Description
[Some of the following is not my writing. Unfortunately, I cannot remember where it came from. My apologies to the unknown author whom I have failed to cite. I’ll rewrite it as soon as I can.]
Onychophorans are small, caterpillar-like animals with a segmented body plan. The Onychophoran dermis lacks a chitinous cuticle or exoskeleton (Jacobs et al. 2000, p. 343). The anterior end is indicated by the antennae and by the ventrally directed mouth, while the posterior end, projecting behind the last pair of walking legs, bears the terminal anus. There are many pairs of legs attached latero-ventrally, these being the only external signs of segmentation. The body is ringed by annuli on which are tubercles set in rows. Each tubercle ends in a tiny chitinous spine.
The body itself is not segmented except for the head, which is divided into three segments. The first contains the two large antennae with an eye at the base (Jamaica and South Africa have cave dwelling species which do not have eyes). Some males also have other appendages believed to be involved in sperm transfer.
The first pair of appendages are the antennae, these are tactile and the chief sense organs. A small eye is situated dorsally behind the base of each antenna and has a spherical lens. The mouth is directed ventrally and surrounded by ridged lips. In the sides of the mouth cavity are a pair of jaws, the appendages of segment 2.
The second segment contains the jaw like mouth which is used for rasping into prey and then sucking out the nutrients. The third segement holds the first pair of parapodia-like legs.
Each jaw is a low papilla with a pair of chitinous teeth. The roof of the anterior end of the mouth cavity is thickened to form the tongue which has a row of small chitinous teeth on its surface. Oral papillae, located lateral to the mouth, are presumed to have a sensory funtion. Defensive slime-glands open through the ends of the oral papillae.
Each of the legs bears a pair of chitinous claws for gripping, although on smooth substrate they walk on walking pads. Variously, the body between the limb insertion points, and the limbs themselves, may be finely annulated.
The legs are conical in shape and terminate distally in a plantigrade foot. The foot is retractable and is usually raised if the going is easy, being brought into play on slippery surfaces. The foot terminates distally in a pair of retractable claws and bears 3 prominent tubercles. Ventrally on the distal ridges of the leg proper are spinous pads; the shape and number of pads are helpful in recognising some species. Basally on the leg a groove runs along the ventral surface of the leg at right angles to the body axis. On all legs except 4 and 5 the excretory pore may be found at the basal end of this groove. If crural glands are present they will be found distal to the end of the groove.
Contemporary Onycophorans are ceolomates and have haemocoel, which means they have a lined body cavity filled with blood, rather than a vascular system. They have a muscular tubular heart which pumps the colorless blood around the body cavity. Locomotion is essentially annelid-like, with the body cavity functioning as a hydrostatic skeleton. The parapodia-like legs are also filled with blood and a valve at the base keeps them firm and muscular coordination can extend them or retract them and make them move forward or make them move backward.
Onycophorans have a cuticle with a-chitin but lacking collagen, which is periodically shed to permit growth (ecdysis). New cuticle is secreted underneath the old one by the ectodermal cells which develop microvilli that are subsequently withdrawn. Ecdysteroids have been found in various tissues but their function remains unknown (Hoffmann 1997; Nielsen 2001, p. 198). Unlike insect dermis, the cuticle of modern representatives is non-articulated, thin and soft and covered in hundreds of papillae and sensory hairs giving them a velvety texture, hence the common name ‘velvet worm.’ However, a characteristic feature of several fossil species is the paired internal sclerotic plates above the limb insertions, which may be variously developed into (presumeably defensive) spikes.
Like insects the Onycophora breathe through spiracles. Spiracles open out to the enviroment and oxygen enters through a system of tubules (trachae) and is absorbed into the tissues across the moist surfaces. However, unlike the insects, onycophorans have no control on the spiracles and they are always open, making the animal extremely vulnerable to dessication, so high levels of humidity are required.
Contemporary onycophorans are able to predate organisms several times larger than themselves by immobilising it with a gluey secretion from glands in its head, projected up to 30cm. The secretion holds the prey while the animal approaches it, bites through the cuticle, and injects a toxic, digestive saliva into the wound.
