VIZUALIZING EARTH HISTORY
By Loren E. Babcock
Chapter 11
Mesozoic World
Timeline of Mesozoic events
Triassic Period
Major marine life forms of the Triassic Period.
Global sea level at the beginning of the Triassic Period
was relatively low, and close to its modern position. It
rose to a level perhaps 100 m above its present position,
then fell again at the end of the period.
Orogenies in Eurasia completed the assembly of Pangea.
Large portions of Pangea, ones far from ocean waters,
formed arid deserts. The high peaks of the Appalachian
Mountains, which arose in the late Paleozoic, were subdued
through erosion in the Triassic Period.
Triassic Period
Triassic Marine Life
Extinction at the end of the Permian Period wiped out great
numbers of marine organisms (trilobites, fusulinid foraminifera,
rugose corals, and lacy bryozoans), leaving a new world to be
exploited by those surviving the crisis.
Other groups suffered substantial declines without
being completely annihilated.
Recovery from the Permian extinction event was a protracted
process for most marine animals other than ammonoid
cephalopods and conodonts.
Triassic Period
Triassic Marine Life
Reef-forming corals, sea urchins expanded during the Triassic
and Jurassic periods.
Reptiles first invaded the sea during the Triassic Period.
Plesiosaur - A Mesozoic marine reptile with a euryapsid skull
type, a broad body, and large paddlelike limbs.
Ichthyosaur - A Mesozoic marine reptile with a dolphin-like body.
At the end of the Triassic Period, mass extinction occurred in
marine ecosystems. The timing of extinction coincides with floral
evidence for a climatic shift to arid conditions in terrestrial
environments of Gondwana, and with a large drop in
sea level.
Triassic Period
Terrestrial life of the Triassic Period
Land plants were little affected by extinction at the end of the
Paleozoic Era. As the world entered the Mesozoic Era,
gymnosperms such as ferns, conifers, cycads, and
ginkgos dominated the landscape.
Three land vertebrates appeared during the Early Triassic,
an amphibian group (frogs), and two reptilian groups
(turtles and primitive archosaurs).
The group known as archosaurs includes crocodiles,
phytosaurs, pterosaurs (flying reptiles), and dinosaurs.
All these groups appeared in the Late Triassic.
Triassic Period
Terrestrial life of the Triassic Period
The earliest dinosaurs were small bipedal animals. It was in the
Jurassic Period that dinosaurs diversified and reached
large proportions.
Primitive synapsids (or “mammal-like reptiles) persisted from
the late Paleozoic. Fossils of one Triassic therapsid,
Lystrosaurus, were helpful in matching Pangea.
Mammals appeared in the Late Triassic. The early
mammals remained small and inconspicuous through the
entire Mesozoic Era.
Triassic Period
Terrestrial life of the Triassic Period
Jurassic Period
The splitting and rifting of Pangea.
The Jurassic Period was a time of great change tectonically
and biotically. The period opened with global sea level at one
of its lowest points in geologic history.
Pangea was beginning to rift apart, and both marine and
terrestrial ecosystems were being reshaped. Separation
of Pangea led not only to the development of the modern
continents, but also to the evolution of separate biotas
associated with them.
Jurassic Period
Jurassic Marine Life
In the Jurassic Period, marine predator-prey systems reached
the top of the Mesozoic Marine Revolution.
Swimming predatory mollusks (ammonoids) and relatives
of the squids called the belemnoids appeared. These
cephalopods enjoyed a varied diet including crustacean
arthropods, fishes, and mollusks. Ammonoids evolved rapidly
in the Jurassic and Cretaceous periods.
Fishes of the Mesozoic Era were mostly carnivorous. Sharks
and rays, holdovers from the Paleozoic Era, increased in
number in the Jurassic and Cretaceous periods.
Jurassic Period
Jurassic Marine Life
Plesiosaurs had developed into a major threat, and they
apparently had a large vertical range in the water column.
