|Reconstruction of the lobopodian Aysheaia pedunculata|
|Cladistically included but traditionally excluded taxa|
The lobopodians, members of the informal group Lobopodia(from the Greek, meaning "blunt feet") or the formally erected phylum Lobopoda Cavalier-Smith (1998) 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, worm-like fossil panarthropods such as Aysheaia and Hallucigenia .
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 scope of the Lobopodian concept varies from author to author. Its most general as well as the most limited sense refers to a suite of mainly Cambrian worm-like panarthropod taxa with lobopods – for example Aysheaia , Hallucigenia and Xenusion , members which were traditionally united as "xenusians" or "xenusiids" (class Xenusia). The dinocaridid genera Pambdelurion and Kerygmachela may also be regarded as lobopodians,eventually referred as "gilled lobopodians" or "gilled lobopod". Under such definitions, "Lobopodia" compose of only extinct taxa, and widely accepted as an informal, paraphyletic grade in correspond to the crown-group of three extant panarthropod phyla: Onychophora (velvet worms), Tardigrada (waterbears) and Arthropoda (arthropods).
An alternative, broader definition of lobopodians would also incorporate the extant phyla Onychophora and Tardigrada,two groups of panarthropod which also bore lobopodous limbs as well. "Lobopodia" may also refer to a possible clade sister to arthropoda, and compose of only Tardigrada and Onychophora. The broadest definition proposes the monophyletic superphylum Lobopodia is equivalent to Panarthropoda.
The better-known genera include, for example, Aysheaia , which was discovered in the Canadian Burgess Shale and Hallucigenia , known from both Chenjiang Maotianshan Shale and the Burgess Shale. Aysheaia pedunculata has morphology apparently basic for lobopodians— for example significantly annulated cuticle, terminal mouth opening, specialized frontalmost appendages and stubby lobopods with terminal claws. Hallucigenia sparsa is famous by 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: reveal the bulbous inprint previously thought to be a head was actually gut contents being expelled from its 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 often found disarticulated and known as an example of small shelly fossils (SSF). Xenusion has the oldest fossil record amongst 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 arthropod s, as they have both lobopodian affinities and characters link to the arthropod stem-group.
Most lobopodians are only a few centimeters in length, while some genera may have grown up to over 20 centimeters. Their bodies are annulated, although the annulation may be difficult to discern, due to their close spacing (~0.2 mm) and low relief on the fossil materials. Body and appendages are circular in cross-section.
The limbs of lobopodians, technically called lobopodsor lobopodous limbs, are loosely conical in shape, tapering from the body to their clawed or claw-lacking tips. Usually the longest and most robust legs are at the middle of the trunk, with those nearer the anterior and posterior more spindly. The claws, if present, are slightly curved, and their length is loosely proportional to the length of the leg to which they are attached. In some genera, the lobopods may bear additional structures such as spines (e.g. Diania ), fleshy outgrowths (e.g. Onychodictyon ), or tubercules (e.g. Jianshanopodia ). There's no signs of arthropodization (development of harden exoskeleton and segmental division on panarthropod appendages) in known members of lobopodians even for those belong to the arthropod stem-group (e.g. gilled lobopodians and siberiids), and the suspected case of arthropodization on the limbs of Diania is considered to be a misinterpretation.
Heads are more or less bulbous,and may bear a pair of pre-ocular, protocerebral appendages – for examples primary antennae or well-developed frontal appendages, which are individualized from the trunk lobopods (with the exception of Antennacanthopodia , which have two pairs of head appendages instead of one ). Mouthparts may consist of rows of teeth or conical proboscis. The eyes may be represented by one or be numerous pairs of simple ocelli as has been shown in Paucipodia , Luolishania , Miraluolishania , Ovatiovermis , Onychodictyon , Hallucigenia , and possibly Aysheaia as well. However in gilled lobopodians like Kerygmachela , the eyes are relatively complex reflective patches.
