"Ancient Life" 542-251mya
1. Cambrian Period (542-488 mya) "Explosion of Invertabrates"
The supercontinent Pannotia had begun to break up as the Iapetus Ocean formed in between Laurentia (North America), Siberia, Baltica (North Europe), and Gondwana (South America, Australia, Africa, India, and Antarctica). Throughout the Cambrian, Siberia remained east of Laurentia, while Baltica was moving south of Siberia and southeast of Laurentia. Also moving south was Gondwana, the largest continent, which stretched from the Equator to the South Pole.
Global warming during the Cambrian period lead to greenhouse conditions. This climate change was probably linked to major changes in the positions of the continents. In Cambrian time, most of the land was in tropical, subtropical, and temperate climatic zones; the ice masses melted and oceans flooded the continental shelves. Marine life flourished in shallow, warm-water continental-shelf environments and limestone deposition became common; Cambrian limestones are known from all parts of the world.
The advent of the Paleozoic Era brought an incredible diversification of multicellular animals having many different body plans, including most of the major groups alive today, such as mollusks, trilobites, other arthropods, brachiopods, echinoderms, corals, sponges, and chordates (our ancestors). Most significantly, the first animals having hard outer skeletons evolved; as a result, many of these shells became fossilized. Most of the Early Cambrian animals having exoskeletons were small, measuring one to five millimeters, and are often called the "Small Shelly Fauna". Another unique feature of the Cambrian fauna is the sponge-like archaeocyaths that formed reefs during Early Cambrian time. This sudden appearance of many different kinds of animals having skeletons (shells) in the fossil record is called the Cambrian Explosion.
The "Age of Trilobites" and the Cambrian Fauna
The most abundant and diverse animals of Cambrian time were the triolobites. Trilobites had long antennae, compound eyes, many jointed legs, and a hard exoskeleton like many of their modern arthropod relatives, such as lobsters, crabs, and insects. The Cambrian is sometimes called the "Age of Trilobites" because of their explosive diversification into all marine environments worldwide. In size, they ranged from a few millimeters (1 mm = 0.25 inches) to 45 centimeters (18 inches).
The diversity of Cambrian life contrasts with that of the late Proterozoic partly because the newly evolved exoskeletons allowed Cambrian animals to become fossilized more easily. The evolution of shells may have allowed for the evolution of a wider variety of body plans. Whatever the reason, Cambrian time saw an "explosion" of diversity in the evolution of life on Earth. Shells also provide protection, and the Cambrian records the first examples of predation in the fossil record. For many different reasons, the Cambrian Explosion is an important event in the history of life.
The Cambrian Period began with an explosion of life forms. It ended in a mass extinction. Advancing glaciers would have lowered the temperature of the shallow seas where so many species lived. Changes in the temperature and the amount of oxygen in the water would have meant the end for any species that could not adapt.
2. Ordovician (488-444) "Verterbrates Appear"
Most of the world's landmasses came together to create the supercontinent of Gondwana, which included the continents of Africa, South America, Antarctica, and Australia. Gondwana drifted south throughout the period, finally settling on the South Pole. The landmass that would become North America was combined into the supercontinent of Laurentia, which was separated from Gondwana by the narrow Iapitus Ocean. Proto-North America straddled the Equator, though to begin with it lay largely underwater.
For the most part the Earth's climate was warm and wet, with sea levels rising as much as 1,970 feet (600 meters) above those of today. But once Gondwana took up its polar position in the late Ordovician, massive glaciers formed over Africa at the supercontinent's center. This heralded a 20-million-year ice age during which shallow, life-rich seas shrank away.
Life at the start of the Ordovician remained confined to the seas with new animals evolving in place of those that didn't survive the Cambrian. Chief among them were the squidlike nautiloids, a type of tentacled mollusk. The nautiloids lifted off from life on the seabed as gas-filled chambers in their conical shells made them buoyant. They were accomplished swimmers, propelling themselves by jetting water through their body cavity. Equipped with grasping tentacles, the nautiloids were effective predators.
Another group of marine hunters were the mysterious conodonts, known mainly from the tiny fossil teeth they left behind. The few complete fossils that have been found suggest they were finned, eel-like creatures with large eyes for locating prey. The conodonts are now thought to have been true vertebrates; however, this line of backboned animals later went extinct.
