Opabinia
Name: Opabinia
(named after Opabin pass, near Lake O'Hara).
Phonetic: O-pa-bin-ee-ah.
Named By: Charles Doolittle Walcott - 1912.
Classification: Arthropoda, Dinocaridida,
Radiodonta, Opabiniidae.
Species: O. regalis (type).
Diet: Carnivore/Detritvore.
Size: Usually between 4 and 7 centimetres
long.
Known locations: Canada, British Columbia -
Burgess Shale.
Time period: Cambrian.
Fossil representation: Many specimens.
Perhaps
the best word to describe Opabinia is bizarre.
With five eyes, a
forward facing proboscis a third of the length of the body and a mouth
that is not only on the underside of the body but faces backwards,
and you end up with a creature like no other we know of today.
However when you look at Opabinia within the
context of other Burgess
Shale creatures such as Hallucigenia,
Wiwaxia
and Anomalocaris,
then Opabinia actually begins to look quite normal.
Opabinia
is loosely defined as a lobopod in that its body was arranged in
segments called 'lobes'. However while superficially similar to
later creatures like trilobites, the lobes of Opabinia
seem to have
been soft, without the hard exoskeleton that later arthropods would
possess. This soft body has often been distorted during the
preservation process resulting in slightly different reconstructions
depending upon the specimen being described. Also because the body
was soft, the internal organs of Opabinia are
sometimes revealed in
impressions. This means it takes an experienced eye to identify these
organs and not to mistake them for external features.
The
lobes that came out from the sides of the main body were flat and
projected at a downwards angle. These lobes also overlapped one
another with the rear edge resting over the top of the front edge of
the lobe behind it. Markings on these lobes are interpreted as being
gills, and are present on all but the first lobes either side.
However these gills have been interpreted to being on both either the
top or lower sides of the lobes. If the gills were underneath they
would have been protected from above. However if the gills were on
the top of lobe they would have been protected from below as well as
having a greater exposure to the flow of sea water due to the
overlapping arrangement of the lobes.
The
lobes of Opabinia also made a ‘V’ shaped tail
formed by two sets of
three lobes that rose up either side from the rear of the tail. This
gave Opabinia a tail loosely reminiscent of modern
day fighter jet,
and may have served a stabilising purpose.
It’s
possible that Opabinia could swim above the bottom
by rippling its
outer lobes in sequence so that it maintained the same level it needed
to be. While this form of movement would not have been fast, or
indeed powerful against a strong current, it would have allowed for
stable positioning over areas where Opabinia had
found suitable food
sources.
The
proboscis of Opabinia terminated in a pincer which
had spines that
pointed forwards and inwards on the inner sides of the claws. What
this pincer was used on is difficult to say as it would have equally
been able to grasp small soft bodied organisms as well as small chunks
of organic matter. The low number of Opabinia
specimens in relation
to other species may point towards the latter carnivore theory.
Assuming this low occurrence is not a result of the true numbers not
being preserved, Carnivorous animals are usually less common than
other kinds of animals so that they do not exhaust their food supply.
The
rest of the proboscis itself was roughly a third of the length of the
body and is striated in multiple cross sections. This suggests that
the proboscis was flexible, and when combined with the length, it
is theoretically possible for the proboscis to pick up a food item and
curl round underneath to deposit the food into the mouth. This would
also explain why the mouth faces backwards instead of just facing
down, as the proboscis could curve along its natural curvature when
delivering food for processing. The length and flexibility of the
proboscis also meant that Opabinia could search
between cracks and
crevices for food where smaller items could be hidden. Because the
mouth on Opabinia faced backwards its gut had a
strong ‘U’ shaped
bend which food would have to pass around. Once inside the food would
make its way down the length of the body as it passed down the gut.
