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
2.1 Ichthyopterygian Diversity
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Paleontology: Ancient Marine Reptiles
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Ichthyopterygians
Welcome back! We hope that you enjoyed the first lesson of the Ancient Marine Reptiles mini MOOC. In lesson one, we discussed a variety of modern and extinct groups of aquatic amniotes. Some of these groups would have been familiar to you, and some you had probably never heard of before. We also explored some of the many differences between living in water and living on land, and we gave examples of how some amniote groups have overcome these challenging differences. As you may have already seen, some of the solutions to the different aspects of the aquatic problem were solved in the same way by unrelated groups- a phenomenon called convergence.
The next three lessons will build on your understanding of the aquatic problem as we investigate it more detail in three extinct groups of marine reptiles: the ichthyopterygians, sauropterygians, and mosasauroids. Each lesson will follow the same general outline. First, we will introduce you to the group and some of its diversity. The second part of each lesson will focus on how that group adapted to solve the aquatic problem, and how this resulted in specializations for feeding, locomotion and reproduction. Finally we will give you an overview of the evolutionary history of the group through time and space, as well as presenting some important fossils and localities from Canada and around the world.
In lesson two, we will study the first of the three major groups of extinct marine amniotes: the ichthyopterygians. The ichthyopterygians were highly specialized animals. Most of the later members looked something like a dolphin or a tuna, a great example of convergent evolution. These reptiles were probably among the fastest aquatic creatures that ever lived. They were found all over the world, from the early Triassic to the mid-Cretaceous, a span of 150 million years.
By the end of this lesson, you should be familiar with the history and diversity of this group, and be able to appreciate some of their impressive specializations to solve the aquatic problem.
Conheça os instrutores
Michael CaldwellProfessor
Department of Biological Sciences
Halle P. StreetSessional Instructor
Faculty of Science
Welcome back to Marine Reptiles.
In lesson two, we'll examine the diversity of form and function for
Ichthyopterygians and their closest relatives.
The Ichthyopterygians are a distinctive and highly specialized group of
marine reptiles that lived from the early Triassic to the mid-cretaceous,
over a span of nearly 150 million years.
They all had the same basic body plan but were specialized
in a variety of ways ranging in length from 1 to 21 meters.
Some were small and snub-nosed, others had long pincer-like jaws and
still others had an overhanging upper jaw like a swordfish.
The derived members were streamlined like the fastest fishes.
They breathe through nostrils located between their eyes and
propel themselves through the water using their crescent shape tail flukes.
They are an incredible example of convergent evolution,
being similar in shape to sharks, tuna, and dolphins.
If a casual observer looked at a drawing of a Icthyosaur and
identified it as a dolphin,
this mistake could easily be forgiven.
You can see that the two look very similar.
However, a close examination will reveal some key morphological differences between them.
If you compare these pictures of an Ichthyosaur and
a dolphin, you might notice three major differences.
On the Ichthyosaur, the breathing holes, or
nares, are in front of the eyes instead of on top of the head.
Also, the tail fluke is vertical like we'd see on a fish.
It's not horizontal as we see on the dolphin or any other marine mammal.
Finally, the Icthyosaur had hind flippers that might of aided in steering.
Even though these animals appear to be very similar, these key difference
indicate that they evolved from different branches of the amniote tree.
Icythyopterygians are best known for
the dolphin-like members of this clade, the Ichthyosaurs.
Although Ichthyosaur means fish-lizard, they were aquatic amniotes, not fish.
Let's take a look at this generalized diagram of an Ichthyosaur, and
see if we can figure out which branch of the amniote tree these animals belong to.
Ichtyopterygians had only one temporal fenestra and
so you might thing that they are synapsid amniotes.
But ichtyopterygians and many other marine reptiles we'll discuss in this course
have a new kind of skull configuration called <b>euryapsid</b>.
Euryapsid skulls were once thought to represent a fourth branch
of amniote diversity.
But are now understood to be a modified diapsid skull.
Therefore, ichthyopterygians evolved from ancestors
with two openings in the skull behind each orbit.
These openings are the upper and lower temporal fenestra.
