Lobopodia
Lobopodians are members of the informal group Lobopodia, or the formally erected phylum Lobopoda Cavalier-Smith. They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as well. While the definition of lobopodians may differ between literatures, it usually refers to a group of soft-bodied, marine worm-like fossil panarthropods such as Aysheaia and Hallucigenia. However, other genera like Kerygmachela and Pambdelurion are often referred to as "gilled lobopodians".
The oldest near-complete fossil lobopodians date to the Lower Cambrian; some are also known from Ordovician, Silurian and Carboniferous Lagerstätten. Some bear toughened claws, plates or spines, which are commonly preserved as carbonaceous or mineralized microfossils in Cambrian strata. The grouping is considered to be paraphyletic, as the three living panarthropod groups are thought to have evolved from lobopodian ancestors.
Definitions
The lobopodian concept varies from author to author. 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". Certain dinocaridid genera, such as Opabinia, Pambdelurion, and Kerygmachela, may also be regarded as lobopodians, sometimes referred to more specifically as "gilled lobopodians" or "gilled lobopods". 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, Tardigrada, and Arthropoda – as well as their most recent common ancestor and all of its descendants. Thus, in this usage, Lobopodia consists of various basal panarthropods. This corresponds to "A" in the image to the left.An alternative, broader definition of Lobopodia would also incorporate Onychophora and Tardigrada, the two living panarthropod phyla which still bear lobopodous limbs. This definition, corresponding to "C", is a morphological one, depending on the superficial similarity of appendages. Thus, it is paraphyletic, excluding the euarthropods, which are descendants of certain lobopodians, on the basis of their highly divergent limb morphology. "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, but would exclude all the euarthropod-line taxa traditionally considered lobopodians. Its validity is uncertain, however, as there are a number of hypotheses regarding the internal phylogeny of Panarthropoda. 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.
Lobopodia has, historically, sometimes included Pentastomida, a group of parasitic panarthropods which were traditionally thought to be a unique phylum, 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 — for example, a significantly annulated cuticle, a terminal mouth opening, specialized frontalmost appendages, and stubby lobopods with terminal claws. Hallucigenia sparsa is famous for having a complex history of interpretation — it was originally reconstructed with long, stilt-like legs and mysterious fleshy dorsal protuberances, and was long considered a prime example of the way in which nature experimented with the most diverse and bizarre body designs during the Cambrian. 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. Xenusion has the oldest fossil record amongst the described lobopodians, which may trace back to Cambrian Stage 2. Luolishania is an iconic example of lobopodians with multiple pairs of specialized appendages. The gill lobopodians Kerygmachela and Pambdelurion shed light on the relationship between lobopodians and arthropods, as they have both lobopodian affinities and characteristics linked to the arthropod stem-group.
Morphology
Most lobopodians were only a few centimeters in length, while some genera grew up to over 20 centimeters. Their bodies are annulated, although the presence of annulation may differ between position or taxa, and sometimes difficult to discern due to their close spacing and low relief on the fossil materials. Body and appendages are circular in cross-section.Head
Due to the usually poor preservation, detailed reconstructions of the head region are only available for a handful of lobopodian species. The head of a lobopodian is more or less bulbous, and sometime possesses a pair of pre-ocular, presumely protocerebral appendages – for example, primary antennae or well-developed frontal appendages, which are individualized from the trunk lobopods. Mouthparts may consist of rows of teeth or a conical proboscis. The eyes may be represented by a single ocellus or by numerous pairs of simple ocelli, as has been shown in Luolishania, Ovatiovermis, Onychodictyon, Hallucigenia, Facivermis, and less certainly Aysheaia as well. However, in gilled lobopodians like Kerygmachela, the eyes are relatively complex reflective patches that may had been compound in nature.Trunk and lobopods
The trunk is elongated and composed of numerous body segments, each bearing a pair of legs called lobopods or lobopodous limbs. The segmental boundaries are not as externally significant as those of arthropods, although they are indicated by heteronomous annulations in some species. The trunk segments may bear other external, segment-corresponding structures such as nodes, papillae, spine/plate-like sclerites or lateral flaps. One member of Lobopodia, Palaeocampa, has spines which seem to have been poisonous as shown by preserved exudates at their tips, with FTIR showing the secretions likely contained aldehydes. The trunk may terminate with a pair of lobopods or a tail-like extension.The lobopods are flexible and loosely conical in shape, tapering from the body to tips that may or may not bear claws. The claws, if present, are hardened structures with a shape resembling a hook or gently curved spine. Claw-bearing lobopods usually have two claws, but single claws are known, as are more than two depending on its segmental or taxonomical association. In some genera, the lobopods bear additional structures such as spines, fleshy outgrowths, or tubercules. There is no sign of arthropodization in known members of lobopodians, even for those belonging to the arthropod stem-group, and the suspected case of arthropodization on the limbs of Diania is considered to be a misinterpretation.
Differentiation between trunk somites barely occurs, except in hallucigeniids and luolishaniids, where numerous pairs of their anterior lobopods are significantly slender or setose in contrast to their posterior counterparts.
Internal structures
The gut of lobopodians is often straight, undifferentiated, 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. In some specimens, parts of the lobopodian gut can be preserved in three dimensions. This cannot result from phosphatisation, which is usually responsible for 3-D gut preservation, because the phosphate content of the guts is under 1%; the contents comprise quartz and muscovite. The gut of the representative Paucipodia is variable in width, being widest at the centre of the body. Its position in the body cavity is only loosely fixed, so flexibility is possible.Not much is known about the neural anatomy of lobopodians due to the spare and mostly ambiguous fossil evidence. Possible traces of a nervous system were found in Paucipodia, Megadictyon and Antennacanthopodia. The first and so far the only confirmed evidence of lobopodian neural structures comes from the gilled lobopodian Kerygmachela in Park et al. 2018 — it presents a brain composed of only a protocerebrum that is directly connected to the nerves of eyes and frontal appendages, suggesting the protocerebral ancestry of the head of lobopodians as well as the whole Panarthropoda.
In some extant ecdysozoan such as priapulids and onychophorans, there is a layer of outermost circular muscles and a layer of innermost longitudinal muscles. The onychophorans also have a third, intermediate, layer of interwoven oblique muscles. Musculature of the gilled lobopodian Pambdelurion shows a similar anatomy, but that of the lobopodian Tritonychus shows the opposite pattern: it is the outermost muscles that are longitudinal and the innermost layer that consists of circular muscles.