A Brief History of Life:
Origins
The
First Life
Multicellularity
The
Invasion of the Land
Human
Evolution
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| 1 | Quaternary (middle) |
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65 |
| 2 | Cretaceous/Tertiary (K/T) |
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| 3 | Triassic/Jurassic |
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| 4 | Permian/Triassic |
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| 5 | Devonian/Missippian |
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| 6 | Ordovician/Silurian |
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| 7 | Cambrian/Ordovician |
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This scene depicts a muddy estuary that occupied what is now southern Quebec, about 370 million years ago. On the left, a group of Cheirolepis (primitive ray-finned fishes) is being followed by Eusthenopteron, a large lobe-finned relative of the tetrapods seen in the center. In the upper left, an early lungfish called Scaumenacia rises to the surface in order to gulp air. In the lower center, box-like armored placoderms belonging to the genus Bothriolepis cruise slowly over the muddy bottom in order to feed on detritus. One of the last armored jawless fishes, Alaspis (an ostracoderm related to Cephalaspis), is also feeding on bottom detritus, at the lower right. To the far right is the fish Elpistostege, an extinct relative of tetrapods.
The Environment
These
fossils are preserved in sandy and silty sediments of the Escuminac Bay
Formation, in what is now Quebec, Canada. In these coarse sediments, fossil
fishes are frequently accompanied by
terrestrial
plant fossils, suggesting that storms or strong temporary currents probably
were responsible for much of the sediment formation. Many of the fishes
are preserved in three dimensions, suggesting that they were overwhelmed
and buried quickly. For most of the time, however, conditions in the estuary
probably were clear and quiet, with much slower rates of sediment accumulation.
Waters in the estuary were probably warm, and supported a rich and taxonomically
diverse population of fishes.
The
Invasion of the Land
The Scenario
In
a small ox-bow lake the large shark Orthacanthus attacks the tetrapod
Eryops. Eryops, which is related to frogs and salamanders, was also
a common predator, lurking in shallow water for prey. On the right are
the enigmatic lepospondyls, consisting of the terrestrial microsaur Pantylus
crawling on a log and the boomerang-skulled Diplocaulus swimming
below. In the right background is the aquatic anthracosaur Cricotus,
a large, crocodile-like predator related to the more terrestrial Diadectes,
seen in the far left background.
The Environment
The Early Permian redbeds of what is now Texas and Oklahoma document a series of changing environments that alternated from arid to wet. This scene represents a wet period, during the formation of a broad delta. Ox-bow lakes, cut from the main river system, provided rich habitats for fish and the early tetrapods that preyed on them. Forests of cycads and tree ferns were being replaced by the relatives of pine trees and other gymnosperms.
During
the Jurassic the great inland seas of what is today Europe were home to
a wide variety of vertebrates. In the foreground here the large marine
crocodile Mystriosaurus chases a tasty-looking school of Dapedium.
On the right, the large shark Hybodus is also after some fish, while
overhead the pterosaur Campylognathoides skims along looking for
its own prey. In the left background of the water are Plesiosaurus
and the ichthyosaur Stenopterygius, large predators of the open
sea.
The Environment
The inland seas of what is now Europe formed at various times during the Mesozoic. At times, they were ringed by coral reefs and pre-Alpine mountain-formation. During the Early Jurassic, a shallow sea formed in the area of central Europe; its sediments are the black shales of the Holzmaden Formation. This sea was very low in oxygen and nutrients, due to limited sea-water circulation, and this prevented animals from living on the sea bottom. Because there were no bottom-dwelling scavengers, animals that died while swimming or flying and fell to the sea's bottom, were preserved.
The Scenario
This scene depicts an attack played out in a shallow stream-bed with forested banks, about 140 million years ago in the area that is now Utah, Colorado, and Wyoming. An attacking group of ceratosaurs has isolated an individual camptosaur, in the foreground, as well as a mother and juvenile barosaur, in the background, while the other barosaurs and camptosaurs flee. Also fleeing in the right foreground is a smaller predator, Ornitholestes.
The Environment
Fossils of these dinosaurs are preserved in rocks of the Morrison Formation. These rocks represent the sediments washed out of canyons by the rivers and streams draining the ancestral Rocky Mountains to the west. The sand and mud were deposited across an immense, relatively flat flood-plain that also contained shallow lakes, ponds, and swamps. The larger rivers probably always ran; however, seasonal rainfall dictated that the smaller streams ran only during the wetter seasons, so that parts of the flood plain were actually quite dry during certain times of the year. The flood plain bordered a shallow sea to the east that, from time to time during the Age of Dinosaurs, extended from the Gulf of Mexico up to the Arctic Ocean. The forested areas were populated primarily by conifers, with other plants on the flood plain, including ginkgoes, ferns, and the cycadlike bennetites.