At some point, the bones behind the eyes got smaller and
smaller until eventually the lower temporal fenestra was lost,
leaving only the upper temporal fenestra.
The second temporal opening may no longer have been necessary
after this group started swallowing their prey whole
since temporal fenestra are associated with chewing muscles.
We can differentiate synapsid and
euryapsid skulls based on which bones surround the fenestra.
Even the earliest ichthyopterygians
show evidence of being well adapted to a fully aquatic lifestyle.
On land, the skeleton is essential for counteracting gravity,
and a key element of the system is the tight <b>articulation</b>, or
close connection, of the leg and hip bones to the spine.
Acquatic organisms supported by the water they live in don't need a pelvic
girdle capable of supporting their bodies.
Even primitive Ichthyopterygians have lost the boney connection
between their pelvic girdle and their spines.
Like whales today,
the pelvic girdle floated in the layers of muscle of the abdominal wall.
You can see this quite clearly in this skeleton.
See how far apart the girdles are from the spine?
The spinal column provides necessary support to an animal.
It needs to provide more support on land and less in the water.
But the spine also limits the flexibility and
type of motion that an animal can perform.
Think of the vertebral column is a string of beads.
Which of these beads strings would be most flexible?
A, B, or C.
Think about a string of beads, if the beads are shaped like long narrow
cylinders like many vertebrae are, the string can only flex
in-between each bead, decreasing the length between each bead
increases flexibility because there's less distance between each joint.
So the shortest vertebrae make the most flexible spinal column
and the long vertical cylindrical vertebrae would result
in the least flexible spine.
That makes A, the correct answer.
Ichthyopterygians have more vertebrae in their torsos and
tails than most terrestrial reptiles do.
They've got more than 40 vertebrae between their heads and pelvic girdle and
up to 80 vertebrae in their tails.
The increased number of vertebrae facilitated the axial undulations
necessary for anguilliform swimming.
Early Ichthyopterygians had vertebrae that were longer than they were wide,
kind of like this mosasaur vertebra.
Throughout their evolutionary history, the vertebrae developed into
the characteristic concave disk shape, like this.
Now in terrestrial vertebrates the vertebrae have complex <b>neural spines</b>,
and these are blade-like processes that come off of the vertebrae like
you can see on this bison vertebra.
And processes on the neural spines allow for
tight articulation to help support the body against gravity.
Ichthyopterygian neural spines are less complex and
poorly attached and this increases the flexibility of the spinal column.
As is typical of many marine reptiles that evolved during the Triassic period,
the exact evolutionary relationships of Ichthyopterygians are still unresolved.
They appear very suddenly in the fossil record, and
the oldest examples are fully adapted aquatic forms.
We have not yet discovered any transitional forms.
So we really don't know exactly how they evolved or from what.
There are three major hypotheses about which
Sauropsid group Ichthyopterygians belong to.
Ichthyosaurs might be basal Diapsids,
Lepidosauromorphs or even Archosauromorphs.
In this course we will use the most conservative hypothesis and
place ichthyopterygians as a separate branch of Diapsida.
Now let's look at some of the members of the group.
Since ichthyopterygians appeared suddenly in the fossil record and
no transitional forms have been found,
it's difficult to know what other reptiles they're most closely related to.
Two possible candidates are the <b>thalattosaurs</b> and the <b>hupehsuchians</b>.
Thalattosaurs are large Triassic marine reptiles with paddle-like feet and
long snouts.
Hupehsuchians are also from the Triassic.
They have fusiform bodies, similar to ichthyosaurs, but
had paddle-like feet instead of flippers.
Bony plates along their backs and
gastralia on their stomachs would have helped them achieve neutral buoyancy.
Now that we've looked at some possible relatives of the ichthyopterygians,
let's take a closer look at <b>ichthyoperygia</b> proper.
The most famous members of the ichthyopterygia are the <b>ichthyosaurs</b>.
And the most derived ichthyosaurs have fusiform bodies with a crescent
shaped vertical tail fluke.
Their vertebrae have a morphology that we call <b>amphicoelous</b>,
which means that both faces of the disk are concave.
