PALEOZOIC-LIFE
Overview
Cambrian Explosion
Geologically sudden appearance of complex animals with shells.
All modern phyla appeared with the possible exception of bryozoans.
Atmospheric oxygen content reached levels necessary for multicellular life.
This "explosion" follows on the heels of a series of severe glaciations and a possible "hot house" climate of extreme temperatures, begging the question of whether there might be a connection.
Cambrian
Seas dominated by trilobites, a type of arthropod.
Small reefs ("patch reefs") constructed by archaeocyathids.
Jawless fish appear.
Ordovician
Seas dominated by brachiopods.
Large reefs appear built by corals, sponges, and bryozoans.
Land invaded by simple plants, including seedless vascular plants, such as ferns.
Late Ordovician mass extinction probably related to glaciation.
Silurian
Major, massive reefs appear.
Appearance of first jawed fishes.
Land invaded by insects.
Major diversification of plants in the Late Silurian.
Devonian
Major reef building continued.
Appearance of cartilaginous and bony fish.
Appearance of amphibians.
Evolution of the seed and appearance of forests in Late Devonian.
Late Devonian mass extinction possibly caused by global cooling.
Mississippian
Abundance of crinoids in Mississippian seas.
Giant insects appeared, possibly due to high levels of atmospheric oxygen.
Abundance of amphibian labyrinthodonts.
Probable evolution of the amniotic egg and reptiles by the Late Mississippian.
Swamp forests composed of lycopsids (scale trees), sphenopsids, and ferns appeared.
Gymonsperms appeared and greatly diversify.
Pennsylvanian
Reptiles appeared in the fossil record.
Vast "coal" forests existed on swampy plains.
Permian
Final assembly of Pangea led to dry conditions and restricted epeiric seas.
Huge reefs developed on the edge of the epeiric sea, for example, in West Texas.
Reptiles diversified and developed mammal-like features.
Gymnosperms dominated the land.
Greatest mass extinction in geologic history occurred at the end of the Permian.
Cambrian Period
Cambrian Explosion
The "Cambrian Explosion" is the name for the geologically rapid appearance of shelled animals during the Cambrian. These animals evolved from earlier animals lacking shells that were poorly preserved. Some may have evolved from the Ediacaran fauna.
This "explosion" occurred after the earth went through a series of massive glaciations and possible "hot house" climate. It is thought that this stress may have driven evolution.
Another factor is that multicellular animals (metazoa) need an oxygen-based metabolism to provide energy and sufficient atmospheric oxygen. Atmospheric oxygen rose significantly before the Cambrian.
A final consideration is that the Cambrian saw vast epeiric seas move over the continents, providing abundant habitat for marine invertebrates.
Marine Invertebrates
Although trilobites were the most abundant invertebrates, there were others, including brachiopods and archaeocyathids, which built small reefs.
The best-preserved Cambrian fossils are found in the Burgess shale.
The two views of the meaning of the diversity of the Cambrian fossils are:
Most if not all of the fossils can be assigned to known phyla. Evolution since then has been a process of the diversification of these phyla.
Many of the creatures belonged to extinct phyla. Those "body plans" that were not successful were "weeded out", leaving the modern phyla.
Possible reasons for the evolution of exoskeltons (like shells) were:
Protection against evolving predators.
Protection from ultraviolet radiation.
An advantage in size, since exoskeltons provide support for larger bodies.
Protection from dehydration when exposed temporarily to the atmosphere.
Marine Vertebrates
The vertebrates evolved from earlier chordates, which had:
Notochord - a support structure that become the backbone of vertebrates.
Hollow nerve cord - evolved into the spinal cord.
Gill slits - their supports evolved into the jaw and other structures in the head.
The first chordates appeared in the early Cambrian and were slender creatures resembling the modern lancelet.
The first vertebrates were the agnathids (jawless fishes). Their appearance at about the same time as the first chordate fossils implies they evolved from even earlier chordates.
Ordovician Period
Marine Invertebrates
There was a great increase in diversity of brachiopods, tabulate corals, rugose corals, and bryozoans, whose fossils appear for the first time. These along with sponges (stromatoporoids) built reefs that were sometimes much larger than those built by archaeocyathids in the Cambrian.
