Lobopodia

Source: Wikipedia, the free encyclopedia.
For a type of cell projection, see
Pseudopod. For an explanation of very similar terms, see Panarthropoda. For Lobopoda, a genus of beetles, see Lobopoda
.
Lobopodia
Temporal range: Cambrian Series 2–Early Pennsylvanian[1] Descendant taxa
Euarthropoda survive to Recent, possible Ediacaran ichnofossils[2]
Reconstruction of various lobopodians. 1: Microdictyon sinicum, 2: Diania cactiformis, 3: Collinsovermis monstruosus, 4: Luolishania longicruris, 5: Onychodictyon ferox, 6: Hallucigenia sparsa, 7: Aysheaia pedunculata, 8: Antennacanthopodia gracilis, 9: Facivermis yunnanicus, 10: Paucipodia inermis, 11: Jianshanopodia decora, 12: Hallucigenia fortis
Reconstruction of various lobopodians. 1: Microdictyon sinicum, 2: Diania cactiformis, 3: Collinsovermis monstruosus, 4: Luolishania longicruris, 5: Onychodictyon ferox, 6: Hallucigenia sparsa, 7: Aysheaia pedunculata, 8: Antennacanthopodia gracilis, 9: Facivermis yunnanicus, 10: Paucipodia inermis, 11: Jianshanopodia decora, 12: Hallucigenia fortis
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
Superphylum: Ecdysozoa
(unranked): Panarthropoda
Phylum: †"Lobopodia"
Snodgrass 1938
Groups included
Cladistically included but traditionally excluded taxa

Crown-group

Euarthropoda

Synonyms

Lobopodians are members of the informal group Lobopodia

panarthropods with stubby legs called lobopods,[5] a term which may also be used as a common name of this group as well.[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.[9][5][10]

The oldest near-complete

Tardigrada and Onychophora
) are thought to have evolved from lobopodian ancestors.

Definitions

Various definitions of lobopodians within Panarthropoda.[8]

The Lobopodian concept varies from author to author.

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.[5][21][22][8][23][24]
This corresponds to "A" in the image to the left.

An alternative, broader definition of Lobopodia would also incorporate Onychophora and Tardigrada,

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. Its validity is uncertain, however, as there are a number of hypotheses regarding the internal phylogeny of Panarthropoda.[25] 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.[26][6]

Lobopodia has, historically, sometimes included

phylogenomic and anatomical studies to be a highly specialized taxon of crustaceans.[30][31][32][33][34]

Representative taxa

Aysheaia pedunculata and Hallucigenia sparsa, two of the most iconic as well as the first described[5] lobopodians.

The better-known

Maotianshan Shale and the Burgess Shale. Aysheaia pedunculata has a morphology apparently basic for lobopodians[21] — 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.[35] 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.[14][22]

small shelly fossils (SSF). Xenusion has the oldest fossil record amongst the described lobopodians, which may trace back to Cambrian Stage 2.[16][10] Luolishania is an iconic example of lobopodians with multiple pairs of specialized appendages.[36] 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.[20][26]

Morphology

Complete fossil of Aysheaia pedunculata, showing overall morphology.
  • Maximum size of the 3 species of Hallucigenia (from top, H. fortis, H. hongmeia and H. sparsa) in scale.
    Maximum size of the 3 species of Hallucigenia (from top, H. fortis, H. hongmeia and H. sparsa) in scale.
  • Fossils of Xenusion, a lobopodian that might have grown up to 20 centimeters.
    Fossils of Xenusion, a lobopodian that might have grown up to 20 centimeters.

Most lobopodians were only a few centimeters in length, while some genera grew up to over 20 centimeters.[7] 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.[37] Body and appendages are circular in cross-section.[37]

Head

Anterior section of Onychodictyon ferox, showing head structures.
Fossil of Jianshanopodia decora, showing head region (upper left) compose of robust frontal appendage (right) and pharynx with rows of teeth (bottom left).