In the skull, ichthyosaurs typically have a long snout and
the teeth are set in a groove rather than in sockets like most marine reptiles.
They also have a large orbit and a reduced cheek region.
Ichthyosaur teeth are distinctive because in most species,
the base of the tooth has a fluted, grooved external appearance.
This easily recognizable external texture reflects an internal pattern
of folding in the dentine of the tooth known as <b>plicidentine</b>.
Of the following pairs of fossils,
select the one in each set that comes from an ichthyosaur.
Remember that ichthyosaur vertebrae are amphicoelous,
meaning that they are concave on both sides like we see in vertebra B.
Ichthyosaur jaw bones have a groove for
teeth instead of sockets, like we see in jaw C.
And finally,
ichthyosaurs have the characteristic feature called plicidentine,
which can be identified by the folding in the base of the tooth, as in E.
Now that you know the key Ichthyopterygian characteristics,
let's go back in their evolutionary history, and trace how they developed.
Ichthyopterigians first appear in the fossil record during the early parts
of the Triassic period.
The earliest ichthyopterigian is a small one meter long animal
called <i><b>Parvinatator</b></i>, which remains poorly known to this day.
However, paleontologists do know that this taxon displayed the beginnings of
a trend that more derived ichthyosaurs would take to an extreme: <b>hyperphalangy</b>.
If you look at your own hand or foot, you will see that you have two or
three segments to each finger or toe, which are known as <b>digits</b>.
Now, each of these segments is an individual bone called a <b>phalanx</b>.
Many terrestrial tetrapods also have two or three phalanges in their digits.
However, it's common for aquatic tetrapods to develop many
extra phalanges as their terrestrial limb lengthens into a fin.
<i>Parvinatator</i> was already well adapted to its aquatic environment,
and had up to five phalanges in each digit.
Some of the more derived ichthyosaurs we'll learn about later
had as many as 30 phalanges per digit.
Other early Triassic ichthyopterigians include the fully
aquatic <i><b>Utatsusaurus</b></i> and <i><b>Grippia</b></i>.
The teeth of <i>Utatsusaurus</i> were still set in shallow sockets,
but the dental groves seen in more derived ichthyosaurs had also started to form.
<i>Utatsusaurus</i> also has a long,
narrow humerus, which is typical for terrestrial tetrapods.
And the more aquatic <i>Grippia</i>, the humerus is wider then it is long.
Unlike more derived ichthyosaurs, <i>Utatsusaurus</i> and <i>Grippia</i> lacked
dorsal fins, probably didn't have prominent tail flukes, and had longer,
more sinuous bodies because they had more vertebrae in their spinal column.
And these traits tend to be representative of anguilliform swimmers.
Related to <i>Grippia</i> is a genus from the early triassic of China, <i><b>Chaohusaurus</b></i>.
This small ichthyopterigian never grew longer than two meters.
<i>Chaohusaurus</i> also had multiple rows of crushing teeth in its upper jaw.
Its limbs had developed into primitive flippers but
the pattern of bones in the fingers more closely resembled terrestrial tetrapods.
Despite its primitive number of phalanges, <i>Chaohusaurus</i> is believed to
have been fully aquatic, as the limbs are well-developed flippers.
New fossils of <i>Chaohusaurus</i> include pregnant females,
including this specimen, which shows three embryos in the mother's abdomen.
This demonstrates that these primitive ichthyopterigian gave
live birth in the water.
Another possible basal ichthyopterigian is <i><b>Omphalosaurus</b></i>.
And this animal is definitively fully aquatic and
shares many features in common with basal ichthyopterygians like <i>Utatsusaurus</i> and
<i>Grippia</i>, but its teeth look very different from other ichthyopterygians.
Instead of rows of conical fish-eating teeth,
<i>Omphalosaurus</i> has a pavement of rounded,
bulbous teeth across most of the front of its lower jaw.
We would normally expect to see teeth like that in the <b>durophagous</b> animal,
one that uses powerful crushing bites to break down hard prey.
But the skull of <i>Omphalosaurus</i> was not suited to biting down hard.
