Wiwaxia
| Wiwaxia | ||||||||||||
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Specimen of Wiwaxia in the National Museum of Natural History |
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| Temporal occurrence | ||||||||||||
| Lower and Middle Cambrian | ||||||||||||
| 525 to 505 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name of the family | ||||||||||||
| Wiwaxiidae | ||||||||||||
| Walcott , 1911 | ||||||||||||
| Scientific name of the genus | ||||||||||||
| Wiwaxia | ||||||||||||
| Matthew , 1899 | ||||||||||||
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Wiwaxia is an extinct species of animal that has been passed down in the fossil sites of the lower and middle Cambrian such as the Burgess slate . Two species that are assigned to this genus are known.
The soft body of the animals was covered with back armor plates ( sclerites ) and long spike-like body appendages, which are mainly preserved in the sedimentary rocks . Completely preserved specimens have a length of 3.4 mm to a little over 5 cm. The exact taxonomic classification of the genus is still controversial among paleontologists .
Discovery story
Wiwaxia was originally described by George Frederic Matthew in 1899 using a single spike found in Ogyopsis shale and classified as a hyolithid . Other specimens were found by the American paleontologist Charles Doolittle Walcott in 1911 during one of his field excursions to the nearby Burgess Shale in the Canadian Rockies . Walcott classified the fossil as a member of the Polychaeta genus within the tribe of annelids .
In 1966 and 1967, a team led by Harry B. Whittington went on another expedition to Burgess Shale and found so many fossils that it took years to evaluate them all. In addition, Wiwaxia was one of the most difficult fossils to analyze. 464 specimens of Wiwaxia are known from the Burgess slate , making up 0.88% of the fauna community. Simon Conway Morris , a member of Whittington's team, published a detailed description in 1985 in which he came to the conclusion that Wiwaxia was not to be placed among the Polychaeta. Before fragments were found in the Georgina Basin in Australia in 1991 , all known individuals of Wiwaxia came either directly from the Burgess slate or from sites in the vicinity. In 2004, further discoveries were made in the same deposit that could represent two new species.
In 1994 another species ( W. taijiangensis ) was described on the basis of fragmentarily preserved fossils. These were found in the Kaili Formation in China, which are dated to the end of the Middle Cambrian.
Occur
Halfway complete specimens were found in the Burgess slate, which represents the middle Cambrian around 505 mya . Fossils that have been preserved in fragments were discovered in slightly older or younger strata near the Burgess Shale, in the Kaili Formation in China, which is dated to the end of the Middle Cambrian, and in the central Cambrian strata of the Buchava Formation in Týřovice nad Berounkou in the Czech Republic ( Moderhof). Further specimens come from the Lower Cambrian Mount Cap Formation in the Mackenzie Mountains in northwest Canada, the Emu Bay Shale on the Kangaroo Island off the south coast of Australia, which arose in the section of the upper botomium in the lower Cambrian, and from the middle botomium of Siberia .
The finds show that Wiwaxia and many of the other life forms of the Burgess slate were widespread.
description
This description is mainly based on the species Wiwaxia corrugata , as 138 complete specimens from the Burgess slate have come down to us, while the second species has only survived in fragments.
body shape
Wiwaxia had a bilaterally symmetrical body that, when viewed from above, was elliptical in shape with no distinct head or tail. When viewed from the front, the animal was almost rectangular in shape. The most fully preserved fossils differ in size. One group has a body length of 2 to 5 cm, which is probably full-grown animals, while the other group is only 3.4 mm to 1.5 cm in length. These specimens are probably young animals. The height of Wiwaxia is difficult to estimate because the animals were pressed together after their death. A specimen with an average length of 3.4 cm could have had a body height of 1 cm without the spine-like appendages. The ratio of body length to height does not seem to have changed during the growth of the animals.
Body surface
The body surface was covered with armor plates, and there were long spikes on the back. The animals probably shed their skin.
Armor plates
The animal's body was covered by small, corrugated armor plates, so-called sclerites, which lay flat on the body in an overlapping manner. The body plates formed five major regions: the top with 8 to 9 rows of sclerites; the upper side part with 11 to 12; the lower side part with 8; the front and the area closest to the sea floor with 12-17 rows. Most of the sclerites were oval in shape, but the ventrolateral ones, closest to the sea floor, were crescent-shaped and formed in a row with their tips pointing downwards. In addition, Wiwaxia had two rows of corrugated spines that protruded upwards on each side of the upper side of the body and had a slight upward curvature at their ends. The extensions could reach a length of 11 to 52 mm. The number of spine-like appendages seems to have been dependent on the size of the individual, but could be up to 12 per side. The number and spacing of the spines was asymmetrical in the specimens found. This may correspond to the natural growth and not be due to the action of predators before or after the death of the animals. Although the spines in the middle of each row were usually the longest, up to 5 cm, some found specimens had relatively short central spines, which may be due to the fact that these were regrowing spines. It seems that the smallest specimens did not yet have long dorsal spines and that these only grew rapidly in larger young animals and more slowly in adult Wiwaxia .
