Wiwaxia

Taxobox
fossil_range = Early–Middle Cambrian


regnum = Animalia
superphylum = Lophotrochozoa
phylum = "incertae sedis
unranked_classis = Halwaxiida
familia = Wiwaxiidae
familia_authority = Walcott, 1911
genus = "Wiwaxia"
binomial = "Wiwaxia corrugata"
binomial_authority = Walcott, 1911

"Wiwaxia" is genus of soft-bodied, scale-covered animals known from Burgess shale type lagerstatte dating from the Early to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from convert|3.4|mm|in to a little over convert|5|cm|in in length. The precise taxonomic affinities of the genus are a matter of ongoing debate amongst palaeontologists.

History of discovery

"Wiwaxia" was originally described by W.D. Matthew in 1899 from an isolated spine that had been found earlier in the Ogyopsis Shale, and classified as a hyolithid. Further specimens were found by in 1911 by American paleontologist Charles Doolittle Walcott as a result of one of his field trips to the nearby Burgess Shale in the Canadian Rocky Mountains, and he classified it as a member of the polychaete group of annelid worms.

In 1966 and 1967 a team led by Harry B. Whittington revisited the Burgess Shale and found so many fossils that it took years to analyze them all, and "Wiwaxia" was one of the most difficult to analyze. [cite book
author= Gould, S.J. | title=Wonderful Life | publisher= Hutchinson Radius
location=London | date=1990 | isbn=0091742714 | page=p. 77 and p. 189] Eventually in 1985 Simon Conway Morris, then a member of Whittington's team, published a detailed description that concluded "Wiwaxia" was not a polychaete.cite journal
author = Conway Morris, S. | year = 1985
title = The Middle Cambrian metazoan Wiwaxia corrugata (Matthew) from the Burgess Shale and Ogygopsis Shale, British Columbia, Canada
journal=Philosophical Transactions of the Royal Society of London, Series B
volume = 307 | pages = 507–582 | doi=10.1098/rstb.1985.0005
url=http://www.jstor.org/pss/2396338 | accessdate=2008-08-04 ] All the known specimens came from in and around the Burgess Shale until 1991, when fragmentary fossils were reported from Australia's Georgina Basin. [ cite journal
author=Southgate, P.N., and Shergold, J.H. | date=1991
title=Application of sequence stratigraphic concepts to Middle Cambrian phosphogenesis, Georgina Basin, Australia
journal=Journal of Australian Geology and Geophysics | volume=12 | pages=119-144] In 2004 additional finds which may represent two different species were reported from the same area. cite journal
author=Porter, S.M. | date=May 2004 | accessdate=2008-08-01
url=http://findarticles.com/p/articles/mi_qa3790/is_200405/ai_n9377598/pg_1?tag=artBody;col1
title=Halkieriids in Middle Cambrian Phosphatic Limestones from Australia
journal=Journal of Paleontology | volume=78 |issue=3 | pages=574-590
doi=10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2]

Occurrence

Reasonably complete specimens have been found in the Burgess Shale, and fragmentary specimens in in nearby strata slightly older than and younger than the Burgess Shale, in China's Kaili Formationcite journal
author = Zhao, Y.L. | coauthors = Qian, Y.; Li, X.S.
year = 1994
title = Wiwaxia from Early-Middle Cambrian Kaili Formation in Taijiang, Guizhou
journal = Acta Palaeontologica Sinica | volume = 33 | issue = 3 | pages = 359–366
url=http://www.jstor.org/pss/1307127 | accessdate=2008-08-04] and in Australia's Georgina Basin. All of these locations are dated to the Middle Cambrian period, and the Burgess Shale has been dated to ma|505. [cite web
title = Age of Burgess Shale
url=http://palaeo.gly.bris.ac.uk/palaeofiles/lagerstatten/Burgess/Setting.html
work = Burgess Shale | publisher = Bristol University | accessdate = 2007-09-05 ] These finds show that "Wiwaxia" and most of the other Burgess Shale type fauna were very widespread. [ cite web
url=http://paleobiology.si.edu/burgess/cambrianWorld.html | title=The Cambrian World
accessdate=2008-08-04 Reconstruction of the Burgess Shale and map of the world in Mid-Cambrian times.]

