[Note:
Portions in bold black and red are required knowledge based on our classes; portions in orange are complementary information that
will not be required for the final exam.]
[Portions in green were added Sunday evening, 12/15/02, for those
who saw the earlier version.]
Overview:
The Mesozoic Era is known as the Age of Dinosaurs or Age of
Reptiles. It is important to note that mammals were around through most of this
interval, and birds during part of it, but they both played a relatively minor
role among vertebrates. It wasn’t until dinosaurs, pterosaurs, and the various
marine reptiles disappeared after the end-Cretaceous extinction that mammals
and birds became dominant.
The Age of Reptiles [Mesozoic Era] was quite long (about 185
million years) compared to the length of the Age of Mammals [Cenozoic Era] (about
65 million years, and which still continues, of course). Technically, the Age
of Reptiles really started in the Paleozoic, by the Permian Period, but the
dominant reptile groups changed quite a lot period to period. Even the dominant
dinosaur groups varied through time.
Record
of imporant events that occur in Mesozoic Era
(Triassic, Jurassic, and Cretaceous Periods)
|
Information to know |
The basis for the information |
Explanatory comments |
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Triassic Period: |
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World recovering from giant mass extinction at Permian-Triassic
boundary, possibly caused by extraterrestrial impact |
there is new evidence from
just the past few months that the extinction was rapid, that there are
chemicals at the boundary believed to be associated with impacts, and that
erosion increased due to loss of plant community |
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Great supercontinent of Pangea exists, with relatively dry
climate |
the rock record, including
paleomagnatism, indicate that all the world’s continents were together |
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“Mammal-like reptiles” (synapsids) and “thecodonts” dominate land ecosystems |
vertebrate records from around
the world, including esp. South Africa |
thecodonts are the ancestors of dinosaurs (and eventually birds) and mammal-like reptiles are ancestors of mammals |
|
Mammals and
dinosaurs both evolve by the mid-Triassic. |
vertebrate fossil record |
mammals remained small during the next 150 million
years, and diversified into larger animals only after the dinosaurs were
extinct |
|
mass extinction occurs at end of Triassic |
|
this mass extinction occurred both in the ocean and on
land; possibly it helped dinosaurs take over from other reptile groups |
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Jurassic Period: |
|
|
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By the end of the Triassic dinosaurs have begun
dominating land ecosystems and Pangea has
begun splitting apart |
dinosaur bones become more numerous and others less
numerous |
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Pangea continues splitting
apart |
among other evidence, there
are fossil rift valleys along the east coast of the U.S. where the continent
ripped during splitting |
these rift valleys contain
most of the dinosaur remains in the Northeastern U.S., particularly including
footprints |
|
Large
dinosaurs, marine reptiles, and pterosaurs all flourishing |
the U.S., Britain, and
northern Europe (and many other parts of the world) contain excellent
Jurassic sequences, both terrestrial and marine |
these include the well-known Morrison Formation where Apatosaurus, Stegosaurus, and other well-known dinosaurs were unearthed |
|
theropod, sauropod, and stegosaur dinosaurs are among
major dinosaurs |
the relative abundance of dinosaurs is estimated from
the relative abundance of their bones, though of course there are some
preservational biases |
|
|
birds appear |
bird skeletons are known from
Germany, China, and elsewhere |
a well-known example of a primitive bird is Archaeopteryx,
which share bird features (feathers, wings bones, beak) and dinosaur features
(most of skeleton nearly identical to small theropods, teeth, long tail) |
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Cretaceous Period |
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The groups of large reptiles continue and grow in size
still further |
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Ceratopsian and hadrosaur dinosaurs become more common
than previously, perhaps filling the role that herds of hoofed mammalian
plant eaters (antelopes, for example) in the Cenozoic |
some well known assemblages have been found in North America, Africa, and Argentina, and elsewhere, including dinosaur nests that suggest herding and caring for young |
|
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Flowering plants become common and start to dominate
floras |
floral fossil remains begin to change |
|
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Time of extremely high sea level and warmth, but
cooling toward end of of period |
a shallow seaway spread from the south up through North
America, leaving behind sediments |
the shallow sea way was a place where sediment accumulated, so that we know a lot about shallow oceans from this point of time, including marine reptiles |
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Cretaceous-Tertiary boundary |
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|
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Large extraterrestrial impact, probable cause for
extinction |
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|
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Numerous other reasons have been suggested for the cause of the extinction |
some of the most problematic hypotheses explain the disappearance of only one group |
|
Major
types of Mesozoic reptiles
You
will be responsible for recognizing the major groups of reptiles and a few
other kinds of animals for which we looked at pictures and videos in class. For
example, you should be able to recognize sauropods, theropods, hadrosaurs,
ceratopsians, stegosaurs, pterosaurs, ichthyosaurs, plesiosaurs, and primitive
birds. Recommendation: To see images of these animals, open a new window in
your browser software and go to www.google.com,
then click on the tab that says “images.”