The Scenario
As
two voracious Deinonychus mount a frontal assault on the heavily
armored nodosaur Sauropelta, a third Deinonychus prepares
to attack from the stream bank at the rear. In the left foreground of this
scene -- which takes place 107 million years ago -- a smaller theropod
relative of Deinonychus, Microvenator, flees the area, as does a
herd of the ornithischian Tenontosaurus in the left middle-ground.
In the far distance, a trio of titanosaurs keeps a watchful eye on the
conflict as they move away from the stream border.
The Environment
The
fossil animals from this scene are preserved in the Cloverly Formation,
which is exposed in north-central Wyoming and south-central Montana. Like
the slightly older Morrison Formation, these sediments were washed out
of the ancestral Rocky Mountains that were being uplifted just to the west.
They were deposited across a broad flood plain that bordered a shallow
continental sea to the east. Sandstones and conglomerates are commonly
preserved in the bottom of ancient stream channels, and finer grained mudstones
and siltstones represent flood debris deposited away from the stream channels
on the adjacent plain. Much of the finer grained mudstone is made up of
altered volcanic ash that presumably erupted out of volcanoes to the west
in the ancestral Rockies. The forest contained abundant conifers, and the
presence of crocodiles suggests that the temperatures rarely, if ever,
dropped below freezing.
The Scenario
This image represents a 75-million-year-old scene from an area around what is now southeast Alberta, Canada. The focal point of this scene involves a confrontation in the middle of a dry stream bed in which an armored dinosaur, Euoplocephalus, tries to fend off the attack of a tyrannosaur, Albertosaurus, with its bony tail club. Keeping a wary eye on the fight is the ornithomimid Struthiomimus, content to try to find a spot to eat a recently caught meal consisting of a small mammal. On the far bank of the stream, small groups of the crested hadrosaur Corythosaurus and the spike-frilled ceratopsian Styracosaurus beat a hasty retreat from the conflict.
The Environment
The fossils are contained in a rock unit called the Judith River Formation. The fine sand, silt, and mud that makes up this rock unit were washed out of the ancestral Rocky Mountains to the west. Meandering rivers transported the eroded debris across a broad flood plain and emptied through a large delta into a shallow continental sea whose shoreline ran along the present border between Alberta and Saskatchewan. Floods were common during the wetter parts of the year and during peak runoff in the mountains. Although a cool dry season occurred, the climate was frost-free, and the air was probably often humid. The lowland forests were dominated by cypress and redwoods, with ferns providing a major portion of the ground cover. Cattails and water lilies grew in and along the lakes and ponds adjacent to the streams.
The Scenario
A part
of the world now occupied by Mongolia provides the setting for this 72-million-year-old
scene. A small family of primitive ceratopsians belonging to the genus
Protoceratops flees over the crest of a large sand dune, pursued
by a pair of fleet-footed theropods from the genus Velociraptor. In
the right foreground, a solitary mother Oviraptor observes the chase
while guarding her nest of eggs. A pack of theropods belonging to the genus
Sauronithoides pursues a flock of the flightless bird Mononykus
in the middle distance, while a pair of ankylosaurs from the genus Pinacosaurus
watches from a distant sand dune.
The Environment
The
fossils are preserved in the sandy sediments of the Djadokhta Formation.
The large, angled strata contained within the thickest major beds suggest
that many of these deposits represent the remains of extensive fields of
wind-blown sand dunes. Less common beds of mud and clay suggest that ponds
occasionally formed between the dunes, and coarse-grained beds of conglomerate
attest to the presence of streams, at least at times, within the general
area. Nonetheless, the climate was rather arid and vegetation was sparse
in relation to the forested flood plains that existed at the same time
in the middle of the North American continent, near present-day Alberta,
Montana, Wyoming, Colorado, and Utah.
The Scenario
This
scene depicts the subtropical forest surrounding the Green River Lake system,
in what is now southwestern Wyoming. The forest is composed of deciduous
hardwood trees, as well as palms and early cacti. In the higher mountains
surrounding the lakes, the forest contains mixed conifers and deciduous
hardwood trees. These lush surroundings supported a diverse fauna of mammals,
both on the ground and in the trees, including early relatives of living
rodents, lemurs, and horses.
