Microdictyon

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Microdictyon
Microdictyon sinicum, in the Chengjiang Museum

Microdictyon sinicum , in the Chengjiang Museum

Temporal occurrence
Cambrian
Locations

Eurasia (China, Siberia, Europe), Australia, North America

Systematics
Tissue animals (Eumetazoa)
Bilateria
Primordial mouths (protostomia)
Molting animals (Ecdysozoa)
Lobopods (Lobopodia)
Microdictyon
Scientific name
Microdictyon
Bengtson , Matthews & Missarzhevsky , 1991

Microdictyon is a genus of lobopods , a group of extinct "legged worms" similar to the recent velvet wormsthat lived on the ocean floor of the lower and middle Cambrian . At first, the genus was only found on the basis of thescreen-like sclerites , whichconsistof phosphate and arepart of the so-called small shelly fauna . In 1995,completely preserved animals were describedwith the species Microdictyon sinicum from the Lower Cambrian Chengjiang fauna .

Microdictyon sinicum

Microdictyon sinicum is an articulated, worm-like animal with, on the surviving specimens, about 10 to 77 millimeters in length. The front of the cylindrical trunk had an imperfectly set off, elongated, rounded, smooth head with a simple mouth opening, without recognizable protrusions or mouthparts, which reached about 15 to 25 percent of the total length of the animal. The fuselage was indistinctly divided into nine sections, which are also divided into flatter ringlets (annuli). Each of the sections carries a sclerotized plate laterally on each side. In addition, underneath (ventral) the plates, he wore an elongated, undivided pair of legs, with the exception of the last section, which had two such pairs of legs, each sitting in front of and behind the plate (a total of ten pairs). The flexible, long and thin legs were arranged on the belly side of the cylindrical body (with an approximately circular cross-section) so that they were about three-quarters of the trunk diameter apart. This suggests that the animal must have walked on them and not pushed forward sideways. The length of the legs decreased slightly from front to back, the longer front three pairs of legs reached about 16 percent of the length of the animal. Only a few specimens show that each leg had a pair of small, curved claws at the end. Inside the fuselage, a simple, continuous intestine can be seen as a dark band (filled with sediment); sometimes there are other traces that are interpreted as longitudinal canals of unknown function. The rear end of the fuselage ends in a very short, slightly downwardly directed "tail" of barely a millimeter in length.

The nine pairs of sclerites were outwardly curved plates. Partially deformed specimens show that, despite the dark color, they were not completely mineralized. They reveal a hexagonal honeycomb pattern, with a round channel in the center of each of these cells. A short, thorn-like extension sits on the corners of the meshes. The shape of the plates was somewhat different along the longitudinal axis of the body, the first pair elongated and elliptical, the following pairs from round and ovoid to rounded and square.

Based on the first findings, there was initially uncertainty about the front and rear ends of the animals. The superficial similarity of sclerites with a compound eye was temporarily the interpretation arise, the animal would have nine obsess over the hull distributed pairs of eyes, it is today but as clearly refuted.

Way of life

Microdictyon sinicum was believed to be a benthic organism that lived on the sea floor. Numerous specimens have been found in spatial connection with fossils of Eldonia (or closely related genera), so that the theory is widespread that they may have lived epizoically mounted on them, possibly only as young animals. Eldonia was a possibly free-swimming (pelagic), possibly bottom-living (benthic) soft-skinned, round, bell-shaped organism in plan, with radially arranged, sac-like structures inside. Many have a structure that is interpreted as a possible lophophore . The assignment of Eldonia to a recent animal strain is controversial. There are various theories about the possible nutrition of Microdictyon , it was interpreted as a possible saprophagous organism or as a microphagous particle eater.

Other kinds

From Microdictyon , apart from the Chinese finds, there are only finds of isolated sclerites. These phosphated structures are usually obtained by dissolving limestone with hydrochloric acid, which exposes the insoluble sclerite. They belong to a globally widespread collection of isolated, sclerite-like structures, in which in many cases the associated organism itself is unknown, they are, somewhat casually, summarized as " Small Shelly Fauna " and are particularly typical of the first series of the Cambrian ( Terreneuvium , formerly "Tommotium"). Microdictyon is just as common in the second series of the Cambrian (the Atdabanium according to the Russian nomenclature), Microdictyon effusum was even suggested as a possible key fossil for its beginning. In the Kaili formation in southwest China, it still reaches the beginning of the third series, which is already part of the Central Cambrian.

The name Microdictyon (from ancient Greek dictyon: net) refers to the net-like or sieve-like structure of this sclerite. From China there are finds of sclerites partially fused together on the flat side, in which a larger sclerite sits on a slightly smaller one. These are interpreted as remnants of an individual that just his moulting was going through. This is direct evidence of molting in the lobopods. Microdictyon is, besides Onychodictyon , the only genus in which isolated sclerites can be assigned to a lobopod organism.