Description

This concentrates on the species "Wiwaxia corrugata", found in the Burgess Shale, since the other specimens consist only of fragments, while the Burgess Shale has provided at least 138 complete ones.

"Wiwaxia" was a bilaterally symmetrical animal. Viewed from the top the body was elliptical with no distinct head or tail, and from the front or rear it was almost rectangular. The most complete fossils fall into two size ranges: convert|2|cm|in to convert|5|cm|in long, which are thought to be adults; and convert|3.4|mm|in to convert|1.5|cm|in, which are thought to be juveniles. Estimating their height is difficult because specimens were compressed after death; a specimen of the average length, convert|3.4|mm|in, may have been convert|1|cm|in high excluding the spines on their backs. The ratio of width to length does not appear to change as the animals grew.

The animal was covered in small ribbed armor plates called sclerites, that lay flat against the body, overlapped so that the rear of one covered the front of the one behind, and formed five main regions — the top, with 8-9 rows of sclerites; the upper part of the sides, with 11-12; the lower part of the sides, with 8; the front; and the area nearest the sea-floor, with 12-17 rows. Most of the sclerites were shaped like oval leaves, but the ventro-lateral ones, nearest the sea-floor, were crescent-shaped, rather like flattened bananas, and formed a single row with the tips pointing down. In addition there were two rows of ribbed spines running from to rear, one along each side of the top surface, and projecting out and slightly upwards, with a slight upwards curve near the tips. Specimens ranging from convert|11|mm|in convert|52|mm|in have about the same number of ventro-lateral sclerites just above the foot. On the other hand the number of spines seems to depend on the size of the specimen, up to about 12 per side. The number and spacing of the spines is asymmetrical in the specimens found, and this may have been natural rather than a result of events in the animal's life or after death. Although the spines in the middle of each row are usually the longest, up to convert|5|cm|in, a few specimens have rather short middle spines, perhaps because these were part-grown replacements. The smallest specimens may have lacked the long dorsal spines, which appear to have grown quickly in larger juveniles and then more slowly in adults.

Annotated image | float=right | caption="Wiwaxia" spine, seen from front and side|

annotations = Annotation|80|40|"Blade" Annotation|80|100|="background-color:#ff8080"> = RootEach sclerite was rooted separately in the body; the roots of body sclerites are 40% of the external length or a little less, while the roots of the spines are a little over 25% of the external length; all were rooted in pockets in the skin, rather like the follicles of mammalian hair. The roots of the body sclerites were significantly narrower than the sclerites, but the spines had roots about as wide as their bases; both types of root were made of fairly soft tissue. The sclerites and spines were not mineralized, and the frayed appearance of some broken ones suggests a fibrous structure. They way they were preserved suggests they were not made of chitin, from which insects' exoskeletons are formed. They may have been made of tanned proteins or of collagen, which is the main component of cartilages and tendons in humans. Since the body sclerites had bases that were narrower than the hard external parts, it is hard to see how they grew. They may have enclosed soft tissue that could have secreted the hard walls, but there is no convincing evidence for this. Butterfield (1990) examined some sclerites under both optical and scanning electron microscopes and concluded that they were not hollow, and that the bases split and spread to form the blades, a pattern that is also seen in monocot leaves.

"Wiwaxia"’s flat underside was soft and unarmored. Little is known of the internal anatomy, although the gut apparently ran straight and all the way from the front to the rear. At the the front end of the gut, about convert|5|mm|in from the animal's front in an average specimen about convert|2.5|cm|in long, there was a feeding apparatus that consisted of two (or in rare large specimens three) rows of backward-pointing conical teeth. The feeding apparatus was tough enough to be frequently preserved, but unmineralized and fairly flexible, as it folded and retracted when not in use. It would have had to pushed forward out of the mouth in order to feed. Even the smallest specimens have this type of apparatus, with two rows containing the same number of teeth as in larger ones. This indicates that "Wiwaxia"’s feeding habits remained the same throughout its life after the larval stage. The feeding apparatus may have acted as a rasp to scrape bacteria off the top of the microbial mat that covered the sea-floor, or as a rake to gather food particles that lay on the sea-floor.

