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
2.3 Stratigraphic Record of Ichthyopterygians
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Paleontology: Ancient Marine Reptiles
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从本节课中
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.
与讲师见面
Michael CaldwellProfessor
Department of Biological Sciences
Halle P. StreetSessional Instructor
Faculty of Science
In this lesson, and the two that follow, we will conclude by exploring
the distribution of marine reptiles through space and
time, by focusing on the localities where their fossils are found today.
Ichthyopterigians and their closet relatives first appear
during the Early Triassic, about 248 million years ago.
The globe looked vastly different at this time,
with oceans flooding continents and forming shallow seas.
Ichthyopterigians diversified in these shallow seas and
spread along coastal waterways quickly evolving morphological <b>disparity</b> or
variations in body plan, which was greatest during the late Triassic.
An extinction event at the end of the Triassic possibly linked to sea level
fluctuations, reduced the number of lineages that survived into the Jurassic.
The species that did survive, diversified again,
into multiple groups of strong oceanic cruisers,
before the entire group went extinct in the early part of the Late Cretaceous.
Now let's discuss the oldest know Ichthyosaurs.
These fossils come from the early Triassic, and
we know nothing about the evolutionary transition
from their terrestrial ancestors, since all of the oldest Ichthyophthirigian
fossils we've discovered are already fully adapted to an aquatic lifestyle.
And we've never found any intermediate forms or terrestrial, proto-icthyosaurs.
We do not know exactly where icthyopterygians originated because
early Triassic species are found all over the world, in Japan, in
Eastern China to British Columbia and the Svalbard Islands in the north Atlantic.
No transitional early ichthyopterygian has been found.
Yet, if we were to find such a fossil,
which are the following characteristic it might have?
Select all that you think might apply.
A, long bodies.
B, a blowhole.
C, carnivorous.
And or D, large orbits?
Ichthyopterygians are descended from terrestrial tetrapods that do not
have blowholes.
Even the most derived ichthyosaurs have nares near their eyes,
not on top of their head.
So B is incorrect.
Large eyes are an adaptation for low light conditions, so
it's unlikely that early ichthyosaurs would have the large orbits
seen in later species that lived in the open ocean, so D is incorrect.
However, early ichthyosaurs would likely have been long slender and
carnivorous.
This body shape and lifestyle are typical
of most early transitional forms of amniotes that returned to the water.
The description also matches the earliest icthyopterygians that we
HAVE found like <i>Grippia</i>.
So it's likely that their ancestors were similar.
So A and C are correct.
About 8 million years into the Triassic period,
during the middle Triassic, the first true ichthyosaurs evolved,
including a large bodied ichthyosaur <i><b>Thalattoarchon</b></i>.
<i>Thalattoarchon</i> was one of the first <b>apex predators</b> in the Mesozoic and
occupied the top rung of its food chain,
consuming prey such as small marine reptiles, sharks, and fish.
Since large apex predators can only evolve if there is large prey for them to eat,
the presence of <i>Thalattoarchon</i> in the fossil record shows that marine
ecosystems must have largely recovered from the end Permian extinction,
which had eliminated about 90% of all life on earth about 8 million years earlier.
The middle Triassic represents the peak in ichthyosaur species diversity and
worldwide distribution.
There were many different species with very disparate morphologies.
It's clear that these diverse animals could only have coexisted
by specializing on different foods, and
occupying different <b>niches</b> or roles in their community.
One of the most abundant groups at this time was the Mixosauridae,
particularly the one to two meter long <i>Mixoasauras</i>.
Mixosaurids have been found all across the northern and
southern hemispheres including Central Europe, the Svalbard Islands,
Western North America, and even Indonesia.
This suggests that in addition to being abundant,
it was one of the most widespread of all Ichthyosaur groups.
Although their tails were not as advanced as later ichthyosaurs,
Middle triassic species had evolved in more compact and fish like shape
indicating they were faster swimmers than their Early Triassic relatives
and this probably made their world wide distribution possible.
By the late Triassic, ichthyosaurs reached their peak size
with imposing giants like <i><b>Shonisaurus</b></i> and <i><b>Shastasaurus</b></i> dominating the oceana.
<i>Shonisaurus sikanniensis</i> was the largest
of all known marine reptiles at 21 meters in length.
Other species of <i>Shonisaurus</i> and
<i>Shastasaurus</i> were still huge, in the range of 10 to 15 meters.