The trunk is elongated and compose of numerous body segments (somites) each bore a pair of lobopod, but the segmental boundaries are not externally significant as those of arthropods. The trunk segments may bear other external structures such as turbecles (e.g. Hadranax ,Kerygmachela ), spine/plate-like sclerites (e.g. armoured lobopodians ) or flaps (e.g. gilled lobopodians ). Differentiation between trunk appendages barely occure, except in luolishaniids and hallucigenids, where numerous pairs of their anterior lobopods are significantly slender and/or setose contrast to their posterior counterparts. The trunk may terminate with a pair of lobopod (e.g. Aysheaia , Hallucigenia ) or tail-like extension (e.g. Siberion , Jianshanopodia ).
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 guts were surrounded by pairs of serially repeated kidney-shaped gut diverticulae (digestive glands). 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, for 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.
No much are known about the neural anatomy of lobopodians due to the spare and mostly ambiguous fossil evidence. Possible traces of brain and ventral nerve cord were found in Megadictyon and Paucipodia , respectively.The first confirmed evidence of lobopodian neural structures comes from the gilled lobopodian Kerygmachela in a 2018 study — it presents a brain compose of only protocerebrum (the frontalmost cerebral ganglion of panarthropods) which were directly connect to the nerves of eyes and frontal appendages, suggest the protocerebral ancestry of the head of lobopodians as well as panarthropods.
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 has a third intermediate layer of interwoven oblique muscles between the two others. But the musculature of a fossil lobopodian Tritonychus show the opposite pattern; it is the outermost muscles that are longitudinal and the innermost layer consisting of circular muscles.
Based on external morphology, lobopdians 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.
Armoured lobopodians referred to xenusiid lobopodians which bore repeated sclerites such as spine or plates on their trunk (e.g. Hallucigenia , Microdictyon , Luolishania ) or lobopods (e.g. Diania). In contrast, lobopodians without sclerites may be referred to as "unarmoured lobopodians".Function of the sclerites were interpreted as protective armor and/or muscle attachment points. In some cases, only the disarticulated sclerites of the animal were preserved, which represented as component of small shelly fossils (SSF). Armoured lobopodians were suggest to be onychophoran-related and may even represent a clade in some previous studies, but their phylogenetic positions in later studies are controversial. (see text)
Gilled lobopodians referred to dinocaridids with lobopodian affinities (e.g. annulation, lobopods) and a pair of flaps on each of their trunk segments, but no signs of arthropodization on their robust frontal appendages like those of a derived dinocaridid taxon: Radiodonta. Gilled lobopodians were recognized by at least two genera: Pambdelurion and Kerygmachela.Opabinia may also fall under this category in a broarder sense, although the present of lobopods in this genus are more or less ambigious. Omnidens , a genus known only by Pambdelurion-like mouth apparatures, may also be a gill lobopodian as well. The body flaps may function as swimming appendages and/or gills, and possibly homologous to the dorsal flaps of radiodonts and exopods of euarthropods. Gilled lobopodians are not considered lobopodians in some usage, and they are widely accepted as stem-group arthropod s just basal to radiodonts.
Siberion , Megadictyon and Jianshanopodia may be grouped taxonomically as Siberiida or siberiids by some studies. centimeters ) xenusiid lobopodians with widen trunk, stout trunk lobopods without evidence of claws, and most notably a pair of robust frontal appendages. With the exception of Siberion, they also have digestive glands like those of a gilled lobopodian and basal euarthropod. Their anatomy represent transitional froms between typical xenusiids and gilled lobopodians, eventually placing them under the basalmost position of arthropod stem-group.They are generally large (body length ranging between 7 and 22
Lobopodians may have occupied a wide range of ecological niche s. Althought most of them had undifferentiated appendages and straight gut, which would suggest a simple sediment-feeding lifestyle,sophicated digestive glands and large size of gilled lobopodians and siberiids would allow them to consume larger food items, and their robust frontal appendages may even suggest a predatory lifestyle. On the other hand, luolishaniids such as Luolishania and Ovatiovermis have elaborate feather-like lobopods that presumably formed 'baskets' for suspension or filter-feeding. Lobopods with curved termial claws may have given some lobopodians the ability to climb on substrances.
