Fuxianhuia

From Wikipedia, the free encyclopedia

Fuxianhuia protensa
Fossil range: Lower Cambrian

Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Yunnanata†
Order: Fuxianhuiida
Family: Fuxianhuiidae
Genus: Fuxianhuia
Species: F. protensa
Binomial name
Fuxianhuia protensa
Hou, 1987

Fuxianhuia protensa is a Lower Cambrian fossil arthropod known from the Chengjiang Fauna in China. Its purportedly primitive features have led to its playing a pivotal role in discussions about the euarthropod stem group. Nevertheless, despite being known from many specimens, disputes about its morphology, in particular its head appendages, have made it one of the most controversial of the Chengjiang taxa, and it has been discussed extensively in the context of the arthropod head problem.

Contents

[edit] Morphology

Complete Fuxianhuia specimens are approximately 4 centimetres long. The anterior of Fuxianhuia is marked by an oval sclerite, from which two stalked eyes emerge. Inserting directly behind this sclerite, on the head shield proper, are two stout antennae. When the head of Fuxianhuia was originally described, two additional head appendages, the "sub-chelate" pair were also described.[1] These are geniculate, backward-pointing appendages that lie in a highly stereotypical position (i.e., their position does not vary much from one specimen to another). Partly because of this, and partly because of their rather indistinct morphology, their status as appendages has been questioned. Indeed, on the grounds that these structures seem to lie between two cuticular layers, Waloszek and colleagues have suggested that they are not appendages at all, but rather gut diverticulae;[2] a reassignment that has however not been universally accepted.[3] Ventrally, a large plate has been interpreted as a hypostome.

The head shield overlaps a tapering series of 16–17 trunk tergites, which lead into a set of limb-bearing segments comprising the thorax. The limbs are simple in form, consisting of a smooth oval exopod and a stout, annulated endopod. There is no one-to-one correspondence between the thoracic tergites and the limbs, but, rather, there appear to be two or three limbs per tergite.

Behind the thorax is a narrower abdominal region consisting of 14 tergites that bears no appendages. The abdomen is terminated by a telson-like spine.

[edit] Classification

Fuxianhuia was first described from incomplete material,[4] and its true nature did not become apparent until the head and limbs were discovered. Its articulated head region, lack of tergite-segment correspondence and undifferentiated limbs have all been taken to indicate a very basal position in the arthropods,[5][6][7] even though an early cladistic analysis suggested, rather, that it was a stem-group chelicerate.[8] The presence of a distinct anterior sclerite bearing the eyes has been taken to suggest that a distinct acron once existed in front of the euarthropod head.[9]

Fuxianhuia is not a unique arthropod: two other taxa, Chengjiangocaris and Shankouia are clearly closely related,[10] although they differ in some details, such as the limbs. A relationship with the Burgess Shale taxon Canadaspis has also been suggested.[11]

[edit] Notes

  1. ^ Chen, J.-y., Edgecombe, G. D., Ramsköld, L. & Zhou, G.-q. (1995). Head segmentation in Early Cambrian Fuxianhuia: implications for arthropod evolution. Science 268, 1339–1343.
  2. ^ Waloszek, D., Chen, J., Maas, A. & Wang, X. (2005). Early Cambrian arthropods – new insights into arthropod head and structural evolution. Arthropod Structure and Development 34, 189–205.
  3. ^ Scholtz, G. & Edgecombe, G. D. (2006). The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence. Development, Genes and Evolution 216, 395–415.
  4. ^ Hou, X.-g. (1987). Three new large arthropods from Lower Cambrian, Chengjiang, Eastern Yunnan [in Chinese]. Acta Palaeontolologica Sinica 26, 272–285.
  5. ^ Hou, X.-g. & Bergström, J. (1997). Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. Fossils & Strata 45, 1–116.
  6. ^ Budd, G. E. (2002). A palaeontological solution to the arthropod head problem. Nature 417, 271–275.
  7. ^ Chen, J.-y., Edgecombe, G. D., Ramsköld, L. & Zhou, G.-q. (1995). Head segmentation in Early Cambrian Fuxianhuia: implications for arthropod evolution. Science 268, 1339–1343.
  8. ^ Wills, M. A. (1996). Classification of the arthropod Fuxianhuia. Science, 272, 746–747.
  9. ^ Chen, J.-y., Edgecombe, G. D., Ramsköld, L. & Zhou, G.-q. (1995). Head segmentation in Early Cambrian Fuxianhuia: implications for arthropod evolution. Science 268, 1339–1343.
  10. ^ Waloszek, D., Chen, J., Maas, A. & Wang, X. (2005). Early Cambrian arthropods – new insights into arthropod head and structural evolution. Arthropod Structure and Development 34, 189–205.
  11. ^ Budd, G. E. (2002). A palaeontological solution to the arthropod head problem. Nature 417, 271–275.

[edit] Further reading

  • Hou, Xian-Guang; Aldridge, Richard J., Bengstrom, Jan; Siveter, David J.; Feng, Xiang-Hong 2004; The Cambrian Fossils of Chengjang, China, Blackwell Science Ltd, 233 pp.