Fossils of these giant beasts have been found in China, the United States,
and Canada.
Late Triassic ichthyosaurs are well represented
by discoveries from the Pardonet Formation in the Pink Mountains of British Columbia.
Dozens of ichthyosaurs have been found here,
sometimes, by helicopter pilots as they fly overhead.
Many of these late Triassic finds have been various species of Shastasaurus
including a new genus called <i>Metashastasaurus</i>, and
the enormous <i>Shonisaurus sikanniensis</i> found near the Sikanni Chief River.
The skull of this massive animal is 3 and a half meters long and
the largest piece of the fossil weighs over 4,000 kilos.
The fossil is so massive that it had to be airlifted using a cargo helicopter.
The end of the Triassic is marked by another mass extinction event,
although this one was far less severe than the end Permian extinction.
This mass extinction might have been triggered by the break up
of the super continent Pangea and
is one of the most significant points in the evolution of the Ichthyosaurus.
It represents an <b>evolutionary bottleneck</b> for the group.
What do you think we might mean by the term evolutionary bottleneck?
Choose all that apply.
As a hint, think about what a bottleneck refers to in traffic,
engineering, or software.
A, a place where a herd of animals get stuck.
B, a reduction of species number, diversity, or disparity.
C, a decrease in the average number of offspring or
D, a drastic change in the environmental conditions.
If you hear the term bottleneck in engineering, software, or
in your morning traffic report, it refers to a point of congestion where
the performance of the system is limited by a small number of components.
When we talk about evolutionary bottlenecks, we're referring to a point
in the evolution of a lineage where the number of individuals crashes,
causing a decrease in species diversity, disparity,
abundance, range, significance, or any combination of these.
So B is the correct answer here.
A decrease in the number of offspring or a drastic change in environmental
conditions could cause a decline in populations that could eventually
lead to an evolutionary bottleneck but they are not bottlenecks by themselves.
So C and D are incorrect.
A place where a herd of animals gets stuck could correspond to a geographical
bottleneck, kind of like when a long line of cars get stuck on the highway
when it's blocked by construction, but it's not an evolutionary bottleneck.
So A is also incorrect.
During the end Triassic extinction event, most Ichthyosaur taxa went extinct.
Only 2 lineages that evolved at the very end of the period
survive the extinction event.
<i><b>Leptonectes</b></i> and <i><b>Ichthyosaurus</i></b> lived on to become the first
in a new wave of Ichthyosaurs that evolve after this extinction.
These early members of the Parvipelvia quickly achieved greater number and
almost as much species diversity as all the Ichthyosaur families of the Triassic.
But they showed low morphological disparity, while the Triassic
was populated by Ichthyosaurs with a variety of body shapes,
including lizard-like, fish like, and whale-size forms,
only small and medium-sized
dolphin like parvipelvian icthyosaurs were present in the Jurassic period.
In the Triassic, ichthyaurs occupied three niches.
fish-eating piscivores, mollusk-crushing durophages, and
predators at the top of the food chain.
After the evolutionary bottleneck caused by the end-Triassic extinction,
ichthyosaurs only occupied one of these niches.
Which one do you think it was?
A, Fish eater, B, Durophage,
or C, Apex predator.
Ichthyosaurs of the Triassic period had a variety of diets,
while all the Jurassic ichthyosaurs were open-ocean fish and
squid eaters whose bodies were specialized to pursue their prey at high speeds.
Some of the roles previously occupied by ichthyopterygeans in the Triassic,
seem to have been taken over in the Jurassic by marine crocodilians,
sauropterygians, sharks, and bony fishes.
So after the end Triassic extinction in the Jurassic period,
the only niche that Ichthyosaurs occupied was the open-ocean fish eater.
Thalattosuchians, sharks, bony fish, and
sauropterygians diversified to fill the empty niches.
Answer A is correct.
An early example of the early Jurassic environment can be found at
Lyme Regis on the southwest coast of England, which is one of the most
famous in best preserved early Jurassic reptile sites in the world.
In these seaside cliffs, many remains of well-preserved,
complete specimens can be found.
And many of the earliest discoveries of prehistoric
reptile remains were made in this area.
Most notably, those discovered by a young woman named <b>Mary Anning</b>
who likely inspired the tongue twister, she sells sea shells by the seashore.
Mary and her brother were not the first to find fossils of Ichthyosaurs.