Not much is known about the physiology of lobopodians. There are evidence suggest that lobopodians moult just like other ecdysozoan taxa, but the outline and ornamentation of the harden sclerite did not vary during ontogeny.The gill-like structures on the body flaps of gilled lobopodians and ramified extensions on the lobopods of Jianshanopodia may provide respiratory function (gills). Pambdelurion may control the movement of their lobopods in a way similar to onychophorans.
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.
|Neutralized phylogeny between lobopodians and other Ecdysozoan taxa. Extant taxa are in bold. Relationship between the total-group of extant panarthropod phyla is unresolve.|
The overall phylogenetic interpertation on lobopodians changed dramatically beyond decades.The reassignments are not only based on new fossil evidence, but also new embryological, neuroanatomical, and genomic (e.g. gene expression, phylogenomics) informations observed from extant panarthropod taxa.
Based on their apparently onychophoran-like morphology (e.g. annulated cuticle, lobopodous appendage with claws), lobopodians were traditionally thought to be present a group of paleozoic onychophorans.This interpretation was flawed after the discovery of lobopodians with arthropod and tardigrade-like characters, suggest the similarity between lobopodians and onychophorans represent deeper panarthropod ancestral trait (plesiomorphies) instead of onychophoran-exclusive characters (synamorphies). For example, The British palaeontologist Graham Budd sees the Lobopodia as representing a basal grade from which the phyla Onychophora and Arthropoda arose, with Aysheaia comparable to the ancestral plan, and with forms like Kerygmachela and Pambdelurion representing a transition that, via dinocaridids to arthropods, would lead to an arthropod body plan. Aysheaia's surface ornamentation, if homologous with palaeoscolecid sclerites, may represent a deeper link connecting it with cycloneuralian outgroups. Many further studies follow and extend the idea, generally agreed that all three panarthropod phyla have lobopodians in their stem lineages. Lobopodians are thus paraphyletic, and include the last common ancestor of arthropods, onychophorans and tardigrades.
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).Their positions within arthropod stem-group are indicated by numerous arthropod groundplans and intermediate forms (e.g. arthropod-like digestive glands, radiodont-like frontal appendages and dorso-ventral appendicular structures link to arthropod biramus appendages). Lobopodian ancestry of arthropods also reinforced by genomic studies on extant taxa — gene expression support the homology between arthropod appendages and onychophoran lobopods, suggests that modern less-segmented arthropodized appendages evolved from annulated lobopodous limbs, with multipodomerous appendages of extinct basal euarthropods (e.g fuxianhuiids) may represent an intermediate form. 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 and developmental pattern of the labrum of extant arthropods.
Diania , a genus of armoured lobopodian with stout and spiny legs, were originally thought to be associated within the arthropod stem-group based on its apparently arthropod-like (arthropodized) trunk appendages.However, this interpretation is questionable as the data provided by the original description are not consistent with the suspected phylogenic relationships. Further re-examination even revealed that the suspcted arthropodization on the legs of Diania was a misinterpretation — although the spine may have hardened, the remaining cuticle of Diania'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. Thus, the re-examination eventually reject the evidence of arthropodization (sclerotization, segmentation and articulation) on the appendages as well as the fundamental relationship between Diania and arthropods.
While Antennacanthopodia is widely accepted as a member of stem-group onychophoran,position of other xenusiid genera that previously though to be onychophoran-related are controversial — in further studies, most of them were either suggest to be stem-group onychophorans or basal panarthropods, with a few species occasionally suggest to be stem-group tardigrades and/or stem-group panarthropods. A study in 2014 suggest that Hallucigenia are stem-group onychophorans based on their claws, which have overlapped internal structures resemble to those of an extant onychophoran. This interpretation was questioned by later studies, as the structures may present panarthropod plesiomorphy.