Then type in a name you wish to see pictures of, and it will give you
“thumbnail” pictures that are available on the Internet. These images may be
sufficient to remind you what, say, an ichthyosaur looks like.
The major groups of Mesozoic “reptiles”: Archosaurs (a
group of diapsids) and synapsids
A tiny bit of anatomy: There are
a number of groups of reptiles that are distinguished in part of the basis of
skull anatomy. One particular distinction involves the number of cavities in
the skull. Those with one hole in particular places behind the eye socket are
known as synapsids (the line including mammals), and those with two holes are
known as diapsids (the line including birds and dinosaurs). These two groups
seem to have diverged early in the history of reptiles (the Carboniferous), and
the Mesozoic and Cenozoic are the story of dominance by these two groups.
Synapsids
- Synapsids are a group including mammals and their ancestors.
You are a synapsid. Synapsids were very successful in the Permian-Triassic, and
included the sail-backed pelycosaurs.
- Mammal-like reptiles are now
known as “protomammals” are not considered true reptiles, but an independent
offshoot of early land-dwelling vertebrates.
- Many possess anatomy that contains characteristics of
both early vertebrates and mammals. This group was relatively abundant and
dominant in many ecosystems into the mid-Triassic, then declined substantially.
- Mammals themselves arise in the mid-Triassic and stay small until the early Cenozoic Era. We know about these animals primarily through teeth, which preserve relatively well compared to weak bones.
Dinosaurs
Dinosaurs: Dinosaurs are a particular group of
reptiles with a common ancestor.
This is evident
from unique aspects of the anatomy of all species of dinosaurs. There are many
such traits, but they include (1), upright stance, with legs held directly
beneath the body (unlike, for example, sprawling lizards) and (2), a particular
kind of ankle (“metatarsal”) bones. They existed from the Midde Triassic to the
end of the Cretaceous, and occupied many ecological niches on land.
Dinosaurs are NOT simply any large prehistoric vertebrates.
Animals that are not dinosaurs include pterosaurs, any marine
reptiles, mastodons and mammoths, and pelycosaurs such as Dimetrodon.
Why is this distinction important? Imagine trying to make sense of human
history if we mistakenly ascribed people to incorrect families or cultures,
maybe even in the wrong intervals of time.
Dinosaurs are divided into two large groups based on the shape of the hip (and other anatomical traits), known as the saurischians and ornithischians; the significance of noting the shape of the hip is simply that it indicates common ancestry.
First dinosaurs:
The very first dinosaurs, ancestors of dinosaurs to follow, do not fit
cleanly into these two dinosaur groups, as they have anatomical characteristics
intermediate between the two groups. The first dinosaurs were small bipedal
(walking on two legs) meat-eaters, similar to the theropods.
Saurischians: “Lizard-hipped” dinosaurs. These include the seemingly unlikely pairing of meat-eating theropods and long-necked behemoths. Early sauropods (prosauropods) do not look much like their descendants, as they are bipedal and relatively small.
Theropods: Meat eating bipedal
dinosaurs. They include everything from Coelophysis, the little late
Triassic dinosaur, to Tyrannosaurus, from the latest Cretaceous. There
were many different groups of theropod dinosaurs, including for example the
“raptors” of Jurassic Park fame.
Birds are, believe it or not, now classified by many as a kind of theropod dinosaur. This is because the building consensus suggests that birds are descendents of these dinosaurs. (A more traditional view is that, even if this evolutionary relationship is correct, it makes more sense to elevate birds to a new group on par with reptiles, rather than classify them a subset of dinosaurs.) The evidence for this relationship involves dozens of aspects of anatomy – the skeletons of primitive birds are almost exactly like theropod dinosaurs – together with new discoveries of feather-like down on small theropod dinosaurs. Feathers thus apparently developed for thermoregulation (to help keep warm-blooded dinosaurs warm) and were only later developed for other purposes, such as flight.
Sauropods:
The sauropods are the “long-necked” giants, including familiar dinosaurs
such as Apatosaurus (formerly Brontosaurus), Brachiosaurus,
Diplodocus, and many others. These dinosaurs had small heads and teeth
adapted for stripping branches of leaves rather than chewing leaves; digestion
apparently occurred in a “gizzard” filled with stones (called gastroliths).