Earlier discoveries can be traced all the way back to the 1600s.
However, these earlier discoveries were thought to be fish or alligators.
Mary's discoveries revealed that ichthyosaurs were actually a unique
group of reptiles and caused scientists to consider the concept of extinction.
The fossil deposits that Mary Anning explored were part of a fossil-rich
layer of rocks called, the Lower Lias, which formed during the earliest Jurassic.
Exposed strata from that time, from both southern England, and
Belgium, contained fossils of parvipelvian ichthyosaurs
such as <i>Temnodontosaurus</i> and <i>Ichthyosaurus</i>.
Leptopterygids like <i>Excalibosaurus</i> and <i>Leptonectes</i>
with their elongate swordfish-like snouts are also found in these localities.
Another of the most famous Ichthyosaur localities in the world is that
Holzmaden in Southern Germany.
The Holzmaden oil shales exquisitely preserved
an amazing variety of Ichthyosaurs from the end of the early Jurassic.
Thousands of perfectly preserved fossils,
particularly of <i>Temnodontosaurus</i> and <i>Stenopterygius</i> have been found here,
including many of the most famous examples of pregnant females.
The sheer number of pregnant females found here suggests that this
area may have served as a breeding or birthing ground for
ichthyosaurs similar to some of the shallow near-shore areas
frequented by grey whales, blue whales, and humpback whales today.
Holzmaden is also famous for another reason.
Most of the time fossils consist of bones, shells, teeth, and other hard parts.
However, some localities provide us with some exceptional fossils.
The fossils of Holzmaden preserved soft tissue like skin,
tendons, and even organs in exquisite detail.
Why are fossils that preserve soft tissues so important to paleontologists?
Check all that you think apply.
A, they can tell about about body shape.
B, they can tell us about mating behavior.
C, they can tell us what color they were and/or D,
they can tell us about internal organs.
Our knowledge of the muscles, skin, eyes, and other soft tissues
of long extinct animals usually consists of educated guesses.
For example, we can use the size and
shape of the interior of the skull to tell us things about the brain.
The fossils of Holzmaden provide some of the best evidence
paleontologists have to determine overall body shapes of
ichthyosaurs because the soft tissues of the body wall, flukes, and
flippers are remarkably preserved here as a dark film of carbon around the skeleton.
These fossils also occasionally preserve parts of the internal organs
which can tell us a lot about the physiology and
internal anatomy of these animals.
With information like, this paleontologists can make more educated guesses
about behaviours involved with hunting and movement.
Unfortunately they cannot tell us anything about mating behavior
or the colour of the animals.
So the correct answers here are A and D.
Ichthyosaur species diversity starts to decline during the Middle Jurassic
and their fossils are not very abundant from this time.
The huge-eyed thunnosaurian genus, <i>Opthalmosaurus</i> first evolved and
became abundant during the Middle Jurassic.
As far as we know, this was one of the only taxa of ichthyosaur
swimming in the Middle Jurassic seas.
By the late Jurassic,
the only surviving lineage of Ichthyopterygians, was the Thunnosauria.
<i>Ophthalmosaurus</i> survived into the late Jurassic, and its fossils are common in
the Oxford Clay of England and in the Sundance formation of the Western United States.
However, both of these localities preserve no other Ichthyosaur genera.
The Solnhofen limestone of Germany, where <i>Archaeopteryx</i> comes from,
has produced an exceptional specimen of the ichthyosaur <i>Aegirosaurus</i>.
Impressions of its soft tissues were preserved all around the body
in showed the typical crescent-shaped tail and four flipper-like limbs.
From the preserved impressions, we know that the graceful forelimbs were
long and narrow, and the hind limbs were shorter and broader.
In this specimen, there were also impressions interpreted
as minute scales covering the whole body.
Impressions of tiny fibers, were also found in some specimens
that are similar to collagen structures found in marine mammals and
sharks, which help keep the skin tight,
smooth, and hydrodynamic.
Our understanding of Cretaceous ichthyosaurs
has greatly improved in the last few years.
A significant extinction event for
marine organisms occurred at the Jurassic Cretaceous boundary,
possibly because of a climate change.
Until very recently,
it was thought that most ichthyosaur lineages went extinct at the end of
the Jurassic, leaving only a single genus <i>Platypterygius</i>, in the Cretaceous.
