Lobopoda

From Palaeos

Parent taxa:

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ECDYSOZOA
Taxonomy	Phylogeny
Kingdom: Metazoa Subkingdom: Bilateria Branch: Protostomia Infrakingdom: ECDYSOZOA Superphylum: "Aschelminthes" Superphylum: Panarthropoda	Protostomia `--Ecdysozoa \|--Scalidophora \|?--Protoconodonta† \| `--Chaetognatha \|--+--Nematomorpha \| `--Nematoda `--o--Lobopoda `--+--Tardigrada `--+--Anomalocarida `--Arthropoda
Ecdysozoa topics: Fossil Record \| Characteristics \| Ecology and Lifestyle \| References \| Links

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[edit] Introduction

The clade Lobopoda ("lobe feet") contains two living animal phyla, the Onychophora (velvet worms) and Tardigrada (water bears), united by the possession of unspecialised, unjointed limbs.

Velvet worms (Onychophora), exemplified by the genus Peripatus, possess a soft chitinous exoskeleton and tracheae. The embryonic development is identical to that of annelids and uniramous arthropods, and these creatures seemed at first to represent a missing link between the two groups. However, modern molecular phylogeny has shown that annelids and arthropods are not at all closely related (contrary to previous opinion), in which case (and contra to the arguments of Ernst Haeckel and many other Darwinists since) embryology is not necessarily a good aid to phylogeny. Conversely it has been suggested note that the Onychophora are actually modified arthropods, rather than the other way around. Some Cambrian forms (the Xenusia), which include the fossil genera Aysheaia, Hallucigenia (spectacularly misinterpreted when first discovered, and by Stephen Jay Gould in Wonderful Life), Microdictyon, and Xenusion) have been placed in this group on the basis of superficial appearance, but the relationship is by no means certain.

In the last decade a more developed picture of Cambrian lobopod morphology has taken shape, due mainly to Ramsköld's work on the Chengjiang lagerstätte. In a recent write up, Ramsköld and Junyuan analyze the Cambrian lobopoda together with Recent Onychophora, and locate the extant onychophorans as ingroup Cambrian "lobopodians" on the basis of four synapomorphies.

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[edit] References

Parker, S. P. (ed.), 1982: Synopsis and classification of living organisms. Vols. 1 & 2 --McGrew-Hill Book Company

Edgecombe, ed. Arthropod Fossils and Phylogeny, chap. 3, "Cambrian Lobopodians: Morphology and Phylogeny" by Lars Ramsköld and Chen Junyuan

Ballard, J. W. O., G. J. Olsen, D. P. Faith, W. A. Odgers, D. M. Rowell, P. W. Atkinson. 1992. - Evidence from 12S Ribosomal RNA sequences that Onychophorans are modified Arthropods. Science 258:1345-1348.