Paleontology: Ancient Marine Reptiles is a four-lesson course teaching a comprehensive overview of the evolutionary changes that occur when air-breathing terrestrial animals return to water. This course examines the diversity, adaptations, convergence, and phylogenetic relationships of extinct marine reptiles. Students will explore three major groups of marine reptiles: ichthyosaurs, plesiosaurs, and mosasaurs. Watch a preview of the course here: https://uofa.ualberta.ca/courses/paleontology-marine-reptiles
4.1 Mosasauroid Diversity - Part 2
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來自 University of Alberta 的課程
Paleontology: Ancient Marine Reptiles
451 個評分
從本節課中
Mosasauroids
Welcome to the final lesson in Palaeontology: Ancient Marine Reptiles. The lesson you just finished gave you an in depth look at one of the most morphologically disparate groups of marine tetrapods to ever live. There were the turtle-shaped placodonts with crushing teeth; the extraordinarily long-neck elasmosaurs; and the pliosaurs with jaws massive enough to take on nearly any prey. Even though sauropterygians evolved many different body plans, they all shared certain adaptations for solving the aquatic problem such as live birth and appendicular locomotion.
In contrast, the final group we will investigate showed convergence on one morphotype. The mosasaurs, though diverse, were all fairly long reptiles, with large jaws, four flippers and a long, lobed tail. You may recognise one of them from Jurassic World, where an enormous genetically-engineered Mosasaurus leaped out of the water to eat a shark.
This last lesson will once again start by examining the diversity of the group, from their terrestrial origins to last days of the Cretaceous when they ruled as apex predators. We will investigate the specific adaptations of the group to the aquatic problem, and finish with an overview of the history of mosasaurs throughout time and space.
Please enjoy the last lesson of Palaeontology: Ancient Marine Reptiles. It is an appropriate way to finish this course since the mosasaurs were among the largest and most powerful marine predators to ever live, and were the last major marine reptile group to evolve during the Mesozoic.
與講師見面
Michael CaldwellProfessor
Department of Biological Sciences
Halle P. StreetSessional Instructor
Faculty of Science
The next group we will discuss is the halisaurinae.
<i><b>Halisaurus</b></i>, which means “sea lizard”,
is an enigmatic genus of mosasaur that first appeared in the Santonian.
The relationships between this genus and
the rest of the derived mosasaurs have been problematic.
Various paleontologists have grouped them on both sides of the mosasaur tree,
or even as separate branches all together.
We will follow a recent phylogenetic analysis which found that
<i>Halisaurus</i> was the sister group to the tethysaurs and yaguarasaurus,
but this is a topic that is still under research.
Even if there is little consensus about
where Halisaurines fit in the mosasaur lineage,
their morphology is well documented.
Halisaurines range from four to six meters in length.
And their skeletons do not exhibit many characteristics
indicative that they were highly adapted for swimming.
Yet they managed to have a large geographic and stratographic distribution,
surviving from the Santonian to the Maastrichtian and
have been found on almost every continent.
The <i><b>Plioplatecarpinae/b></i> were the longest lived group of mosasaurs
arising early in the evolution of this lineage and
surviving until the end-Cretaceous extinction event.
Plioplatecarpines exhibit a very generic mosasaur body plan
with few unique specializations.
Their skulls and bodies were usually not very elongated.
Their pelvic girdle was reduced and no longer articulated to the spinal colum,
and their paddle like limbs only exhibit a modest degree of hyperphalangy
with only up to five or six phalanges in their longest digits.
One of the earlier mosasaur in this group is <i>Platecarpus</i>
that lived during the Santonian.
Though medium sized by mosasaurus standards at six and a half meters,
<i>Platecarpus</i> were exceptionally fast and flexible.
Their teeth were fairly small suggesting that they specialized
in chasing down fish and cephalopods.
This is supported by finds of these types of prey
associated with <i>Platecarpus</i> fossils.
