Tylosaurus
Name:
Tylosaurus
(Knob lizard).
Phonetic: Tie-lo-sore-us.
Named By: Othniel Charles Marsh - 1872.
Classification: Chordata, Reptilia, Squamata,
Scleroglossa, Anguimorpha, Mosasauridae, Tylosaurinae.
Species: T. prioriger (type), T.
capensis, T. kansasensis, K. nepaeolicus, K. pembinensis,
K. saskatchewaensis.
Diet: Carnivore.
Size: Up to 15 meters long.
Known locations: Canada. USA, particularly the
central states.
Time period: Maastrichtian of the Cretaceous.
Fossil representation: Many Specimens as well as
numerous teeth.
The Discovery and Naming
of Tylosaurus
The
discovery of Tylosaurus harks back to the 'bone
wars', a fierce
rivalry between the palaeontologists Edward Drinker Cope and Othniel
Charles Marsh that took place towards the end of the nineteenth
century. As such the naming history of Tylosaurus
is muddled and can
cause confusion to those who are not aware of the full history of
Tylosaurus's naming.
The
first specimen of a skull and vertebrae discovered in Kansas in 1868
was given the name Macrosaurus prioger by Cope.
Approximately a year
later Cope re-assigned the material to Liodon.
Marsh described a more
complete individual in 1872, however his name Rhinosaurus
(Nose
lizard) had already been used, and its replacement Rhamposaurus
(Beak lizard) was also already in use by another animal. Both of
these proposed names referenced the strong snout and this is loosely
reflected in the final name of Tylosaurus (Knob
lizard). This
included all of his specimens as well as the material named by cope to
produce the type species T. prioger.
There
are a large number of fossils associated with Tylosaurus,
mostly from
the central United States which was once the sea floor of the
Western interior Seaway, but also in other areas of North America.
This has resulted in a great number of species being erected for the
genus, although as often proves the case with prehistoric animals
whose discovery dates back to the nineteenth century, many of these
have been found to be synonymous with existing older species as well as
other mosasaur genera. This is why modern listings of Tylosaurus
usually have much smaller species listings than older ones.
One
often mentioned species of Tylosaurus, T.
haumuriensis described
from material in New Zealand has since been declared to be a synonym of
the Taniwhasaurus
type species T. oweni.
Tylosaurus as
a Mosasaur
Tylosaurus
was one of the larger mosasaurs that lived towards the end of the
Cretaceous period, something which has secured its frequent inclusion
in popular media such as books and television documentaries. Rivals
to Tylosaurus in terms of upper size include Mosasaurus
and Hainosaurus.
Unlike
the earlier pliosaurs which relied upon their flippers for locomotion,
mosasaurs relied upon their tails to propel themselves though the
water, and Tylosaurus was certainly no
exception. The tail
consisted of more that eighty vertebrae, each with a tall neural
spine that projected upwards and a deep 'V' shaped chevron on the
underside. These vertebrae resulted in a tail that was laterally
compressed but deep so that the maximum surface area available could be
used for pushing against the water and propelling Tylosaurus
forward.
Although
not entirely fusiform like a fish, the overall body shape was adapted
to be as hydrodynamic as possible. When viewed from the side
Tylosaurus was a very big animal, but when viewed
from the front
Tylosaurus would have presented a very small
cross-section. This,
combined with the long sloping skull, resulted in a large reduction
in the amount of resistance that Tylosaurus
experienced as it swam
through the water. Also the flippers that were proportionately
smaller than those found in pliosaurs were used more for steering than
actual swimming, again resulting in a reduction of drag.
The
specialised and occasionally damaged snout of Tylosaurus
has been
suggested to being a weapon used by Tylosaurus
against others of its
kind during territorial combat. While only speculation, no
ecosystem is capable of supporting a high number of large predators in
the same area indefinitely, and it is at least possible that
Tylosaurus may have been territorial and fought for
the right to be
active in an area where food was plentiful.
For
a long time Tylosaurus was often reconstructed with
a dorsal crest
which was based upon fossils that appeared to show one running down the
length of the animal. Later study revealed this to have been
cartilage from the trachea, basically the windpipe that air passes
down from the nostrils to the lungs. In life the cartilage would have
kept the shape of the trachea preventing it from closing off when food
was being swallowed just like in other animals. While it is rare for
cartilage to be preserved, many sharks which have cartilaginous
skeletons have been preserved on many occasions proving that when
conditions are right it is possible for cartilage to be preserved,
although with varying degrees of success. This is why accurate
reconstructions omit the dorsal crest.
