Lobopodians are members of the informal group Lobopodia (), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as well. The grouping is considered to be paraphyletic, as the three living panarthropod groups (Arthropoda, Tardigrada and Onychophora) are thought to have evolved from lobopodian ancestors.
Definitions
thumb|left|300px|Various definitions of lobopodians within [[Panarthropoda. Certain dinocaridid genera, such as Opabinia, Pambdelurion, and Kerygmachela, may also be regarded as lobopodians, This traditional, informal usage of "Lobopodia" treats it as an evolutionary grade, including only extinct panarthropods near the base of crown Panarthropoda. Crown Panarthropoda comprises the three extant Panarthropod phyla – Onychophora (velvet worms), Tardigrada (waterbears), and Arthropoda (arthropods) – as well as their most recent common ancestor and all of its descendants. Thus, in this usage, Lobopodia consists of various basal panarthropods. The broadest definition treats Lobopodia as a monophyletic superphylum equivalent in circumscription to Panarthropoda. By this definition, represented by "D" in the image, Lobopodia is no longer treated as an evolutionary grade but as a clade, containing not only the early, superficially "lobopodian" forms but also all of their descendants, including the extant Panarthropods. but revealed by subsequent phylogenomic and anatomical studies to be a highly specialized taxon of crustaceans.
Representative taxa
The better-known genera include Aysheaia, which was discovered in the Canadian Burgess Shale, and Hallucigenia, known from both the Chenjiang Maotianshan Shale and the Burgess Shale. Aysheaia pedunculata has a morphology apparently basic for lobopodians However, further discoveries showed that this reconstruction had placed the animal upside-down: interpreting the "stilts" as dorsal spines made it clear that the fleshy "dorsal" protuberances were actually elongated lobopods. More recent reconstruction even exchanged the front and rear ends of the animal: it was revealed that the bulbous imprint previously thought to be a head was actually gut contents being expelled from the anus.
Microdictyon is another charismatic as well as the speciose genus of lobopodians resembling Hallucigenia, but instead of spines, it bore pairs of net-like plates, which are often found disarticulated and are known as an example of small shelly fossils (SSF). Xenusion has the oldest fossil record amongst the described lobopodians, which may trace back to Cambrian Stage 2. or well-developed frontal appendages, (with the exception of Antennacanthopodia, which have two pairs of head appendages instead of one or a conical proboscis. that may had been compound in nature.
Trunk and lobopods
The trunk is elongated and composed of numerous body segments (somites), each bearing a pair of legs called lobopods bear claws. The claws, if present, are hardened structures with a shape resembling a hook or gently curved spine. is considered to be a misinterpretation. and sometimes preserved in the fossil record in three dimensions. In some specimens the gut is found to be filled with sediment. The gut consists of a central tube occupying the full length of the lobopodian's trunk, which does not change much in width - at least not systematically. However, in some groups, specifically the gilled lobopodians and siberiids, the gut is surrounded by pairs of serially repeated, kidney-shaped gut diverticulae (digestive glands). because the phosphate content of the guts is under 1%; the contents comprise quartz and muscovite.
Categories
Based on external morphology, lobopodians may fall under different categories — for example the general worm-like taxa as "xenusiid" or "xenusian"; xenusiid with sclerite as "armoured lobopodians"; and taxa with both robust frontal appendages and lateral flaps as "gilled lobopodians". Some of them were originally defined under a taxonomic sense (e.g. class Xenusia), but neither any of them are generally accepted as monophyletic in further studies. Opabinia may also fall under this category in a broader sense, although the presence of lobopods in this genus is not definitively proven. Omnidens, a genus known only from Pambdelurion-like mouthparts and distal parts of the frontal appendages, may also be a gilled lobopodian. The body flaps may have functioned as both swimming appendages and gills, However, they are widely accepted as stem-group arthropods just basal to radiodonts. jianshanopodians by some literatures. They are generally large — body length ranging from Lobopods with curved terminal claws may have given some lobopodians the ability to climb on harder substrates like rocks, sponges, or animal carcasses.
Distribution
During the Cambrian, lobopodians displayed a substantial degree of biodiversity. One species is known from each of the Ordovician and Silurian periods, with a few more known from the Carboniferous (Mazon Creek) — this represents the paucity of exceptional lagerstatten in post-Cambrian deposits.
Phylogeny
The overall phylogenetic interpretation on lobopodians has changed dramatically since their discovery and first description.
Based on their apparently onychophoran-like morphology (e.g. annulated cuticle, lobopodous appendage with claws), lobopodians were originally thought to be present a group of paleozoic onychophorans. suggesting that the similarity between lobopodians and onychophorans represents deeper panarthropod ancestral traits (plesiomorphies) instead of onychophoran-exclusive characteristics (synapomorphies). Aysheaia's surface ornamentation, if homologous with palaeoscolecid sclerites, may represent a deeper link connecting it with cycloneuralian outgroups. Further re-examination even revealed that the suspected arthropodization on the legs of Diania was a misinterpretation — although the spine may have hardened, the remaining cuticle of Diania<nowiki/>'s legs were soft (not harden nor scleritzed), lacking any evidence of pivot joint and arthrodial membrane, suggest the legs are lobopods with only widely spaced annulations.]]
Lobopodian taxa of the tardigrade stem-group is unclear.