Fish started becoming more widespread in the fossil record. They were small and had downward-pointing, jawless mouths, indicating they lived by sucking and filtering food from the seabed. Bony shields covered the front of their bodies—the beginnings of a fashion for armor plating among fish. Lampreys and hagfish are these fishes' living descendants.
The archaic sponge reef-dwellers of the Cambrian gave way to bryozoans—tiny, group-living animals that built coral-like structures. Ordovician reefs were also home to large sea lilies, relatives of sea stars. Anchored to the bottom inside calcareous tubes, they collected food particles with feathery arms that waved in the ocean currents.
From Sea to Land
The hard-bodied arthropods started eyeing opportunities on land. Edging into freshwater and shallow lagoons, they likely included horseshoe crabs, which, despite their name, are more closely related to spiders and scorpions. A few species of these "living fossils" still survive today, such as along the eastern seaboard of the United States, where each spring horseshoe crabs crawl ashore to spawn.
There is also evidence that the first primitive plants began to appear on the previously barren land.
These first steps toward life on land were cut short by the freezing conditions that gripped the planet toward the end of the Ordovician. This resulted in the second largest mass extinction of all time, wiping out at least half of all marine animal species .
At the end of the Ordovician Period, the world entered an intense ice age, possibly brought about by the location of the supercontinent Gondwana over the southern pole.
The formation of large ice sheets meant sea levels fell dramatically, perhaps by as much as70-100m. This particularly affected the corals and bryozoans that were living in shallow inland seas, which drained of water. Global cooling spelt disaster for warm-adapted species that had nowhere to migrate to.
3. Silurian Period (444-416 mya) "Animals & Plants Emerge on Land"
The supercontinent of Gondwana was still positioned over the South Pole but the vast icecaps of the late Ordovician period melted almost to nothing.
The recovery soon got under way in the oceans as climbing temperatures and rising sea levels reproduced the shallow, marine environments of earlier times. The climate remained warm and stable throughout most of the Silurian. Sediments formed from massive quantities of broken shells suggest violent storms were triggered by the warmth of tropical oceans.
The Paleozoic era's Silurian period saw animals and plants finally emerge on land. But first there was a period of biological regrouping following the disastrous climax to the Ordovician.
Huge reef systems flourished in the clear, tropical seas—evident today in the many limestone rock formations that date to this period. Previous reef-builders such as bryozoans were joined by corals and prehistoric sponges known as stromatoporoids, which formed a hard outer skeleton. They survived solely by eating microscopic animals trapped by their stinging tentacles.
The growth of corals and other marine organisms was stoked by oceans teeming with tiny planktonic creatures. Waiting at the other end of the food chain were the fearsome eurypterids, or sea scorpions. Some species grew to more than six feet (two meters) in length and are considered the largest arthropods ever to have lived. Bearing a resemblance to their scorpion descendents, eurypterids had a pair of huge, compound eyes for searching out primitive fish, which they grabbed with their powerful, clawlike pincers.
Fish were now diversifying and extending their feeding options beyond simply vacuuming meals off the seabed. A group called acanthodians, or spiny sharks, evolved more menacing mouths with jaws. While these fish, the first true jawed fish, reached no great size during the Silurian, they were on their way to becoming one of the planet's top predators.
Creepy-crawlies also began to appear on land. Starting off small, they measured no more than a few centimeters long. These terrestrial pioneers were arthropods, such as primitive centipedes and arachnids, the ancestors of spiders.
First True Plants
The first true plants began to take root on land some 430 million years ago. They evolved rigid stems, enabling them to stand upright, and the tubular tissues common to all vascular plants that allowed the transport of water and nutrients. These first colonizers lacked leaves, but mosses and other plants followed, providing a thin layer of waterside vegetation that encouraged more aquatic animals to make the transition to land.
The Silurian drew to a close with a series of extinction events linked to climate change; however, these were relatively minor compared with those of other geologic periods
4. Devonian Period (416-359 mya) " Age of the Fishes"
The great supercontinent of Gondwana was headed steadily northward, away from the South Pole, and a second supercontinent began to form that straddled the Equator. Known as Euramerica, or Laurussia, it was created by the coming together of parts of North America, northern Europe, Russia, and Greenland.
Red-colored sediments, generated when North America collided with Europe, give the Devonian its name, as these distinguishing rocks were first studied in Devon, England.
A vast ocean covered the rest of the globe, and the land was comparatively arid. Global climate was relatively warm and dry, and there was less of an equator-to-pole temperature gradient than today. There were no glaciers until the Late Devonian, when ice began to cover parts of the South Polar region.