Members of the genus <i>Platecarpus</i> eventually gave rise
to the more specialized form, <i><b>Plioplatecarpus</b></i>.
<i>Plioplatecarpus</i> was probably an inhabitant of shallow inshore waters and
had the largest brain of any mosasaur.
It's needle sharp backwards curved teeth suggest
that it drew in prey by what is known as <b>ratchet feeding</b>.
This type of feeding is where the jaw walks down the prey first.
The top jaw, and then the bottom jaw until it has been completely swallowed.
The most derived mosasaurs on this side of the tree are the <i><b>Tylosaurinae</b></i>.
The first recognized Tylosaurine was <i>Tylosaurus proriger</i>,
which would have a skull very similar to this.
It was up to 15 meters long, had heavy jaws,
robust, sharp, conelike teeth; and paddle-like fore and hind flippers.
It preyed on fish, shellfish, diving birds,
and probably smaller mosasaurs and plesiosaurs.
<i>Tylosaurus</i> tranlates to knob lizard, and
<i>prorigor</i> means prow bearing.
Names that both refer to the unique elongated rostrum
that projected beyond the teeth in the upper jaw.
It's been hypothesized that this knob was similar in function to
the ram that ancient Greeks and
Romans mounted on the prows of their warships to ram enemy vessels.
Tylosaurines may have used their snout rams to stun prey,
to defend against predators, or to battle rivals of their own species.
Weighing in at an estimated eight tons
this would have been a devastating blow for any creature on the receiving end.
One of the largest of the Tylosaurinae mosasaurs was <i>Hainosaurus</i>.
It could reach a length of over 15 meters with a skull up to two meters long.
It had 40 vertebrae not including the tail, compared to the 35 of <i>Tylosaurus</i>.
One particular specimen from Belgium
demonstrates the degree that which these animals could open their jaws:
the carapace of a large turtle was found in its guts.
Before we move on to the other major mosasaur lineage,
take a minute to review the groups you've just learned about,
how they relate to each other, and
their progression of adaptations from terrestrial to aquatic.
We have introduced you to multiple genera from one side of the mosasaur lineage.
Even though they were closely related, a wide variety of forms,
functions, and behaviours evolved within this sub family.
For example compare <i>Platecarpus</i> and <i>Tylosaurus</i>.
<i>Tylosaurus</i> was larger, with a large tail and
a greater degree of hyperphalangy than <i>Platecarpus</i>.
Their teeth were also different.
<i>Platecarpus</i> had small, narrow, smooth teeth,
while <i>Tylosaurus</i> had large, sturdy teeth with cutting edges.
What do these differences in morphology say about
the differences in the way these two mosasaurs swam and hunted?
A, both <i>Platecarpus</i> and <i>Tylosaurus</i> ambushed small prey.
B, <i>Platecarpus</i> ambushed small prey, and
<i>Tylosaurus</i> pursued large prey.
C, <i>Tylosaurus</i> ambushed small prey, and <i>Platecarpus</i> pursued large prey.
D, Both <i>Tylosaurus</i> and <i>Platecarpus</i> pursued large prey.
<i>Tylosaurus</i> exhibited greater hyperphalangy then <i>Platecarpus</i>,
with nine or ten phalanges in their longest digits, resulting in longer,
stiffer flippers that were better adapted for changing direction at speed.
Their tails were longer and more powerful which allowed them to accelerate quickly.
The mouth of <i>Tylosaurus</i> was filled with large thick teeth that had cutting edges.
These teeth are from the cutting guild we talked about in lesson 2 and
are perfect for tearing chunks off of large prey.
<i>Platecarpus</i> had smaller more pointed teeth,
similar to those in the smash guild.
These teeth were too delicate for tearing off large prey but
they were well suited for catching smaller slippery prey like fish and squid.
Additionally, <i>Platecarpus</i> had smaller flippers containing
fewer phalanges and a less powerful tail.
<i>Platecarpus</i> was well adapted for
swimming long distances in search of schools of fish or squid
that they could ambush.