Each plate was individually rooted in the body. The roots of the body sclerite comprised 40%, the roots of the spines comprised approximately 25% of the outer length of the sclerite. They sat in bags in the skin, similar to the hair follicles of mammals . The roots of the body sclerite were significantly narrower than the actual sclerite, but the roots of the spiky sclerite were about as wide as their bases. Both types of roots were made up of relatively soft tissue. The sclerites and spines were not calcified, and some broken specimens are frayed, so it is believed that they had a fibrous structure. The way of preservation suggests that the sclerits not like the exoskeletons of insects from chitin passed. They were probably made from proteins or collagen , which is also found in human cartilage and tendons . Since the body sclerites had bases that were narrower than the hard outer parts, their growth is difficult to trace. After examining some of the sclerites under both an optical and scanning electron microscope in 1990, Butterfield concluded that the sclerites are not hollow and that the bases split and spread, forming the body plates. This is a pattern reminiscent of the structure of the leaves of monocotyledons .
Molting
A young specimen seems to have passed down in the state of molting , it has not yet completely shed its old armor. The new set of spines appears less rigid than the older and slightly underdeveloped, as if the spines were being pumped up with body fluid in the next step and then hardened. The new armor presumably had a 50 to 70% larger internal volume than the old one. The molting appears to have taken place in one step, as adult specimens do not show any breaks in the armor that indicate molting of parts of the carapace or of individual sclerites. Since the bases of the sclerite were relatively narrow and there was no evidence of the sclerite fanning during molting, the withdrawal of the soft tissue from the sclerite may required it to break down into a liquid form, as is done in the claws of crabs and lobsters when molting happens. The skin also had to be shed because the shed armor appears as a unit and was not made up of scattered sclerites. In the juvenile specimen that was moulting when it died, the eating apparatus also seems to have renewed itself, as this specimen has a row of teeth facing forward.
Spines
The long dorsal spines in all likelihood acted as a defense against predators , especially since finds of broken spines indicate that Wiwaxia was attacked. The creature probably crawled across the ocean floor, feeding on particles that had sunk from higher layers of water. Wiwaxia shows no signs of legs and was probably too big to move with the help of cilia . The most plausible is locomotion through muscle contraction. Young animals could have buried themselves in the sea floor. The remains of the brachiopod Diraphora bellicostata have been found on the ventrolateral sclerites of an individual , which suggests that adult representatives of Wiwaxia did not dig the seabed or plow it too deeply while they were moving. Two other specimens of Diraphora bellicostata adhered to the dorsal sclerites. Wiwaxia seems to have lived solitary and not in groups .
Ingestion
Wiwaxia's flat underside was soft and not armored. Little is known about the internal anatomy . The digestive tract obviously ran straight from front to back. At its front end, in an animal with an average size of 2.5 cm, about 5 mm from the mouth opening, were the eating tools, which consisted of two (in a few specimens even three) rows of inwardly directed, conical teeth . This eater was tough enough to last dozens of times, but it wasn't calcified and fairly flexible as it could be folded and retracted when not needed. When the animal ate, the eater was pushed forward out of the mouth. Two rows with the same number of teeth as in larger individuals are present even in the smallest specimens. This indicates that Wiwaxia's feeding habits did not change after the larval stage . The eating tools presumably acted like rasps that were used to scrape bacteria off the carpet of microbes that covered the ocean floor. It is also possible that it was used to collect food particles from the sea floor, like a rake .
Since there are no signs of tentacles or eyes, it is believed that Wiwaxia relied primarily on their chemical senses, taste, and smell . The way the breathing of wiwaxia is also unknown.
Systematics
During the Cambrian, most of the tribes known today arose . Consequently, many later extinct lineages appear intermediate to two or more modern groups or do not have all of the characteristics that the members of a modern group have. They therefore fall into the " core groups " of a modern taxon . The debate about whether Wiwaxia should be classified in a modern crown group or a core group continues. When Walcott described Wiwaxia first, he assumed that it was a poly-bristle from the class of annelids and that his sclerites were elytres similar to those of annelids. The debate has recently been intense. Suggestions for classification included: Wiwaxia as a member of an extinct phylum that was distantly related to the mollusks ; belonging to a crown group of the Polychaeta ; as a member of a core group of the Annelida ; as a problematic representative of the Bilateria department ; as a parent group or possibly primitive representative of a crown group of molluscs.