Ornithischians:
The “bird-hipped” dinosaurs are entirely plant-eaters, including all the
armored dinosaurs and the bipedal plants eaters, the “ornithopods.” Don’t be confused
– although described as “bird-hipped” in the late 1800’s. their hips are only
superficially bird-like; it is the lizard-hipped dinosaurs that gave
rise to birds.
Stegosaurs: plates running down the back.
Ankylosaurs: armor over the back.
Ceratopsians: frills over the neck and horns
Ornithopods: bipedal plant eaters
Hadrosaurs: bipedal plant eaters with duck-shaped faces
Other
things to know about dinosaurs:
Dinosaurs were diverse.
We tend to see a limited number of types of dinosaurs that are especially
well-known and familiar to the public showing up in popular books, but we have discovered over 600 species and we
continue to find new ones every year. The most important limiting factor to
continuing discovery is simply the human and financial resources available for
finding and studying their remains.
Dinosaurs were very successful.
There is a media tradition of using dinosaurs as a symbol of something outdated
and perhaps somehow slow, stupid, and poorly adapted. In fact, dinosaurs dominated
land ecosystems around the world for 150 million years – they are by far the
most dominating group of land animals of all time.
Dinosaur species did not all live
simultaneously. Firstly, it is evident that there were trends in
abundance of different groups of dinosaurs. Dinosaurs of late Triassic,
Jurassic, and Cretaceous were very different. Furthermore, species evolve and
go extinct on a regular basis, geologically speaking. There is a misconception
that perhaps all dinosaur species (or even all dinosaur individuals!) that had
ever existed until the Cretaceous-Tertiary extinction; this is not at all true.
Flying
reptiles
The primary
group of flying reptiles was the “pterosaurs” (pronounced “TER-o-saurs). These
reptiles share some anatomical traits with dinosaurs that put them both in the
group archosaurs. Pterosaurs are not birds, however, just as bats are not
birds. Some pterosaurs had wing spans far exceeding any bird every known, and
most seem to have been lived on fish. Recent discoveries have revealed that at
least some pterosaurs had a thin veneer hair- or down-like material over parts
of their bodies. You needn’t memorize the
names, but pterosaurs that we saw in the videos included Rhamphorhynchus,
the crested pterosaur Tapejara, and the gigantic pterosaur Ornithocheirus.
Marine
reptiles
Marine reptiles belonged to several groups of unrelated animals. All the groups evolved from reptiles that had dwelled on land, and in that sense “returned” to the ocean.
Ichthyosaurs seem to have been the most fully developed to marine life, and evidence suggests that they even gave birth to live young in theier aquatic environment. Their bodies have qualities that give them the appearance of dolphins (paddles and snouts) and sharks (vertical tail). They presumably moved their tail like a shark to swim. They seem to have been especially successful in the Triassic, but to have declined in importance through the rest of the Age of Dinosaurs. The particular genus we saw in the video was Opthalmosaurus.
Plesiosaurs were
perhaps the dominant marine reptiles. Some were the largest predators in Earth
history, in sea or on land. Others were (somewhat) smaller, with remarkable
snake-like necks, giving many the idea that “Nessie” the “Lochness Monster” of
Scotland must be a surviving plesiosaur. Their unique bodies were stout with
short tails, but large flippers from which they got their propulsion, unlike
the vast majority of marine vertebrates that are powered by their tails. It is
thought pleiosaurs laid eggs on land like a returning marine turtle, but moving
awkwardly like a seal. The gigantic predatory
plesiosaur in the video was Liopleurodon and the smaller long-necked
plesiosaur Cryptoclidus.
Mosasaurs are a group of marine lizards that were more common toward the end of the Mesozoic. [We didn’t see any of these in the videos.] Mosasaurs had a tail that tapered near the end, and was presumably the main form of locomotion.
Mammals
Mammals
first arise about the same time as dinosaurs, about 220 million years ago in
the middle Triassic. Mammals remain small and rodent-like through the Mesozoic;
soon after dinosaurs disappear, mammals diversify to fill the niches of most
land and marine reptiles. It seems likely that mammals were not able to
diversify during the Mesozoic when dinosaurs already filled the niches of large
land vertebrates. Note that the greatest diversity of mammals is still today in
rodent-sized animals, a niche that dinosaurs never filled.
Mesozoic-Cenozoic invertebrate marine life
The history of the diversity of marine animals has been broadly divided
into three categories, based in part on statistically analysis of
the plot of marine animal families through time.
1. organisms that dominate the Cambrian
Period, including trilobites, rise in diversity through the Cambrian,
then gradually decline through the rest of the Paleozoic.