Fossil records of Microdictyon -Skleriten are common in rocks of the lower and middle Cambrian right. The following species have been described based on the shape of the sclerite, its size and the shape of the mandrels attached:

  • Microdictyon effusum Bengtson, Matthews & Missarzhevsky, 1991. Kazakhstan, Siberia, England, Sweden
  • Microdictyon rhomboidale Bengtson, Matthews & Missarzhevsky, 1986. Kazakhstan and (Sekwi Formation, Mackenzie Mountains) Canada.
  • Microdictyon robisoni Bengtson, Matthews & Missarzhevsky, 1986. Utah, USA
  • Microdictyon tenuiporatum Bengtson, Matthews & Missarzhevsky, 1986. Siberia
  • Microdictyon sphaeroides Hinz, 1987. Shropshire, England.
  • Microdictyon depressum Bengtson, 1990. South Australia
  • Microdictyon chinense (Hao et Shu, 1987). China and Siberia
  • Microdictyon jinshaense Zhang & Aldridge, 2007. China and Australia
  • Microdictyon rozanovi Demidenko, 2006. Siberia
  • Microdictyon fuchengense Li et Zhu, 2001. China

There are also other finds, some only of broken sclerites, which were only assigned to the genus but not to any species, for example from Denmark and Turkey. Finds that have not been assigned to any species are also available from the Chinese Central Cambrian Kaili fauna . The assignment of sclerites to species is generally viewed critically, as the intraspecific and ontogenetic variability is only insufficiently understood.

Phylogeny

There are numerous different hypotheses about the relationships within the lobopods. It has been suggested that the lobopods might be paraphyletic . According to this theory, Microdictyon belongs to a clade that would also include the recent Onychophora ( stumpworts ).

Individual evidence

  1. ↑ a b Chen Jun-Yuan, Zhou Gui-Qing, Lars Ramsköld (1995): The Cambrian Lobopodian Microdictyon sinicum. Bulletin of National Museum of Natural Science 5: 1-93.
  2. ^ A b Xian-guang Hou, Richard Aldridge, Jan Bergström, David J. Siveter, Derek Siveter, Xiang-Hong Feng: The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life. John Wiley & Sons, 2008. ISBN 978-0-470-99994-3 . on pages 90–91.
  3. ↑ Xiaoya Ma, Xianguang Hou, Richard J. Aldridge, David J. Siveter, Derek J. Siveter, Sarah E. Gabbott, Mark A. Purnell, Andrew R. Parker, Gregory D. Edgecombe (2012): Morphology of Cambrian lobopodian eyes from the Chengjiang deposit and their evolutionary significance. Arthropod Structure & Development 41: 495-504. doi: 10.1016 / j.asd.2012.03.002
  4. ↑ Chen Jun-Yuan, Zhu Mao-Yan, Zhou Gui-Qing (1995): The Early Cambrian medusiform metazoan Eldonia from the Chengjiang deposit. Acata Palaeontologica Polonica 40 (3): 213-244.
  5. ↑ Xian-guang Hou, Richard Aldridge, Jan Bergström, David J. Siveter, Derek Siveter, Xiang-Hong Feng: The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life. John Wiley & Sons, 2008. ISBN 978-0-470-99994-3 . on page 200–201.
  6. ^ Stefan Bengtson (2004): Early Skeletal Fossils. In: Lipps, JH and Wagoner, BM (editors): Neoproterozoic- Cambrian Biological Revolutions. Paleontological Society Papers 10: 67-78.
  7. ^ S. Peng, LE Babcock, RA Cooper: The Cambrian Period. Chapter 19 in Felix M. Gradstein, James G. Ogg, Mark Schmitz, Gabi Ogg (editors): The Geologic Time Scale. Elsevier, 2012. doi: 10.1016 / B978-0-444-59425-9.00019-6
  8. ^ Jean-Bernard Caron, Martin R. Smith Thomas HP Harvey (2013): Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians. Proceedings of the Royal Society B 280: 20131613. doi: 10.1098 / rspb.2013.1613
  9. ↑ a b Yu. E. Demidenko (2006): New Cambrian Lobopods and Chaetognaths of the Siberian Platform. Paleontological Journal 40 (3): 234-243. doi: 10.1134 / S0031030106030026
  10. ^ Xi-Guang Zhang, Richard J. Aldridge (2007): Development and Diversification of Trunk Plates of the Lower Cambrian Lobopodians. Paleontology 50 (2): 401-415. doi: 10.1111 / j.1475-4983.2006.00634.x
  11. ↑ Zhao Yuanlong, Zhu Maoyan, Loren E. Babcock, Yuan Jinliang, Ronald L. Parsley, Peng Jin, Yang Xinglian, Wang Yue (2005): Kaili Biota: A Taphonomic Window on Diversification of Metazoans from the basal Middle Cambrian: Guizhou, China. Acta Geologica Sinica 79 (6): 751-765.
  12. ↑ Timothy P. Topper, Glenn A. Brock, Christian B. Skovsted, John R. Paterson (2011): Microdictyon plates from the lower Cambrian Ajax Limestone of South Australia: Implications for species taxonomy and diversity. Alcheringa 35 (3): 427-443. doi: 10.1080 / 03115518.2011.533972
  13. ↑ Ma XiaoYa, Hou XianGuang, David Baines (2010): Phylogeny and evolutionary significance of vermiform animals from the Early Cambrian Chengjiang deposit. Science China Earth Sciences 53: 1774-1783. doi: 10.1007 / s11430-010-4084-y

Web links

Commons : Lobopodia  - collection of images, videos and audio files