The general consensus was that the Cretaceous ichthyosaurs
were not very diverse, and gradually tapered off
until the just disappeared completely around the middle Cretaceous.
New discoveries and new studies however,
are changing our ideas about the last ichthyosaurs.
Isolated and fragmentary ichthyosaur remains were known from Early Cretaceous
strata across Europe,
but many of these fossils could not be precisely identified.
Since 2000, new fossils have been found
and specimens in museum collections have been reevaluated.
They've revealed, that ichthyosaurs from this time,
were more diverse than Paleontologists understood just a decade ago.
New species have been found across Europe, Argentina, Iraq, and Russia.
All the new species belong to the Thunnosauria, and
most of them are closely related to either <i>Ophthalmosaurus</i> or <i>Platypterygius</i>.
Ophthalmosaurs and Platypterygians are also relatively abundant in Canada
from the time when the western interior sea bisected North America.
Two new species of opthalmosaurs
have also been found in Canadian middle Cretaceous strata.
One is <i><b>Athatbascasaurus bitumineus</b></i>,
which was found in an oil sand mine in Northern Alberta.
<i><b>Bitumineus</b></i>, which is a species name, was inspired because this specimen
still has tar, or bitumen, oozing out of its bones.
The other new ophthalmosaur is <i><b>Maiaspondylus lindoei</b></i>.
This specimen was found in the Northwest Territories.
<i>Maiaspondylus</i> means good mother vertebrae
and it refers to the pair of preserved embryos found inside the mother,
near her backbone.
<i>Lindoei</i>, the species name,
is in honour of the University of Alberta technician who discovered the fossils.
The recent discoveries of many early Cretaceous ichthyosaurs
are changing the way paleontologists think about their extinction.
Which Hypothesis do these discoveries support?
A.
All species of ichthyosaurs went extinct at the end of the Jurassic.
B.
Early Cretaceous ichthyosaurs were few stragglers
left over from a greater dynasty.
Or, C.
There were many species of early Cretaceous ichthyosaurs
that occupied a variety of ecosystems.
The unexpected species diversity of Cretaceous ichthyosaurs
supports the updated hypothesis that they weren't
just a few stragglers left over from a greater dynasty.
The last ichthyosaurs did not go extinct in the Jurassic.
They were still relatively diverse and
played important parts in their ecosystems.
So, the correct answer is C.
By the Mid-Cretaceous ichthyosaur diversity was dramatically reduced.
<i>Platypterygius</i> is the only representative of the Ichthyosaur lineage
that has been found in any strata younger than 100 millions years old.
These remaining Ichthyosaurs still had a fairly extensive range across
the Northern Hemisphere.
However, it appears that early paleontologists were correct
in thinking that ichthyosaurs died off
in the middle of the Cretaceous, long before the extinction event
that killed other marine reptiles and the dinosaurs.
So, why did the ichthyosaurs go extinct
in the middle of the Cretaceous period about 93 million years ago?
Why didn't they survive until the end-Cretaceous extinction
like the Plesiosaurs, Mosasaurs, and dinosaurs?
When we thought that Cretaceous ichthyosaurs were just a few stragglers,
it seemed like maybe other marine reptiles slowly replaced them.
It's an intriguing coincidence that mosasaurs originated around the same time
the ichthyosaurs finally went extinct,
so it's been proposed that the mosasaurs out competed the ichthyosaurs for
resources.
However, the earliest Mosasaurs were quite small, and would've lived near shore
so it is unlikely that they came into direct conflict with the ocean cruising
<i>Platypterygius</i>.
The answer to ichthyosaur extinction will require
Paleontologists to find additional fossils that will allow a better
understanding of mid-Cretaceous ocean ecosystem dynamics.
The ichthyosaurs are among the most specialized reptiles that have ever lived.
They were superbly adapted for life in the oceans with their long tooth-studded
mouth, large eyes, powerful tails, and two sets of steering flippers.
Ichthyopterygians came in all sizes from small to extra large but
most solved the aquatic problem by converging on a similar body
plan to modern tuna. They may even have been endothermic.
The limitations imposed by having to lay eggs on land
were bypassed by giving birth to live young
and it's possible that they raised these young in social groups.
They were rulers of the oceans for nearly 150 million years.
But what happened to them?
We can only speculate.
Perhaps, the answers to their demise lie in studying
the other marine reptiles that shared the oceans with them.
The plesiosaurs and mosasaurs, that we will learn about in the following lessons.
