Shonisaurus
Name:
Shonisaurus
(Shoshone lizard - After Shoshone Mountain).
Phonetic: Show-nee-sore-us.
Named By: Camp - 1976.
Classification: Chordata, Reptilia,
Ichthyosauria, Shonisauridae.
Species: S. popularis (type).
Diet: Piscivore/carnivore, thought to specialise
in soft bodied cephalopods.
Size: 15 meters long.
Known locations: USA, Nevada - Luning
Formation.
Time period: Norian of the Triassic.
Fossil representation: Many known individuals, Shonisaurus
is one of the better known ichthyosaurs.
Body features, swimming and
feeding behaviour
Shonisaurus
is a firm favourite amongst researchers of marine reptiles, not just
because of its large size but because it is known from so many
remains, best known from the Berlin-Ichthyosaur State Park in
Nevada, USA. These represent individuals of all ages and also
reveal a rather surprising age related development regarding the
teeth. In very small juveniles teeth are present at the forward
portions of the jaws, while older and larger individuals do not have
teeth at all. This is not a case of the teeth falling out, but
growing out and the presence of teeth as a juvenile characteristic
suggests that they were only there to give young Shonisaurus
a head
start in life.
While
Shonisaurus was a large and in certain aspects,
specialised
ichthyosaur
it also appears to be quite primitive. Study of the tail
reveals that it most likely had a tail fluke similar to a fish although
in a slightly different orientation. The main muscular area that
allowed for controlled movement of the tails dipped downwards into what
is termed the lower lobe of the caudal (tail) fin, whereas
fish like sharks however have bodies that curve upwards into the fin.
Usually the part of the caudal fin that the body curves into is the
most developed and so reconstructions of living Shonisaurus
often show
the lower portion of the tail to be larger and more developed than the
upper portion. This can also suggest that Shonisaurus
may have been a
comparatively slow swimmer as fast swimming animals usually have tails
where the upper and lower portions are roughly equal in both size and
proportion.
Another
area of possible inaccuracy in some depictions of Shonisaurus
is the
dorsal fin, in that it seems that Shonisaurus
didn’t have one. No
dorsal fin bone has ever been recovered in association with Shonisaurus
remains, and other similar ichthyosaurs are also known to be lacking
dorsal fin fossils as well, but later and more advanced ichthyosaurs
are known to have had them. Current thinking explains this as either
a complete lack of dorsal fins in living Shonisaurus,
or that the
fins were composed of a softer material like cartilage, similar to
how they are in sharks. As ichthyosaurs developed over successive
generations the cartilage could have ossified to form a permanent bony
structure seen in more advanced forms. Without a fossil imprint
however this remains speculation.
The
lack of teeth in the jaws suggests that Shonisaurus
had a preference
for soft bodied animals such as cephalopods like squid. This is
actually in quite strong contrast to other large deep water
ichthyosaurs such as Temnodontosaurus
which have deep rooted teeth for
cracking the shells of animals like ammonites. Like in other large
ichthyosaurs the large size of Shonisaurus meant
that it could take in more air at the surface and stay down deeper for
longer,
increasing hunting time. Squid would have also been an abundant food
supply for Shonisaurus to exploit as they breed in
large numbers and
quickly attain adult size ensuring that they quickly replaced their
numbers.
Key
features of Shonisaurus are its four limbs which
interestingly for a
fusiform marine creature are all the same size, although similar
sized front and rear limbs are seen in other basal ichthyosaurs.
Usually the front limbs are much larger and serve to counter the
downwards effect of swimming by the tail. Other sea creatures
(especially fast swimming ones) suffer from a downwards pitching
effect caused by propelling themselves from the rear with their tails,
and without their front fins they would nosedive into the ocean
abyss whenever they swam. This is also why on a side note the size of
the front fins can be taken as a guide to the animals behaviour, as
the faster the animals could swim, the larger the front fins need to
be to counter the downward pitch.
All
four limbs being the same size however may have meant that Shonisaurus
had greater vertical plane control, basically meaning it could
pitch to swim up or down much more quickly. This is a
considerable advantage when you consider that cephalopods like squid
usually swim up and down between different depths rather than side to
side. As such feeding behaviour for Shonisaurus
could see it
cruising around until it located a pocket of squid and then swimming up
and down to follow them.
Shonisaurus
was once regarded as the largest known ichthyosaur, but the largest
fossil specimen once attributed to Shonisaurus
has actually been
found to be that of another previously named genus called
Shastasaurus.
As such Shastasaurus is now considered the largest
known ichthyosaur, and is also a contender for the largest known
marine reptile.
Shonisaurus
and the Kraken -
Problems and explanation
In
late 2011 Shonisaurus made headlines around the
world, but not
because of the fact that it was a magnificent ichthyosaur, but
because a large number of them had been killed and eaten by a giant
cephalopod such as an octopus or squid, or at least this was the
hypothesis proposed by Mark McMenamin and Dianna Schulte McMenamin in a
lecture to a meeting of the Geological Society of America. The basis
of this theory comes from the observation that one assemblage of
Shonisaurus remains bears a striking visual
similarity to an octopus.
The explanation for this similarity was that an unknown prehistoric
cephalopod had arranged the Shonisaurus bone into a
form of
self-portrait.
Fellow
geologists and palaeontologists were for the most part unimpressed by
this proposition. Cephalopods are one of the most intelligent life
forms on this planet, and octopuses in particular do exhibit problem
solving skills. Aside from the classic example of an octopus
unscrewing and opening a jaw to get food within, any keeper of
octopuses will tell you that they are extraordinary escape artists,
and if not secured properly they can escape their own tanks and even
enter other nearby aquariums. Despite this level of intelligence
however, cephalopods have never been observed to create
self-portraits. In fact an octopus shown its reflection in a mirror
may even exhibit aggression towards its own mirror image.
