103-506: Tritonychus phanerosarkus is a Cambrian lobopodian , exceptionally preserved in the Orsten fashion by phosphate deposition, additionally preserving muscle fibres . Its name loosely translates to "Three-clawed animal with well displayed flesh". Phylogenetic analysis suggests that it was a close relative of the Onychophora , possibly even a member of the main lineage. The fossil was discovered in
206-460: A GSSP, but it is expected to be defined in strata marking the first appearance of trilobites in Gondwana . There was a rapid diversification of metazoans during this epoch, but their restricted geographic distribution, particularly of the trilobites and archaeocyaths , have made global correlations difficult, hence ongoing efforts to establish a GSSP. The Miaolingian is the third series/epoch of
309-431: A few species ( Aysheaia or Onychodictyon ferox ) occasionally suggested to be stem-group tardigrades. A study in 2014 suggested that Hallucigenia are stem-group onychophorans based on their claws, which have overlapped internal structures resembling those of an extant onychophoran. This interpretation was questioned by later studies, as the structures may be a panarthropod plesiomorphy. Lobopodian taxa of
412-436: A group of paleozoic onychophorans. This interpretation was challenged after the discovery of lobopodians with arthropod and tardigrade -like characteristics, suggesting that the similarity between lobopodians and onychophorans represents deeper panarthropod ancestral traits ( plesiomorphies ) instead of onychophoran-exclusive characteristics ( synapomorphies ). For example, The British palaeontologist Graham Budd sees
515-486: A pair of flaps on each trunk segment, but otherwise no signs of arthropodization, in contrast to more derived dinocaridids like the Radiodonta that have robust and sclerotized frontal appendages. Gilled lobopodians cover at least four genera: Pambdelurion , Kerygmachela , Utahnax and Mobulavermis . Opabinia may also fall under this category in a broader sense, although the presence of lobopods in this genus
618-513: 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. Lobopodians possibly occupied a wide range of ecological niches . Although most of them had undifferentiated appendages and straight gut, which would suggest
721-778: A paper in 2023 found luolishaniids to be the closest relatives of tardigrades using various morphological characteristics. 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; while other studies rather suggest luolishaniid and hallucigenid, two lobopodian taxa which had been resolved as members of stem-group onychophorans as well. As of 2018, over 20 lobopodian genera have been described. The fossil materials being described as lobopodians Mureropodia apae and Aysheaia prolata are considered to be disarticulated frontal appendages of
824-619: A profound change in life on Earth ; prior to the Period, the majority of living organisms were small, unicellular and poorly preserved. Complex, multicellular organisms gradually became more common during the Ediacaran, but it was not until the Cambrian that the rapid diversification of lifeforms, known as the Cambrian explosion , produced the first representatives of most modern animal phyla . The Period
927-502: A proposed sister clade to Arthropoda, consisting of the extant Onychophora and Tardigrada, as well as their most recent common ancestor and all of its descendants. This definition renders Lobopodia a monophyletic taxon, if indeed it is valid (that is, if Tardigrades and Onychophora are closer to one another than either is to Arthropoda), but would exclude all the Euarthropod-line taxa traditionally considered Lobopodians. Its validity
1030-502: 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 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 terminal claws may have given some lobopodians
1133-507: A sliver of continental terrane rifted from Laurentia with the narrow Taconic seaway opening between them. The remains of this terrane are now found in southern Scotland, Ireland, and Newfoundland. Intra-oceanic subduction either to the southeast of this terrane in the Iapetus, or to its northwest in the Taconic seaway, resulted in the formation of an island arc . This accreted to the terrane in
SECTION 10
#17328693198721236-542: A subduction zone was closing the narrow seaway between the North West Kunlun region of Tarim and the South West Kunlun terrane. North China lay at equatorial to tropical latitudes during the early Cambrian, although its exact position is unknown. Much of the craton was covered by shallow seas, with land in the northwest and southeast. Northern North China was a passive margin until the onset of subduction and
