Ecdysozoa: Onychophora

Onychophora

Taxa on This Page

1. Antennacanthopodia X
2. Onychophora

Introduction

Onychophorans, the group that includes the extant velvet worms and their immediate fossil relatives, are a group of shy and retiring invertebrates that frequent the wet tropics and rather resemble elongate caterpillars. They possess a soft chitinous exoskeleton, and share a number of characteristics with both annelids (segmented worms), and arthropods (unbranched limb and insect-like tracheae). Variously included with annelid worms under the Articulata, and with myriapods and inscets under the Uniramia, The embryonic development of is identical to that of annelids and uniramious arthropods, so and it was for a long time thought that these creatures seem to represent a missing link between the two groups. It has even been suggested by Ballard et al 1992 that the Onychophora are actually modified arthropods, rather than the other way around. However, modern molecular phylogeny has shown that annelids and arthropods are not at all closely related, and the consensus now is that they belong to the clade of moulting animals (Ecdysozoa). They share a common ancestry to arthropods, but are neither ancestral to nor descended from them; or to put it differently, tardigrades and onychophores are stem-groups (early side branches) on the main arthropod line (Budd 2001, Edgecombe 2009). Similarities with both annelids, myriapods, and insects are therefore the result of evolutionarty convergance. They are variously given class (if included under Arthropoda), subphylum (Cavalier-Smith (1998)) or phylum ( Neilsen 2001) rank. MAK120426

Fossil History

A number of Cambrian fossils have been described which look more or less like onychophorans. Some, such as the Middle Cambrian form Aysheaia are rather similar to living forms and have been traditionally included in the Onychophora. Others were armored with various plates and spines which, disarticulated, contribute substantially to the "small shelly fauna." The majority opinion now is that Cambrian lobopods represent a paraphyletic assemblage of early panarthropods, many of which were not even on the ancestral line to onychophores, although some may be (see phylogenetic review). All of these Cambrian forms differed from living onychophorans in being marine.

The earliest 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).. Several Cretaceous and Teriary species hacve also been found preserved in amber - Chris Clowes - Peripatatus. upodated MAK120426

Loss of exoskeleton

Euonychophorans differ from Early Paleozoic lobopods in lacking the spines and armour plates of their ancient marine ancestors. This can be related to changes in one of the Hox genes called engrailed. To quote (Jacobs et al. 2000,

"Onychophorans are thought to be the sister taxon of 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). - via Chris Clowes - Peripatatus

Descriptions

Onychophora Phylogenetic definition: here defined as all species closer to .Peripatus (Euonychophora) than to Echiniscus (Tardigrades) or Drosopholia (Arthropoda)

Phylogeny: Panarthropoda : Mureropodia + (Aysheaia + (Tardigrada + ((Kerygmachela + (Dinocaridida + Arthropoda)))) + * : Antennacanthopodia + Eunychophora))

Characters: "Primitively, homonomous annulation (present in crown-node; one pair of differentiated frontal appendages (present in crown and widespread in outgroups); a terminal mouth; and none of the specialist features of the extant Onychophora such as slime papillae; feet and jaws (none of these structures are known from any immediate outgroups)". Budd 2001

Comments: "The class Onychophora was used by Hou & Bergström 1995 to include the terrestrial forms: the modern onychophorans and the single known fossil terrestrial species, Helenodora. These authors also noted their belief that the fossil marine form, Onychodictyon, "is closer to modern onychophorans than any of the other Cambrian lobopodians" (p. 11). Their cladogram (Hou & Bergström 1995, fig. 7) depicts Onychodictyon, Helenodora, and modern onychophorans together comprising a well-formed clade. Yet in their systematic section (p. 17) Onychodictyon is left outside Onychophora, in the class Xenusia, on morphologic grounds. In the spirit of a more cladistic taxonomy, the class Onychophora is here broadened to include the marine Onychodictyidae (Onychodictyon)." - Chris Clowes - Peripatatus However cladistic analysis generally presents Onychodictyon in a more derived position (see link for references). Other canditates for sister taxon of extant onychophora are Orstenotubulus (Edgecombe 2010 fig.1) and Antennacanthopodia (Ou et al 2011) MAK120426

Links: UC Berkeley; Wikipedia

Antennacanthopodia gracilis Ou et al 2011

Horizon: Chengjiang Lagerstätte, Yunnan, southwestern China., (Early Cambrian)

Phylogeny: Onychophora : Eunychophora + *

Characters: two muscular and slender appendage pairs (prominent frontal and second antenna), simple lateral eyes situated basal of frontal antennae; trunk devoid of obvious annuli and sclerotized plates; a straight, voluminous midgut, diminutive spines arrayed on the leg and the trunk, stout euonychophore-like lobopods with well-developed legs armed with annuli of thorn-shaped spines, highly sclerotized disc-shaped terminal leg pads, pair of putative cirriform appendicules attached to terminal projection of trunk (Ouetal2011)

Comments: an unarmoured form that seems to be intermediate between Cambrian lobopodians and extant Onychophora. (Ou et al 2011) We have tentatively included it here as a stem-group onychophore. MAK120426

Illustration: Life reconstruction of Antennacanthopodia gracilis from (Ou et al 2011. The anetnnae in teh name are the frontal and second antennae at the left; what look like long wavy arthropod antennae at the rear are actually a pair of cirriform structures. Scale bar: 1 mm.