Stem-group panarthropods
It is unclear which lobopodians represent members of the panarthropod stem-group, and which were branched just before the last common ancestor of extant panarthropod phyla. Aysheaia may have occupied this position based on its apparently basic morphology; Miraluolishania was suggested to be synonym of Luolishania by some studies. Palaeocampa, formerly thought to be a fireworm, was also found as a lobopodian.
|| Paucipodiidae
|| An unarmoured lobopodian from the Chengjiang Biota. Probably a scavenger, it is equipped with paired claws at the end of each lobopod. Some are preserved alongside symbiotic epibionts.
| data-sort-value="518" |Cambrian Stage 3
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| Lenisambulatrix
| data-sort-value="2018" |
- L. humboldti <small>Ou & Mayer, 2018</small>
|| Cardiodictyidae
|| Lobopodian with extremely elongate body, with roughly 25 pairs of lobopod limbs, each associated with a saddle-shaped trunk sclerite.
| data-sort-value="518" |Cambrian Stage 3
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| Xenusion
| data-sort-value="1927" |
- X. auerswaldae <small>Pompeckj, 1927</small>
|| Onychodictyidae
|| The body of this lobopodian was covered in numerous elongate papillae, each trunk segment bearing a pair of netted sclerites ending in a pointed tip. O. spiniferum (=Onychomicrodictyon spiniferum) is known from Greenland, it is considered by some to be a junior synonym.
| data-sort-value="518" |Cambrian Stage 3
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| Orstenotubulus
| data-sort-value="2007" |
- O. evamuellerae <small>Maas et al., 2007</small>
|| (unassigned)
|| Known from microscopic, three-dimensionally preserved Orsten-type fossils, which show the detailed ornament of the cuticle, and retractable spines that protrude from the inner surface of the limbs.
| data-sort-value="506" |Furongian
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| Microdictyon
| data-sort-value="1981" |
- M. effusum <small>Bengston, Matthew, & Missarzhevsky, 1981</small>
- Q. tenuiporatum <small>Bengtson, Matthews & Missarzhevsky, 1986</small>
|| Eoconchariidae
|| Eoconchariid lobopodian similar to Microdictyon. Known only from isolated plates.
| data-sort-value="518" |Cambrian Stage 3
||
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|-
| Fusuchoncharium
| data-sort-value="1987" |
- F. typicum <small>Hao & Shu, 1987</small>
|| Luolishaniida (Collinsovermidae)
|| A collinsovermid lobopodian with internal sclerite plates which acted similarly to arthropod tergites or vertebrae, reinforcing the body and allowing body erection for filter feeding.
| data-sort-value="506" |Wuliuan–Drumian
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| Collinsovermis
| data-sort-value="2020" |
- C. monstruosum <small>Caron & Aria, 2020</small>
|| Luolishaniida (Collinsovermidae)
|| Collinsovermid lobopodian with an elongate body bearing 15 pairs of lobopod limbs and sclerite spine sets.
| data-sort-value="518" |Cambrian Stage 3
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| Acinocricus
| data-sort-value="1988" |
- A. stichus <small>Conway-Morris & Robison, 1988</small>
|| Luolishaniida (Collinsovermidae)
|| Highly armoured collinsovermid lobopodian with hundreds of spines borne on whorls, set between the limbs rather than above them.
| data-sort-value="507" |Wuliuan
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| Aysheaia
| data-sort-value="1911" |
- A. pedunculata <small>Walcott, 1911</small>
|| Onychophora
|| A putative velvet worm from Mazon Creek, whether or not it lived on land or in the ocean is debated.
| data-sort-value="308" |Moscovian
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| Siberion
| data-sort-value="2011" |
- S. lenaicus <small>Dzik, 2011</small>
|| Kerygmachelidae
|| A large pelagic lobopodian, probably related to Kerygmachela. Its lobopodous appendages are flattened as to form flaps.
| data-sort-value="504" |Drumian
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| Pambdelurion
| data-sort-value="1997" |
- P. whittingtoni <small>Budd, 1997</small>
|| (Unassigned)
|| The largest known Cambrian animal, reaching an estimated ~1.7 metres in length, based on the closely related Pambdelurion. Known from large sclerotized jaws and the terminal, sclerotized talons of the frontal appendages.
| data-sort-value="518" |Cambrian Stage 3
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| Opabinia
| data-sort-value="1912" |
- O. regalis <small>Walcott, 1912</small>
|| Opabiniidae
|| The first new opabiniid described since 1912, this species is known from a single specimen from the Wheeler Shale of Utah.
| data-sort-value="504" |Drumian
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| Mieridduryn
| data-sort-value="2022" |
- M. bonniae <small>Pates et al, 2022</small>
|| Opabiniidae
|| A probably opabiniid from the Ordovician of Wales, found at the Castle Bank Lagerstätte. This species apparently lacks eyes, with its head covered instead by a small cephalic sclerite, but does possess both flaps and lobopods, and a frontal proboscis.
| data-sort-value="462" |Middle Ordovician
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| Youti?
| data-sort-value="2024" |
- Y. yuanshi <small>Smith et al., 2024</small>
|| (Unassigned)
|| An embryonic panarthropod from the Yu'anshan Formation, preserved three-dimensionally in Orsten-style, this fossil provides a remarkable view into the internal anatomy of early panarthropods.
| data-sort-value="518" |Cambrian Stage 3
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|| 200px
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