In 1985, Simon Conway Morris Walcott admitted that there were similarities with the poly-bristles, but considered the sclerites of Wiwaxia to be different from the elytra of the annelids. He was more impressed by the similarities between Wiwaxia's eating apparatus and the radula of some molluscs and placed the animal in the new taxon Molluscata, which, according to his proposal, should also contain the mollusks and the Hyolitha . When he later described the specimen from Halkieria , which was only halfway preserved , he took the view that these were closely related to Wiwaxia .
Nick Butterfield, a 1990 graduate student in paleontology at Harvard , agreed with Morris that the sclerites were shaped like elytra, the winglets of beetles , that are relatively fleshy and soft. However, he concluded that Wiwaxia could not be placed in the taxon "Coeloscleritophora" due to the hard sclerite and therefore could not be more closely related to Halkieria , which had hollow sclerite. Instead, he believed that these resembled the chitinous bristles ( setae ) that protrude from the body of modern annelids and, in some genera, form leaf-shaped scales that cover the back like bricks. The similarities concerned the composition, the structure, the rooting by follicles in the skin and the general appearance of the sclerite. A number of modern annelids also develop bristles on both sides of the body, which both Walcott and Butterfield viewed as comparable to the dorsal spines of Wiwaxia .
Butterfield also argued that Wiwaxia's eater could just as easily have sat in two parts on the sides of the "head" instead of in the middle of the "head", as is common with polychaetes. He went so far as to classify Wiwaxia as a member of the modern order Phyllodocida , and pointed out that the apparent lack of body segmentation in Wiwaxia is no obstacle, since some modern representatives of the polychaetes also have no segmentation except during their development. He later found that Wiwaxia lacked some features of the polychaetes that were expected to fossilize easily , and therefore considered Wiwaxia to be a parent group annelid, an early ancestor of the modern annelids.
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Conway Morris and Peel (1995) broadly accepted Butterfield's arguments and treated Wiwaxia as an ancestor of the polychaetes. They said Butterfield informed them that the microscopic structure of the Wiwaxia sclerites were identical to the bristles of Burgessochaeta and Canadia , two polychaetes made from the Burgess slate. Conway Morris and Peel also found that a specimen of Wiwaxia showed traces of a small shell , possibly a holdover from an earlier stage in the evolution of the genus. The authors noted that a certain group of modern polychaetes also have stunted remnants of former shells. Nevertheless, they still took the view that the feeding apparatus of wiwaxia was more like the radula of mollusks. They also argued that Wiwaxia was quite closely related to, and strictly descended from, the Halkieriidae because the sclerites were similarly grouped, although those of the Halkieriidae were much smaller and more numerous. They also indicated that in 1994 Butterfield found sclerites from Wiwaxia which were hollow. They presented an extensive cladogram according to which
- the early Halkieriidae represented a "sister" group of the mollusks. Both were therefore descendants of a fairly closely related common ancestor.
- the Halkieriidae, which Conway Morris had found in the Sirius Passet deposit in Greenland , formed a sister group to the brachiopods ; Animals whose modern forms have double-lobed shells, but differ from the mollusks by a muscular handle and a characteristic eating device with tentacles, the lophophore .
- another genus of the Halkieriidae, Thambetolepis , the "great aunt" and Wiwaxia the "aunt" of the Annelids.
The marine biologist Amélie H. Scheltema found in 2003 that Wiwaxia's eating apparatus of the radula resembled the modern, shell-less worm mollusks and that the sclerites of the two groups were very similar. She concluded that Wiwaxia should be placed in a clade that also includes the mollusks.
The Danish zoologist Danny Eibye-Jacobsen claimed in 2004 that Wiwaxia had no characteristics that would allow it to be safely related to the polychaetes or the annelids. Eibye-Jacobsen considered the bristles to be a property shared by mollusks, annelids, and brachiopods. Even if the sclerites were very similar to the bristles, which the scientist doubted, this does not therefore prove that Wiwaxia's closest relatives are the annelids. He also showed that the very different numbers of sclerites in different areas of Wiwaxia's body were not associated with any plausible pattern of segmentation. Although Eibye-Jacobsen did not assume that this would prevent Wiwaxia from being classified as a polychaet, he considered the lack of distinctive features of the polychaetes to be a serious objection. In his opinion, there were no convincing reasons why Wiwaxia should be classified as a protoannelid or a protomollusk.
Butterfield returned to the debate in 2006 and reiterated the arguments he presented in 1990 about why Wiwaxia should be seen as a polychaet. Although bristles are a feature of various groups, they only appear as back cover in the polychaetes.
Web links
Individual evidence
- ↑ a b c d Zhao, YL, Qian, Y .; Li, XS: Wiwaxia from Early-Middle Cambrian Kaili Formation in Taijiang, Guizhou . In: Acta Palaeontologica Sinica . 33, No. 3, 1994, pp. 359-366. Retrieved August 4, 2008.