1339-623: A suite of mainly Cambrian worm-like panarthropod taxa possessing lobopods – for example, Aysheaia , Hallucigenia , and Xenusion – which were traditionally united as "Xenusians" or "Xenusiids" (class Xenusia). Certain Dinocaridid genera, such as Opabinia , Pambdelurion , and Kerygmachela , may also be regarded as lobopodians, sometimes referred to more specifically as "gilled lobopodians" or "gilled lobopods". This traditional, informal usage of "Lobopodia" treats it as an evolutionary grade , including only extinct Panarthropods near
1442-690: A tail-like extension (e.g. Paucipodia , Siberion , Jianshanopodia ). The lobopods are flexible and loosely conical in shape, tapering from the body to tips that may or may not bear claws. The claws, if present, are hardened structures with a shape resembling a hook or gently-curved spine. Claw-bearing lobopods usually have two claws, but single claws are known (e.g. posterior lobopods of luolishaniids ), as are more than two (e.g. three in Tritonychus , seven in Aysheaia ) depending on its segmental or taxonomical association. In some genera,
1545-484: A term which may also be used as a common name of this group as well. While the definition of lobopodians may differ between literatures, it usually refers to a group of soft-bodied, marine worm-like fossil panarthropods such as Aysheaia and Hallucigenia . However, other genera like Kerygmachela and Pambdelurion (which have features similar to other groups) are often referred to as “gilled lobopodians”. The oldest near-complete fossil lobopodians date to
1648-529: Is a layer of outermost circular muscles and a layer of innermost longitudinal muscles. The onychophorans also have a third, intermediate, layer of interwoven oblique muscles. Musculature of the gilled lobopodian Pambdelurion shows a similar anatomy, but that of the lobopodian Tritonychus shows the opposite pattern: it is the outermost muscles that are longitudinal and the innermost layer that consists of circular muscles. Based on external morphology, lobopdians may fall under different categories — for example
1751-451: Is also unique in its unusually high proportion of lagerstätte deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells. By the end of the Cambrian, myriapods , arachnids , and hexapods started adapting to the land, along with the first plants . The term Cambrian is derived from the Latin version of Cymru ,
1854-441: Is characterised by complex, sediment-penetrating Phanerozoic-type trace fossils , and its upper part by small shelly fossils. The second series/epoch of the Cambrian is currently unnamed and known as Cambrian Series 2 . It lasted from c. 521 Ma to c. 509 Ma. Its two stages are also unnamed and known as Cambrian Stage 3 , c. 521 Ma to c. 514 Ma, and Cambrian Stage 4 , c. 514 Ma to c. 509 Ma. The base of Series 2 does not yet have
1957-846: 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 . Whether these genera were true lobopodians is still contested by some. However, they are widely accepted as stem-group arthropods just basal to radiodonts. Siberion , Megadictyon and Jianshanopodia 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 (2¼ to 8⅔ inches) — xenusiid lobopodians with widen trunk, stout trunk lobopods without evidence of claws, and most notably
2060-399: 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,
2163-484: Is oriented perpendicular to the body axis, presumably the remains of a more extensive layer of circular muscles. The preservation of the animal is unusual in several respects. Firstly, in Orsten-type microfossils muscle preservation is very rare and it is much more common that other tissue is preserved in phosphate in this way. Secondly, when onchyophorans are rotted in seawater or saline solution , body wall muscles are
SECTION 20
#17328693198722266-602: Is the first geological period of the Paleozoic Era, and the Phanerozoic Eon . The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordovician Period 485.4 Ma. Most of the continents lay in the southern hemisphere surrounded by the vast Panthalassa Ocean . The assembly of Gondwana during the Ediacaran and early Cambrian led to
2369-467: Is uncertain, however, as there are a number of hypotheses regarding the internal phylogeny of Panarthropoda. The broadest definition treats Lobopodia as a monophyletic superphylum equivalent in circumscription to Panarthropoda . By this definition, represented by "D" in the image, Lobopodia is no longer treated as an evolutionary grade but as a clade, containing not only the early, superficially "Lobopodian" forms but also all of their descendants, including
2472-627: The Altai-Sayan terranes. Some models show a convergent plate margin extending from Greater Avalonia, through the Timanide margin of Baltica, forming the Kipchak island arc offshore of southeastern Siberia and curving round to become part of the Altai-Sayan convergent margin. Along the then western margin, Late Neoproterozoic to early Cambrian rifting was followed by the development of a passive margin. To