The Environment
The
climate was relatively warm, and rainfall was high, although it showed
variation with the seasons. The summer months were the driest part of the
year. In the basin, freezing temperatures were unheard of, judging by the
presence of animals like crocodiles, but cool winter temperatures and frost
may have been present at higher elevations.
The Scenario
In
the foreground, two Baena turtles are feeding on insects on a partially
sunken log while the crocodile Leidyosuchus attacks a trionychid
turtle. Varanid lizards of the genus Saniwa in the left background are
feeding on the snake Boavus, another example of which is foraging
in a tree on the right. The fishes Diplomystus are in the water.
The Environment
During the Eocene, a series of large, freshwater lakes were present in the area of what today is Wyoming and Utah. These lakes accumulated fine sediments which form the Green River Formation. At the same time, rivers and swamps near these lakes accumulated sands and clays that form the Bridger Formation. The scene shows a margin of a large lake where foliage and water provided diverse habitats for vertebrate life.
This
scene depicts a landscape about 32 million years ago, in what is now the
northern Great Plains at Badlands National Monument, South Dakota. The
flood plain of one of many shallow streams in the area passes southeast
from the Black Hills through a terrain of low relief. The forest, primarily
mixed deciduous trees and conifers, is confined to the vicinity of the
stream and nearby floodplain. Hoplophoneus, a saber-tooth carnivore,
uses the the cover provided by the brush to stalk a group of unsuspecting
early horses, as Archaeotherium, a very distant relative of pigs,
keeps a wary eye on the predator.
The Environment
Away
from the streams more open areas supported an association of low trees,
shrubs, and perhaps some grasses. Rainfall and temperature were seasonally
driven. The winters were cooler than during the Eocene; however, temperatures
did not drop below freezing for any extended period of time. Alligators
still lived in the streams. The summers were warm, with long dry periods.
The range of temperatures was narrower than it had previously been in the
interior of North America, and much narrower than in this same area today.
The Scenario
This scene depicts late spring
in the Northern Great Plains about 10 million years ago, in what is now
southern South Dakota. The Black Hills appear in the background, and a
shallow stream with braided sandbars winds through the foreground. The
water in the stream is deep enough to support a group of beavers. Grasses,
shrubs, and trees growing along the streamside provide ample cover for
a wolf-size carnivore, distantly related to modern dogs, to stalk the beavers
and horses drinking from the stream.
The Environment
Trees were concentrated along
the stream border, while further away, more open spaces had a lighter covering
of shrubs, brush, and grasses. The climate was temperate with fairly mild
winters, during which the temperature rarely fell below freezing and snow
was quite unusual. Summers were hot and dry except for occasional thundershowers.
At this time of year, water was sparse away from the stream, and stream-flow
diminished to form waterholes frequented by most members of the local fauna.
While the climate was drier, and the annual range of temperatures was narrower
than in the same area today, the seasonal extremes were greater than in
the Oligocene.
The Scenario
This scene depicts spring in the Los Angeles Basin about 19,000 years before present. The San Gabriel Mountains, in the background, are still covered with snow. A tributary of the Los Angeles River, in the foreground, often floods high enough in this season to leave a sheet of water on top of the tar-soaked sands on the bottom of the stream, so that when animals come to drink, they get mired in the tar. Predators then attempt to prey on these trapped animals.
A lone
Smilodon stalks along the stream searching for prey. The giant ground
sloth has noticed the cat and looks on warily. The horses have also noticed
and begun to move away. Further in the distance, the mammoths go about
their foraging undisturbed because they are too large for the cat to bring
down. The dire wolves watch with anticipation, hoping to reap the spoils
of the sabertooth's meal.
The Environment
The
Ice Age vegetation was somewhat more lush than that found in the Los Angeles
Basin today. The larger snowfields in the San Gabriel Mountains supplied
moisture to feed more permanent streams and rivers, as well as the pine
forests that grew along their steep slopes. Willows, live oaks, and scattered
Monterey pines grew along the streams, whereas chaparral covered the low
hills and flatlands of the basin. These shrubs provided plenty of cover
for predators, as well as plenty of open grassland for herbivores. The
climate was more extreme than that found in this area today, with colder
winters and cooler summers. Snow caps were present at lower elevations.
Most precipitation occurred during the winter, as it does today.
Human
Evolution
Australopithecines,
the first true hominids, could stand and walk upright.
 |
Evolutionary
Connections:
Why
Do Humans Walk Upright?
Back to the Biology 101 Syllabus 