2. animals that dominate the rest of the Paleozoic Era, including many of the fossils common in New York State, for example:
* brachiopods
* corals
* sea lilies (crinoids)
* [trilobites were still common in certain times and places, but not a dominant part of the fauna like in Cambrian]
3. animals that dominate the
Mesozoic and Cenozoic Era. Some of these are animals you are more
familiar with, such as:
* clams and snails (“rudist” clams were an important
reef-builder in the Mesozoic)
* ammonites (from the Mesozoic Era)
* crabs and lobsters
* (“bony”) fish and sharks
This is interesting, in
part because the kind of marine faunas that look not unlike today’s began well
back into the Mesozoic, whereas the terrestrial fauna looked very little like
today’s. It is also interesting that diversity seems to be higher today that it
ever has been; paleontologists wonder if this has something to do with the kind
of dominant marine organisms, such as the increased predation that occurs in
the Mesozoic.
Preservation
What
we know about fossil animals – in this case, dinosaurs and other vertebrates
alive during the Mesozoic Era – depends on the circumstances of preservation. While
you may have no personal experience considering how bones become fossilized and
then found and excavated, you can imagine from intuition that it is a bit like
tracing the history of civilization: some kinds of information is better known
than others, depending on circumstances such as whether someone wrote down the
information, whether records were preserved or destroyed, and so on. Likewise,
some kinds of organisms more naturally find themselves at the right time and
place to be preserved. You should be able to apply the following basic
principles to any example:
1)
animals with hard parts (skeletons) naturally preserve as fossils better than
those without, since flesh usually rots quickly after death
2)
robust hard parts like big bones fair better than small, delicate bones
3)
remains of life must be buried in sediment to become a fossil; otherwise,
water, wind, and other forces will eventually weather and destroy them
4)
sediment accumulates primarily in aquatic settings, thus most fossils form in
aquatic settings, including oceans, lakes, and big rivers
5)
some animals are more likely to be in or around an aquatic setting, so
naturally their remains are more likely to be found as fossils
6)
some animals are ecologically more abundant than others; for example, though we
hear a lot about lions, tigers, and bears, and about meat eaters like T rex, in
the news, in fact carnivores are usually outnumbered 10:1 by herbivores. This
is simply part of the food pyramid – each succeeding level of the pyramid is
less numerous than the one below it.
7)
it is not possible to see all ancient life in any one place, just as you can’t
see all living animals by going to any one place (other than a zoo); thus
paleontologists study sediments from a particular age in many different
localities to try to get a full picture.
While
the Hyde Park mastodon, which is about 12,000 years ago, was preserved in soupy
mud that had been neither compacted nor cemented together, dinosaurs and other
animals of that age are normally found in rock. This makes excavation and
transport substantially more difficult. In addition, Mesozoic-aged bones have
generally been filled in and sometimes replaced by minerals other than the
original calcium phosphate that bones are made of.
Instances of exceptional preservation
While
usually flesh rots after death and fragile bones and shells break and disappear
during weathering, once in awhile special ecological circumstances form to
allow exceptional preservation. Here are some examples; not all are Mesozoic.
flesh
is preserved in rocks:
Flesh
may be preserved in ice, but this only occurs for the most recent glacial
advances, which are within the last million years; this includes the Jarkov
mammoth (20,000 years ago) and the ice man (a 5000 year-old person in the Alps
of Austria); the Mesozoic Era is far too old to have fossils in ice
soft parts are preserved in unusually biogeochemical conditions.
The Cretaceous-Tertiary extinction
This
extinction is best known to most people as the time the dinosaurs died, but in
fact it is also a time when many other organisms went extinct. They include:
most or all vertebrates >100 pounds, including all nonavian dinosaurs,
pterosaurs, all marine reptiles all rudist clams, all ammonites, most
planktonic foraminifera and the kind of unicellular algae that make chalk, many
corals (etc.)
There are many explanations for the extinction, most of
which suffer from one of two problems:
1) the explanation considers only one of the group of
organisms that went extinct, without taking into account all of the data
2) the proposal lacks available evidence to show that
it’s true or false, meaning that no one can do research on it (it could be
right, but we may never know)
Some of the ideas have included:
a) mammals ate all the dinosaur eggs
b) diseases spread by migrating dinosaurs killed off the
dinosaurs
c) cooling was too hard on the cold-blooded dinosaurs
d) volcanoes erupted, causing the spread of clouds of ash
that had an affect not unlike an extraterrestrial impact
e) an extraterrestrial impact
The extraterrestrial impact, as strange as it sounds, is
favored. This is because many people did research on evidence left behind by
impacts, and found that there is evidence of the impact all over the world.
Also, a crator has been found near Mexico that is the right age. The impact
would have thrown dust into the air, which would have traveled via the jet
stream around the world, blocking out light and killing the base of the food
chain.