2575-572: The Arequipa-Antofalla block united with the South American sector of Gondwana in the early Cambrian. The Kuunga Orogeny between northern ( Congo Craton , Madagascar and India ) and southern Gondwana ( Kalahari Craton and East Antarctica ), which began c. 570 Ma, continued with parts of northern Gondwana over-riding southern Gondwana and was accompanied by metamorphism and the intrusion of granites . Subduction zones , active since
2678-547: The Jiangshanian c. 494 Ma to c. 489.5 Ma, which have defined GSSPs; and the unnamed Cambrian Stage 10 , c. 489.5 Ma to 485.4 ± 1.9 Ma. The GSSP for the Cambrian–Ordovician boundary is at Green Point , western Newfoundland , Canada, and is dated at 485.4 Ma. It is defined by the appearance of the conodont Iapetognathus fluctivagus . Where these conodonts are not found the appearance of planktonic graptolites or
2781-592: The Lower Cambrian ; some are also known from Ordovician , Silurian and Carboniferous Lagerstätten . Some bear toughened claws, plates or spines, which are commonly preserved as carbonaceous or mineralized microfossils in Cambrian strata. The grouping is considered to be paraphyletic , as the three living panarthropod groups ( Arthropoda , Tardigrada and Onychophora ) are thought to have evolved from lobopodian ancestors. The Lobopodian concept varies from author to author. Its most general sense refers to
2884-614: The continental flood basalts of the Kalkarindji large igneous province (LIP) began to erupt. These covered an area of > 2.1 × 10 km across northern, central and Western Australia regions of Gondwana making it one of the largest, as well as the earliest, LIPs of the Phanerozoic. The timing of the eruptions suggests they played a role in the early to middle Cambrian mass extinction . The terranes of Ganderia , East and West Avalonia , Carolinia and Meguma lay in polar regions during
2987-400: The radiodonts Caryosyntrips and Stanleycaris , respectively. Miraluolishania was suggested to be synonym of Luolishania by some studies. The enigmatic Facivermis was later revealed to be a highly specialized genus of luolishaniid lobopodians. Cambrian The Cambrian ( / ˈ k æ m b r i . ə n , ˈ k eɪ m -/ KAM -bree-ən, KAYM - )
3090-564: The trilobite Jujuyaspis borealis can be used. The boundary also corresponds with the peak of the largest positive variation in the δ C curve during the boundary time interval and with a global marine transgression. Major meteorite impact structures include: the early Cambrian (c. 535 Ma) Neugrund crater in the Gulf of Finland , Estonia, a complex meteorite crater about 20 km in diameter, with two inner ridges of about 7 km and 6 km diameter, and an outer ridge of 8 km that formed as
3193-682: The 19 km diameter Glikson crater (c. 508 Ma) in Western Australia; the 5 km diameter Mizarai crater (500±10 Ma) in Lithuania; and the 3.2 km diameter Newporte structure (c. 500 Ma or slightly younger) in North Dakota , U.S.A. Reconstructing the position of the continents during the Cambrian is based on palaeomagnetic , palaeobiogeographic , tectonic , geological and palaeoclimatic data. However, these have different levels of uncertainty and can produce contradictory locations for
Tritonychus - Misplaced Pages Continue
3296-536: The Amazonia region of Gondwana with a narrow Iapetus Ocean that only began to open once Gondwana was fully assembled c. 520 Ma. Those not in favour of the existence of Pannotia show the Iapetus opening during the Late Neoproterozoic, with up to c. 6,500 km (c. 4038 miles) between Laurentia and West Gondwana at the beginning of the Cambrian. Of the smaller continents, Baltica lay between Laurentia and Gondwana,
3399-509: The Cambrian and Early Ordovician. Gondwana was a massive continent, three times the size of any of the other Cambrian continents. Its continental land area extended from the south pole to north of the equator. Around it were extensive shallow seas and numerous smaller land areas. The cratons that formed Gondwana came together during the Neoproterozoic to early Cambrian. A narrow ocean separated Amazonia from Gondwana until c. 530 Ma and
3502-533: The Cambrian, Laurentia lay across or close to the equator. It drifted south and rotated c. 20° anticlockwise during the middle Cambrian, before drifting north again in the late Cambrian. After the Late Neoproterozoic (or mid-Cambrian) rifting of Laurentia from Gondwana and the subsequent opening of the Iapetus Ocean, Laurentia was largely surrounded by passive margins with much of the continent covered by shallow seas. As Laurentia separated from Gondwana,
3605-710: The Cambrian, lasting from c. 509 Ma to c. 497 Ma, and roughly identical to the middle Cambrian in older literature [1] . It is divided into three stages: the Wuliuan c. 509 Ma to 504.5 Ma; the Drumian c. 504.5 Ma to c. 500.5 Ma; and the Guzhangian c. 500.5 Ma to c. 497 Ma. The name replaces Cambrian Series 3 and was ratified by the IUGS in 2018. It is named after the Miaoling Mountains in southeastern Guizhou Province , South China, where
