Wiwaxia
Name: Wiwaxia
Phonetic: Wee-wax-ee-ah.
Named By: Charles Doolittle Walcott - 1911.
Classification: Animalia, Lophotrochozoa.
Species: W. corrugata (type), W.
herka, W.
taijiangensis.
Diet: Bottom feeder.
Size: Varying sizes from 3.4 to 50.8
millimetres.
Known locations: Worldwide distribution, but
particularly well-known from Canada, British Columbia - Burgess
shale. Also, China - Kaili Formation, Australia - Emu Bay
Shale, Czech Republic - Buchava Formation.
Time period: Early to Middle Cambrian.
Fossil representation: Hundreds of known specimens,
but sometimes only the spines are recovered.
Wiwaxia
was originally described back in 1899 by W. D. Matthew.
However this description was based upon just one of the dorsal spines
that rise up from the back of Wiwaxia, something
that was realised
in 1911 when more complete specimens were discovered by Charles
Doolittle Walcott. However in Doolittle's description, Wiwaxia
was
classed as a polycheate worm, something that would remain until
1985 when Simon Conway Morris published a more detailed description.
Wiwaxia
possessed a small squat body that when seen from above looked
elliptical, like a slightly squashed circle. When Wiwaxia
is viewed
in cross section from either the front or the rear however it was
distinctly rectangular, with a small protuberance around the bottom.
This has resulted in the sides being identified into three portions of
upper lateral, lower lateral (where the protuberance meets the
upper lateral) and the ventro-lateral where the body meets the
ground. From above there is no clear growth that indicates a head,
but the direction the sclerites overlap in may indicate which end is
the front.
The
sclerites are ridged scales that covered the body of Wiwaxia
forming
basic armour. The root of the sclerite was about the equivalent of
forty per cent the external size and anchored into a skin pocket. The
way the sclerites overlapped each other meant that the root of an
individual sclerite was easily covered by the sclerite that overlapped
it. This meant that the sclerites formed a firm yet flexible
defence, especially from predators that had suction type mouths
intended to work on soft bodied animals.
Most
of these sclerites were oval shaped with the exception of the ones on
the ventro-lateral edge. Instead of being oval these formed a
crescent shape, presumably because of their close contact with the
seafloor. The number of sclerites on any given Wiwaxia
individual
seemed to vary, but the easiest way is to count the number of rows.
Dorsal rows (the ones than ran across the back) number between
eight and nine. The upper lateral areas (roughly top two thirds of
the sides) have between eleven and twelve. The lower lateral area
(the thickened base portion that faced slightly upwards) had eight.
The ventro-lateral area (which faced slightly downwards and had the
crescent shaped sclerites) had between twelve and seventeen rows.
It
should be remembered that the number of sclerites was not necessarily
determined by the age of the individual, as study of different sized
specimen’s revealed sclerites that grew proportionately with the
increased size. In essence this means that Wiwaxia
simply grew bigger
rather than growing extra parts. As for the overall size of Wiwaxia,
specimens that are between two and five centimetres long are
considered to be adults, with smaller specimens juveniles. Because
Wiwaxia had a soft body it is always preserved flat
which has made it
difficult to determine how tall Wiwaxia was in
life. However gauging
the height is not just dependent upon the orientation of the fossil,
but can be estimated from how large the associated dorsal spines are
and
where they attach to the body. Height estimates for a Wiwaxia
individual of thirty-four millimetres length is often quoted at being
ten millimetres high.
The
features of Wiwaxia that are most immediately
visible are the dorsal
spines that run along the back in two rows. While the smaller
sclerites provided a closer internal defence these spines seem to have
been the front line defence for Wiwaxia. Not only
do these spines
show signs of damage and irregular growth suggesting predation,
Wiwaxia does not seem to have had eyes making it
impossible for the
spines to have been used for display.
The
spines themselves were rooted in pits in the skin like the sclerites,
however only about twenty-five per cent of their external length was
the root. Usually the spines in the middle of the rows were tallest,
but are sometimes smaller through damage and replacement. Like many
other animals that lose body parts, the spines were replaced if
lost, although also like other animals that have this ability,
replacement is faster and more complete in younger individuals,
especially juveniles that are still in their active growth stage. On
top of this the actual number of spines can vary between individuals,
including those that do not appear to have lost any. At up to five
centimetres long, the dorsal spines are also the most common fossils
of Wiwaxia, being considerably more numerous than
complete specimens.