The Devonian, part of the Paleozoic era, is otherwise known as the Age of Fishes, as it spawned a remarkable variety of fish. The most formidable of them were the armored placoderms, a group that first appeared during the Silurian with powerful jaws lined with bladelike plates that acted as teeth. Early placoderms fed on mollusks and other invertebrates, but later species developed into ferocious, fish-slicing monsters measuring up to 33 feet (10 meters) long. Other types of bone-plated fish that lacked jaws developed a range of bizarre forms. Fossil specimens include species with horseshoe-shaped heads and others that looked like rounded shields.
Despite their heavy protection, these primitive fishes weren't built to last. The Devonian ancestors of fishes living today belonged to two main nonarmored groups. The cartilaginous fish, so-called because cartilage formed their skeletons, later gave rise to sharks and rays. They had small, rough scales, fixed fins, and sharp, replaceable teeth. The second group, the bony fish, were covered in scales and had maneuverable fins and gas-filled swim bladders for controlling their buoyancy. Most modern fishes are bony fish.
The bony fish included lobefins. Named after the thick, fleshy base to their fins, lobefins are credited with the giant evolutionary stride that led to the amphibians, making lobefins the ancestors of all four-limbed land vertebrates, including dinosaurs and mammals. The fossils of these remarkable animals come from the red rocks of Devon. Some lobefins are still around today, such as the famous "living fossil" fish, the coelacanth.
A recently discovered fossil creature from the Devonian has been hailed as a vital link between fish and the first vertebrates to walk on land. Found in the Canadian Arctic in 2004, Tiktaalik had a crocodile-like head and strong, bony fins that scientists think it used like legs to move in shallow waters or even on land. The fish showed other characteristics of terrestrial animals, including ribs, a neck, and nostrils on its snout for breathing air.
The first amphibians breathed through simple lungs and their skin. They may have spent most of their lives in the water, leaving it only to escape the attentions of predatory fish.
The first ammonoids also arose during the Devonian. Related to octopuses and squid, these marine animals survived until the end of the Cretaceous period, 65 million years ago.
Plants began spreading beyond the wetlands during the Devonian, with new types developing that could survive on dry land. Toward the end of the Devonian the first forests arose as stemmed plants evolved strong, woody structures capable of supporting raised branches and leaves. Some Devonian trees are known to have grown 100 feet (30 meters) tall. By the end of the period the first ferns, horsetails, and seed plants had also appeared.
The new life burgeoning on land apparently escaped the worst effects of the mass extinction that ended the Devonian. The main victims were marine creatures, with up to 70 percent of species wiped out. Reef-building communities almost completely disappeared. Theories put forward to explain this extinction include global cooling due to the re-glaciation of Gondwana, or reduced atmospheric levels of the greenhouse gas carbon dioxide because of the foresting of the continents. A major asteroid impact has also been suggested.
5. Carboniferous (359-299) "The Coal Age"
In the U.S., scientists divide the Carboniferous into two parts: the earlier Mississipian (359.2 million to 318.1 million years ago) and the later Pennsylvanian (318.1 million to 299 million years ago).
During the Mississipian, Euramerica, or Laurussia, which included North America, northern Europe, and Greenland, remained separate from the larger, cooler supercontinent of Gondwana to the south. To the east, parts of Asia, including China, were surrounded by warm oceans.
Africa collided with eastern North America in the late Pennsylvanian, an event that formed the Appalachian Mountains. Vast coal swamps stretched across the lowlands to the west of the rising mountains.
While Gondwana became progressively colder as it began another poleward migration, the tropical landmasses remained wet and humid.
Carboniferous coal was produced by bark-bearing trees that grew in vast lowland swamp forests. Vegetation included giant club mosses, tree ferns, great horsetails, and towering trees with strap-shaped leaves. Over millions of years, the organic deposits of this plant debris formed the world's first extensive coal deposits—coal that humans are still burning today.
The growth of these forests removed huge amounts of carbon dioxide from the atmosphere, leading to a surplus of oxygen. Atmospheric oxygen levels peaked around 35 percent, compared with 21 percent today. It also may explain the giant creepy-crawlies that now emerged—the size reached by insects and similar creatures is thought to be limited by the amount of air they are able to breathe.