3708-597: The Chenjiang Maotianshan Shale and the Burgess Shale. Aysheaia pedunculata has a morphology apparently basic for lobopodians — for example, a significantly annulated cuticle, a terminal mouth opening, specialized frontalmost appendages, and stubby lobopods with terminal claws. Hallucigenia sparsa is famous for having a complex history of interpretation — it was originally reconstructed with long, stilt-like legs and mysterious fleshy dorsal protuberances, and
3811-533: The Ediacaran Timanian Orogeny was coming to an end. In this region the early to middle Cambrian was a time of non-deposition and followed by late Cambrian rifting and sedimentation. Its southeastern margin was also a convergent boundary , with the accretion of island arcs and microcontinents to the craton, although the details are unclear. Siberia began the Cambrian close to western Gondwana and north of Baltica. It drifted northwestwards to close to
3914-568: The GSSP marking its base is found. This is defined by the first appearance of the oryctocephalid trilobite Oryctocephalus indicus . Secondary markers for the base of the Miaolingian include the appearance of many acritarchs forms, a global marine transgression , and the disappearance of the polymerid trilobites, Bathynotus or Ovatoryctocara. Unlike the Terreneuvian and Series 2, all the stages of
4017-599: The ICS ratify rock units based on a Global Boundary Stratotype Section and Point (GSSP) from a single formation (a stratotype ) identifying the lower boundary of the unit. Currently the boundaries of the Cambrian System, three series and six stages are defined by global stratotype sections and points. The lower boundary of the Cambrian was originally held to represent the first appearance of complex life, represented by trilobites . The recognition of small shelly fossils before
4120-470: The Iapetus and from Gondwana by the Ran Ocean. It was composed of two continents, Fennoscandia and Sarmatia , separated by shallow seas. The sediments deposited in these unconformably overlay Precambrian basement rocks. The lack of coarse-grained sediments indicates low lying topography across the centre of the craton. Along Baltica's northeastern margin subduction and arc magmatism associated with
4223-555: 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 the dinocaridids , 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 followed and extended
Tritonychus - Misplaced Pages Continue
4326-419: The Miaolingian are defined by GSSPs . The olenellids , eodiscids , and most redlichiids trilobites went extinct at the boundary between Series 2 and the Miaolingian. This is considered the oldest mass extinction of trilobites. The Furongian , c. 497 Ma to 485.4 ± 1.9 Ma, is the fourth and uppermost series/epoch of the Cambrian. The name was ratified by the IUGS in 2003 and replaces Cambrian Series 4 and
4429-423: The Neoproterozoic, extended around much of Gondwana's margins, from northwest Africa southwards round South America, South Africa , East Antarctica , and the eastern edge of West Australia. Shorter subduction zones existed north of Arabia and India. The Famatinian continental arc stretched from central Peru in the north to central Argentina in the south. Subduction beneath this proto- Andean margin began by
4532-452: The Ran Ocean (an arm of the Iapetus) opening between it and Gondwana. Siberia lay close to the western margin of Gondwana and to the north of Baltica. Annamia and South China formed a single continent situated off north central Gondwana. The location of North China is unclear. It may have lain along the northeast Indian sector of Gondwana or already have been a separate continent. During
4635-542: The Sedgwick's "Upper Cambrian", claiming all fossilised strata for "his" Silurian series. Matters were complicated further when, in 1852, fieldwork carried out by Sedgwick and others revealed an unconformity within the Silurian, with a clear difference in fauna between the two. This allowed Sedgwick to now claim a large section of the Silurian for "his" Cambrian and gave the Cambrian an identifiable fossil record. The dispute between
4738-761: The Welsh name for Wales, where rocks of this age were first studied. It was named by Adam Sedgwick in 1835, who divided it into three groups; the Lower, Middle, and Upper. He defined the boundary between the Cambrian and the overlying Silurian, together with Roderick Murchison , in their joint paper " On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales ". This early agreement did not last. Due to
