Amiskwia
| Amiskwia Temporal range:
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| Genus: | Amiskwia
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| Amiskwia sagittiformis Walcott 1911
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Amiskwia is a genus of large, soft-bodied invertebrate of unknown affinity known from fossils of the Middle Cambrian Lagerstätten both in the Burgess shale formation in British Columbia and the Maotianshan shales of Yunnan Province, China.
Very few specimens of this organism have been found, only eighteen of the Burgess shale species, A. sagittiformis,[1][2] and a single specimen of the Maotianshan species, A. sinica — which may be a reflection of its genuine rarity, but is more likely to be due to taphonomic (preservational) or behavioural factors. The fossils reach 25 mm (1 in) in length. The head is rounded, tipped with two tentacles, and appears to contain a four-ganglion brain; the body flattens out and broadens in the trunk, which appears to have been fairly muscular. Where the trunk meets the head there is a small tubular opening, which can be interpreted as the mouth; the gut terminates where the trunk narrows and meets the tail, which is broad and paddle shaped. The body morphology suggests a free energetic swimmer, which may be consistent with the dearth of fossils (in other words, a free-swimming animal would be able to avoid being buried alive in a mudflow on a regular basis, compared to a benthic animal).
Amiskwia was originally categorized by paleontologist Charles Walcott. Walcott thought he saw three buccal spines in the fossils, and therefore categorized Amiskwia as a chaetognath worm (arrow worm). However, Amiskwia appears to lack the characteristic grasping spines and teeth of other Burgess fossil arrow worms. Later scientists suggested an affinity with the nemerteans (ribbon worms), but the evidence for this was somewhat inadequate.[1] Conway Morris, on re-examining of the Burgess Shale fauna in the 1970s, described it as being the single known species in an otherwise unknown phylum, given that it has two tentacles near its mouth, rather than the characteristic single tentacle of true nemerteans. (Nemerteans do not have a single tentacle. However, a pair of antero-lateral tentacles is present in 2 of the many genera of pelagic nemerteans. Nemerteans do have a single eversible—normally internal—proboscis, which when everted could resemble an anterior median tentacle if fossilized. Whether retracted or everted, the proboscis is the only structure in pelagic nemerteans likely to fossilize, as it is the only structure with substantial connective tissue and muscle. The body wall has almost no muscle or connective tissue and is exceedingly unlikely to fossilize; hence, a pelagic nemertean fossil would be only the proboscis).[1] Butterfield implies from the appearance of the fossils that the organisms may have lacked a cuticle:[3] while this is also true of the nemerteans, these organisms lack a coelom and are thus unlikely to fossilise. He goes on to argue that the absence of cuticle is characteristic of the Chaetognaths; whilst teeth would be expected, a similar fossil, Wiwaxia, shows such structures in only 10% of the expected instances, and Anamolocarids are often found detached from their mouthparts, so the absence may be taphonomic rather than genuine. The absence of spines could simply mean that the fossils represent young organisms — or that later chaetognath evolution involved paedomorphosis.[4]
While more work on undescribed specimens is required before firm consensus is reached, the discovery of an undisputed chaetognath, Protosagitta spinosa, which strongly resembles modern Chaetognaths, in the similarly aged Chengjiang biota places doubts on Amiskwia's position in the Chaetognath crown group.[5] It has also been likened to the molluscs,[6] and bears limited resemblance to unusual forms such as Pterotrachea. Of course, many similarly enigmatic Cambrian fossils probably represent stem groups to living taxa, and there is no reason to assume that this organism must fall into an extant crown group at all.[7]
The scientific name Amiskwia sagittiformis derives from the nearby Amiskwi River, and its sagittiform shape. "Sinica," of A. sinica, refers to that species' origin from China.
References
- ^ a b c Conway Morris, S. (1977). "A redescription of the Middle Cambrian worm Amiskwia sagittiformis Walcott from the Burgess Shale of British Columbia" (PDF). Paläontologische Zeitschrift. 51 (3–4). Springer Berlin / Heidelberg: 271–287. doi:10.1007/BF02986576. ISSN 0031-0220.
- ^ Caron, Jean-Bernard; Jackson, Donald A. (October 2006). "Taphonomy of the Greater Phyllopod Bed community, Burgess Shale". PALAIOS. 21 (5): 451–65. doi:10.2110/palo.2003.P05-070R. JSTOR 20173022.
- ^ Butterfield, N. J. (1 July 1990). "Organic Preservation of Non-Mineralizing Organisms and the Taphonomy of the Burgess Shale". Paleobiology. 16 (3). Paleontological Society: 247–399. ISSN 0094-8373. JSTOR 2400788.
- ^ Kasatkina, A. P. 1982. Ŝetinkočelustnyje morej SSSR i sopredel'nyh vod.
136 pp. Nauka, Leningrad.
- Cited in Doguzhaeva, L. A.; Mutvei, H.; Mapes, R. H. (2002). "Chaetognath grasping spines from the Upper Mississippian of Arkansas (USA)" (PDF). Acta Palaeontologica Polonica. 47 (3): 421–430. Retrieved 2007-08-19.
- ^ Vannier, J.; Steiner, M.; Renvoisé, E.; Hu, S. X.; Casanova, J. P. (2007). "Early Cambrian origin of modern food webs: evidence from predator arrow worms" (PDF). Proceedings of the Royal Society B: Biological Sciences. 274 (1610): 627–633. doi:10.1098/rspb.2006.3761. PMC 2197202. PMID 17254986. Retrieved 2007-08-19.
- ^ Chen, J. Y.; Huang, D. Y.; Bottjer, D. J. (2005). "An Early Cambrian problematic fossil: Vetustovermis and its possible affinities". Proceedings of the Royal Society, Part B. 272 (1576): 2003–2007. doi:10.1098/rspb.2005.3159. PMC 1559895. PMID 16191609.
- ^ Budd, G. E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews. 75 (2): 253–295. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID 10881389.