Ichthyosaurs were among the top predators of Mesozoic seas.
Pterosaurs, or flying reptiles, probably relied on fishes,
crustaceans, and cephalopods swimming close to the water’s
surface for a large part of their nutrition.
Jurassic Period
Jurassic Marine Life
Other benthic or nektobenthic carnivores included some
snails and crustaceans.
Brachiopods and stalked echinoderms fell into great decline,
and they never again achieved more than a minor role
in marine ecosystems.
Jurassic Period
Jurassic Marine Life
A group of floating algae, called the dinoflagellates, which
produce durable organic cysts, emerged in the Jurassic
Period as important contributors to the phytoplankton
biomass.
Dinoflagellates have an evolutionary history
extending back to the Proterozoic Era (where some
are referred to under the collective term “acritarchs”),
but they achieved a more impressive diversity
in the Jurassic Period.
Jurassic Period
Terrestrial Jurassic Life
Gymnosperms were the most common land plants of the
Jurassic Period. Cycads, conifers, ferns, and ginkgos
dotted the landscape and provided much of the food
for herbivorous dinosaurs and mammals.
Dinosaurs became the most conspicuous land animals of the
Jurassic Period, and they remained so through the
Cretaceous Period.
Jurassic Period
Dinosaurs Inherit the Land
Dinosaurs are characterized by an upright posture, carrying
their legs below the body, and by having a skull with two
openings behind the eye. There are two major groups, each
distinguished principally on the basis of hip bone structure.
Saurischian dinosaur - An archosaur characterized, at least
primitively, by a lizard-like hip.
Ornithischian dinosaur - A Mesozoic archosaur characterized
by a bird-like hip; most species were herbivores.
Jurassic Period
Dinosaurs Inherit the Land
Jurassic Period
Dinosaurs Inherit the Land
Saurischian dinosaurs are of two basic types: theropods and
sauropodomorphs.
Theropod - A clade of saurischian dinosaur having bipedal gait
and, at least primitively, teeth adapted for carnivory.
The clade includes birds.
Sauropodomorphs - were herbivores, and basically
quadrupedal. The more derived sauropods were the
largest animals to ever live on land.
Jurassic Period
Dinosaurs Inherit the Land
Theropods often had long strides, meaning they could move
quickly. This is consistent with the interpretation that they were
endothermic, or “warm-blooded,” rather than ectothermic, or
“cold-blooded,” like present-day lizards and crocodiles.
Jurassic Period
Dinosaurs Inherit the Land
Ornithischian dinosaurs, which range from the latest Triassic
to the end of the Cretaceous Period, were mostly herbivores.
Genera include Protoceratops, Triceratops,
and others, all of which had giant bony shields covering
the head and neck.
Jurassic Period
Dinosaurs Inherit the Land
Another group of ornithischian dinosaurs is the bipedal
ornithopods, which include Iguanodon, the duckbills
(Hadrosaurs), Parasaurolophus, Corythosaurus, and
Maiasaura, among others.
Nests and eggs laid by dinosaurs are known from many
areas of the world. One of the most revealing discoveries
is from Cretaceous strata of Montana, where nesting
grounds of the hadrosaur Maisaura (literally “good mother
lizard”) were rapidly buried under shifting desert sands.
Jurassic Period
Origin of Birds
The oldest known bird fossils, Archaeopteryx (literally,
“ancient wing”), are from the Jurassic of Germany.
Cretaceous Period
The origin of chalk and black shale in Cretaceous strata.
During the Cretaceous Period, Pangea continued
fracturing into its modern drifting continents.
High sea level, greenhouse climatic conditions, driven by
undersea volcanoes spewing their content, warmed the
oceans beyond the tropics, and planktonic microorganisms
bloomed in the seas until the end of the period.
The Earth experienced a long interval of normal polarity
(about 40 million years), during the mid-Cretaceous.