4841-534: The Xiaotan section, Yongshan , Yunnan Province in the Yu'anshan Formation . The muscle fibres preserved in its legs establish peripheral musculature as a characteristic of all panarthropods , but are arranged in an unusual pattern (detailed below). The fossil found was small, with only one segment of the animal found. This was 1 millimeter long, and folded at the mid-point, with one pair of lobopods preserved and much of
4944-480: The ability to climb on substrances. Not much is known about the physiology of lobopodians. There is evidence to suggest that lobopodians moult just like other ecdysozoan taxa, but the outline and ornamentation of the harden sclerite did not vary during ontogeny . 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
5047-410: The appendages as well as the fundamental relationship between Diania and arthropods. While Antennacanthopodia is widely accepted as a stem-group onychophoran, the position of other xenusiid genera that were previously thought to be onychophoran-related is controversial — in further studies, most of them were either suggested to be stem-group onychophorans or basal panarthropods, with
5150-408: The arthropod stem-group. 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. Due to
5253-591: 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 biramous appendages). Lobopodian ancestry of arthropods also reinforced by genomic studies on extant taxa — gene expression support
SECTION 50
#17328693198725356-495: The base of crown Panarthropoda. Crown Panarthropoda comprises the three extant Panarthropod phyla – Onychophora (velvet worms), Tardigrada (waterbears), and Arthropoda (arthropods) – as well as their most recent common ancestor and all of its descendants. Thus, in this usage, Lobopodia consists of various basal Panarthropods. This corresponds to "A" in the image to the left. An alternative, broader definition of Lobopodia would also incorporate Onychophora and Tardigrada,
5459-435: The body wall. There were three layers of fibrous muscular tissue, each around 10μm thick. The outermost layer of muscle consisted of 5-10μm thick fibres, which part to leave a 60μm gap between the lobopods, thought to be a gonopore and part again near the outer edge of each lobopod, possibly for leg elevator muscle insertion. Inside this, there was a layer of much thinner, interwoven oblique fibres. The innermost layer of fibres
5562-604: The development of new convergent plate boundaries and continental-margin arc magmatism along its margins that helped drive up global temperatures. Laurentia lay across the equator, separated from Gondwana by the opening Iapetus Ocean . The Cambrian was a time of greenhouse climate conditions, with high levels of atmospheric carbon dioxide and low levels of oxygen in the atmosphere and seas. Upwellings of anoxic deep ocean waters into shallow marine environments led to extinction events, whilst periods of raised oxygenation led to increased biodiversity . The Cambrian marked
5665-637: The development of the Bainaimiao arc in the late Cambrian. To its south was a convergent margin with a southwest dipping subduction zone, beyond which lay the North Qinling terrane (now part of the Qinling Orogenic Belt ). South China and Annamia formed a single continent. Strike-slip movement between it and Gondwana accommodated its steady drift northwards from offshore the Indian sector of Gondwana to near
5768-400: The dorsal body surface missing. This missing surface was actually an advantage to the scientists studying it as it revealed the muscle fibres within the body. The skin has circumferential wrinkles spaced at 10 μm apart, which divide and merge irregularly. These wrinkles bear small papillae which are thought to correspond to the dermal papillae of other extinct Onchyophora. Along the two lobopods,
5871-455: The early Cambrian, and high-to-mid southern latitudes by the mid to late Cambrian. They are commonly shown as an island arc-transform fault system along the northwestern margin of Gondwana north of northwest Africa and Amazonia, which rifted from Gondwana during the Ordovician. However, some models show these terranes as part of a single independent microcontinent , Greater Avalonia, lying to
5974-552: The equator as the Ægir Ocean opened between it and Baltica. Much of the continent was covered by shallow seas with extensive archaeocyathan reefs . The then northern third of the continent (present day south; Siberia has rotated 180° since the Cambrian) adjacent to its convergent margin was mountainous. From the Late Neoproterozoic to the Ordovician, a series of island arcs accreted to Siberia's then northeastern margin, accompanied by extensive arc and back-arc volcanism. These now form
6077-546: 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 (= Miraluolishania ), Ovatiovermis , Onychodictyon , Hallucigenia , Facivermis , and less certainly Aysheaia as well. However, in gilled lobopodians like Kerygmachela ,
