Lobopods

Lobopods
Type specimen of Xenusion auerswaldae in the Berlin Museum of Natural History
Temporal occurrence
Cambrian to Asselian ( Lower Permian )
Systematics
Multicellular animals (Metazoa) Tissue animals (Eumetazoa) Bilateria Primordial mouths (protostomia) Molting animals (Ecdysozoa) Lobopods
Scientific name
Lobopodia
Snodgrass , 1938

As lobopodia (Lobopodia from ancient Greek λοβός Lobos , German , rags' and ancient Greek πούς pous , Gen. ποδός podos , German , Foot ' , literally, Lappenfüßer') refers to a now extinct group of marine fauna from the geological ages from the Cambrian to the Permian . Since they resemble the modern columbus (Onychophora) in many of their properties , they are often regarded as their ancestors and assigned to this animal phylum ; however, this is controversial. The term is sometimes used to denote the broader group of Cambrian and Ordovician forms on the one hand and the later stumpworts on the other.

features

The earliest known whole-body fossils come from the Lower Cambrian. They are particularly similar to modern forms due to the elongated, worm-shaped body, on which there are stub-shaped body appendages, the stump legs. However, unlike the columbus living today, they ended up in a variable number of hardened claws. The number of clearly identifiable head attachments is not the same for all genres, and it is also unclear how they relate to the antennae ( feelers ) of modern forms.

Many lobopods had armor plates on the back, so-called sclerites , which apparently covered the entire body including the head and continued in long, pointed dorsal spines. Numerous very similar structures can be identified as part of the so-called small shelly fauna (SSF) before the onset of the Cambrian , detached from their carriers , which indicates the formation of the lobopods or their immediate precursors in the late Proterozoic ; they were the first animals to walk on legs .

Way of life

All lobopods apparently lived in shallow lagoon water and are only known from the tropical and temperate belts from the Cambrian to the Lower Permian, which then contained the continents of Baltica, "China" and "North America", which are now further north due to continental drift . Some species seem to have fed on sponges (Porifera) or the mysterious Eldonia species, which are sometimes placed with the sea ​​rollers (Holothuroidea), and were therefore carnivorous (carnivorous). It is very likely that lobopods have already shed their moult, like all modern velvet worms.

biodiversity

Aysheaia

Kerygmachela

The following 18 genera are known in detail, but their affiliation to the lobopods is not always undisputed.

