The lobopodians, members of the informal group Lobopodia (from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998),
are
panarthropods
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphyletic gr ...
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
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
'' and ''
Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
''.
The oldest near-complete
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
lobopodians date to the
Lower Cambrian
The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...
; some are also known from
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
,
Silurian and
Carboniferous Lagerstätten.
Some bear toughened claws, plates or spines, which are commonly preserved as
carbonaceous
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up ...
or
mineralized microfossils
A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...
in Cambrian strata.
The grouping is considered to be
paraphyletic, as the three living panarthropod groups (
Arthropoda
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...
,
Tardigrada
Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...
and
Onychophora
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
) are thought to have evolved from lobopodian ancestors.
Definitions
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
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
'', ''
Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
'' and ''
Xenusion'', members which were traditionally united as "
xenusians" or "
xenusiids" (class
Xenusia).
The
dinocaridid genera ''
Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' and ''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
'' 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
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
(velvet worms),
Tardigrada
Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...
(waterbears) and
Arthropoda
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...
(arthropods).
An alternative, broader definition of lobopodians would also incorporate the extant phyla
Onychophora
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
and
Tardigrada
Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...
,
two groups of panarthropod which bear lobopodous limbs as well.
"Lobopodia" may also refer to a possible clade sister to Arthropoda, and compose of only Tardigrada and Onychophora. Lobopodia sometimes also included
Pentastomida,
a group of parasitic panarthropod which traditionally though to be a unique
phylum, but revealed by subsequent
phylogenomic
Phylogenomics is the intersection of the fields of evolution and genomics. The term has been used in multiple ways to refer to analysis that involves genome data and evolutionary reconstructions. It is a group of techniques within the larger fields ...
and anatomical studies as a highly specialized taxon of
crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...
arthropods. The broadest definition proposes the
monophyletic superphylum Lobopodia is equivalent to
Panarthropoda
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphylet ...
.
Representative taxa
The better-known
genera include, for example, ''
Aysheaia
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
'', which was discovered in the Canadian
Burgess Shale
The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fo ...
and ''
Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
'', known from both the Chenjiang
Maotianshan Shale
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 ...
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
''Microdictyon'' is an extinct armoured worm-like animal coated with net-like scleritic plates, known from the Early Cambrian Maotianshan shale of Yunnan China and other parts of the world. ''Microdictyon'' is part of the ill-defined taxon &ndas ...
'' 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
Stage 2 of the Cambrian is the unnamed upper stage of the Terreneuvian Series. It lies atop the Fortunian and below Stage 3 of the Cambrian. It is commonly referred to as the Tommotian, after the Cambrian stratigraphy of Siberia. Neither the uppe ...
.
''
Luolishania
''Luolishania'' is an extinct genus of lobopodian worm and known from the Lower Cambrian Chiungchussu Formation (Maotianshan Shales) of the Chengjiang County, Yunnan Province, China. A monotypic genus, it contains one species ''Luolishania longi ...
'' is an iconic example of lobopodians with multiple pairs of specialized appendages.
The gill lobopodians ''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
'' and ''
Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' shed light on the relationship between lobopodians and
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s, as they have both lobopodian affinities and characteristics linked to the arthropod stem-group.
Morphology
File:20210000 Hallucigenia diagrammatic reconstruction.png, Maximum size of the 3 species of ''Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
'' (from top, ''H. fortis'', ''H. hongmeia'' and ''H. sparsa'') in scale.
File:Xenusion auerswaldae.jpg, 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.
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
(with the exception of ''
Antennacanthopodia'', which have two pairs of head appendages instead of one
). 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
''Luolishania'' is an extinct genus of lobopodian worm and known from the Lower Cambrian Chiungchussu Formation (Maotianshan Shales) of the Chengjiang County, Yunnan Province, China. A monotypic genus, it contains one species ''Luolishania longi ...
''
(=''
Miraluolishania''
), ''
Ovatiovermis'',
''
Onychodictyon
''Onychodictyon'' is a genus of extinct lobopodian known from the Lower Cambrian Chengjiang Maotianshan Shales in the Yunnan Province in China. It was characterized by a stout body covered by fleshy papillae and pairs of sclerotized plates with ...
'',
''
Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
'',
''
Facivermis
''Facivermis'' (meaning "torch worm" ) is a genus of sessile lobopodian from the Lower Cambrian Maotianshan shales of China
Anatomy
''Facivermis'' was a worm-like creature up to 90 mm long. Its body was divided into three sections. The ante ...