6180-516: The extant Panarthropods. Lobopodia has, historically, sometimes included Pentastomida , a group of parasitic panarthropod which were traditionally thought to be a unique phylum , but revealed by subsequent phylogenomic and anatomical studies to be a highly specialized taxon of crustaceans . The better-known genera include Aysheaia , which was discovered in the Canadian Burgess Shale , and Hallucigenia , known from both
6283-437: The eyes are relatively complex reflective patches that may had been compound in nature. The trunk is elongated and composed of numerous body segments ( somites ), each bearing a pair of legs called lobopods or lobopodous limbs. The segmental boundaries are not as externally significant as those of arthropods, although they are indicated by heteronomous annulations (i.e., the alternation of annulation density corresponding to
SECTION 60
#17328693198726386-475: The first parts to decay, well before organs such as gonads or the gut and before harder parts such as the hollow claws present in this species, of which only the inner bases remain. Previously, due to the rarity both of onchyophoran fossils and preserved musculature, it was uncertain whether peripheral muscle was a characteristic of all panarthropods or just of primitive euarthropods and tardigrades (modern members of this clade have evolved skeletal muscle from
6489-501: The first trilobites, and Ediacara biota substantially earlier, has led to calls for a more precisely defined base to the Cambrian Period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland
6592-455: The front and rear ends of the animal: it was revealed that the bulbous imprint previously thought to be a head was actually gut contents being expelled from the anus. Microdictyon is another charismatic as well as the speciose genus of lobopodians resembling Hallucigenia , but instead of spines, it bore pairs of net-like plates, which are often found disarticulated and are known as an example of small shelly fossils (SSF). Xenusion has
6695-686: 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
6798-513: The globe that corresponded to the base of the Cambrian. An early date of 570 Ma quickly gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate. A more precise analysis using modern radiometric dating yields a date of 538.8 ± 0.2 Ma. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in
6901-399: 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
7004-411: The homology between arthropod appendages and onychophoran lobopods, suggests that modern less-segmented arthropodized appendages evolved from annulated lobopodous limbs. 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
7107-431: The idea, generally in agreement 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
7210-410: The impressions of these claws denote a hollow claw slightly longer than the 25μm impressions, hence the figure of 30μm. The lobopods had a circular cross-section, flattened in places, and are thought to be at the posterior end of the specimen with the claws pointing towards the anterior, as in other lobopodians. The muscles preserved were those inside the body cavity of the specimen, mostly making up
7313-512: The innermost longitudinal, reversing the order of those in Tritonychus. This leaves the homology of muscle layers unclear, to say the least. Tritonychus is clearly given a place within an 'onchyophoran-like' clade, however, due to skin similarities to other 'onchyophoran-like' lobopodians and modern onchyophorans. These include bifurcating circumferential wrinkles, papillae mounted on solid bases, and (in parts) hexagonal patterning. This extends
7416-455: 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
7519-540: The late Cambrian, triggering southeast-dipping subduction beneath the terrane itself and consequent closure of the marginal seaway. The terrane collided with Laurentia in the Early Ordovician. Towards the end of the early Cambrian, rifting along Laurentia's southeastern margin led to the separation of Cuyania (now part of Argentina) from the Ouachita embayment with a new ocean established that continued to widen through
7622-592: The late Cambrian. Along the northern margin of Gondwana, between northern Africa and the Armorican Terranes of southern Europe, the continental arc of the Cadomian Orogeny continued from the Neoproterozoic in response to the oblique subduction of the Iapetus Ocean. This subduction extended west along the Gondwanan margin and by c. 530 Ma may have evolved into a major transform fault system. At c. 511 Ma
7725-423: The lobopods bear additional structures such as spines (e.g. Diania ), fleshy outgrowths (e.g. Onychodictyon ), or tubercules (e.g. Jianshanopodia ). 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