The
only unarmoured part of Wiwaxia was the underside
and this is also
where the mouth was located. The mouth was located roughly one fifth
of the total body length from the front of Wiwaxia
and usually housed
two rows of teeth, although large specimens are known to have three
rows. The teeth in these rows were conical and faced backwards. The
teeth were also packed tightly together in their rows together forming
a single scraper that seemed to be folded in a 'V' shape when at
rest, but extended out into a straight edge when used for feeding.
It
is possible that Wiwaxia used this mouth to feed
upon the microbial
mats that covered the sea floors at the Beginning of the Cambrian. As
the Cambrian period continued a process that is referred to as the
Cambrian substrate revolution occurred. This was basically newly
evolving animals burrowing beneath the microbial mats and slowly
exposing the anoxic sediment underneath to a combination of water and
oxygen. This resulted in the underlying sediment becoming habitable
to more animals which in turn increased the process further which
resulted in the eventual loss of the microbial mats. The loss of the
microbial
mats towards the end of the Cambrian may explain why Wiwaxia
is so far
only known from the early and middle stages of the Cambrian.
Further reading
- The Middle Cambrian metazoan Wiwaxia corrugata
(Matthew) from the
Burgess Shale and Ogygopsis Shale, British Columbia, Canada. -
Philosophical Transactions of the Royal Society of London B 307 (1134):
507–582. - S. Comway Morris - 1985.
- A reassessment of the enigmatic Burgess Shale fossil Wiwaxia
corrugata (Matthew) and its relationship to the polychaete
Canadia
spinosa. Walcott. - Paleobiology 16 (3): 287–303. - N. J. Butterfield -
1990.
- Wiwaxia from Early-Middle Cambrian Kaili
Formation in Taijiang,
Guizhou. - Acta Palaeontologica Sinica 33 (3). - Y. L. Zhao, Y. Qian
& X. S. Li - 1994.
- Original Molluscan Radula: Comparisons Among Aplacophora,
Polyplacophora, Gastropoda, and the Cambrian Fossil Wiwaxia
corrugata.
- Journal of Morphology 257 (2). - A. H. Scheltema, K. Kerth &
A. M. Kuzirian - 2003.
- A reevaluation of Wiwaxia and the polychaetes of
the Burgess Shale. -
Lethaia 37: 317. D. Eibye-Jacobsen - 2004.
- Mouthparts of the Burgess Shale fossils Odontogriphus
and Wiwaxia:
Implications for the ancestral molluscan radula. - Proceedings of the
Royal Society B 279 (1745): 4287–4295. - M. R. Smith - 2012.
- Ontogeny, morphology and taxonomy of the soft-bodied Cambrian
‘mollusc’ Wiwaxia. - Palaeontology vol 57, issue 1.
- Martin R. Smith -
2013.
- New Wiwaxia material from the Tsinghsutung
Formation (Cambrian Series
2) of Eastern Guizhou, China. - Geological Magazine 151 (02): 339–348.
- H. J. Sun, Y. L. Zhao, J. Peng & Y. N. Yang - 2014.
- Articulated Wiwaxia from the Cambrian Stage 3
Xiaoshiba Lagerst�tte.
- Scientific Reports 4:4643. - J. Yang, M. R. Smith, T. Lan, J.-B. Hou
& X.-G. Zhang - 2014.
- Ontogeny, morphology and taxonomy of the soft-bodied Cambrian
'mollusc' Wiwaxia. - Palaeontology 57 (1). - M. R.
Smith - 2014.
- First report of Wiwaxia from the Cambrian
Chengjiang Lagerst�tte. -
Geological Magazine 152 (2): 378–382. - F. C. Zhao, M. R. Smith, Z. -J.
Yin, H. Zeng, S. -X Hu, G. -X Li & M. - Y. Zhu - 2015.
- New reconstruction of the Wiwaxia scleritome,
with data from
Chengjiang juveniles. - Scientific Reports. 5. - Zhifei Zhang, Martin
R. Smith & Degan Shu - 2015.
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