'',
and less certainly ''
Aysheaia
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
'' as well.
However, in gilled lobopodians like ''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
'', the eyes are relatively complex reflective patches that may had been
compound
Compound may refer to:
Architecture and built environments
* Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall
** Compound (fortification), a version of the above fortified with defensive struc ...
in nature.
Trunk and lobopods
The trunk is elongated and composed of numerous body segments (
somites
The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide i ...
), each bearing a pair of legs technically called lobopods
or lobopodous limbs.
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.
[Chen, J.Y., Zhou, G.Q., Ramsköld, L. (1995a)]
The Cambrian lobopodian ''Microdictyon sinicum''
Bulletin of the National Museum of Natural Science 5, 1–93 (Taichung, Taiwan). The trunk segments may bear other external, segment-corresponding structures such as nodes (e.g. ''
Hadranax'',
''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
''
), papillae (e.g. ''
Onychodictyon
''Onychodictyon'' is a genus of extinct lobopodian known from the Lower Cambrian Chengjiang Maotianshan Shales in the Yunnan Province in China. It was characterized by a stout body covered by fleshy papillae and pairs of sclerotized plates with ...
''
), spine/plate-like sclerites (e.g. armoured lobopodians
) or lateral flaps (e.g. gilled lobopodians
). The trunk may terminate with a pair of lobopods (e.g. ''
Aysheaia
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
'', ''
Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
sparsa'')
or a tail-like extension (e.g. ''
Paucipodia'', ''
Siberion'', ''
Jianshanopodia
''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 repea ...
'').
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 2 claws, but single claws are known (e.g. posterior lobopods of
luolishaniids), as are more than 2 (e.g. 3 in ''
Tritonychus'',
7 in ''
Aysheaia
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
''
) depending on its segmental or taxonomical association.
In some genera, the lobopods bear additional structures such as spines (e.g. ''
Diania
''Diania'' is an extinct genus of lobopodian 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 be ...
''
), fleshy outgrowths (e.g. ''
Onychodictyon
''Onychodictyon'' is a genus of extinct lobopodian known from the Lower Cambrian Chengjiang Maotianshan Shales in the Yunnan Province in China. It was characterized by a stout body covered by fleshy papillae and pairs of sclerotized plates with ...
''
), or tubercules (e.g. ''
Jianshanopodia
''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 repea ...
''
). 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
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
stem-group (e.g. gilled lobopodians and siberiids), and the suspected case of arthropodization on the limbs of ''
Diania
''Diania'' is an extinct genus of lobopodian 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 be ...
''
is considered to be a misinterpretation.
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.
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 (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,
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
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
'' in Park et al. 2018 — it presents a brain composed of only a protocerebrum (the frontal-most cerebral
ganglion of
panarthropods
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphyletic gr ...
) 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
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphylet ...
.
In some extant
ecdysozoan such as
priapulid
Priapulida (priapulid worms, from Gr. πριάπος, ''priāpos'' 'Priapus' + Lat. ''-ul-'', diminutive), sometimes referred to as penis worms, is a phylum of unsegmented marine worms. The name of the phylum relates to the Greek god of fertility ...
s and
onychophora
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
ns, 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
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' 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.
Categories
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
Armoured lobopodians referred to
xenusiid lobopodians which bore repeated sclerites such as spine or plates on their trunk (e.g. ''
Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
'', ''
Microdictyon
''Microdictyon'' is an extinct armoured worm-like animal coated with net-like scleritic plates, known from the Early Cambrian Maotianshan shale of Yunnan China and other parts of the world. ''Microdictyon'' is part of the ill-defined taxon &ndas ...
'', ''
Luolishania
''Luolishania'' is an extinct genus of lobopodian worm and known from the Lower Cambrian Chiungchussu Formation (Maotianshan Shales) of the Chengjiang County, Yunnan Province, China. A monotypic genus, it contains one species ''Luolishania longi ...
'') or lobopods (e.g. ''
Diania
''Diania'' is an extinct genus of lobopodian 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 be ...
''). 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
Dinocaridids with lobopodian affinities (due to shared features like annulation and lobopods) are referred to as "gilled lobopodians"
or "gilled lobopods".
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
Radiodonta is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the last two origina ...
that have robust and sclerotized frontal appendages. Gilled lobopodians cover at least two genera: ''
Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' and ''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
.''
