Onychophora

Abstract

This page presents a brief overview of the Onychophora, from Cambrian to Recent.

Keywords: Onychophora, velvet worm, Aysheaia, Hallucinogenia, Microdictyon, Peripatoides, Peripatus, Xenusion

Introduction

Onychophorans, of which there are several endemic species in New Zealand, share a number of characteristics with both annelids (segmented worms) and arthropods, although they are more closely related to the latter and are sometimes, as here, regarded as a subphylum of the Arthropoda. Other authors (e.g. Nielsen 2001) regard the Onychophora as a phylum.

Currently there are around 10 genera and 110 species recognized within two extant families: the Peripatidae (known from the circumtropical regions of Mexico, Central and northern South America, equatorial West Africa, and South East Asia) and the Peripatopsidae (found in Chile, South Africa, Australia including Tasmania, and New Zealand).

Phylogeny

Fossil History

A number of fossils from the Cambrian have been described which look more or less like onychophorans. Some, such as the Middle Cambrian form Aysheaia are rather similar to living forms. Others were armored with various plates and spines which, disarticulated, contribute substantially to the "small shelly fauna." All of these Cambrian forms differed from living onychophorans in being marine.

Hallucigenia ...

Xenusion, from early Cambrian sandstones of eastern Europe. This form was also armed with spines, although they were shorter than those of Hallucigenia. Only two specimens have been found so far.

Microdictyon ...

The earliest (and the only) fossil terrestrial onychophoran was found in the Pennsylvanian deposits of  Mazon Creek, near Chicago, a locality that has yielded a great many fossils of soft-bodied organisms. The fossil is not very different from living onychophorans (Thompson and Jones 1980).

Segmentation

"Onychophorans are thought to be the sister taxon of [eu]arthropods and are segmented. However, onychophorans lack engrailed expression in their dermis. Instead, expression is observed in the posterior half of the developing limb and in a segmental pattern in the lateral mesoderm. The limb staining suggests shared ancestry of the onychophoran and arthropod limbs. However, given the close relationship of Arthropoda and Onychophora, and their segmented body plans, the lack of segmental ectodermal expression in Onychophora suggests that the ancestral role of engrailed was not segmentation; this absence may be a consequence of evolutionary loss of skeletons. Onychophoran dermis lacks a chitinous cuticle; thus Onychophora lack an exoskeleton. Furthermore, Cambrian fossils thought to be stem group onychophorans, such as Microdictyon, Hallucinogenia, and Xenusion, bear skeletal elements above the limb on each segment. Therefore, the absence of engrailed transcription in the ectoderm of modern Onychophora could well be a consequence of evolutionary loss of exoskeletal elements..." (Jacobs et al. 2000, pp. 343-345; full text).

Systematics

Phylum Arthropoda Siebold & Stannius 1895

Subphylum Protarthropoda Lankester 1904

Type: Peripatus Guilding 1825

Also includes the subphyla Pentastomida and Tardigrada.

Class Onychophora Grube 1853

Order Protonychophora Hutchinson 1930

The Order Protonychophora includes the problematical fossil Aysheaia, first described from the Burgess Shale by Walcott.

Family Aysheaiidae Walcott 1911

Genus Aysheaia Walcott 1911

Aysheaia pedunculata Walcott 1911

This problematical fossil was first described from the Burgess Shale by Walcott in 1911.

Family Uncertain

Genus Xenusion Pompeckj 1927

Order Euonychophora Hutchinson 1930

Onychophorans themselves have few predators, except perhaps insect carnivores such as centipedes, birds and rodents.

Family Peripatidae Evans 1901

Type: Peripatus Guilding 1825

The members of this family are generally coloured red-brown and possess 22 to 43 pairs of legs.

Family Peripatopsidae Bouvier 1907

This family contains organisms which are generally coloured blue-green and possess 14 to 25 pairs of legs.

Peripatoides novaezealandiae

Occurrence: Never abundant but "not uncommon" at Titirangi, near Auckland (Gill 1998).

Habit: The Peripatus preys on small arthropods by squirting and entrapping them in a glue fired from openings beside the mouth. It then injects saliva into the prey, dissolving the inner contents, and thus enabling the Peripatus to suck them out. Peripatus live in damp areas such as under moss, in rotting logs, behind the bark of trees, and in leaf litter.

Discussion: ...

References

Budd, Graham E. 2001: Why are Arthropods Segmented? Evolution and Development, vol. 3 (5): 332-342.

Gill, B.J. (ed.) 1998: Powell's Native Animals of New Zealand. 4th edition. Bateman. 94 pp.

Hoffmann, K. 1997: Ecdysteroids in Adult Females of a 'Walking Worm' Euperipatoides leuckartii (Onychophora, Peripatopsidae). Invert. Reprod. Dev. 32: 27-30.

Jacobs, David K.; Wray, Charles G.; Wedeen, Cathy J.; Kostriken, Richard; DeSalle, Rob; Staton, Joseph L.; Gates, Ruth D.; Lindberg, David R. 2000: Molluscan engrailed Expression, Serial Organization, and Shell Evolution. Evolution & Development, vol. 2 (6): 340-347.

Nielsen, Claus 2001: Animal Evolution. Second Edition. 563 pp. Oxford University Press.

Ramskold, L. 1992: The Second Leg Row of Hallucigenia Discovered. Lethaia 25: 221-224.

Thompson, I. and Jones, D. 1980: A Possible Onychophoran from the Middle Pennsylvanian Mazon Creek beds of Northern Illinois. Journal of Paleontology 54: 588-596.