ''Namacalathus'' (Namibia,

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

, Oman,

Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive region, geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a ...

Paraguay Paraguay (; ), officially the Republic of Paraguay ( es, República del Paraguay, links=no; gn, Tavakuairetã Paraguái, links=si), is a landlocked country in South America. It is bordered by Argentina to the south and southwest, Brazil to th ...

) 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 In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

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.

Morphology

The skeleton is believed to have consisted of high-magnesium calcite. 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. 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. 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 of shapes, including closed and open circles, irregular hexagons or heptagons, as well as heart and moon shapes.

Ecology

''Namacalathus'' was an ecological generalist, able to colonise a variety of settings in the mid- to off-ramp environs, adapting its size to suit the local conditions.

Affinity

''Namacalathus'' has typically been considered to represent a cnidarian-grade organism, due in part to its propensity for asexual reproduction by budding. Most recently, however (2015), it been interpreted as a lophophorate based on detailed observations of its skeletal construction, which point to accretionary growth in the manner of brachiopods and bryozoans.

Notes

References

{{Taxonbar, from=Q554023 Ediacaran life Enigmatic prehistoric animal genera Fossils of Namibia Precambrian Africa Precambrian fossils Fossil taxa described in 2000

Morphology

Ecology

Affinity

See also

Notes

References