In the 20 million years prior, quick polarity reversals (less
than 1 million year intervals), were the rule, just as they had
been through much of the Mesozoic Era.
Cretaceous Period
Cretaceous Tectonics, Sea Level, and Ocean Circulation
Supercontinent breakup and rifting accelerated in the
Cretaceous Period.
Oxygen minimum zone - Interval in a water body in which the
amount of dissolved oxygen is less than that above or below it.
At the end of the Cretaceous Period, beginning about 67
million years ago, sea level dropped precipitously. Oxygen
isotopic ratios recorded in marine foraminifera, and a change
in land plant communities in high latitudes, point to
cooling of the poles.
Cretaceous Period
Cretaceous Marine Life
Sea level reached also its highest levels during the Cretaceous,
and warm conditions meant that the oceans were teeming with
life. Microorganisms forming the bases of food webs were
major contributors to marine sediments.
Diatom - Single-celled phytoplankton having an ornate
microscopic skeleton (test) composed of two valves
impregnated with opaline silica.
Cretaceous Period
Cretaceous Marine Life
Dinoflagellates experienced a series of adaptive radiations
in the Phanerozoic Era, including in the Cretaceous Period.
Chalk - Soft marine limestone composed mostly
of calcitic coccolithophorid plates.
Mollusks continued to evolve quickly in the Cretaceous.
Rudist - A type of Mesozoic clam having unequal valve sizes
that often formed reefs in the Cretaceous.
Many modern snails appeared in the Cretaceous Period.
Cretaceous Period
Cretaceous Marine Life
Swimming carnivorous mollusks, the belemnoids, account for
some of the most conspicuous Cretaceous fossils. Ammonoids
evolved quickly during this period.
Ammonite - A cephalopod having ammonitic sutures.
Teleost fishes, which are the dominant group of fishes today,
became prevalent in Cretaceous seas.
The Cretaceous Period witnessed an arthropod predator: the
modern crabs. With their claws, crabs break shells or
skeletons of their prey obtaining nutritious soft tissues inside.
Cretaceous Period
Cretaceous Terrestrial Life
Dinosaurs ruled the land during the Cretaceous Period.
The giant sauropods were all gone, but ceratopsians
and ornithopods expanded in numbers. Carnivorous
theropods also prowled the landscape.
By the Late Cretaceous, the two modern groups of mammals,
placentals and marsupials, had evolved, yet they
remained small and inconspicuous.
Flying reptiles and birds were the
largest animals in the air.
Cretaceous Period
Origin of flowering plants.
Close to the Jurassic-Cretaceous transition, a group of
gymnosperms gave rise to the flowering plants (angiosperms).
Angiosperms have a tremendous ecological advantage over
gymnosperms partly because they enclose their seeds in a
special reproductive chamber (an ovary).
The evolutionary success of angiosperms reinforced by their
development of the flower. Showy flowers are attractive to
animals, especially insects, but also birds and mammals. All of
these animals help with the fertilization process.
Cretaceous Period
End-Cretaceous Extinction
The impact of a bolide from outer space at the end of
the Cretaceous may have crippled ecosystems
that were already in fragile condition.
Streaking toward Earth 65.5 million years ago was probably
an asteroid that had fallen from its orbit, finally crashing into
the northern tip of the Yucatan Peninsula of Mexico.
The Late Cretaceous was a time of major volcanic activity,
where gases and dust were emitted that blocked sunlight.
With less sunlight reaching the Earth’s surface, global
temperatures should have declined.
Cretaceous Period
End-Cretaceous Extinction
Sea level dropped markedly at the end of the
Cretaceous Period.
Impact of the Chicxulub bolide was felt globally. Shock
waves reverberated from the impact site, and dust and
debris were sent high into the atmosphere.
Shocked quartz - Quartz grain showing distinctive parallel
sets of welded microscopic planes, called shock lamellae.
The aftereffects of the Chicxulub impact, added to an
already stressed ecosystem, may have been enough
to drive many groups to extinction.