''
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 a ''Pambdelurion''-like mouth apparatus, may also be a gilled lobopodian.
The body flaps may have functioned as both swimming appendages and gills,
and are possibly homologous to the dorsal flaps of radiodonts and exites of
Euarthropoda
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...
.
Whether these genera were true lobopodians is still contested by some. However, they are widely accepted as stem-group
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s just basal to radiodonts.
''Siberion'' and similar taxa
''
Siberion'', ''
Megadictyon'' and ''
Jianshanopodia
''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 repea ...
'' may be grouped as siberiids (order
Siberiida),
jianshanopodians
or "giant lobopodians" by some literatures. They are generally large (body length ranging between 7
and 22 centimeters
) xenusiid lobopodians with widen trunk, stout trunk lobopods without evidence of claws, and most notably a pair of robust frontal appendages.
With the possible exception of ''Siberion'',
they also have digestive glands like those of a gilled lobopodian and basal euarthropod.
Their anatomy represent transitional forms between typical xenusiids and gilled lobopodians,
eventually placing them under the basalmost position of arthropod stem-group.
Paleoecology
Lobopodians possibly occupied a wide range of
ecological niche
In ecology, a niche is the match of a species to a specific environmental condition.
Three variants of ecological niche are described by
It describes how an organism or population responds to the distribution of resources and competitors (for ...
s.
Although most of them had undifferentiated appendages and straight gut, which would suggest a simple sediment-feeding lifestyle,
sophisticated 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
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
lifestyle.
On the other hand, luolishaniids such as ''
Luolishania
''Luolishania'' is an extinct genus of lobopodian worm and known from the Lower Cambrian Chiungchussu Formation (Maotianshan Shales) of the Chengjiang County, Yunnan Province, China. A monotypic genus, it contains one species ''Luolishania longi ...
'' 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
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
of lobopodians. There are evidence suggest that lobopodians
moult
In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
just like other
ecdysozoan taxa, but the outline and ornamentation of the harden sclerite did not vary during
ontogeny
Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the s ...
.
The gill-like structures on the body flaps of gilled lobopodians and ramified extensions on the lobopods of ''
Jianshanopodia
''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 repea ...
'' may provide
respiratory
The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gre ...
function (
gills
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
).
''
Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' may control the movement of their lobopods in a way similar to
onychophorans.
Distribution
During the Cambrian, lobopodians displayed a substantial degree of
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
. One species is known from each of the
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
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 changed dramatically beyond decades.
The reassignments are not only based on new fossil evidence, but also new
embryological
Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and ...
,
neuroanatomical, and
genomic (e.g.
gene expression,
phylogenomics) informations observed from extant
panarthropod taxa.
Based on their apparently
onychophoran
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
-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
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
and
tardigrade
Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...
-like characters,
suggest the similarity between lobopodians and onychophorans represent deeper
panarthropod ancestral trait (
plesiomorphies
In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades.
Plesiomorphy, symplesiomorphy, apomorphy, and ...
) instead of onychophoran-exclusive characters (
synapomorphies).
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
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
'' comparable to the ancestral plan, and with forms like ''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
'' and ''
Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' 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.
As 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
''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 repea ...
'' occupied the basalmost position, gilled lobopodians ''
Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' and ''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
'' branch next, and finally lead to a clade compose of ''
Opabinia'',
Radiodonta
Radiodonta is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the last two origina ...
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 biramous 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.
File:20191201 Radiodonta Amplectobelua Anomalocaris Aegirocassis Lyrarapax Peytoia Laggania Hurdia.png, Radiodonts are stem-group arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s with gilled lobopodian-like body flaps, arthropodized frontal appendages and stalked compound eye
A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...
s.
File:Alacaris restoration.jpg, The multisegmented limbs of fuxianhuiid may represent intermediate form between lobopods and modern arthropod appendages.
''
Diania
''Diania'' is an extinct genus of lobopodian 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 be ...
'', 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 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.
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.
As stem-group onychophorans
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 (''
Aysheaia
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
''
or ''
Onychodictyon
''Onychodictyon'' is a genus of extinct lobopodian known from the Lower Cambrian Chengjiang Maotianshan Shales in the Yunnan Province in China. It was characterized by a stout body covered by fleshy papillae and pairs of sclerotized plates with ...
ferox''
) occasionally suggest to be stem-group tardigrades. A study in 2014 suggest that ''
Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
'' 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.
As stem-group tardigrades
Lobopodian taxa of tardigrade stem-group is unclear.