Due to the usually poor preservation, detailed reconstructions of the head region are only available for a handful of lobopodian species.[38][22] The head of a lobopodian is more or less bulbous,[5] and sometime possesses a pair of pre-ocular, presumely protocerebral[23] appendages – for example, primary antennae[39][36][23][40] or well-developed frontal appendages,[41][19][42][7][5] which are individualized from the trunk lobopods[23][43] (with the exception of Antennacanthopodia, which have two pairs of head appendages instead of one[39]). Mouthparts may consist of rows of teeth[37][22][42][7][44] or a conical proboscis.[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.[38] However, in gilled lobopodians like Kerygmachela, the eyes are relatively complex reflective patches[48] that may had been compound in nature.[49]

Trunk and lobopods

Paucipodia inermis, a lobopodian with featureless, undifferentiated trunk region.
Luolishania longicruris, showing clear differentiation between trunk segments and lobopods.

The trunk is elongated and composed of numerous body segments (

somites), each bearing a pair of legs called lobopods[5] or lobopodous limbs.[19] The segmental boundaries are not as externally significant as those of arthropods, although they are indicated by heteronomous annulations (i.e., the alternation of annulation density corresponding to the position of segmental boundaries) in some species.[50][36][21] The trunk segments may bear other external, segment-corresponding structures such as nodes (e.g. Hadranax,[6] Kerygmachela[19]), papillae (e.g. Onychodictyon[38]), spine/plate-like sclerites (e.g. armoured lobopodians[5]) or lateral flaps (e.g. gilled lobopodians[19][44]). The trunk may terminate with a pair of lobopods (e.g. Aysheaia, Hallucigenia sparsa)[22] or a tail-like extension (e.g. Paucipodia, Siberion, Jianshanopodia).[50][37][7][27]

The lobopods are flexible and loosely conical in shape, tapering from the body to tips that may [21][5] or may not [39][51][10] bear claws. The claws, if present, are hardened structures with a shape resembling a hook or gently-curved spine.[37][52][36][21][5] Claw-bearing lobopods usually have two claws, but single claws are known (e.g. posterior lobopods of luolishaniids[36][45][40]), as are more than two (e.g. three in Tritonychus,[53] seven in Aysheaia[41]) depending on its segmental or taxonomical association.[21] In some genera, the lobopods bear additional structures such as spines (e.g. Diania[51]), fleshy outgrowths (e.g. Onychodictyon[38]), or tubercules (e.g. Jianshanopodia[7]). There is no sign of arthropodization (development of a hardened exoskeleton and segmental division on panarthropod appendages) in known members of lobopodians, even for those belonging to the arthropod stem-group (e.g. gilled lobopodians and siberiids), and the suspected case of arthropodization on the limbs of Diania[54] is considered to be a misinterpretation.[51][10]

Differentiation (tagmosis) between trunk somites barely occurs, except in hallucigenids and luolishaniids, where numerous pairs of their anterior lobopods are significantly slender (hallucigenids) or setose (luolishaniids) in contrast to their posterior counterparts.[5][22][45][24][40]

Internal structures

Fossilized posterior trunk region of Jianshanopodia decora, showing traces of lobopods, gut diverculae and lobe-like terminal extension.

The gut of lobopodians is often straight, undifferentiated,[55] and sometimes preserved in the fossil record in three dimensions. In some specimens the gut is found to be filled with sediment.[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. 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).[7][42][55] 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,[56] because the phosphate content of the guts is under 1%; the contents comprise quartz and muscovite.[37] 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.

Eyes (deep blue), brain (light blue) and digestive system (yellow) of Kerygmachela.

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

panarthropods) 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.[49]

In some extant

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,[57] 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.[53]

Categories

Based on external morphology, lobopdians may fall under different categories — for example the general worm-like taxa as "

monophyletic in further studies.[21][8][23]

Armoured lobopodians

Fossil of Microdictyon sinicum, showing pairs of sclerite and trace of trunk and lobopods.
Diania cactiformis, an armoured lobopodian covered by myriad spines.

Armoured lobopodians referred to

small shelly fossils (SSF).[50][14] Armoured lobopodians were suggest to be onychophoran-related and may even represent a clade in some previous studies,[58] but their phylogenetic positions in later studies are controversial. (see text
)

Gilled lobopodians

The gilled lobopodians Pambdelurion (top left) and Kerygmachela (bottom right).

Euarthropoda.[19][60] Whether these genera were true lobopodians is still contested by some.[62] However, they are widely accepted as stem-group arthropods just basal to radiodonts.[8][21][22][23]

Siberion and similar taxa

The siberiid lobopodians Siberion (upper left), Megadictyon (bottom center) and Jianshanopodia (upper right).

