Terminonatator
Name:
Terminonatator
(Last swimmer).
Phonetic: Ter-min-o-nay-tay-tor.
Named By: Tamaki Sato - 2003.
Classification: Chordata, Reptilia,
Sauropterygia, Plesiosauria, Elasmosauridae.
Species: T. ponteixensis
(type).
Diet: Piscivore.
Size: Estimated at 7 meters long. Possibly
9 meters long if it had a neck of proportionate length to some other
elamosaurids. Skull is 26.8 centimetres long.
Known locations: Canada, Saskatchewan -
Bearpaw Formation.
Time period: Campanian of the Cretaceous.
Fossil representation: Skull and partial articulated
skeleton, possibly of a sub adult.
With
remains that were found in Campanian age rocks Terminonatator
was
possibly one of the last elasmosaurid plesiosaurs to swim in the
ocean. This is a far cry away from the Jurassic heyday of the
plesiosaurs when they were one of the most common types of marine
reptile in the ocean. In the Jurassic plesiosaurs had to contend with
being hunted by large pliosaurs such as Pliosaurus
and Kronosaurus,
but the Late Cretaceous waters of the Western Interior Sea Way which
submerged large portions of central North America were probably even
more dangerous. Here Terminonatator would have
had to contend with
mosasaurs like Tylosaurus
which was possibly larger and faster than the
previous pliosaurs, as well as large sharks such as Cretoxyrhina
which seem to have eaten anything in front of them, including
marine reptiles.
Evidence
for how dangerous these waters were comes from the right thigh bone of
Terminonatator which was broken but managed to
heal. It is uncertain
how this injury occurred but could quite possibly have been caused by
an impact with another marine reptile. On a related note the femur
(upper leg bone) of the rear flippers is longer than the equivalent
humerus (upper arm bone) of the front flippers, which is
different to the usual elasmosaurid configuration. It remains
uncertain what effect a more developed set of rear limbs would have had
on swimming, but it may have allowed for a more powerful stroke which
could have granted Terminonatator a boost in speed
and possibly
manoeuvrability, both making it easier for Terminonatator
to evade
predators.
The
skull of Terminonatator displays most of the
typical elasmosaurid
features of prey capture; typically long conical teeth that
intermeshed together when the jaws were closed. Seventeen to eighteen
teeth were in the each side of the dentary (lower jaw), while at
least thirteen teeth were in each side of the maxilla (main upper
jaw). The premaxilla (front of the upper jaw) had nine teeth,
but without further specimens it is impossible to say if this is
typical of the genus or just the individual. Together this means that
Terminonatator had around sixty-nine to
seventy-one teeth in its
mouth, although the actual figure may still have been a little higher.
The
overall appearance of the skull and lower jaw for Terminonatator
is
quite unique compared to other elasmosaurids. The snout was quite
short compared to other elasmosaurid plesiosaurs and the pineal
foramen, a hole in the skull where a light sensitive organ is
sometimes found in reptiles is completely closed. This suggests that
Terminonatator did not rely upon sensing the
direction of the light
above which raises the question of was it nocturnal? Without the
scleral rings of the eyes it would be impossible to say if were
nocturnal or not, but by being so Terminonatator
might have been able
to avoid the larger predators of the day, possibly explaining why it
was still managing to stay alive when the plesiosaurs and particularly
elasmosaurids had greatly declined in numbers.
The
lower jaw of Terminonatator also has an extended
coronoid process, a
bone that rises from the lower jaw into the skull and serves as an area
for muscle attachment. A more developed bone suggests a more
developed muscle system that may not necessarily mean a stronger bite
but a faster one. A shorter snout would also reduce resistance when
opening in the water which could have increased jaw opening times.
Fast jaw opening and closing could mean a specialisation in hunting
faster moving fish, with a speed boost from slightly more developed
rear limbs helping in the pursuit of prey. Unfortunately the rear
portions of the skull of the holotype are damaged and incomplete and as
such it is difficult to infer the full workings of the skull with
accuracy.
Despite
the incompleteness of the skull however, partial impression of the
brain have been found, giving palaeontologists a rare glimpse at
elasmosaurid biology. One area that stands out seems to be a
well-developed olfactory area. This connects to a wider theory about
how marine reptiles could sniff the water to pick up smells that were
drifting in the currents and how some marine reptiles are thought to
have had directional smell that helped them to locate things like prey.
The
actual size of Terminonatator is still something
that is not known as
an absolute as there are two main factors to consider. The type
specimen of Terminonatator has vertebrae that are
fused to the neural
arches, something which is taken to suggest that it had reached
adulthood. However other parts of the skeleton that should be fused
are not which suggests that this specimen of Terminonatator
may in fact
be a sub adult that had reached almost adult size, but still had bit
to go yet. The second thing to consider is the true length of the
neck. If Terminonatator had an exceptionally long
neck similar to
that seen in some like Mauisaurus
then it may have been even longer
with a total length of nine rather than the seven meters usually
attributed to it. However it’s worth remembering that this extra
length would just be extra neck and that Terminonatator
would still be
towards the smaller end of the elasmosaurid size scale.
Plesiosaurs
are thought to have swallowed gastroliths, and clear evidence for
this can be seen in the Terminonatator holotype
which has over
one-hundred and fifty pebbles (some six centimetres across) which
were found inside the stomach area of the body. While these pebbles
would have weighted Terminonatator down in the
water so that it was
neutrally buoyant, another main reason for these stones would have
been to aid digestion. You must realise that while the sharp teeth of
Terminonatator were excellent for catching prey like
fish, they could
not cut through flesh and so fish would have likely been swallowed
whole. The grinding action of the stones in the stomach would have
helped with the digestion of this fish, and further support for this
comes from ground up fish bones amongst the gastroliths in the belly of
a specimen of an elasmosaurid called Styxosaurus.
The
type species name of T. ponteixensis means
‘from Ponteix’, the
town near where the remains were recovered.
Further reading
- Terminonatator ponteixensis, a new elasmosaur (Reptilia:
Sauropterygia) from the Upper Cretaceous of Saskatchewan. - Journal of
Vertebrate Paleontology 23(1):89-103. - Tamaki Sato - 2003.
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