''
Aysheaia
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
''
or ''
Onychodictyon
''Onychodictyon'' is a genus of extinct lobopodian known from the Lower Cambrian Chengjiang Maotianshan Shales in the Yunnan Province in China. It was characterized by a stout body covered by fleshy papillae and pairs of sclerotized plates with ...
ferox''
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).
Although not widely accepted, there are even suggestions that Tardigrada itself representing the basalmost panarthropod or branch between the arthropod stem-group.
As 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
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
'' may have occupied this position based on its apparently basic morphology;
while other studies rather suggest luolishaniid and hallucigenid,
two lobopodian taxa which had been resolved as members of stem-group onychophorans as well.
Described genera
As of 2018, over 20 lobopodian genera have been described.
The fossil materials being described as lobopodians ''
Mureropodia apae'' and ''
Aysheaia
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
prolata'' are considered to be disarticulated
frontal appendages of the
radiodonts ''
Caryosyntrips'' and ''
Stanleycaris'', respectively. ''
Miraluolishania'' was suggested to be
synonym of ''
Luolishania
''Luolishania'' is an extinct genus of lobopodian worm and known from the Lower Cambrian Chiungchussu Formation (Maotianshan Shales) of the Chengjiang County, Yunnan Province, China. A monotypic genus, it contains one species ''Luolishania longi ...
'' by some studies.
The enigmatic ''
Facivermis
''Facivermis'' (meaning "torch worm" ) is a genus of sessile lobopodian from the Lower Cambrian Maotianshan shales of China
Anatomy
''Facivermis'' was a worm-like creature up to 90 mm long. Its body was divided into three sections. The ante ...
'' was later revealed to be a highly specialized genus of luolishaniid lobopodians.
*''
Antennacanthopodia''
*''
Aysheaia
''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm.
Anatomy
''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate ...
''
*''
Carbotubulus''
*''
Cardiodictyon''
*''
Collinsium
''Collinsium'' was a genus of lobopodian from the Early Cambrian. It is represented by a single fossil species, ''Collinsium ciliosum'', found in the Xiaoshiba Lagerstätte (Hongjingshao Formation) of China. Similar to the later ''Hallucigenia'' ...
''
*''
Collinsovermis
''Collinsovermis'' is a genus of extinct panarthropod belonging to the group Lobopodia and known from the middle Cambrian Burgess Shale in British Columbia, Canada. It is monotypic having only one species, ''Collinsovermis monstruosus''. After i ...
''
*''
Diania
''Diania'' is an extinct genus of lobopodian 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 be ...
''
*''
Facivermis
''Facivermis'' (meaning "torch worm" ) is a genus of sessile lobopodian from the Lower Cambrian Maotianshan shales of China
Anatomy
''Facivermis'' was a worm-like creature up to 90 mm long. Its body was divided into three sections. The ante ...
''
*''
Fusuconcharium''
*''
Hadranax''
* ''
Hallucigenia
''Hallucigenia'' is a genus of Cambrian animal resembling worms, 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 ...
''
*''
Jianshanopodia
''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 repea ...
''
*''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
''
*''
Lenisambulatrix''
*''
Luolishania
''Luolishania'' is an extinct genus of lobopodian worm and known from the Lower Cambrian Chiungchussu Formation (Maotianshan Shales) of the Chengjiang County, Yunnan Province, China. A monotypic genus, it contains one species ''Luolishania longi ...
''
(=''
Miraluolishania'')
*''
Megadictyon''
*''
Microdictyon
''Microdictyon'' is an extinct armoured worm-like animal coated with net-like scleritic plates, known from the Early Cambrian Maotianshan shale of Yunnan China and other parts of the world. ''Microdictyon'' is part of the ill-defined taxon &ndas ...
''
*''
Onychodictyon
''Onychodictyon'' is a genus of extinct lobopodian known from the Lower Cambrian Chengjiang Maotianshan Shales in the Yunnan Province in China. It was characterized by a stout body covered by fleshy papillae and pairs of sclerotized plates with ...
''
*''
Orstenotubulus''
*''
Ovatiovermis''
*''
Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
''
*''
Paucipodia''
*''
Quadratapora''
*''
Siberion''
*''
Thanahita''
*''
Tritonychus''
*''
Xenusion''
References
{{Taxonbar, from=Q5189
Prehistoric protostomes
Cambrian Series 2 first appearances
Paraphyletic groups
Panarthropoda