Charnia

Charnia Temporal range: Ediacaran, 575–545 Ma[1] PreЄ Є O S D C P T J K Pg N
A cast of the holotype of Charnia masoni. Metric scale.
Scientific classification
Order:	Rangeomorpha?
Genus:	Charnia
Type species
C. masoni
Species
C. masoni Ford 1958 C. wardi Narbonne & Gehling 2003 C. antecedens Laflamme et al. 2007
Synonyms
Rangea sibirica=Glaessnerina sibirica=Charnia sibirica = C. masoni Rangea grandis=Glaessnerina grandis = C. masoni

Charnia is the genus name given to a frond-like Ediacaran 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 in Leicestershire, England, where the first fossilised specimen was found.

Diversity

Three species, C. masoni Ford 1958, C. wardi Narbonne & Gehling 2003, and C. antecedens Laflamme et al. 2007 can be distinguished on the basis of length/width ratios and the degree of attachment of adjacent branches.^[2]

Charnia masoni^[3] was brought to the attention of scientists by Roger Mason, a schoolboy who later became a professor of metamorphic petrology, in 1957 in what is now a protected fossil site in Central England, and named by Trevor Ford a local geologist. Tina Negus, a 15 year old schoolgirl at the time, had seen this fossil a year previously^[4] but her geography schoolteacher flatly denied the possibility of Precambrian fossils.^[5] The holotype of this species now resides, along with its sister taxon Charniodiscus, in New Walk Museum & Art Gallery, Leicester.

Charnia wardi,^[6] 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 2 m. The holotype is a fragmentary specimen with a C. masoni like structure. It was defined as a new species on the basis of long and narrow shape.

Charnia antecedens^[2] has more irregular and higher angles of branching than C. masoni.^[7]

A number of Ediacaran form taxa are thought to represent Charnia (or Charniodiscus) at varying levels of decay; these include the Ivesheadiomorphs Ivesheadia, Blackbrookia, Pseudovendia and Shepshedia.^[8]

Significance

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 1930s (in Namibia) and the 1940s (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,^{[9] [10]} and the current "state of the art" is something of a "statement of ignorance".^[11]

An increasingly popular theory has arisen since the mid 1980s, 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,^[12] a clade whose position in the tree of life is unclear, perhaps united by its construction via unipolar iterations of one cell family.

Distribution

Charnia is both temporally and geographically the most widespread Ediacaran fossil.^[11] The greatest abundance of specimens, which are also the oldest reliably dated Ediacaran fossils, are found along the southeast coast of Newfoundland.^[6]

Ecology

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.^[13]

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.^[11]

See also

Paleontology portal

List of Ediacaran genera

References

1. ^ Grazhdankin, Dima (2004). "Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution". Palæobiology 30 (2): 203–221,. doi:10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2. http://paleobiol.geoscienceworld.org/cgi/reprint/30/2/203.pdf. 
2. ^ ^{a b} M. Laflamme; G. M. Narbonne1, C. Greentree & M. M. Anderson (2007). "Morphology and taphonomy of an Ediacaran frond: Charnia from the Avalon Peninsula of Newfoundland". Geological Society, London, Special Publications 286 (1): 237–257. doi:10.1144/SP286.17. http://sp.lyellcollection.org/cgi/content/abstract/286/1/237. 
3. ^ Ford, T.D. (1958). "Precambrian fossils from Charnwood Forest". Yorkshire Geological Society Proceedings 31: 211–217. doi:10.1144/pygs.31.3.211. 
4. ^ Ford, Trevor. "The discovery of Charnia". http://www.charnia.org.uk/newsletter/2007/discovery_charnia_2007.htm. 
5. ^ Negus, Tina. "An account of the discovery of Charnia". http://www.charnia.org.uk/newsletter/2007/discovery_charnia_2007.htm. 
6. ^ ^{a b} Narbonne, G.M.; Gehling, J.G., (2003). "Life after Snowball: the oldest complex Ediacaran fossils". Geology 31: 27–30. doi:10.1130/0091-7613(2003)031<0027:LASTOC>2.0.CO;2. http://geol.queensu.ca/people/narbonne/LifeAfterSnowball1.pdf. 
7. ^ Hofmann, H J, O'Brien, S J, King, A F: EDIACARAN BIOTA ON BONAVISTA PENINSULA, NEWFOUNDLAND, CANADA in Journal of Paleontology, Jan 2008
8. ^ Liu, A. G.; McIlroy, D.; Antcliffe, J. B.; Brasier, M. D. (2011). "Effaced preservation in the Ediacara biota and its implications for the early macrofossil record". Palaeontology: In press. doi:10.1111/j.1475-4983.2010.01024.x.  edit
9. ^ 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. 
10. ^ Gary C. Williams. Aspects of the Evolutionary Biology of Pennatulacean Octocorals. http://research.calacademy.org/research/izg/EvolutionaryBiology.htm. 
11. ^ ^{a b c} Antcliffe, J.B.; Brasier, M.D. (2008). "Charnia at 50: Developmental Models for Ediacaran Fronds". Palaeontology 51 (1): 11–26. doi:10.1111/j.1475-4983.2007.00738.x. 
12. ^ 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.
13. ^ Narbonne

External links

For pictures of Charnia, see:

• http://geol.queensu.ca/museum/index.php?option=com_content&view=article&id=55&Itemid=62

An article on the discovery of Charnia masoni:

• http://www.charnia.org.uk/newsletter/brit_assoc_2002.htm