Since there is no sign of eyes or tentacles, "Wiwaxia" may have relied mainly on chemical senses such as smell and taste. Its respiratory system is also unknown.

One juvenile specimen appears to be preserved while molting and not yet completely detached from its discarded armor. Its new set of spines seem less rigid than the old ones and slightly underdeveloped, as if the next stages were going to be inflation by body fluids and then hardening. The new armor may have had an internal volume 50% to 70% larger than the old one. Molting appears to have occurred all at once, as adult specimens shows no signs of interruptions in the sclerite armor that would indicate molting of parts of the armor or of individual sclerites. Since the bases of the body sclerites are relatively narrow and these is no sign of sclerites splitting during molting, withdrawing soft tissue from the old sclerites would probably have required the tissues to be broken down in to a more fluid form, as happens in the claws of lobsters and crabs when they molt. The skin must also have been shed, since the discarded armor appears as a complete unit rather than scattered sclerites. In the juvenile that was apparently molting when it died, the feeding apparatus also appears to have been shed, as half of one tooth row is pointing forwards.

The long dorsal spines may have been a defense against predators, and finds of broken spines suggest that "Wiwaxia" was attacked. The animal appears to have crawled on the surface of the sea-floor feeding on particles that fell from higher levels of the sea. "Wiwaxia" shows no signs of legs and was probably too large to move on cilia, so it probably moved by muscular contraction that made its foot ripple. Juveniles may have burrowed into the sea-floor. In one specimen a small brachiopod, "Diraphora bellicostata", appears to be attached to one of the ventro-lateral sclerites. This suggests that adult "Wiwaxia" did not burrow or even plough much into the sea-floor as they moved. Two other specimens of "Diraphora bellicostata" have been found attached to dorsal sclerites. "Wiwaxia" appears to have been solitary rather than gregarious.

Classification

During the Cambrian, most of the main groupings of animals recognised today were beginning to diverge. Consequently, many lineages (that would later become extinct) appear intermediate to two more more modern groups, or lack features common to all modern members of a group, and hence fall into the "stem group" of a modern taxon. [ cite journal
author=Budd, G.E. | title=The Cambrian Fossil Record and the Origin of the Phyla
journal=Integrative and Comparative Biology | date=2003 | volume=43 | issue=1 | pages=157-165
doi=10.1093/icb/43.1.157 | url=http://icb.oxfordjournals.org/cgi/content/full/43/1/157
accessdate= 2006-08-20 ] Debate is ongoing as to whether "Wiwaxia" can be placed within a modern crown group and, if it cannot, in which group's stem it falls. When Walcott first described "Wiwaxia", he regarded it as a polychaete annelid worm, and its sclerites as similar to the elytra ("scales") of annelids.cite journal
author = Walcott, C.D. | authorlink = Charles Doolittle Walcott | year = 1911
title = Middle Cambrian annelids. Cambrian geology and paleontology, II
journal = Smithsonian Miscellaneous Collections | volume = 57 | pages = 109–144] More recently the debate has been intense, and proposed classifications include: a member of an extinct phylum distantly related to the molluscs; a crown-group polychaete; a stem-group annelid; a problematic bilaterian; a stem- or possibly primitive crown-group mollusc.

