Charnia

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Charnia
Fossil range: Ediacaran — 575–545 Ma^[1]

A cast of the holotype of Charnia masoni. Metric scale.

Order:	Rangeomorpha?
Genus:	Charnia

Species

Charnia wardi
Charnia masoni

Charnia is the genus name given to a frond-like Precambrian lifeform with segmented ridges branching alternately to the right and left from a zig-zag medial suture. The genus Charnia was named after Charnwood Forest, where the first specimen was found. There are two species, Charnia masoni and Charnia wardi.

Charnia masoni was discovered by Roger Mason, a schoolboy who would later become a professor of metamorphic petrology, in 1957 in what is now a protected fossil site in Central England. The holotype of this species now resides, along with its sister taxon Charniodiscus, in the Leicester City Museum.

Charnia wardi, later discovered in 1978 in southeast Newfoundland, was first described in 2003. This is the longest known Ediacaran age fossil reaching in some instances over 2m. The holotype is a fragmentary specimen with a Charnia masoni like structure. It was defined as a new species on the basis of long and narrow shape.

Charnia is a highly significant fossil for several reasons. Firstly it is the first fossil that was ever described that came from undoubted Precambrian rocks. Until this point the Precambrian was thought to be completely devoid of fossils and consequently possibly of life. Despite similar fossils being unearthed in the 1930's (in Namibia) and the 1940's (in Australia) these forms were assumed to be of Cambrian age and so were considered unremarkable at the time. Secondly, Charnia has become an enduring image of Precambrian animals. Originally interpreted as an alga (Ford), it was spectacularly recast as a sea pen (a sister group to the modern soft corals) from 1966 onwards (Glaessner). With this image of Precambrian sea pens in mind, the gates were open for the recognition of many other of the major animal groups in the Precambrian. However, this sea pen interpretation has recently been discredited,^[2] and the current "state of the art" is something of a "statement of ignorance".^[3]

An increasingly popular theory that has arrisen since the mid 1980's, following the work of Prof Adolf Seilacher who suggested that Charnia belongs to an extinct group of unknown grade that was confined to the Ediacaran Period. This theory suggests that almost all the forms that have been postulated to be members of many and various modern animal groups are actually more closely related to each other than anything else. This new group was termed the Vendobionta,^[4] a clade whose position in the tree of life is unclear, perhaps united by its construction via unipolar iterations of one cell family.

Charnia is both temporaly and geographically the most widespread Ediacaran fossil.^[3] The greatest abundance of specimens, which are also the oldest reliably dated Ediacaran fossils, are found along the southeast coast of Newfoundland.^[5]

Little is known of the ecology of Charnia. It was benthic, anchored to the sea floor. According to one currently popular hypothesis, it probably lived in deep waters, below the wave base (perhaps a great deal below the wave base); this means it could not have photosynthesised. Further it has no obvious feeding apparatus (mouth, gut etc) and so its ecology remains somewhat of an enigma. Some have speculated that it survived either by filter feeding or directly absorbing nutrients and this is source of considerable and significant current research. ^[6]

The growth and development of the Ediacara biota is also the source of continued research and it was this that was used to discredit the sea pen hypothesis. In contrast to the sea pens (that grow by basal insertion), Charnia grew by the apical insertion of new buds.^[3]

[edit] References

^ Grazhdankin, Dima (2004). "Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution". Palæobiology 30 (2): 203-221,.
^ Antcliffe, J.B.; Brasier, M.D. (2007). "Charnia and sea pens are poles apart". Journal of Geological Society 164 (1): 49. doi:10.1144/0016-76492006-080.
^ ^a ^b ^c doi:10.1111/j.1475-4983.2007.00738.x
^ SEILACHER, A. 1984. Late Precambrian and Early Cambrian Metazoa: preservational or real extinctions? 159–168. In HOLLAND, H. D., TRENDAL, A. F. and BERNHARD, S. (eds). Patterns of change in Earth evolution. Springer Verlag, New York, NY, 450 pp.
^ Guy M. Narbonne and James G. Gehling (2003). "Life after snowball: The oldest complex Ediacaran fossils". Geology 31 (1): 27-30.
^ Narbonne