Siberion, Megadictyon and Jianshanopodia may be grouped as siberiids (order Siberiida),[27] jianshanopodians[21] or "giant lobopodians"[63] by some literatures. They are generally large — body length ranging between 7[27] and 22 centimeters[42] (2¼ to 8⅔ inches) — xenusiid lobopodians with widen trunk, stout trunk lobopods without evidence of claws, and most notably a pair of robust frontal appendages.[8] With the possible exception of Siberion,[27][21] they also have digestive glands like those of a gilled lobopodian and basal euarthropod.[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.[7][42][8][23]

Paleoecology

predator
.

Lobopodians possibly occupied a wide range of

predatory lifestyle.[7][55] On the other hand, luolishaniids such as Luolishania and Ovatiovermis have elaborate feather-like lobopods that presumably formed 'baskets' for suspension or filter-feeding.[36][45] Lobopods with curved terminal claws may have given some lobopodians the ability to climb on substrances.[5]

Not much is known about the

onychophorans.[57]

Distribution

During the Cambrian, lobopodians displayed a substantial degree of biodiversity. One species is known from each of the Ordovician and Silurian periods,[12][64] with a few more known from the Carboniferous (Mazon Creek) — this represents the paucity of exceptional lagerstatten in post-Cambrian deposits.

Phylogeny

Ecdysozoa
Cycloneuralia

Nematoda
and relatives

Panarthropoda

(Lobopodian taxa controversial)

(Lobopodian taxa controversial)

Crown-group

Tardigrada

(Lobopodian taxa controversial)

Neutralized phylogeny between lobopodians and other
Ecdysozoan taxa.[21][8][22][60][23][45][24]
Extant panarthropod taxa are in bold. Relationship between the total-group of extant panarthropod phyla is unresolved.

The overall phylogenetic interpretation on lobopodians has changed dramatically since their discovery and first description.

panarthropod taxa.[5][23][65]

Based on their apparently

paraphyletic, and include the last common ancestor of arthropods, onychophorans and tardigrades.[5]

Stem-group arthropods

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).[21][8][22][60][23][45][24] 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 biramous appendages).[8][23] 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.[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.[43][23]

  • Radiodonts are stem-group arthropods with gilled lobopodian-like body flaps, arthropodized frontal appendages and stalked compound eyes.
    Radiodonts are stem-group arthropods with gilled lobopodian-like body flaps, arthropodized frontal appendages and stalked compound eyes.
  • Restoration of Pambdelurion a "gilled lobopodian" related to arthropods, which has both pairs of lobopods and lateral flaps.
    Restoration of Pambdelurion a "gilled lobopodian" related to arthropods, which has both pairs of lobopods and lateral flaps.

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.[54] However, this interpretation is questionable as the data provided by the original description are not consistent with the suspected phylogenic relationships.[69][70] 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'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.[51][10] 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.[51][10]

Stem-group onychophorans

Antennacanthopodia gracilis, a lobopodian suggested to be a stem-group onychophoran.

While

panarthropod plesiomorphy.[24][40]

Stem-group tardigrades

Artistic reconstruction of the “Orsten” tardigrade, a parasitic stem-lineage tardigrade, known from fossils recovered from the Cambrian Kuonamka Formation of Siberia.[71]

Lobopodian taxa of the tardigrade stem-group is unclear.[5] Aysheaia[45][24][40] or Onychodictyon ferox[21][22] had been suggest to be a possible member, based on the high claw number (in Aysheaia) and/or terminal lobopods with anterior-facing claws (in both taxa).[21] Although not widely accepted, there are even suggestions that Tardigrada itself representing the basalmost panarthropod or branch between the arthropod stem-group.[68]

Stem-group panarthropods

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 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.[5][21][22][53][47]

Described genera

Fossil of Aysheaia pedunculata.
Fossil of Microdictyon sinicum.
Fossil of "Mureropodia apae", which may be in fact frontal appendage of Caryosyntrips cf. camurus.
Reconstruction of Facivermis, an unusual lobopodian with limbless posterior region that lived like a tube worm

As of 2018, over 20 lobopodian genera have been described.[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.[72][73][74] Miraluolishania was suggested to be synonym of Luolishania by some studies.[36][75][47] The enigmatic Facivermis was later revealed to be a highly specialized genus of luolishaniid lobopodians.[27][45][47]

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