Lobopodian taxa of tardigrade stem-group is unclear.Aysheaia or Onychodictyon ferox had been suggest to be a possible member. Althought not widely accepted, there are even suggestions that Tardigrada itself representing the basalmost panarthropod or branch between the arthropod stem-group.
It is unclear that which lobopodians represent members of the panarthropod stem-group, which were branched just before the last common ancestor of extant panarthropod phyla. Aysheaia may have occupied this position based on its apparently basal morphology;while other studies rather suggest luolishaniid and hallucigenid, two lobopodian taxa which had been resolved as members of stem-group onychophorans as well.
Until 2018, there are over 20 lobopodian genera had been described.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. Miraluolishania was suggested to be synonym of Luolishania by some authors. The enigmatic Facivermis was later revealed to be a highly specialized genus of luolishaniid lobopodians.
Hallucigenia is a genus of Cambrian xenusiids known from articulated fossils in Burgess Shale-type deposits in Canada and China, and from isolated spines around the world. The generic name reflects the type species' unusual appearance and eccentric history of study; when it was erected as a genus, H. sparsa was reconstructed upside down and back to front. Hallucigenia is now recognized as a "lobopodian worm". It is considered by some to represent an early ancestor of the living velvet worms, although other researchers favour a relationship closer to arthropods.
The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their Konservat Lagerstätten, deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales form one of some forty Cambrian fossil locations worldwide exhibiting exquisite preservation of rarely preserved, non-mineralized soft tissue, comparable to the fossils of the Burgess Shale. They take their name from Maotianshan Hill in Chengjiang County, Yunnan Province, China.
Opabinia regalis is an extinct, stem group arthropod found in the Middle Cambrian Burgess Shale Lagerstätte of British Columbia, Canada. It flourished from 505 million years ago to 487 million years ago during the Cambrian Period of the Paleozoic Era. It measured 2-3 inches in length and is presumed to have been a carnivore. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed, where they constitute less than 0.1% of the community. Opabinia was a soft-bodied animal, averaging about 5.7 cm in length, and its segmented body had lobes along the sides and a fan-shaped tail. The head shows unusual features: five eyes, a mouth under the head and facing backwards, and a proboscis that probably passed food to the mouth. Opabinia probably lived on the seafloor, using the proboscis to seek out small, soft food.
Aysheaia was a genus of Cambrian-aged soft-bodied, caterpillar-shaped fossil organisms with average body lengths of 1–6 cm.
Dinocaridida is a proposed taxon of extinct fossil arthropod-like marine animals found, with one exception, in the Cambrian and Ordovician. The name of Dinocaridids comes from Greek, "deinos" and "caris", meaning "terror shrimp" or "terror crab", due to their crustacean-like appearance and the hypotheses suggesting that members of this class were amongst the dominating and most diverse apex predators of their time. Dinocaridids occasionally referred as 'AOPK group' by some literatures, as the group compose of Radiodonta, Opabinia, Pambdelurion and Kerygmachela. It is probably paraphyletic, with Kerygmachela and Pambdelurion more basal than the Opabinia and Radiodonta clade.
Sirius Passet is a Cambrian Lagerstätte in Greenland. The Sirius Passet Lagerstätte was named after the Sirius sledge patrol that operates in North Greenland. It comprises six places in Nansen Land, on the east shore of J.P. Koch Fjord in the far north of Greenland. It was discovered in 1984 by A. Higgins of the Geological Survey of Greenland. A preliminary account was published by Simon Conway Morris and others in 1987, but since then, expeditions led by J. S. Peel and Simon Conway Morris have returned to the site several times between 1989 and the present. A field collection of perhaps 10,000 fossil specimens has been amassed. It is a part of the Buen Formation.
Kerygmachela kierkegaardi was a gilled lobopodian from the Buen Formation of the Sirius Passet Lagerstätte, in northern Greenland. Its anatomy strongly suggests that it, along with its relative Pambdelurion whittingtoni, was a close relative of anomalocaridid (radiodont) and euarthropod. The specific name, "kierkegaardi" honors Danish philosopher Søren Kierkegaard.
