Name:
Shastasaurus
(Shasta lizard - After Mount Shasta).
Phonetic: Shas-tah-sore-us.
Named By: Merriam - 1895.
Synonyms: Shastasaurus alexandrae,
Shastasaurus osmonti, Guanlingsaurus liangae, Shonisaurus
sikanniensis.
Classification: Chordata, Reptilia,
Ichthyosauria, Shastasauridae.
Species: S. pacificus
(type), S.
liangae, S. sikanniensis.
Diet: Specialist of soft bodied cephalopods like
squid.
Size: 21 meters long for S.
sikanniensis,
other species like S. liangae seem to be 4
to 9 meters long
based upon available fossils.
Known locations: USA, California - Mount
Shasta. Canada, British Columbia - Pardonet Formation. China.
Time period: Carnian to Norian of the Triassic.
Fossil representation: Many specimens, although
often incomplete and fragmentary.
Shastasaurus
species and fossil distribution
Shastasaurus
hit the record books in 2011 with the re-examination of a set of
ichthyosaur
remains from British Columbia that were initially described
as a new species of Shonisaurus
back in 2004. Study of the remains
has found that while they are similar, the remains are a better match
for Shastasaurus, and so the remains were moved
from Shonisaurus to
create a new third species, Shastasaurus sikanniensis.
This
confusion is not surprising however when you consider that these two
ichthyosaurs are thought to be very similar to each other in not just
form but predatory behaviour as well.
Although
the material referable to S. sikanniensis has
allowed for a new upper
size of the genus to be established, it is Chinese fossil material
referable to the second species, S.liangae named
in 2000 that has
allowed for considerably more accurate reconstructions. The type
species of Shastasaurus was only ever known from
fragmentary remains
with additional fossils being attributed to it later. More complete
S. liangae material however revealed that Shastasaurus
is a short
snouted ichthyosaur, and as such much of the long snouted ichthyosaur
fossils associated with the type species were found to have actually
been placed in the wrong genus, and have since been moved to other
genera.
Today
Shastasaurus remains are usually associated with the
Northern coast
lines of the Pacific Ocean, but it needs to be remembered that back
in the Triassic today’s continents were still very close to one another
and surrounded by a single large ocean. While the Pacific Rim would
have formed much of this coastline, the gradually breaking up land
masses created new seas and it is thought by some palaeontologists that
Shastasaurus may have ventured into these new areas
as well.
Shastasaurus also has two dubious species
associated with the genus,
S. carinthiacus from Austria and S.
neubigi from Germany.
Central Europe at the time of the Triassic was more like an Island
chain surrounded by shallow seas, not only well away from other
Shastasaurus fossil sites, but different from the
deep water
environments where Shastasaurus is thought to have
hunted. If these
remains are not those of Shastasaurus then they may
yet represent a
similar type of ichthyosaur, whereas if they are the same genus then
they may reveal that Shastasaurus was either more
flexible in its
hunting, or that it was possibly migratory and heading to shallow
water birthing areas or moving on to new feeding grounds.
Shastasaurus
body features at a glance
Shastasaurus
was what is now termed a primitive ichthyosaur. Key to this is what
appears to be a lack of a dorsal fin, something which is clearly seen
in advanced ichthyosaurs but is lacking in basal forms. The lack of a
dorsal fin bone does not necessarily mean that a dorsal fin was not
present; it may have existed in the form of softer tissue such as
cartilage which rapidly decomposes in sea water. Still without more
direct evidence the wider assumption is that Shastasaurus
did not have
a dorsal fin.
The
tail fluke of Shastasaurus may not have been as
well formed as in later
ichthyosaurs either. The rear of the body that grew into the tail is
seen to have strongly arced downwards indicating that the lower tail
lobe was larger and more developed than the upper lobe. This
suggests that Shastasaurus was better at cruising
rather than active
pursuit of prey.
Early
art depicting ichthyosaurs often depicts them to have large rounded
bodies, and while modern reconstructions suggest that ichthyosaurs in
general are much slimmer than what they were classically depicted to
be, the ribs in proportion to total body length prove that this was
definitely not the case for Shastasaurus. The
distance between fore
and aft flippers is roughly seven meters while the ribs were barely two
meters tall. This gives Shastasaurus a profile
more like that of a
torpedo which would have helped to streamline Shastasaurus
as it swam
through the water. This effectively means that Shastasaurus
faced
reduced resistance and experienced less drag so that it did not have to
expend so much energy on swimming, something that is especially
important given the underdevelopment of the tail fluke.
