Crown-group Euarthropoda Lobopodians are members of the informal group Lobopodia[3] (from Ancient Greek λοβός [lobós] 'lobe' and πόδα [podá] 'foot'), or the formally erected phylum Lobopoda Cavalier-Smith (1998).

[6][7] While the definition of lobopodians may differ between literatures,[8] it usually refers to a group of soft-bodied, marine worm-like fossil panarthropods such as Aysheaia and Hallucigenia.

[8] Its most general sense refers to a suite of mainly Cambrian worm-like panarthropod taxa possessing lobopods – for example, Aysheaia, Hallucigenia, and Xenusion – which were traditionally united as "Xenusians" or "Xenusiids" (class Xenusia).

[20] 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.

An alternative, broader definition of Lobopodia would also incorporate Onychophora and Tardigrada,[16][19][4] the two living panarthropod phyla which still bear lobopodous limbs.

[5] "Lobopodia" has also been used to refer to a proposed sister clade to Arthropoda, consisting of the extant Onychophora and Tardigrada, as well as their most recent common ancestor and all of its descendants.

This definition renders Lobopodia a monophyletic taxon, if indeed it is valid (that is, if Tardigrades and Onychophora are closer to one another than either is to Arthropoda), but would exclude all the Euarthropod-line taxa traditionally considered Lobopodians.

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.

[26][6] Lobopodia has, historically, sometimes included Pentastomida,[27] a group of parasitic panarthropod which were traditionally thought to be a unique phylum,[28][29] but revealed by subsequent phylogenomic and anatomical studies to be a highly specialized taxon of crustaceans.

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.

[14][22] 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).

[38][5][45] The eyes may be represented by a single ocellus or by numerous[46] pairs of simple ocelli,[5] as has been shown in Luolishania[36] (=Miraluolishania[46][47]), Ovatiovermis,[45] Onychodictyon,[38] Hallucigenia,[22] Facivermis,[47] and less certainly Aysheaia as well.

[37] The gut consists of a central tube occupying the full length of the lobopodian's trunk,[7] which does not change much in width - at least not systematically.

[20] These forms sport a pair of flaps on each trunk segment, but otherwise no signs of arthropodization, in contrast to more derived dinocaridids like the Radiodonta that have robust and sclerotized frontal appendages.

[7][42][8][55] Their anatomy represent transitional forms between typical xenusiids and gilled lobopodians,[27] eventually placing them under the basalmost position of arthropod stem-group.

There is evidence to suggest that lobopodians moult just like other ecdysozoan taxa, but the outline and ornamentation of the harden sclerite did not vary during ontogeny.

One species is known from each of the Ordovician and Silurian periods,[12][65] with a few more known from the Carboniferous (Mazon Creek) — this represents the paucity of exceptional lagerstatten in post-Cambrian deposits.

[5] The reassignments are not only based on new fossil evidence, but also new embryological, neuroanatomical, and genomic (e.g. gene expression, phylogenomics) information observed from extant panarthropod taxa.

[5][23][66] 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.

[5] Compared to other panarthropod stem-groups, suggestion on the lobopodian members of arthropod stem-group is relatively consistent — siberiid like Megadictyon and Jianshanopodia occupied the basalmost position, gilled lobopodians Pambdelurion and Kerygmachela branch next, and finally lead to a clade compose of Opabinia, Radiodonta and Euarthropoda (crown-group arthropods).

[43] On the other hand, primary antennae and frontal appendages of lobopodians and dinocaridids may be homologous to the labrum/hypostome complex of euarthropods, an idea support by their protocerebral origin[8][23][49] and developmental pattern of the labrum of extant arthropods.

Aysheaia may have occupied this position based on its apparently basic morphology;[58][21][22][47] while other studies rather suggest luolishaniid and hallucigenid,[45][24][40] two lobopodian taxa which had been resolved as members of stem-group onychophorans as well.

[10] The fossil materials being described as lobopodians Mureropodia apae and Aysheaia prolata are considered to be disarticulated frontal appendages of the radiodonts Caryosyntrips and Stanleycaris, respectively.