Pambdelurion whittingtoni is an extinct, blind, nektonic organism from the Sirius Passet Lagerstätte, from Cambrian Greenland. Its anatomy strongly suggests that it, along with its relative Kerygmachela kierkegaardi, was either an anomalocarid or a close relative thereof.
The (pan)arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other. While the dispute has historically centered on the exact make-up of the insect head, it has been widened to include other living arthropods such as the crustaceans and chelicerates; and fossil forms, such as the many arthropods known from exceptionally preserved Cambrian faunas. While the topic has classically been based on insect embryology, in recent years a great deal of developmental molecular data has become available. Dozens of more or less distinct solutions to the problem, dating back to at least 1897, have been published, including several in the 2000s.
A number of assemblages bear fossil assemblages similar in character to that of the Burgess Shale. While many are also preserved in a similar fashion to the Burgess Shale, the term "Burgess Shale type fauna" covers assemblages based on taxonomic criteria only.
The Wheeler Shale is a Cambrian (c. 507 Ma) fossil locality world-famous for prolific agnostid and Elrathia kingii trilobite remains and represents a Konzentrat-Lagerstätten. Varied soft bodied organisms are locally preserved, a fauna and preservation style normally associated with the more famous Burgess Shale. As such, the Wheeler Shale also represents a Konservat-Lagerstätten.
Megacheira is an extinct class of predatory arthropods that possessed a pair of great appendages, hence the class' name as well as the common name "great appendage arthropods". Their neural structures and deutocerebral appendages resemble those of chelicerates. Most of them were found in marine environments throughout the world from the lower to middle Cambrian. Megacheirans were important components of several faunas, including the Burgess, Wheeler and Maotianshan Shales Lagerstatten.
Class Xenusia, the Xenusiids, represents the subset of lobopodian worms that fall in the stem-lineage of Onychophora. Their type genus is Xenusion. They have relatively large, annulated, cylindrical bodies. Their lobopod legs have tubercles at their bases. Some have large frontal appendages, although these may represent taphonomic artefacts. Their mouth is terminal or subterminal, and they are marine. They probably represent a grade rather than a clade.
Jianshanopodia decora is a Cambrian lobopodian. Its frontal, grasping appendages bear wedge-shaped plates. Its limbs branch, instead of being tipped with claws as many lobopods' are. It has a sediment-filled gut surrounded by serially repeated diverticulae. It is thought to have sucked up prey with its short 'trunk'. It mainly crawled on the sea floor, but could swim when necessary. Its mouth resembles those of anomalocaridids and priapulids.
Radiodonta is an order of stem-group arthropods that was successful worldwide during the Cambrian period, and included the earliest large predators known. They may referred as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the latter originally refer to the family Anomalocarididae, which previously included all but recently only a few species of this order. Some of the most famous species of radiodonts are the Cambrian taxa Anomalocaris canadensis, Hurdia victoria, Peytoia nathorsti, and Amplectobelua symbrachiata, the Ordovician Aegirocassis benmoulai and the Devonian Schinderhannes bartelsi.
Diania is an extinct genus of animal found in the Lower Cambrian Maotianshan shale of China, represented by a single species D. cactiformis. Known during its investigation by the nickname "walking cactus", this organism belongs to a group known as the armoured lobopodians and has a simple worm-like body with robust, spiny, and apparently jointed legs. Its significance is that jointed legs are the defining character of the arthropods and Diania may thus be very close to the origins of the most diverse group of animals on the planet.
Euperipatoides kanangrensis is a species of velvet worm of the Peripatopsidae family, described in 1996 from specimens collected in Kanangra-Boyd National Park, New South Wales. It is endemic to Australia. The embryonic development of Euperipatoideskanangrensis has been described. This species is used as model organism for the last common ancestor of the Panarthropoda. It resembles fossil Cambrian lobopodians.
Megadictyon is a genus of cambrian lobopodian with similarites to Jianshanopodia and Siberion. Occasionally mis-spelt Magadictyon.
The protocerebrum is the first segment of the panarthropod brain.