As
mentioned above, the fossil specimens of S. liangae
from China
allowed for the first accurate reconstruction and analysis of
Shastasaurus, particularly the skull. The skull
itself is just a
little bit over eight per cent of the total length of adult
Shastasaurus, and displays extreme shortening
adaptations. While
the jaws are reduced, they seem to have reached their minimisation
limit as the nasal bone which is usually mid-way in the skull is on the
very tip of the jaws. The skull is also laterally compressed which
means that when viewed from the front the skull would look narrow like
it was pushed in from the sides.
Shastasaurus
hunting and feeding specialisation
The
addition of new fossil material to create the species S.
sikanniensis
has made Shastasaurus the largest currently known
marine reptile at
twenty-one metres long. This is much larger than others such as the
Jurassic predator Liopleurodon
which is often incorrectly labelled as
the largest and credited as being up to twenty-five meters long, even
though the largest fossil specimens for this genus are barely six and
half meters long. Shastasaurus is thought to have
grown to such a
large size so that it was better able to dive to deep depths, as well
as being larger than most of the marine predators of the time
effectively taking full grown adults off the menu list. Large size
meant that it could take in a greater amount of air and stay down
longer than a much smaller ichthyosaur.
Shastasaurus
has one of the most specialised mouths of all known ichthyosaurs in
that the jaws are both short and toothless. This toothless adaptation
is also seen in the similar Shonisaurus although
this ichthyosaur has
comparatively longer jaws. Why Shastasaurus
should have such short
jaws has puzzled many, but analysis of the skulls, particularly the
attachments for the jaw opening muscles has given rise to the theory
(proposed by M.P. Sander, X. Chen, L. Cheng & X.
Wang in 2011) that Shastasaurus was a suction
feeder. Like some
fish Shastasaurus opening its mouth wide so fast
that it would have
created a point where the space and existing water inside its mouth
would be reduced to such a low pressure it would create a vacuum inside
its mouth. Water immediately in front of the mouth, as well as
anything in it such as squid, would get sucked into this vacuum as
the pressure within was equalised. Further evidence for this feeding
behaviour comes from the large hyoid bones that would have allowed the
tongues to have also been quickly drawn low and back, further
increasing the vacuum effect. The very size of the jaws themselves
also support this as shorter jaws would have much less resistance when
moving through the water than longer jaws, further increasing the
speed that they can be opened which in turn increases the vacuum effect
inside the mouth.
If
this proposed method of feeding is correct then it’s a wonderful case
of nature reverse engineering itself. This is because squid use jets
of water pressure squirted out of their bodies to propel themselves
through the water, especially when escaping predators. Shastasaurus
on the other hand basing its feeding behaviour on using the hydro
dynamics of an aquatic vacuum would have denied the squid their
advantage, making prey capture for Shastasaurus
quite easy.
1 - Basilosaurus (whale), 2 - C. megalodon - lower average estimate (shark), 3 - Livyatan melvillei - lower estimate (whale), 4 - Pliosaurus funkei, a.k.a Predator X (pliosaur), 5 - Plesiosuchus (thalattosuchian), 6 - Thalattoarchon (ichthyosaur), 7 - Dunkleosteus (arthrodire placoderm), 8 - Shastasaurus (ichthyosaur), 9 - Tylosaurus (mosasaur), 10 - Leedsichthys - upper estimate (fish)), 11 - Brygmophyseter (whale), 12 - Rhizodus (lobe finned fish). |
Further reading
- Giant ichthyosaurs of the Triassic - a new species of Shonisaurus
from the Pardonet Formation (Norian: Late Triassic) of British
Columbia. - Journal of Vertebrate Paleontology 24 (3): 838–849. - E. L.
Nicholls & M. Manabe - 2004.
- On the occurrence of the ichthyosaur Shastasaurus
in the Guanling
Biota (Late Triassic), Guizhou, China. - Vertebrata PalAsiatica 47 (3):
178–193. - Shang Qing-Hua & Li Chun - 2009.
- Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic
Diversification of Suction Feeding Ichthyosaurs. - PLoS ONE 6 (5):
e19480. - P. M. Sender, X. Chen, L. Cheng & X. Wang - 2011.
- Absence of Suction Feeding Ichthyosaurs and Its Implications for
Triassic Mesopelagic Paleoecology. - PLoS ONE. - R. Motani, T. Tomita,
E. Maxwell, D. Jiang & P. Sander - 2013.