Opabinia
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2008-06-23T11:31:30Z
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{{Taxobox
|name = ''Opabinia''
|image = Opabinia BW.jpg
|fossil_range = Middle [[Cambrian]]
|regnum = [[Animal]]ia
|phylum = [[Lobopodia]]
|classis = [[Dinocarida]]
|ordo = [[Radiodonta]]
|genus = '''''Opabinia'''''
|genus_authority = [[Charles Walcott|Walcott]], 1912
|species = '''''O. regalis'''''
|binomial = ''Opabinia regalis''
|binomial_authority = [[Charles Walcott|Walcott]], 1912
}}
'''''Opabinia''''' is a fossil animal found in [[Cambrian]] [[fossil]] deposits. Its sole species, '''''Opabinia regalis''''', is known from the Middle Cambrian [[Burgess Shale]] of [[British Columbia]]. ''[[Myoscolex]]'', from the Lower Cambrian [[Emu Bay shale]] of [[South Australia]], is a possible relative. The discoverer of ''Opabinia'', [[Charles Doolittle Walcott]], named it after a local mountain, [[Opabin Peak]] in the [[Canadian Rockies]]. Fewer than twenty good specimens exist.
==Morphology==
The animal was segmented and had an unmineralized exoskeleton. Total body length ranged from 40 to 70 millimeters. The head carried five stalked eyes that would have given their owner a range of vision of almost 360°, and a long, flexible, hose-like [[proboscis]]. At the end of the proboscis were grasping spines; these are theorised to have served as a grab to catch prey, which would then be brought to the mouth, which was located underneath the head, behind the base of the proboscis.
The body segments each featured a set of [[gill]]s and a pair of flap-like appendages that are also dissimilar to other known animals of the time, save ''[[Anomalocaris]]''. The three rearmost flaps formed the tail. Unlike known [[arthropod]]s, the head does not appear to be formed from fused segments.
''Opabinia'' is thought to have lived on top of the soft sediment on the seabed, although it presumably could have swum after prey using its side flaps. On the bottom, the proboscis could have plunged into burrows after worms. It could also have been used to rapidly stir up sea floor sediment in search of food. If this were the case, ''Opabinia'' would then have folded back the proboscis to bring the food to its mouth on the underside of its head.
==History of classification==
The unusual morphology of ''Opabinia'' led to several attempts at classification in the last 100 years. For example, Walcott, the original describer, considered it to be an anostracan crustacean, whilst [[Leif Størmer]], following earlier work by [[Percy Raymond]], thought that it belonged to the so-called "[[trilobitoid|trilobitoids]]". However, modern studies of ''Opabinia'' were initiated by the comprehensive redescription by [[Harry Blackmore Whittington]] in 1975, who showed that the frontal structure was a flexible, probably fluid-filled nozzle. Whittington could find no evidence for arthropodan jointed limbs, despite dissecting through the body of the animal to where they would be expected to be found.
The lack of limbs, the annulated but not articulated body, and the unusual lateral lobes with gills led Whittington to conclude that ''Opabinia'' was no arthropod. Although he left its classification above the family level open, he considered that it may have been a representative of the ancestral stock from which both the annelids and arthropods arose. The demonstration that ''Opabinia'', and other taxa such as ''[[Marrella]]'' and ''[[Yohoia]]'', cannot be accommodated in modern groups was one of the primary reasons that [[Stephen J. Gould]] in his book on the [[Burgess Shale]] ''[[Wonderful Life (book)|Wonderful Life]]'', considered that Cambrian life was much more diverse and "experimental" than the extant fauna.
At the time that Whittington's redescription of ''Opabinia'' was published, the animal appeared unique. However, a decade later in 1985, [[Derek Briggs]] and Whittington published a description of ''[[Anomalocaris]]'', also from the Burgess Shale. The Swedish palaeontologist [[Jan Bergström]] soon suggested that the two animals were related, based on their shared lateral flaps with gills, stalked eyes and other features. In 1996, [[Graham Budd]] suggested that ''Opabinia'' was in fact a lobopod, and that it fell within the [[stem group]] of the [[Euarthropoda]], along with the anomalocaridids. The details of Budd's reconstruction have recently been criticised by Zhang and Briggs, although they retain Budd's original placement of ''Opabinia'' and ''Anomalocaris'' in the euarthopod stem group. The morphology of ''Opabinia'' thus remains controversial in several details.
The resemblance of ''Opabinia'' to some of the so-called "great appendage" arthropods such as'' [[Yohoia]]'' has led some workers either to place these arthropods at the top of the euarthropod stem group, or to return ''Opabinia'' to the euarthropod [[crown group]], in the stem group of the [[chelicerates]]. Thus, despite general agreement that ''Opabinia'' belongs within the total group of the arthropods, its exact placement remains disputed.
==See also==
[[Image:Opabinia_regalis2.jpg|thumb|350px|Reconstruction of ''Opabinia regalis'']]
* [[Body plan]]
==References==
* Bergström, J. (1986). ''Opabinia'' and ''Anomalocaris'', unique Cambrian arthropods. ''Lethaia'' '''19''', 241-246.
* Bergström, J. (1987). The Cambrian ''Opabinia'' and ''Anomalocaris''. ''Lethaia'' '''20''', 187-188.
* Briggs, D. E. G. & Whittington, H. B. (1987). The affinities of the Cambrian animals ''Anomalocaris'' and ''Opabinia''. ''Lethaia'' '''20''', 185-186.
* Budd, G. E. (1996). The morphology of ''Opabinia regalis'' and the reconstruction of the arthropod stem-group. ''Lethaia'' '''29''', 1-14.
* Whittington, H. B. (1975). The enigmatic animal ''Opabinia regalis'', Middle Cambrian Burgess Shale, British Columbia. ''Philosophical Transactions of the Royal Society of London'' '''B 271''', 1-43.
* Zhang, X. & Briggs, D. E. G. (2007). The nature and significance of the appendages of ''Opabinia'' from the Middle Cambrian Burgess Shale. ''Lethaia'' '''40''', 161-173.
==Further reading==
*Stephen Jay Gould, ''Wonderful Life: The Burgess Shale and the Nature of History''. (Norton: 1989)
*Xian-Guang Hou, ''The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life''. (Blackwell, 2004)
==External links==
* [http://paleobiology.si.edu/burgess/opabinia.html Smithsonian page on Opabinia, with photo of Burgess Shale fossil]
[[Category:Dinocarida]]
[[Category:Cambrian animals]]
[[Category:Prehistoric invertebrates]]
[[Category:Burgess Shale fossils]]
[[Category:Incertae sedis]]
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