The more powerfully built <i>Tylosaurus</i> had the ideal adaptations for
chasing down large prey like turtles and
other marine reptiles that they could tear apart and consume.
Therefore the correct answer is B.
The other main branch of the mosasauroid tree, the sub family Mosasaurinae,
also is thought to have a primitively limbed agialosaur at the base.
<i><b>Dallasaurus</b></i> from the Turonian of Texas had feet typical of the terrestrial lizard
as well as a primitive pelvis indicating that both major mosasaur lineages
made the transition from land to sea independently of each other.
<i><b>Clidastes</b></i>, the most basal flipper bearing mosasaurine
is also the smallest at a maximum length of three and a half meters.
<i>Clidastes</i> appears in the fossil record at the end of the Coniacian, or
beginning of the Santonian,
and has a thin elongated body, and
the shortest relative tail length of the all the mosasaurs.
It has a short skull with smooth sharp tip teeth, and
a low triangular fin on the upper surface of its tail.
It exhibits little hyperphalangy having only as many as five phalanges.
This basal mosasaurine would have very much resembled
some of the earliest ichthyopterygians and sauropterygians.
And this is yet another example of convergence
as all these groups seem to get their start in the same way.
They first lived in the shallow seas, had long bodies and tails, and four feet
that slowly developed into flippers.
<i><b>Prognathodon</b></i> was a genus of medium-sized mosasaur, averaging around five meters,
though some individuals might have been twice that size.
This genus was named for having an unusual <b>premaxilla</b>,
which is a bone that is found in the front portion of the upper jaw.
<i>Prognathodon</i> means “projecting jaw teeth”, which refers to
the way the premaxillary teeth stick out from the front of the mouth.
The rest of the mouth was filled with large, ridged jaw teeth and
a nasty set of backwards-projecting pterygoid teeth.
Prognathadons are the bulldogs of the mosasaur lineage,
with relatively short blunt skulls.
However, the proportions of these skulls to the rest of their bodies, is unusual.
While their skulls are massive, and heavily constructed,
the rest of their skeletons are much more delicate.
It often looks like the skulls shouldn't belong to the bodies.
<i><b>Globidens</b></i>, meaning round or globular teeth,
is another mosasaur with a very robust skull.
<i>Globidens</i> are known for
having rounded teeth instead of the mosasaur typical conical spikes.
These mosasaurs were durophagous,
and probably ate only hard shelled invertebrates.
They must have had a lot of them since these animals could reach
over six meters in length.
A modern analog for
this lifestyle is the horn shark, which has plates in the back of its jaws
that are used specifically to crush the hard shells of oysters and clams.
Closely related to <i>Globidens</i> and having similarly rounded teeth, was <i><b>Carinodens</b></i>.
These mosasaurs generally considered the sister group to <i>Globidens</i>
were only three to four meters long and
probably searched the sea floor for small mollusks and sea urchins to eat.
Most phylogenetic analyses find <i>Globidens</i> and <i>Carinodens</i> to be closely related
to prognathodons due to the similarity of their quadrate morphology.
<i>Globidens</i> and <i>Carinodens</i> are the first marine reptiles
since the sauropterygian placodonts and the ichthyosaur <i>Grippia</i>
of the Triassic that were specialized for feeding on shelled animals.
They're also the only mosasaurs that do not have pterygoid teeth
in the roof of their mouths, which were likely lost
since their shelled prey was not slippery and did not struggle as it was eaten.
Other mosasaurs such as <i>Prognathodon</i> have also been found
with the occasional hard-shelled animal remains in their stomachs,
but in these forms, it was probably opportunistic and
part of a wider feeding strategy.
The namesake of the Mosasaurinae, the group that includes <i>Clidastes</i>,
<i>Prognathodon</i> and <i>Globidens</i> is the genus <i><b>Mosasaurus</b></i> which includes
the original Maastricht species we discussed at the beginning of this lesson.
This genus includes large mosasaurs,
usually ranging in length from 7 to 15 meters long.