• Antennacanthopodia with the only species Antennacanthopodia gracilis from the Chengjiang fauna has neither sclerotized spines nor plates and its physique largely corresponds to the recent columbus, in whose direct trunk group it could belong. The species has two pairs of head appendages, the first pair homologous to the antennae and the second homologous to the mucous glands of the velvet worms.
• Aysheaia is a palaeontologically famous representative of the Cambrian fauna of the Burgess schist . The species looked particularly similar to today's forms and had a pair of head appendages, which are interpreted as homologies of the velvet antennae. The recognizable terminal , i.e. terminal position of the mouth is considered to be original. Two species are known, Aysheaia pedunculata and Aysheaia prolata , both of which lived in the Middle Cambrian.
• Carbotubulus from the Mazon Creek Formation in Illinois is 296 million years old and thus extends the lobopod's lifespan to the early subperm .
• Cardiodictyon had a total of 23 paired heavy armor plates (sclerites) and two to three head attachments. The only known species, Cardiodictyon catenulum , belongs to the subgroup of Scleronychophora, which is characterized by particularly pronounced back armor.
• Collinsium is a genus from the early Cambrian of southern China, which, like Hallucigenia, was well protected by dorsal spines and belongs to the stem group of the velvet worms.
• Diania is known with the only species Diania cactiformis (because of the cactus-like appearance of the fossil) from the Lower Cambrian Maotianshan schist in China .
• Hadranax is known with the only species Hadranax augustus from the sub-Cambrian Buen Formation in northern Greenland .
• Hallucigenia is arguably the most famous fossil lobopod genus. The fossils have a pair of head appendages and a total of seven pairs of sclerites, which continue in long dorsal spines; the genus is therefore also counted among the Scleronychophora. It got its name from the paleontologist Simon Conway Morris , who described the animals as "bizarre, dreamlike and enigmatic". However, this was at least partly due to his now notorious (but technically serious) reconstruction, which swapped the top and bottom - the back spines became mysterious leg-like stilts that were not known from any other animal species, while strange fleshy "back protrusions" were initially not considered Legs were recognizable; For a long time, there was also no consensus on where the front and rear of the animals were. Two species, Hallucigenia fortis and Hallucigenia sparsa, are known from the Burgess schist and from the Chinese Chengjiang faunal community ; they may have been predatory.
• Jianshanopodia with the species Jianshanopodia gracilis reaches the size of 22 centimeters, which is unusual for a lobopod. The species has a pair of long appendages
• Kerygmachela is known from the lower Cambrian of northern Greenland, also from the Buen formation. The genus is monotypical, that is, there is only one species, Kerygmachela kierkegaardi . It is sometimes placed among the early forerunners of the arthropods (Arthropoda).
• Luolishania with the only species Luolishania longicruris is known from the Chengjiang fauna.
• Microdictyon is distributed as an isolated sclerite in Sub-Cambrian sediments almost worldwide. Whole animal fossils ofone species, Microdictyon sinicum , have also been found in Chengjiang, China.
• Miraluolishania with the species Miraluolishania haikouensis from the Chengjiang Fauna Community of China, with a body length of about 14 millimeters, 14 ringed, unlinked pairs of legs and back spines. The species is remarkable for its easily recognizable eyes on the head. Possibly synonymous with Luolishania longicruris .
• Onychodictyon has paired sclerites that continue in spines and is also included in the already mentioned group of Scleronychophora. The only known species, Onychodictyon ferox , had a pair of antennae and claws each and is sometimes regarded as the closest relative of the later Euonychophora.
• Orstenotubulus with the species Orstenotubulus evamuellerae was found in Sweden. Unlike the other fossil species, this animal is physically preserved. Its body wall and even parts of the internal organs were replaced by phosphate and then enclosed in limestone, from which it can be freed after dissolving with hydrochloric acid (so-called "Orsten" fossils). The entire animal (only fragments are preserved) was probably only five millimeters long, it was protected on the back by pairs of spines (similar to Hallucigenia ). The unlinked, ringed legs were attached to the sides of the body.
• Paucipodia , represented by the species Paucipodia inermis from the Chengjiang fauna, has no back plates in contrast to many other lobopods. The name indicates the small number of only nine pairs of legs. Again, the position of the front and rear ends is controversial.
• Tamisiocaris is only known from a single, incompletely preserved specimen from Greenland.
• Xenusion is only preserved in two specimens of the species Xenusion auerswaldae from Sweden and Eastern Europe in early Cambrian sandstone. The animals wore a pair of head hangers and sclerites on their backs that continued outward in thorns. It is speculated that the species was able to curl itself up with its thorns facing out, thereby deterring potential predators.

Tribal history

The number of fossils handed down from the Cambrian is large enough to be able to reconstruct the drifting of the continents of that time with their help . However, it already decreased very strongly in the Ordovician and since the Silurian at the latest the lobopods have been considered extinct in the narrower sense. Whether one can therefore speak of a post-Cambrian decline in biodiversity, as the American paleontologist Stephen Jay Gould suspected, depends on the exact systematic relationship between the lobopods and modern colibuses.