- ↑ a b c d e f g h i j k l Conway Morris, S .: The Middle Cambrian metazoan Wiwaxia corrugata (Matthew) from the Burgess Shale and Ogygopsis Shale, British Columbia, Canada . In: Philosophical Transactions of the Royal Society of London, Series B . 307, 1985, pp. 507-582. doi : 10.1098 / rstb.1985.0005 . Retrieved August 4, 2008.
- ^ GF Matthew: Studies on Cambrian Faunas, No. 3. - Upper Cambrian Fauna, Mount Stephen, British Columbia. - The Trilobites and Worms . In: Transactions of the Royal Society . 5, 1899, pp. 39-68.
- ↑ a b Walcott, CD : Middle Cambrian annelids. Cambrian geology and paleontology, II . In: Smithsonian Miscellaneous Collections . 57, 1911, pp. 109-144.
- ^ Gould, SJ : Wonderful Life . Hutchinson Radius, London 1990, ISBN 0-09-174271-4 , p. 77 and p. 189
- ↑ Southgate, PN, and Shergold, JH: Application of sequence stratigraphic concepts to Middle Cambrian phosphogenesis, Georgina Basin, Australia . In: Journal of Australian Geology and Geophysics . 12, 1991, pp. 119-144.
- ↑ a b c Porter, SM: Halkieriids in Middle Cambrian Phosphatic Limestones from Australia . In: Journal of Paleontology . 78, No. 3, May 2004, pp. 574-590. doi : 10.1666 / 0022-3360 (2004) 078 <0574: HIMCPL> 2.0.CO; 2 . Retrieved August 1, 2008.
- ^ Age of Burgess Shale . In: Burgess Shale . Bristol University. Retrieved September 5, 2007.
- ↑ Andrey Yu. Ivantsov, A. Yu. Zhuravlev, AV Legutaa, VA Krassilova, LM Melnikovaa and GT Ushatinskaya: Palaeoecology of the Early Cambrian Sinsk biota from the Siberian Platform . In: Palaeogeography, Palaeoclimatology, Palaeoecology . 220, No. 1-2, 2005, pp. 69-88. doi : 10.1016 / j.palaeo.2004.01.022 .
- ^ The Cambrian World . Retrieved August 4, 2008. Reconstruction of the Burgess Shale and map of the world in Mid-Cambrian times.
- ↑ a b Eibye-Jacobsen, D .: A reevaluation of wiwaxia and the polychaetes of the Burgess Shale . In: Lethaia . 37, No. 3, September 2004, pp. 317-335. doi : 10.1080 / 00241160410002027 .
- ^ A b c Butterfield, NJ: A reassessment of the enigmatic Burgess Shale fossil Wiwaxia corrugata (Matthew) and its relationship to the polychaete Canadia spinosa . Walcott . In: Paleobiology . 16, No. 3, 1990, pp. 287-303. Retrieved August 5, 2008.
- ↑ Budd, GE: The Cambrian Fossil Record and the Origin of the Phyla . In: Integrative and Comparative Biology . 43, No. 1, 2003, pp. 157-165. doi : 10.1093 / icb / 43.1.157 . Retrieved August 20, 2006.
- ^ A b Butterfield, NJ: Hooking some stem-group “worms”: fossil lophotrochozoans in the Burgess Shale . In: Bioessays . 28, No. 12, 2006, pp. 1161-6. doi : 10.1002 / bies.20507 . PMID 17120226 .
- ^ Conway Morris, S., and Peel, JS: Articulated halkieriids from the Lower Cambrian of north Greenland . In: Nature . 345, June 1990, pp. 802-805. doi : 10.1038 / 345802a0 . Retrieved July 31, 2008. A short but free account is given at Showdown on the Burgess Shale . Retrieved July 31, 2008.
- ^ Butterfield, NJ: Exceptional Fossil Preservation and the Cambrian Explosion . In: Integr. Comp. Biol . 43, 2003, pp. 166-177. doi : 10.1093 / icb / 43.1.166 . Retrieved December 2, 2006.
- ^ A b c Conway Morris, S., and Peel, JS: Articulated Halkieriids from the Lower Cambrian of North Greenland and their Role in Early Protostome Evolution . In: Philosophical Transactions of the Royal Society: Biological Sciences . 347, No. 1321, 1995, pp. 305-358. doi : 10.1098 / rstb.1995.0029 . Retrieved July 31, 2008.
- ↑ Scheltema, AH, Kerth, K., and Kuzirian, AM: Original Molluscan Radula: Comparisons Among Aplacophora, Polyplacophora, Gastropoda, and the Cambrian Fossil Wiwaxia corrugata . In: Journal of Morphology . 257, No. 2, 2003, pp. 219-245. doi : 10.1002 / jmor.10121 . PMID 12833382 .