This group has been found in Campanian and Maastrichtian strata all over the world.
<i>Mosasaurus</i> had strong teeth with a pair of cutting edges running
along their lengths like some carnivorous dinosaurs such as <i>Tyrannosaurus rex</i>.
The limb bones and phalanges of <i>Mosasaurus</i> were large and
blocky indicating that the paddle would have been quite rigid and
they had a moderate degree of hyperphalangy.
<i>Mosasaurus hoffmanni</i> was one of the latest and
largest of the mosasaurs reaching lengths of over 17 meters or more.
Its skull was pointed with the nostrils retracted from the end of the snout
to facilitate breathing.
The multi-faceted teeth were more complex than those of earlier mosasaurs,
and extremely effective at cutting through or dismembering prey.
Its enormous size, along with its powerful jaws and
teeth meant that almost any creature was a potential meal.
One of the most derived Mosasaurs,
known from the Maastrichtian of California is <i>Plotosaurus</i>.
The exact position of <i>Plotosaurus</i> in the mosasaur phylogeny has been
a source of conflict, but it appears to be closely related to <i>Mosasaurus</i>,
and this mosasaur is quite unusual.
The skull is elongated, and the teeth are narrow and densely packed in the jaw.
The pterygoid teeth in the roof of the mouth do not point down and
back into the throat, but out to the sides of the mouth instead.
The limbs are true flippers
with a relatively high aspect ratio for a mosasaur and
up to 16 phalanges in the longest digit.
The vertebrae in the tail are also unusually specialized for
propulsion and likely supported a significant fin.
Overall, <i>Plotosaurus</i> might have looked more like an ichthyosaur than
a typical mosasaur.
Who knows?
It may have even behaved like one,
swimming the oceans in search of schools of fish.
The terms <i>Mosasaurus</i>, Mosasaurinae, mosasaur, and mosasauroid
all share the same root, Mosasaur, which means lizard from the Mosa river.
Even though they sound the same, these words each refer to more and
less general groups of the lizards we have been discussing.
Think back to what you have already learned in this lesson and
see if you can match each term to its definition.
Let's take a minute to think about the four terms,
<i>Mosasaurus</i>, Mosasaurinae, mosasaur, and mosasauroid.
The most general of the four terms is mosasauroid.
This word refers to all the marine reptiles we discussed so
far in this lesson.
It includes all of the well adapted aquatic predators in addition to
all of their ancestors and relatives within the mosasauroids are the mosasurs.
Mosasaur is a general term used for
all the large predators with advanced aquatic adaptations.
So, the small, semi-aquatic aigialosaurs
would not be considered mosasaurs, even though they are mosasauroids.
Remember that there are two major lineages of mosasaurs,
one of which is the sub-family Mosasaurinae.
This group includes genera such as <i>Prognathadon</i>, <i>Clidastes</i>, and <i>Mosasaurus</i>.
<i>Mosasaurus</i> is the least inclusive of the terms since it contains only five species.
Any species of <i>Mosasaurus</i> is found in the sub-family Mosasaurinae,
which is a type of mosasaur which all belong in the mosasauroid lineage.
Since the discovery of the first mosasaur fossils in the 1700s it was always assumed
that the various groups of mosasauroids we have just introduced you to,
all descended from one ancestral lizard that returned to the ocean.
However, recent research calls this long held assumption into question.
As we have discussed in this lesson, at the base of each of the branches of derived
mosasaur clades there are one or two species whose limbs or
pelvic girdles retained some terrestrial capabilities.
Paleontologists are now working on the hypothesis
that the two major mosasaurs lineages, had different semi-aquatic ancestors and
that the two groups evolved independently, but parallel to each other.
If this is the case it would be an example of how convergent evolution
in closely related groups can lead to similar species
responding to similar challenges in similar ways.
Whether they are a monophyletic clade or sister groups,
they face the same environmental challenges and develop similar solutions.
Let us now go on to explore how these marine reptiles
solved the aquatic problem.