Lobopods as velvet worms

Traditionally, lobopods have been associated with modern collous pods because of their similar morphology. A possible hypothesis on the exact relationships is given by the following diagram (without Paucipodia , Hadranax and Kerygmachela ; the names in brackets are sometimes used alternatively):

Onychophora ( Lobopodia , Oncodophora )	NN     †  Protonychophora    † Aysheaia      ? † Xenusion   †  Scleronychophora    † Cardiodictyon      † Hallucigenia      † Microdictyon   †  Paronychophora     Udeonychophora    † Helenodora inopinata      † Succinipatopsis balticus      † Tertiapatus dominicanus      Euonychophora (modern forms) including Cretoperipatus burmiticus      † Onychodictyon      †  Luolishania

The genus Luolishania is therefore considered to be the sister group of all other forms . Among the remaining genera, three major groups can be distinguished: a not very well characterized taxon Protonychophora with the genera Aysheaia and Xenusion , a taxon Scleronychophora , to which the heavily armored species with long robust dorsal spines are counted, and a taxon Paronychophora, which in addition to Onychodictyon also includes all includes terrestrial species including the modern velvet worms. Occasionally Onychodictyon is also placed among the Scleronychophora.

Alternatively, the scientific name Lobopodia or Oncodophora is sometimes used to describe terrestrial velvet worms and lobopods, while the name Onychophora is restricted to the forms identified above as Paronychophora.

Lobopods as representatives of the Tactopoda lineage

Even if the morphological similarity of the lobopods to today's velvet worms is striking, their affiliation to this taxon is not undisputed. An alternative theory sees most lobopods instead as members of the stem line of either a group known as Panarthropoda, composed of columbus, water bear (Tardigrada) and arthropod (Arthropoda), or the tactopoda, the common taxon of the latter two groups.

A possible cladogram in this case looks like this (without Hadranax ; Opabinia and Anomalocaris are generally not considered to be lobopods):

Panarthropoda	NN    Colibus (Onychophora)   NN    † Luolishania   NN    † Xenusion   NN    † Onychodictyon   NN    † Microdictyon      † Paucipodia   NN    † Hallucigenia      † Cardiodictyon   NN    † Kerygmachela   Tactopoda    Tardigrade (Tardigrada)      † Opabinia   NN    † anomalocaris      Arthropod (arthropoda)      † Aysheaia

The still largely undifferentiated form Aysheaia , which looks particularly similar to the modern velvet worms , thus forms the sister group of all Panarthropoda ; many of its features, such as the segmentation, which is inconspicuous except for the pairs of legs, a row of unspecialized long legs, fine body and leg rings, and the head with two antennas, would therefore also be “primitive”, i.e. original features of the Panarthropoda.

These are then divided into the actual velvet worms (including terrestrial fossils; described as Udeonychophora in the alternative diagram) and a group made up of the remaining lobopods, tardigrades and arthropods as well as some closely related fossil species. This taxon is now divided into an extinct lobopod branch, which includes all forms with the exception of Kerygmachela , and another group that splits into Kerygmachela and the Tactopoda species.

One advantage of the relationship hypothesis given above is that from Aysheaia to arthropods, a trend towards more and more pronounced segmentation can be identified, in which the originally very fine body and leg ringing, which in Aysheaia does not yet correlate with the segment boundaries, is increasingly on the Segmentation is coordinated.

A possible problem is that the concept presented is difficult to reconcile with a very widespread theory of the large systematics of animals, the Articulata hypothesis. According to this, the velvet worms in general and the lobopods in particular form the evolutionary link between annelids (Annelida) and arthropods (Arthropoda), two groups of animals, both of which are characterized by clear external segmentation. Since this segmentation remains inconspicuous in modern colibuses and can only be identified by means of the legs that emerge at regular intervals, it has been lost secondarily in the course of evolutionary history according to the articulata hypothesis in the colobus.

Since the weak external segmentation according to the concept presented above is not a secondary, but an original, "primitive" feature of all Panarthropoda, it would have to arise in the development from the annelid to the stubby to the arthropod, then disappeared and finally a second Times, which would seriously affect the credibility of the Articulata hypothesis.