The animal 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.
Onychophorans themselves have few predators, except perhaps insect carnivores such as centipedes, birds and rodents.
Phylogeny and Evolution
“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” (Jacobs et al. 2000, p. 343).
Much has been written about enigmatic lobopodan taxa from the great Cambrian lagerstätten, especially the Burgess Shale and Chengjiang sites, and it is probably the most widely held view that many of these are stem-group onychophorans, as asserted by (for example) Jacobs et al. 2000:
“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...” (p. 343-345).
However, it would be wrong to say this view is held universally. The Cambrian lobopods are exclusively marine, whereas all modern representatives are exclusively terrestrial, and their integument seems poorly suited to undertaking the transition from sea to land. The earliest known terrestrial onychophoran(s) is/are Carboniferous (
Antennipatus montceauensis
from Montceau-les-Mines, France, and, less convincingly,
Helenodora inopinata
, from the Mazon Creek lagerstätte of the USA), and there are no putative onychophoran fossils of Ordovician to Devonian age. At least some of us are still highly sceptical that the Cambrian lobopods (“priapulids on legs”, in the words of Dzik & Krumbiegel 1989) have anything to do with modern onychophorans.
Nevertheless, they will be included here, though only because readers will be expecting it.
Fossil History
A number of fossils from the Cambrian have been described which look more or less like onychophorans. The Cambrian forms are marine, however, and are incompletely understood. It is possible they are not really related closely at all. Some, such as the Middle Cambrian form
Aysheaia
are rather similar to living forms (but see Whittington [find REF] for a contrary view). 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
is most widely known from the Middle Cambrian, Burgess Shale form,
Hallucigenia sparsa
(Walcott), famously misinterpreted by Conway Morris (and later commentators) upside-down and back-to-front. Subsequently, another species,
Hallucigenia fortis
, has been described from the Lower Cambrian Chengjiang fauna.
Maas & Waloszek 2001, reports an undescribed “lobopodian” from the Upper Cambrian ‘Orsten’ beds of Sweden. Although only about a tenth the size of the better-known Cambrian onychophorans, the Swedish “‘Orsten’ lobopodian shares with the Lower to Middle Cambrian lobopodians not only the annulated segmental limbs but also the segmental paired dorsal outgrowths on the finely annulated tubular body, which has a diameter of about 100 to 120 mm. ... The body and limbs are virtually cylindrical, and the limbs were apparently stretched virtually laterally due to a thicker bridge linking right and left legs. [The cuticle] shows a cell-like surface microstructure that resembles the onychophoran condition” (Maas & Waloszek 2001, p. 457).
Another Cambrian fossil organism which might belong within this clade is
Kerygmachela
, known from the Lower Cambrian Sirius Passet locality (Budd 1993).
The earliest candiate terrestrial onychophorans are two fossils of Late Carboniferous (Pennsylvanian) age. They are
Antennipatus montceauensis
from Montceau-les-Mines, France (Garwood et al. 2016) and
Helenodora inopinata
, from the Mazon Creek lagerstätte near Chicago, USA, a locality that has yielded a great many fossils of soft-bodied organisms (Thompson & Jones 1980). The onychophoran affinity of both is contentious, though it is probably fair to say that advocates for one or other of these taxa outnumber those who are sceptical of both.
Systematics
The systematics adopted here mostly follows Hou & Bergström 1995, though with some minor changes as noted.
Superphylum Gnathopoda
Lankester 1877
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1877 |
Gnathopoda Lankester |
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1979 |
Lobopoda Boudreaux |
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1995 |
Panarthropoda Nielsen |
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1996 |
Podophora Waggoner |
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1998 |
Lobopoda de Haro |
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2001a |
Arthropoda; Budd |
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2001 |
Arthropoda s.l.; Maas & Waloszek |
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2006 |
Aiolopoda Hou & Bergstrom |
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Discussion:
Three groups within the ecdysozoans, the onychophora, tardigrades and “euarthropods”, have received variable treatment over time. Probably most often, today, they are treated as separate phyla (e.g., Nielsen 2001). However, they are obviously closely related, and various taxa have been erected to group them, one of the best known being the Panarthropoda of Nielsen 1995, probably because that work is comparatively recent.