Deadly poisonous centipedes some six feet (two meters) in length crawled in the company of mammoth cockroaches and scorpions as much as three feet (one meter) long. Most impressive of all were dragonflies that grew to the size of seagulls. One exquisitely detailed fossil of a dragonfly that died 320 million years ago shows it had a wingspan of 2.5 feet (0.75 meters).
How insects first became airborne remains a mystery. Theories suggest their wings may have developed from structures used to regulate temperature (such as by catching the sun's rays to heat up), or perhaps they evolved from brightly colored signals used to attract mates and ward off rivals. Another idea is that insect wings developed from appendages used to glide between trees in the Carboniferous forests.
Amphibians were also growing in size and diversity. There were predatory species that resembled modern-day crocodiles. Armed with vicious teeth, they reached lengths of almost 20 feet (6 meters). Some amphibians developed a thicker, scaly skin, solving the problem of them drying out if away from water too long. They also reduced their reliance on wetland habitats through a crucial evolutionary adaptation known as the amniote egg. This protected the embryo inside with a fluid-retaining membrane while still allowing in air. In time, the earliest reptiles appeared. Identified from remains found inside fossilized Carboniferous tree stumps, they were small, agile, lizard-like animals.
The end of the Carboniferous period is marked by global climate changes due to the glaciers that covered the South Pole. The mass extinctions that mark the end of other geologic periods were not present, yet many species did go extinct during this time. The marine environments were most affected by these climate changes, so the extinctions were mostly invertebrates that spent their lives in the seas. Horn corals, trilobites and some forms of crinoids were on the road to extinction that would come in the next period,
6. Permian Period (299-251) "Age of the Reptiles"
The emerging supercontinent of Pangaea is formed.
Pangaea presented severe extremes of climate and environment due to its vast size. The south was cold and arid, with much of the region frozen under ice caps. Northern areas suffered increasingly from intense heat and great seasonal fluctuations between wet and dry conditions.
The lush swamp forests of the Carboniferous were gradually replaced by conifers, seed ferns, and other drought-resistant plants.
Early reptiles were well placed to capitalize on the new environment. Shielded by their thicker, moisture-retaining skins, they moved in where amphibians had previously held sway. Over time, they became ideally suited to the desert-type habitats in which they thrive today.
Being cold-blooded, reptiles had to find ways to deal with big daily variations in temperature, from below freezing at night to over 100 degrees Fahrenheit (38 degrees Celsius) during the day. Some of the primitive pelycosaurs, which measured up to ten feet (three meters) long, had sail-like structures on their backs that are thought to have acted as heat exchangers, catching the sun in the morning to help warm the sluggish creatures.
Later, other mammal-like reptiles known as therapsids found an internal solution to keeping warm—scientists suspect they eventually became warm-blooded, conserving heat generated through the breakdown of food. These more metabolically active reptiles, which could survive the harsh interior regions of Pangaea, became the dominant land animals of the late Permian.
The therapsids flourished during the Permian, rapidly evolving many different forms, ranging from dinosaur-like fanged flesh-eaters to plodding herbivores. Some species reached a huge size, weighing in at over a ton. In the latter part of the Permian, smaller varieties emerged, likely warm-blooded and covered in insulating hair. From them, mammals would arise.
The Permian seas came to be dominated by bony fishes with fan-shaped fins and thick, heavy scales. There were large reef communities that harbored squidlike nautiloids. Ammonoids, with their tightly coiled, spiral shells, are also widespread in the Permian fossil record.
The Permian, however, represented the last gasp for much early prehistoric life. The period, and the Paleozoic era, came to a calamitous close 251 million years ago, marking a biological dividing line that few animals crossed. The Permian extinction—the worst extinction event in the planet's history—is estimated to have wiped out more than 90 percent of all marine species and 70 percent of land animals.
Various theories seek to explain this mass extinction. Some scientists think a series of volcanic eruptions pumped so much debris into the atmosphere that the sun was blocked out, causing a significant drop in temperature and preventing plant photosynthesis, which in turn caused food chains to collapse.
Other scientists point to global climate change, citing evidence for a period of sudden warming and cooling. These rapid extremes of conditions may have meant species were unable to adjust. Other theories include a catastrophic release of methane gas stored under the seabed, triggered by earthquakes or global warming, or a massive asteroid impact.
Perhaps a combination of factors was to blame. But whatever the cause, new animals and plants would evolve to fill the void. Not least among them: the dinosaurs.