Study
of another mosasaur named Platecarpus
has revealed
that at least some
mosasaurs may have had a tail fluke similar to the caudal (tail)
fin of a shark. It is still a matter of debate if the same could be
suggested for Tylosaurus, but some artistic
renderings now include a
vertical tail fluke.
Tylosaurus
- The Apex
Predator of the Late Cretaceous Seas
Whereas
Tyrannosaurus
is dubbed 'the' land predator of
the late
Cretaceous, Tylosaurus would have equally been
'the' predator of
the late Cretaceous seas, specifically the Western Interior Seaway
that once submerged the central portion of the United States and
Canada. The large size of Tylosaurus meant that
it easily attained
apex predator status and as such absolutely nothing was off the menu.
Fossil evidence exists to support this as stomach contents for
Tylosaurus are well known and include fish,
sharks, flightless
birds like Hesperornis,
large marine reptiles such as plesiosaurs and
even other mosasaurs
There
is also evidence to suggest that Tylosaurus fed
upon dinosaurs, as
evidenced by a specimen dubbed the 'Talkeetna Mountains Hadrosaur'
from the Matanuska Formation in Alaska. The bones associated with
this hadrosaur have puncture marks that are very similar to the tooth
size and arrangement, in fact detailed study has shown that a feeding
Tylosaurus is the most likely cause of the marks.
However
this case of a Tylosaurus feeding upon a dinosaur
is most likely a case
of scavenging, with a Tylosaurus discovering the
body of a dinosaur
that had drowned and been swept out to sea. When animals die, gases
given off by the decomposition processes build up inside the body
causing it bloat and float up to the surface. Had a mosasaur fed from
the carcass before this, the gashes and puncture wounds from the
teeth would have caused the gases to escape causing the dinosaur to
sink straight to the bottom rather than being washed out with the tides
and currents. The carcass then seems to have sunk to the bottom
through the release of the gas, either through natural
decomposition, or more likely scavenging of other ocean predators
such as sharks like Squalicorax
or perhaps even the Tylosaurus itself.
Study
of the sea floor where the hadrosaur skeleton was found also shows that
the largest concentration of flesh was on the side which was in contact
with the ground. This shows that while the Tylosaurus
was feeding
from the carcass it was unable to reach the bottom side. If the
Tylosaurus had attacked a living hadrosaur you would
expect to see
signs of flesh removal and tooth marks all over. The greatest number
of tooth marks are on the outer extremities of the hadrosaur such as
the lower legs which a mosasaur would have had an easier time
manipulating rather than the bulk of the main body. Also the amount
of flesh on the bone of the lower legs would have been much less,
resulting in a higher incidence of bite marks as the teeth bit through
the thin muscle and straight into the bone.
Although
Tylosaurus had large teeth and powerful jaws,
these may not have been
the primary weapons of attack, at least not in the expected sense.
The most forward areas of the jaws have a severe reduction in teeth to
the point of being toothless. The snout is also reinforced making it
stronger than other marine reptiles. Additionally some Tylosaurus
snouts show signs of compression damage that seem to have been caused
by a violent impact. These all point to Tylosaurus
relying upon brute
force to ram prey at high speeds. Such an attack probably would not
kill prey outright, but could quite easily stun it so that it floated
hopelessly in the water. It would have been particularly effective
against other marine reptiles as they approached the surface to breathe
air. Stunning them by ramming would not only make them a sitting
target, but without a fresh supply of air they could have possibly
drowned if Tylosaurus did not immediately return
to finish its prey
off.
Despite
the devastating nature of such an impact, such as attack would
actually require patience and timing to execute properly, as while
the prey was smaller, it was almost certainly more manoeuvrable than
a fully grown Tylosaurus. This makes attacks in
the same depth plane
unlikely because despite its speed, prey would have likely seen the
attack coming and taken avoiding action. This is why it makes the
most sense for a marine predator to approach its target from below as
not only can it home in unseen, other sensory adaptations such as the
pressure and electro receptors of fish tend to work in sideways
directions, and any scent from the predator would also drift sideways
in the ocean currents, not up. This style of attack has also been
implied for the giant shark C.
megalodon as well as predatory whales
such as Livyatan,
and suggests that while the predators and prey
were different throughout the ages, the actual method
of attack was principally the same.
Smaller
prey items like flightless birds such as Hesperornis
and sea turtles
probably did not require such special hunting strategies and may be
cases of opportunistic feeding. The one drawback of being the biggest
predator in the area is that you have to take every opportunity you
can to feed, or you risk starvation. Cases of Tylosaurus
killing
sharks and other mosasaurs could be presented as a case of intraguild
predation. This is where a predator not only kills for food, but
also removes potential competition from the ecosystem in the process.
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