The relationships between these phyla are still controversial. The name Arthropoda was coined by von Siebold (1848) but, despite common usage to the contrary, it is clear from his diagnosis that he intended this group to include only the tardigrades and euarthropods; not onychophorans (see Ortega-Hernández 2016). Other names intended for the same purpose, and therefore junior synonyms, include Arthropoidea [de Haro 1999] and Tactopoda [Budd 2001c].
Sister relationships have also been also proposed for onychophorans+euarthropods (e.g. the names Arthropoda sensu Lankester 1904 and Antennopoda [de Haro 1998] were intended for this purpose) and, finally, for onychophorans+tardigrades (e.g. Pararthropoda [Vandel 1949] and, with the addition of the Pentastomida, Oncopoda [Weber 1954, p. 198]).
The three together are most likely monophyletic, so for the present we will simply treat them as a polytomy. Common characteristics of the Gnathopoda include the presence of legs and claws, a ventral nervous system, and a segmented body.
Phylum Onychophora
Grube 1853
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1853 |
Onychophora Grube |
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1904 |
Onychophora Lankester, p. 529 |
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Discussion:
The Onychophora as used by Hou & Bergström 1995 includes 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. This seems appropriate, given the still rather weak evidence that the Cambrian marine forms are Onychophorans at all.
Order Euonychophora
Hutchinson 1930, p. 23
Family Peripatidae
Evans 1901
Type:
Peripatus
Guilding 1826
Discussion:
The members of this family are generally coloured red-brown and possess 22 to 43 pairs of legs.
Genus
Peripatus
Guilding 1826
...
Family Peripatopsidae
Bouvier 1907
This family contains organisms which are generally coloured blue-green and possess 14 to 25 pairs of legs.
Genus
Peripatoides
Pocock 1894
Peripatoides novaezealandiae
Description:
Peripatoides novaezealandiae
has 15 pairs of legs with hooks at the end, two robust feelers, and is velvety in appearance and comes in colours of blue, green, grey and brown; and may reach 80 mm in length.
Occurence:
Never abundant but “not uncommon” at Titirangi, near Auckland (Gill 1998).
Habit:
Peripatus live in damp areas such as under moss, in rotting logs, behind the bark of trees, and in leaf litter.
Order incertae sedis
Genus
Antennipatus
Garwood et al. 2016, p. 183
Description:
Vermiform taxon with annulated paired appendages resembling lobopods (maximum length is 2.8 mm), serially repeated along a body covered by an annulated cuticle; each segment with eight plicae; each plica with papillae, their size and spacing similar to onychophoran primary papillae. Head with long onychophoran-like antennae (with alternation of wide and narrow annuli) and possible slime papillae represented by ventrolateral appendages shorter than the other appendages. Length and total number of trunk segments not available; a maximum of five trunk segments preserved. (Claws and spinous pads of legs not observed; jaws not observed; genital opening and anal pore not preserved).
Antennipatus montceauensis
Garwood et al. 2016, p. 184-185
Discussion:
The onychophoran affinities of
Antennipatus montceauensis
… are indicated by the form of the trunk plicae and the shape and spacing of their papillae, details of antennal annuli, and the presence of putative slime papillae. The poor preservation of several key systematic characters for extant Onychophora, however, prohibits the precise placement of the Carboniferous fossil in the stem or crown of the two extant families, or the onychophoran stem-group as a whole. Nevertheless, A. montceauensis is the most compelling candidate to date for a terrestrial Paleozoic onychophoran (after Garwood et al. 2016, Abstract).
Class Xenusia
Dzik & Krumbiegel 1989
Order Protonychophora
Hutchinson 1930, p. 23
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 Xenusiidae
Dzik & Krumbiegel 1989
Description:
[VERIFY] >20 leg-bearing segments; paired, rounded sclerites on each segment; spiny legs.