Having crushing teeth far away form the jaw joint
means that the jaw isn't optimized for powerful bites.
It's unclear exactly what it would have eaten.
Even though the external shape of the teeth is different from most ichthyosaurs,
the presence of plicidentine inside the teeth suggest
that it is probably an ichthyopterygian.
Imagine a basal ichthyosaur that was 10 meters long with
an incredible 60 vertebrae in front of the pelvis.
Now imagine it also had a tail without a fluke, and no dorsal fin.
Finally, imagine that it had very small eyes and
a long snout full of robust, conical teeth.
Which of the three descriptions below do
you think would best describe this ichthyosaur?
Remember, we covered anguilliform, carangiform, and
thunniform swimming in lesson one.
A, An anguilliform swimmer that grazed in shallow water, B,
A carangiform swimmer that hunted in deep water, or
C, A thunniform swimmer that hunted in shallow waters?
The ichthyosaur we just described is called <i>Cymbospondylus</i>.
Its large, conical teeth meant that it was a hunter that probably preyed on fish and
cephalopods.
The lack of a tail fluke indicates that it was probably not a thunniform swimmer.
So the correct answer here is B,
it probably hunted in deep water where there was abundant fish and
cephalopods, and it was most likely a carangiform swimmer.
Now, let's go on to discuss this genus and others like it.
The most derived ichthyopterygians formed the clade <b>Ichthyosauria</b> and are united by
the presence of nares that face laterally rather than being on top of the skull.
Some of the earliest true ichthyosaurs were genera like
<i><b>Cymbospondylus</b></i> and <i><b>Mixosaurus</b></i>.
<i>Cymbospondylus</i> was the first large ichthyosaur, growing to
ten meters long, about the size of an adult male killer whale.
This ichthyosaur was very elongate. It had 60 vertebrae in front of its pelvis and
its very long tail would have assisted in propulsion.
<i>Cymbospondylus</i> also had a much longer snout then earlier ichthyopterygians.
It also has the smallest orbit and therefore eye of any known ichthyosaur.
<i>Mixosaurus</i> represents further steps towards solving the aquatic problem.
Instead of a lizard-like body with a paddle-like tail,
it had a more fusiform, streamlined body, a small dorsal fin for
stability, and a lower tail fluke for added propulsion.
The amphicoelous shape of the vertebrae gave greater flexibility
to the spine and probably resulted in an anguilliform swimming
style similar to more primitive ichthyopterygians.
The biggest ichthyosaur of them all has been found in British Columbia and
is named <i><b>Shonisaurus</b></i>.
It was at least 21 meters long, which is about the size of a finned whale.
The skull alone would have been over three meters long.
<i>Shonisaurus</i> and other large ichthyosaurs known as shastasaurids
may or may not have had dorsal fins.
But paleontologists think they likely did have
a low tail fluke to generate additional thrust.
the vertebrae in this group had developed into the disc-like morphology
characteristic of the derived ichthyosaurs.
And they also lacked teeth as adults, and so what they ate is a mystery.
Some paleontologists think they may have swallowed soft-bodied cephalopods whole.
Shastasaurids and their early relatives,
though not as derived as later ichthyosaurs, had already evolved
to solve many of the issues associated with the aquatic problem.
Based on this reconstruction of a shastasaurid, select which aspect
of the aquatic problem these ichthyopterygians had probably solved.
A, Drag, B, Stability,
C, Propulsion, or D,
All of the above?
Early ichthyosaurs were already extremely well adapted to their aquatic environment.
Their bodies were streamlined to counteract drag.
They had dorsal fins for stability in a 3D environment.
They had laterally compressed tails to generate propulsion, and their nares
had migrated to the top of their heads to facilitate breathing at the surface.
So the correct answer is D, All of the above.
Ichthyosaur is more derived than the shastasaurids form the clade
<i><b>Euichthyosauria</b></i>, and
are united by the loss of the bony attachment between teeth and jaws.
The majority of Euichthyosaurians are in the clade <i>Parvipelvia</i>.
Ichthyosaurs of this clade are united by straight as opposed to plate-like scapula,
a radius wider than it is long, and a reduced, narrow pubis.
