VIZUALIZING EARTH HISTORYBy Loren E. Babcock

Chapter 11

Mesozoic World

Timeline of Mesozoic events

Triassic PeriodMajor 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 PeriodTriassic 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 PeriodTriassic 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 PeriodTerrestrial 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 PeriodTerrestrial life of the Triassic PeriodThe 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 PeriodTerrestrial life of the Triassic Period

Jurassic PeriodThe 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 PeriodJurassic 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 PeriodJurassic 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 PeriodJurassic 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 PeriodJurassic 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 PeriodTerrestrial 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 PeriodDinosaurs 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 PeriodDinosaurs Inherit the Land

Jurassic PeriodDinosaurs 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 PeriodDinosaurs 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 PeriodDinosaurs 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 PeriodDinosaurs 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 PeriodOrigin of Birds

The oldest known bird fossils, Archaeopteryx (literally,

“ancient wing”), are from the Jurassic of Germany.

Cretaceous PeriodThe 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 PeriodCretaceous 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 PeriodCretaceous 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 PeriodCretaceous 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 PeriodCretaceous 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 PeriodCretaceous 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 PeriodOrigin 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 PeriodEnd-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 PeriodEnd-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.