Genus
Xenusion
Pompeckj 1927
Xenusion auerswaldae
Pompeckj 1927
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.
Order Paronychophora
Hou & Bergström 1995
Family Onychodictyidae
Hou & Bergström 1995
Genus
Onychodictyon
Hou et al. 1991
Type Species:
Onychodictyon ferox
Hou et al. 1991
Order Scleronychophora
Hou & Bergström 1995
Family Eoconchariidae
Hao & Shu 1987
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1987 |
Eoconchariidae Hao & Shu |
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1988 |
Eoconchariidae Shu & Chen |
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1989a |
Microdictyonidae Chen et al. |
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Genus
Microdictyon
Bengtson et al. 1981 emend. Chen et al. 1989a
Species:
Microdictyon effusum
(type?), M. rhomboidale, M. robisoni, M. sinicum (Chen et al. 1989a), M. sphaeroides, M. tenuiporatum
Fig. 1:
Microdictyon
sp. specimen from Chengjiang region. Image courtesy of The Natural Canvas.
Family Hallucigeniidae
Conway Morris 1977
Genus
Hallucigenia
Conway Morris 1977, p. 624
Type Species:
Hallucigenia sparsa
(Walcott 1911) Conway Morris 1977
Other Species:
Hallucigenia fortis
Hou & Bergström 1995
Family Cardiodictyidae
Hou & Bergström 1995
Genus
Cardiodictyon
Hou et al. 1991
Type Species:
Cardiodictyon catenulum
Hou et al. 1991
Class incertae sedis
Order Archonychophora
Hou & Bergström 1995
Hou & Bergström 1995 retain Archonychophora within the Xenusia. However, they also note their belief that
Luoishania
, is a sister taxon to all other onychophorans (e.g. their fig. 7). If correct, then a cladistic approach to onychophoran taxonomy suggests promoting the Archonychophora to class rank.
Family Luolishaniidae
Hou & Bergström 1995
Description:
The Luolishaniidae are multi-segmented having a single type of segmentally arranged, small, rounded sclerites usually arranged in sets of three per segment.There are numbers of annuli between each two sclerites decreasing towards both ends of the animal. (After Hou et al. 2004b.)
Genus
Luolishania
Hou & Chen 1989b
Type Species:
Luolishania longicruris
Hou & Chen 1989b [monotypic]
Luolishania longicruris
Hou & Chen 1989b
A rare lobopodian of ~15 mm overall length. The head is rounded and slightly elongate; trunk slender and tapering posteriorly; 16 pairs of long annulated legs (after Hou et al. 2004a).
Genus
Antennacanthopodia
Ou et al. 2011, p. 588
Type Species:
Antennacanthopodia gracilis
Ou et al. 2011, p. 588
Family Paucipodiidae
Hou et al. 2004b
Discussion:
“The Paucipodiidae differ from other known Cambrian xenusians in being devoid of sclerites and of lobopod differentiation, and in having an unusually fine annulation and a low number of lobopods” (Hou et al. 2004b, p. 238).
Genus
Paucipodia
Chen et al. 1995 emend. Hou et al. 2004b
Type Species:
Paucipodia inermis
Chen et al. 1995 [monotypic]
Diagnosis:
Paucipodiids with nine pairs of of lobopods, each with two distal claws. Head and tail elongated and tapering.
Paucipodia inermis
Chen et al. 1995 emend. Hou et al. 2004b
Diagnosis:
Currently the same as for the genus, as the latter is monotypic. It will have to change if/when another
Paucipodia
species is described.
Description:
This animal is one of the largest Cambrian xenusians. Hou et al. (2004b) report a complete specimen as 106.9 mm long and up to 4.3 mm wide.
Discussion:
Paucipodia inermis
was described by Chen et al. (1995) based on four incomplete specimens from the Chengjiang fauna locality at Maotianshan. The emended diagnosis by Hou et al. (2004b) is based on a further 20 specimens collected from Anshan section near the village of Mafang.
Order incertae sedis
Family incertae sedis
Genus
Helenodora
Thompson & Jones 1980
Helenodora inopinata
Thompson & Jones 1980
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