7828-531: The lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms. The Terreneuvian is the lowermost series/ epoch of the Cambrian, lasting from 538.8 ± 0.2 Ma to c. 521 Ma. It is divided into two stages: the Fortunian stage, 538.8 ± 0.2 Ma to c. 529 Ma; and
7931-436: The major continents. This, together with the ongoing debate around the existence of the Neoproterozoic supercontinent of Pannotia , means that while most models agree the continents lay in the southern hemisphere, with the vast Panthalassa Ocean covering most of northern hemisphere, the exact distribution and timing of the movements of the Cambrian continents varies between models. Most models show Gondwana stretching from
8034-1597: 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. Priapulida [REDACTED] , Nematoda [REDACTED] and relatives (Lobopodian taxa controversial) Antennacanthopodia [REDACTED] Crown-group Onychophora [REDACTED] (Lobopodian taxa controversial) Crown-group Tardigrada [REDACTED] (Lobopodian taxa controversial) Megadictyon [REDACTED] and Jianshanopodia [REDACTED] Pambdelurion [REDACTED] and Kerygmachela [REDACTED] Opabinia [REDACTED] Radiodonta [REDACTED] Euarthropoda [REDACTED] [REDACTED] [REDACTED] The overall phylogenetic interpretation on lobopodians has changed dramatically since their discovery and first description. The reassignments are not only based on new fossil evidence, but also new embryological , neuroanatomical , and genomic (e.g. gene expression , phylogenomics ) information observed from extant panarthropod taxa. Based on their apparently onychophoran -like morphology (e.g. annulated cuticle, lobopodous appendage with claws), lobopodians were originally thought to be present
8137-403: The oldest fossil record amongst the described lobopodians, which may trace back to Cambrian Stage 2 . Luolishania is an iconic example of lobopodians with multiple pairs of specialized appendages. The gill lobopodians Kerygmachela and Pambdelurion shed light on the relationship between lobopodians and arthropods , as they have both lobopodian affinities and characteristics linked to
8240-544: The only confirmed evidence of lobopodian neural structures comes from the gilled lobopodian Kerygmachela in Park et al. 2018 — it presents a brain composed of only a protocerebrum (the frontal-most cerebral ganglion of 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 . In some extant ecdysozoan such as priapulids and onychophorans , there
8343-404: The peripheral muscle in their ancestors). Tritonychus phanerosarkus shows that peripheral muscle was also a feature of onchyophorans, and thus of panarthropods as a whole. However, the evolution of three-layered muscle from the two-layered muscle of more ancestral fossils is not straightforward. In living onchyophorans, the outermost layer of muscle is circular, the middle interwoven oblique and
8446-424: The position of segmental boundaries) in some species. The trunk segments may bear other external, segment-corresponding structures such as nodes (e.g. Hadranax , Kerygmachela ), papillae (e.g. Onychodictyon ), 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 , Hallucigenia sparsa ) or
8549-465: The record of these features into the lower Cambrian , along with the multiple layers of peripheral muscle and the possible gonopore . Lobopodian Crown-group Euarthropoda Lobopodians are members of the informal group Lobopodia (from the Greek , meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods ,
8652-478: 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
8755-465: The result of an impact of an asteroid 1 km in diameter; the 5 km diameter Gardnos crater (500±10 Ma) in Buskerud , Norway, where post-impact sediments indicate the impact occurred in a shallow marine environment with rock avalanches and debris flows occurring as the crater rim was breached not long after impact; the 24 km diameter Presqu'ile crater (500 Ma or younger) Quebec , Canada;
8858-409: The scarcity of fossils, Sedgwick used rock types to identify Cambrian strata. He was also slow in publishing further work. The clear fossil record of the Silurian, however, allowed Murchison to correlate rocks of a similar age across Europe and Russia, and on these he published extensively. As increasing numbers of fossils were identified in older rocks, he extended the base of the Silurian downwards into
8961-616: 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 ) Dinocaridids with lobopodian affinities (due to shared features like annulation and lobopods) are referred to as "gilled lobopodians" or "gilled lobopods". These forms sport
9064-568: The south of North China. To the south of these the Tarim microcontinent lay between Gondwana and Siberia. Its northern margin was passive for much of the Paleozoic, with thick sequences of platform carbonates and fluvial to marine sediments resting unconformably on Precambrian basement. Along its southeast margin was the Altyn Cambro–Ordovician accretionary complex, whilst to the southwest