The
other obvious problem about this theory is that there is no body or
other form of physical evidence to suggest the presence of a giant
cephalopod. Immediately some people would say that there is no body
because cephalopods have only soft tissue and this does not fossilise.
Back in the early days of palaeontology this would have been accepted
fact, but today we know better. Fossils of the early fish,
specifically the shark Cladoselache
have been found to include the
presence of mineralised soft tissue which has revealed everything from
muscle fibres to some internal organs. These remains also date back
to the Devonian period, making them much older than the Triassic age
remains of Shonisaurus. On top of this a handful
of fossilised
impressions of octopus bodies are actually known to science. At the
very least soft tissue preservation for a cephalopod is possible if the
conditions were correct, although the lack of soft tissue
preservation on the Shonisaurus may suggest that
this is not the case in this instance.
One part of the octopus that should almost certainly preserve however
is its beak. Made from keratin the beak is the hardest part of the
body and is developed for biting through the shells of armoured prey
like crustaceans. Tentacle hooks might also be preserved, but the
presence of these is dependent upon the specific genus, not the
cephalopod group.
So
without physical evidence of a body, and a proposal for behaviour so
far not observed in living cephalopods, we are left with a set of
Shonisaurus remains that resemble a picture of a
cephalopod. The
answer is very simple; you just have to think of a loaf of sliced
bread. When the loaf is taken out of the packet the ends fall off to
one side with the slices in-between falling on top of one another in an
overlapping arrangement. Now you don’t actually need a loaf of bread
to see this, you can test it with books stood on end and allowed to
fall over, the resulting observation is still the same.
While
we only know Shonisaurus from bones we must
remember it was a living
animal first, and when it died its body would have sunk to the bottom
and rested upon its side as is usually seen when animals like fish and
whales die. In this arrangement the ribs and vertebrae would have
actually been in an upright vertical arrangement within because the
body was keeping them in place together. As the body decayed and was
presumably eaten by scavengers however this support would have been
lost, resulting in the ribs and vertebrae falling and overlapping one
another as the resulting fossils are. From here the bones were
probably quickly covered in sediment as exposed bones do not survive
for long in sea water. As the bones fossilised they may have been
subject to an even greater amount of distortion that pushed the ribs
into a more focused area giving the appearance of tentacles. Evidence
for this comes from the ribs being broken in similar places suggesting
that they were all subject to the same forces. Over two-hundred
million years later and with only a part of the original animal
surviving and you have a conclusion that the remains are most probably
a case of simulacrum, and only appear to be a giant cephalopod to
human eyes, much in the same way clouds can sometimes be interpreted
as looking like other things.
Unfortunately
many news sources picked up upon the announcement of this theory but
did not bother to pick apart the details so that all they presented to
the wider public wasn’t much more than just a headline to grab people
attention. While others have since tried to talk about the theory in
its proper context of fossil interpretation and possible extent of
animal, specifically cephalopod, intelligence, the story still
appeared as ‘proof’ of the existence of giant cephalopods. It is
probably only a matter of time before it is used as the basis for a
straight to home release B movie.
However
the real significance about the Berlin-Ichthyosaur State Park bone bed
is actually the mystery as to why so many Shonisaurus
of all different
ages have ended up being so close to one another, and if they all
died together as a group. If the group theory is correct then it
brings the suggestion that ichthyosaurs may not have just looked like
dolphins, but lived in social groups like them as well.
Alternatively and just as likely is that the assemblage of Shonisaurus
individuals was caused by environmental circumstances.
First
and perhaps the simplest is that the area had an abundant supply of
squid, possibly Shonisaurus’s preferred prey,
and that a greater
number of Shonisaurus converged upon the same
area. This could easily
explain the presence of older individuals that may have died from old
age but it does not explain the presence of juveniles unless disease
and possible predation is a factor. Another theory is they all became
beached and died soon after not being able to return to the sea,
something that is quite easy to imagine as mass beaching has been seen
in whales. Poisoning is also another viable theory which sees the
Shonisaurus exposed to a prehistoric red tide. Red
tides are
essentially mass algal blooms that can result in large scale releases
of toxins that poison all kinds of marine life, in this case not just
affecting Shonisaurus but also the animals that it
preyed upon as
well. These are but a small number of theories that could explain the
presence of the Shonisaurus together, with some
including underwater
ocean currents causing the bodies to drift into submerged catch areas
that would have been sheltered to further disruption from currents.
The truth of the matter is just as likely to either be none or even a
combination of the above factors, but new fossil discoveries of
Shonisaurus and other related marine reptiles may
one day reveal a
greater insight into this mystery.
Further reading
- A revision of the skeletal reconstruction of Shonisaurus
popularis
(Reptilia: Ichthyosauria). - Journal of Vertebrate Paleontology 10 (4):
512–514. - Bradley F. Kosch - 1990.
- Taphonomy and Paleoecology of Shonisaurus popularis
(Reptilia:
Ichthyosauria). - PALAIOS 7 (1): 108–117. - J. A. Hogler - 1992.
- A reinterpretation of the Upper Triassic ichthyosaur Shonisaurus.
-
Journal of Vertebrate Paleontology 19: 42–49. - Chris McGowan &
Ryosuke Motani - 1999.
- Triassic kraken: the Berlin Ichthyosaur death assemblage interpreted
as a giant cephalopod midden. - Geological Society of America Abstracts
with Programs 43 (5): 310. - M. A. S. McMenamin & D. L.
Schulte-McMenamin - 2011.
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