As
stated above, Opabinia had five eyes, something
that is very
unusual as animals today tend to have even numbers of eyes. These
eyes are arranges into two rows on the head with two on the front row,
three on the back row behind them. These eyes where situated on
short stalks that pointed upwards at slight angles. The middle eye of
the back row of three had a stalk that was shorter than the rest
suggesting that it just pointed directly upwards. How these eyes
worked is not entirely known. Some have suggested that they were
compound eyes, although there is no fossil evidence to suggest this.
However all five eyes working together may have actually had a result
similar to the basic concept of a single compound eye.
Given
the position of Opabinia in the fossil record its
eyes would have been
probably been very basic with no way near the ability to produce images
like we see them today. They would however probably have been able to
distinguish between shapes of light and dark, essentially a simple
photoreceptor. By having five photoreceptors positioned together and
pointing in slightly different directions to one another (the same
concept as individual lenses of a compound eye), Opabinia
would be
able to 'see' dark shapes passing over head as the object passed
over the receptors preventing surface light from reaching them.
These 'objects' may have been the forms of potential predators
that Opabinia could interpret as being the sign to
take cover to avoid
being seen. Further support for this theory comes from the fact that
the eyes predominantly point at upwards angles rather than just forward.
Perhaps
the most confusing feature about Opabinia, or at
least the part that
produces the strongest debate amongst researchers are the small
triangular structures that are on the underside of the main body.
These were initially interpreted in 1975 by H. B. Whittington
as being extensions of the gut, with an alternative suggestion in
1994 by Chen et al. being that they were part of side lobes.
However
in 1996 G. E. Budd stated that these triangular forms could not
have been internal structures, and that they were also separate from
the lobes being lower down the body. This resulted in the first
speculation that these conical structures were actually legs,
something that seemed to be confirmed when he found mineralised
patches that seemed to correspond to hard claws on the ends of the legs.
Although
this seems plausible a new study undertaken in 2007 by X. Zhang
and B. E. G. Briggs stated that the conical 'legs' actually
had the same chemical composition as the gut, seemingly confirming
the original interpretation by Whittington. They also suggested that
the lobes and gills of Opabinia were biramous
as seen in
crustaceans, and that Opabinia may be
representative of the form that
originated this design. However a subsequent 2011 paper by G.
E. Budd and A. C. Daley suggested that the chemical composition
similarity is not exclusive to the gut, but is in fact indicative of
the mineralisation that occurs in fluid filled cavities like you would
find in other lobopod legs.
If
Opabinia had working legs then it would not have
needed to always swim
to get about, although this in itself would not suggest that Opabinia
was incapable of swimming by moving the side lobes in a wave like
fashion. The presence of legs if confirmed beyond dispute would
probably lead to a change in traditional reconstructions of Opabinia
slowly swimming just above the bottom.
Because
Opabinia does not feature any defining
characteristics its exact
position among other creatures is still uncertain, although most
people agree that it is similar to members of both the Onychophoria and
the Tardigrades. Also while Opabinia is more
advanced than more basal
lobopods, it is not as advanced as the later arthropods which had
hard exoskeletons. Further fossil evidence may yet clarify this
matter, but until then the phylogenetic position of Opabinia
will
continue to be decided by debate.
Further reading
- The enigmatic animal Opabinia regalis, Middle
Cambrian Burgess Shale,
British Columbia. - Philosophical Transactions of the Royal Society B
271 (910): 1–43 271. - H. B. Whittington - 1975.
- Opabinia and Anomalocaris,
unique Cambrian arthropods. - Lethaia 19
(3): 241–246. - J. Bergstr�m - 1986.
- The morphology of Opabinia regalis and the
reconstruction of the
arthropod stem-group. - Lethaia 29 (1): 1–14. - G. E. Budd - 1996.
- The nature and significance of the appendages of Opabinia
from the
Middle Cambrian Burgess Shale. - Lethaia 40 (2): 161–173. - X. Zhang
& D. E. G. Briggs - 2007.
- The lobes and lobopods of Opabinia regalis from
the middle Cambrian
Burgess Shale. - Lethaia 45: 83. - G. E. Budd & A. C. Daley -
2011.
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