9167-465: The south polar region to north of the equator. Early in the Cambrian, the south pole corresponded with the western South American sector and as Gondwana rotated anti-clockwise, by the middle of the Cambrian, the south pole lay in the northwest African region. Laurentia lay across the equator, separated from Gondwana by the Iapetus Ocean . Proponents of Pannotia have Laurentia and Baltica close to
9270-455: 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
9373-411: The suspected case of arthropodization on the limbs of Diania 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. The gut of lobopodians
9476-465: The tardigrade stem-group is unclear. Aysheaia or Onychodictyon 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. However,
9579-670: The then north, Siberia was separated from the Central Mongolian terrane by the narrow and slowly opening Mongol-Okhotsk Ocean . The Central Mongolian terrane's northern margin with the Panthalassa was convergent, whilst its southern margin facing the Mongol-Okhotsk Ocean was passive. During the Cambrian, the terranes that would form Kazakhstania later in the Paleozoic were a series of island arc and accretionary complexes that lay along an intra-oceanic convergent plate margin to
9682-559: The traditional "Upper Cambrian". The GSSP for the base of the Furongian is in the Wuling Mountains , in northwestern Hunan Province , China. It coincides with the first appearance of the agnostoid trilobite Glyptagnostus reticulatus , and is near the beginning of a large positive δ C isotopic excursion. The Furongian is divided into three stages: the Paibian , c. 497 Ma to c. 494 Ma, and
9785-664: The two geologists and their supporters, over the boundary between the Cambrian and Silurian, would extend beyond the life times of both Sedgwick and Murchison. It was not resolved until 1879, when Charles Lapworth proposed the disputed strata belong to its own system, which he named the Ordovician. The term Cambrian for the oldest period of the Paleozoic was officially agreed in 1960, at the 21st International Geological Congress . It only includes Sedgwick's "Lower Cambrian series", but its base has been extended into much older rocks. Systems , series and stages can be defined globally or regionally. For global stratigraphic correlation,
9888-516: The two living panarthropod phyla which still bear lobopodous limbs. This definition, corresponding to "C", is a morphological one, depending on the superficial similarity of appendages (the "lobopods"). Thus, it is paraphyletic , excluding the Euarthropods, which are descendants of certain Lobopodians, on the basis of their highly divergent limb morphology. "Lobopodia" has also been used to refer to
9991-553: The unnamed Stage 2, c. 529 Ma to c. 521 Ma. The name Terreneuvian was ratified by the International Union of Geological Sciences (IUGS) in 2007, replacing the previous "Cambrian Series 1". The GSSP defining its base is at Fortune Head on the Burin Peninsula, eastern Newfoundland, Canada (see Ediacaran - Cambrian boundary above). The Terreneuvian is the only series in the Cambrian to contain no trilobite fossils. Its lower part
10094-400: 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
10197-415: The west of Baltica and aligned with its eastern ( Timanide ) margin, with the Iapetus to the north and the Ran Ocean to the south. During the Cambrian, Baltica rotated more than 60° anti-clockwise and began to drift northwards. This rotation was accommodated by major strike-slip movements in the Ran Ocean between it and Gondwana. Baltica lay at mid-to-high southerly latitudes, separated from Laurentia by
10300-497: The world, and to the disappearance of distinctive Ediacaran fossils ( Namacalathus , Cloudina ). Nevertheless, there are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata – which would mean that dates from other sections, ranging from 544 to 542 Ma, are more suitable. *Most Russian paleontologists define
10403-478: The wrinkles become smaller and eventually give way to raised polygonal boundaries, which could mark cell borders in the skin tissue. The papillae continue along the lobopods, with bases of around 5μm across and about 7μm high. The fossil found had two lobopods preserved, each around 800μm long and with a uniform diameter of 80μm, not tapering. At the end of each lobopod there are three 30μm claws preserved, with an angle of 45° between each one. The raised centres of
10506-412: 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
10609-444: Was settled upon as a formal base of the Cambrian Period, which was to be correlated worldwide by the earliest appearance of Treptichnus pedum . Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, and it is the T. pedum ichnofossil assemblage that is now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across
#871128