A unique family within this clade is <b>Leptopterygiidae</b>.
They have converged on a similar morphology with swordfish,
having a very narrow elongate upper jaw,
sometimes greatly overhanging the shorter lower jaw.
One species was named <i><b>Excalibosaurus</b></i>, both for
its sword-like jaw and for the fact that it was found in England
near where King Arthur's legendary sword was supposed to have emerged.
In another species, <i><b>Eurhinosaurus</b></i>,
the upper jaw bears teeth even beyond the contact with the lower jaw.
We're not certain what these strange animals ate, but they probably had
similar feeding habits to swordfish that use their extended <b>rostrum</b> or
snout to slash at and injure prey making it easier to catch and swallow.
The genus <i><b>Temnodontosaurus</b></i> contains many species,
including some of the first ichthyosaur fossils ever discovered.
Unlike most ichthyosaurs within Parvipelvia,
species of <i>Temnodontosaurus</i> had fewer than five digits in their flippers.
The phalanges in the first digit were notched, possibly for
the attachment of the soft tissues that covered their limbs.
The final ichthyosaur group we will discuss is the Thunnosauria.
Paleontologists consider this clade to be the best adapted
to the marine environment.
Take a look at this reconstruction.
Why do paleontologists think this?
A, They eat only fast moving prey such as tuna, B, Like tuna,
they evolved gills so they could stay underwater indefinitely,
C, When they were first discovered, early paleontologists mistook them for
tuna, or D, Their body plan resembles a tuna,
which are some of the strongest swimmers alive today.
Since ichthyosaurs are reptiles, they have different skeletons than fish,
and they never evolve gills, so B and C are incorrect.
Thunnosaurs certainly ate fast moving prey, but
like any predator they would have eaten anything they could including slow prey.
So A is incorrect.
The correct answer is D.
Tuna are some of the strongest swimmers in today's oceans.
Thunnosaurs looked very similar to tuna and
therefore were probably the fastest, best-adapted of all the Ichthyosaurs.
As noted in the previous question,
the most highly derived clade are the <b>Thunnosauria</b>,
named for
their resemblance to the strongest open ocean cruisers of today: tuna.
The species in this clade are united by fore flippers
twice as long as their hind flippers.
These ichthyosaurs also have much more exaggerated
down turn bends to their tails.
The tail bend is formed by a few wedge shaped vertebrae, and the vertebrae behind
this kink would have supported the bottom half of a crescent shaped tail fluke.
The most basal thunnosaur is <i><b>Ichthyosaurus</b></i> found
near Lyme Regis on the south coast of England in the early 1800's.
These ichthyosaurs had flippers, well-adapted to an aquatic life.
They had advanced hyperphalangy, and also a condition called <b>hyperdactyly</b>.
Not only did they have at least 25 bones in an individual digit, but
they could have nine or more digits in each flipper.
These adaptations would have made the flippers larger, stiffer and solid
for stronger, faster, and more precise changes of direction.
The genus <i><b>Stenopterygius</b></i> and all the more derived Ichthyosaurs
are characterized by fusion of the Pubis and Ischium.
into a single pelvic element.
<i>Stenopterygius</i> is known from thousands of beautifully preserved fossils,
including numerous pregnant females.
<i><b>Ophthalmosaurus</b></i> is another thunnosaurid, well-known from around the world.
It had the largest
eyes relative to it's skull of any vertebrates that has every lived.
It's eyes were the same size as those of a blue whale,
an animal almost seven times larger.
The most derived genus of Ichthyosaur,
in one of only a few that survived into the Cretaceous is <i><b>Platyperygius</b></i>
and there's several species within this genus but
they all exhibit extreme hypertadactyly and hyperflangy.
With at least seven digits containing up to 30 short
blocky phalanges in their fore flippers.
In this first section I presented you with a sample of Ichthyopterygian diversity.
Before we go one to discuss some of the ways Ichthyosaurs
solved the aquatic problem, take a moment to review the species,
included in the interactive file of genetic tree.
This is the same interactive resource you used in lesson one.
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