In 1985 Simon Conway Morris agreed that there were similarities to polychaetes, but considered that "Wiwaxia"’s sclerites were different in construction to annelids' elytra. He was more impressed by the similarites between "Wiwaxia"’s feeding apparatus and a molluscan radula, and assigned the animal to a new taxon Molluscata, which he proposed should also contain the molluscs and hyolithids. When he later described the first fairly complete specimens of "Halkieria", he suggested that these were closely related to "Wiwaxia". cite journal
title=Articulated halkieriids from the Lower Cambrian of north Greenland
author=Conway Morris, S., and Peel, J.S. | journal=Nature | volume=345 | pages=802–805
date=June 1990 | doi=10.1038/345802a0
url=http://www.nature.com/nature/journal/v345/n6278/abs/345802a0.html | accessdate=2008-07-31 A short but free account is given at cite web
url=http://www.stephenjaygould.org/library/naturalhistory_cambrian.html
title=Showdown on the Burgess Shale | accessdate=2008-07-31 ]

Nick Butterfield, then a postgraduate paleontologist at Harvard inspired by Stephen Jay Gould's lectures, agreed that the sclerites were not like elytra, which are relatively fleshy and soft. However, since the sclerites were solid, he concluded that "Wiwaxia" could not be a member of the "Coeloscleritophora", a taxon that had been proposed in order to unite organisms with hollow sclerites, and could not be closely related to the halkieriids, which have hollow sclerites. Instead he thought that they were very similar in several ways to the
chitinous bristles (setae) that project from the bodies of modern annelids and in some genera form leaf-like scales that cover the back like roof tiles - in composition, in detailed structure, in how they were attached to the body via "follicles" and in overall appearance. Some modern annelids also develop on each side rows of longer bristles, which both Walcott and Butterfield considered similar to "Wiwaxia"’s dorsal spines.cite journal
author=Butterfield, N.J. | year=1990
title=A reassessment of the enigmatic Burgess Shale fossil "Wiwaxia corrugata" (Matthew) and its relationship to the polychaete "Canadia spinosa". Walcott
journal=Paleobiology | volume=16 | pages=287–303
url=http://www.jstor.org/stable/2400789 | accessdate=2008-08-05 ] including the halkieriids.

Butterfield also contended that "Wiwaxia"’s feeding apparatus, instead of being mounted in the middle of its "head", was just as likely to be mounted in two parts on the sides of the "head", an arrangement that is common in polychates. He went so far as to classify "Wiwaxia" as a member of a modern order, Phyllodocida, and pointed out that "Wiwaxia"’s lack of obvious segmentation is no barrier to this, as some modern polychaetes also show no segmentation except during development. He later noted that Wiwaxia lack some polychate features which he would expect to be easily preserved in fossils, and therefore a stem-group annelid, in other words an evolutionary "aunt" of modern annelids.cite journal
author=Butterfield, N.J. |year=2003
title=Exceptional Fossil Preservation and the Cambrian Explosion |journal=Integr. Comp. Biol.
volume=43 | pages=166–177
url=http://icb.oxfordjournals.org/cgi/content/full/43/1/166#I1540-7063-043-01-0166-BUTTERFIELD
accessdate=2006-12-02 | doi=10.1093/icb/43.1.166 ]

clade
1=clade
1=MOLLUSCA
2=clade
1="Siberian halkieriid"
2=clade
1=clade
1=clade
1=clade
1=ANNELIDA
2="Canadia"
2="Wiwaxia"
2="Thambetolepis"
(halkieriid)
2=clade
1="Halkieria evangelista"
2=BRACHIOPODA

Cladogram:Conway Morris & Peel (1995)

Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated "Wiwaxia" as an ancestor or "aunt" of the polychaetes, and said Butterfield had informed them that the microscopic structure of "Wiwaxia"’s sclerites was identical to that of the bristles of two Burgess Shale polychaetes "Burgessochaeta" and "Canadia". Conway Morris and Peel also wrote that one specimen of "Wiwaxia" showed traces of a small shell, possibly a vestige left over from an earlier stage in the animal's evolution, and noted that one group of modern polychaetes also has what may be a vestigial shell. However they maintained that "Wiwaxia"’s feeding aparatus was much more like a molluscan radula. They also argued that "Wiwaxia" was fairly closely related to and in fact descended from the halkieriids, as the sclerites are divided into similar groups, although those of halkieriids were much smaller and more numerous; they also said that in 1994 Butterfield had found "Wiwaxia" sclerites that were clearly hollow. They presented a large cladogram according to which: cite journal
title=Articulated Halkieriids from the Lower Cambrian of North Greenland and their Role in Early Protostome Evolution
author=Conway Morris, S., and Peel, J. S.
journal=Philosophical Transactions of the Royal Society: Biological Sciences | volume=347
issue=1321 | pages=305–358 | doi=10.1098/rstb.1995.0029
url=http://journals.royalsociety.org/content/32l541667hj3071k/ | accessdate=2008-07-31
year=1995 ]
*The earliest halkieriids were a "sister" group to the molluscs, in other words descendants of a fairly closely-related commmon ancestor.
*The halkieriids which Conway Morris had found in Greenland's Sirius Passet lagerstätte were a "sister" geroup to brachiopods, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the lophophore.
*Another halkieriid genus, Thambetolepis, was a "great aunt" of annelids and "Wiwaxia" was an "aunt" of annelids.

Marine biologist Amélie H. Scheltema "et al" (2003) argued that "Wiwaxia"’s feeding apparatus is very similar to the radulas of some modern shell-less aplacophoran molluscs, and that the sclerites of the two groups are very similar. They concluded that "Wiwaxia" was a member of a clade that includes molluscs. [ cite journal
title=Original Molluscan Radula: Comparisons Among Aplacophora, Polyplacophora, Gastropoda, and the Cambrian Fossil "Wiwaxia corrugata"
author=Scheltema, A.H., Kerth, K., and Kuzirian, A.M.
journal=Journal of Morphology | volume=257 |pages=219–245 | date=2003 | doi= 10.1002/jmor.10121
url=http://www3.interscience.wiley.com/journal/104528759/abstract?CRETRY=1&SRETRY=0
accessdate=2008-08-05 ]

Danish zoologist Danny Eibye-Jacobsen argued in 2004 that "Wiwaxia" lacks any characters that would firmly place it as a polychaete or annelid. Eibye-Jacobsen regarded bristles as a feature shared by molluscs, annelids and brachiopods. Hence even if "Wiwaxia"’s sclerites closely resembled bristles, which he doubted, this would not prove that "Wiwaxia"’s closest relative were annelids. He also pointed out that the very different numbers of sclerites in the various zones of "Wiwaxia"’s body do not correspond to any reasonable pattern of segmentation; while Eibye-Jacobsen did not think that this alone would prevent classification of "Wiwaxia" as a polychaete, he thought it was a serious objection given the lack of other clearly polychaete features. In his opinion there were no strong grounds for classifying "Wiwaxia" as a proto-annelid or a proto-mollusc, although he thought the ojections against classification as a proto-annelid were the stronger.cite journal
title=A reevaluation of Wiwaxia and the polychaetes of the Burgess Shale
author=Eibye-Jacobsen, D. | journal=Lethaia | volume=37 | issue=3 | pages=317–335
month=September | year=2004 |doi=10.1080/00241160410002027
url=http://www.ingentaconnect.com/content/tandf/slet/2004/00000037/00000003/art00011;jsessionid=383hjtjp6370h.alexandra
accessdate = 2008-08-04 ]

Butterfield returned to the debate in 2006, repeating the arguments he presented in 1990 for regarding "Wiwaxia" as an early polychaete and adding that, while bristles are a feature of several groups, they appear as a covering over the back only in polychaetes.cite journal
author = Butterfield, N.J. | year = 2006
title = Hooking some stem-group ‘‘worms’’: fossil lophotrochozoans in the Burgess Shale
journal = Bioessays | volume = 28 | issue = 12 | pages = 1161-6 | doi = 10.1002/bies.20507
url=http://www3.interscience.wiley.com/journal/113471993/abstract?CRETRY=1&SRETRY=0
accessdate=2008-08-06 ]

ee also

* Coeloscleritophoran

References

External links

*Pharyngula [http://scienceblogs.com/pharyngula/2007/03/orthozanclus.php entry] on "Orthrozanclus reburrus"