The concept fits well with an alternative systematic theory, the Ecdysozoa hypothesis. According to this, annelids on the one hand and arthropods and stumpefoots on the other hand are not particularly closely related to each other. Instead, the closest relatives of the latter group, and thus also the lobopods, lie in a taxon of worm-like organisms called cycloneuralia, which include thread worms (Nematoda) and string worms (Nematomorpha) as well as a number of obscure forms such as the corset animals (Loricifera). They share about the terminal ( terminal ) position of the mouth and around the throat around situated, eponymous nerve ring, which can still be found in modified form in modern Stummelfüßern. However, their mouth is not terminal, but subterminal , a little behind the foremost tip of the body on the abdomen. From the embryonic development of the head nerves, however, it is concluded that the mouth has shifted there secondarily. If the above diagram describes the relationships correctly, it is confirmed by the very original genus Aysheaia , in which a terminal mouth position could be proven, that the ventral position of the mouth in the modern columbus was secondary.

Whether one of the two scenarios presented is correct, and if so, which one cannot yet be conclusively assessed; The answer is likely to have an impact far beyond the narrower question of the exact systematics of the lobopods on the entire large systematics of animals. At the same time, it will determine whether the above-mentioned assumption by the American paleontologist Stephen Jay Gould, that biodiversity decreased drastically after the Cambrian, is correct: If lobopods are direct precursors of the velvet worms, the variety of forms of the group is in fact at the latest collapsed at the end of the Ordovician; If, on the other hand, it is the representatives of the Tactopoda lineage, i.e. tardigrade and arthropod, the lobopods or their parent species are the ancestors of one of the most successful animal groups on earth.

literature

• GE Budd: The morphology of Opabinia regalis and the reconstruction of the arthropod stem group. In: Lethaia. Volume 29, 1996, p. 1.
• X. Hou, J. Bergström: Cambrian lobopodians - ancestors of extant onychophorans. In: Zoological Journal of the Linnean Society. Volume 114, 1995, p. 3.
• J. Monge-Najera: Phylogeny, biogeography and reproductive trends in the onychophora. In: Zoological Journal of the Linnean Society. Volume 114, 1995, p. 21.

Individual evidence

1. ^ ^{A b} Joachim T. Haug, Georg Mayer, Carolin Haug, Derek EG Briggs: A Carboniferous Non-Onychophoran Lobopodian Reveals Long-Term Survival of a Cambrian Morphotype. Current Biology, August 2012, doi : 10.1016 / j.cub.2012.06.066
2. ↑ Qiang Ou, Jianni Liu, Degan Shu, Jian Han, Zhifei Zhang, Xiaoqiao Wan, Qianping Lei: A Rare Onychophoran-Like Lobopodian from the Lower Cambrian Chengjiang Deposit, Southwestern China, and its Phylogenetic Implications. In: Journal of Paleontology. 85 (3) 2011, pp. 587-594. doi : 10.1666 / 09-147R2.1
3. ^ Jie Yang et al .: A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora. PNAS , 2015, doi : 10.1073 / pnas.1505596112
4. ↑ Jianni Liu, Degan Shu, Jian Han, Zhifei Zhang, Xingliang Zhang: A large xenusiid lobopod with complex appendages from the Lower Cambrian Chengjiang deposit. In: Acta Palaeontologica Polonica. 51 (2), 2006, pp. 215-222. (on-line)
5. ↑ Jianni Liu, Degan Shu, Jian Han, Zhifei Zhang (2004): A rare lobopod with well-preserved eyes from Chengjiang Lagerstätte and its implications for origin of arthropods. Chinese Science Bulletin 49 (10): 1063-1071.
6. ↑ Andreas Maas, Georg Mayer, Reinhardt M. Kristensen, Dieter Waloszek: A Cambrian micro-lobopodian and the evolution of arthropod locomotion and reproduction. In: Chinese Science Bulletin. 52 (24) 2007, pp. 3385-3392. doi : 10.1007 / s11434-007-0515-3

Web links

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