Floating colonies of graptolites were often fossilized as they died and their hard parts settled on the sea floor. They serve as guide fossils for the Ordovician.
A mass extinction at the end of the Ordovician wiped out much of the marine fauna. It was likely caused by glaciation in Gondwana, which lowered sea level worldwide as the ice sheets built up, leading to the disappearance of the epeiric seas.
Marine Vertebrates
Only jawless fishes swam in the Ordovician seas.
Land Plants
The first land plants appear to have evolved from green algae sometime during the Ordovician.
Silurian Period
Marine Invertebrates
The Silurian saw the appearance of large reefs, larger than any of the Ordovician. These reefs were built by corals (rugose and tabulate) and sponges and were extremely important in providing habitat for many other species.
A new, scorpion-like carnivorous arthropod, the eurypterid, appeared.
Land Invertebrates
Insects and some other terrestrial arthropods evolved by the end of the Silurian.
Marine Vertebrates
The first jawed fish appeared in the Early Silurian. Heavily armored jawed fish, the placoderms, appeared by the Late Silurian. They had no real teeth but bony jaws that served the same purpose.
The most widely accepted theory is that jaws evolved from the bony arches that supported the front-most gills.
Land Plants
Vascular plants evolved in the Late Ordovician or Early Silurian.
Vascular plants have a vascular system that transports water and nutrients.
The first vascular plants, such as Cooksonia from the Middle Silurian, reproduced by spores, which made them rely on water for reproduction. This restriction was overcome by the evolution of the seed in the Devonian.
Devonian Period
Marine Invertebrates
An important subclass of the cephalopods, the ammonoids (coiled cephalopods), evolved and diversified. They are noted for being excellent guide fossils for three reasons. The suture patterns on their shells vary from species to species and are readily recognized; many had short stratigraphic ranges (existed for a fairly small amount of geological time); and, being marine swimmers, they were widely distributed over the seas.
A mass extinction occurred near the end of the Devonian, which affected tropical species more than others. About 70% of all species disappeared. The cause is essentially unknown, but the affect on tropical species implies global cooling may have played a role.
Land Invertebrates
Land arthropods (spiders, insects, scorpions, etc.) were well-evolved in the Devonian.
Marine Vertebrates
Both cartilaginous fish (sharks, rays, etc.) and bony fish evolved in the Devonian, apparently from placoderm ancestors. The Devonian is often called the "Age of Fish".
The mass extinction at the end of the Devonian killed off the placoderms.
From the fossil record it is clear that the first amphibians, the labyrinthodonts, evolved from lobe-finned fish, the crossopterygians. The similarities between crossopterygians that could walk underwater and the labyrinthodonts involve both bone and tooth structure. It is unlikely that limbs initially evolved to walk on land, but to aid in moving in shallow water choked with vegetation where swimming would be difficult.
Land Vertebrates
Necessary developments for living at least part of the time on land were stronger limbs and necks, eyelids, inner ears for balance, and more efficient lungs.
The labyrinthodonts did not look much like modern amphibians but were often quite large and sluggish. Because they needed water to reproduce and avoid dessication, they could not venture far from water.
Land Plants
The Devonian saw the first forests and seed plants. Gymnosperms evolved by the end of the Devonian.
Mississippian Period
Marine Invertebrates
Many types of organisms (e.g., brachiopods and ammonoids) quickly recovered from the Late Devonian extinction.
Crinoids became especially abundant during the Mississippian, leading some to dub it the "Age of Crinoids".
The large reefs of earlier periods mostly disappeared due to the decline of corals and reef-building sponges.
Land Invertebrates
Insects diversified and grew to gigantic sizes, possibly as a result of increased oxygen in the air.
Land Vertebrates
Part amphibian-part reptile creatures (cotylosaurs) had appeared by the late Mississippian.
The critical steps from amphibian to reptile were: the amniotic egg (which allows the passage of gases but not water), stronger limbs, internal fertilization and scales to avoid dessication. These allowed reptiles to break the ties to water that restricted the amphibians. Small, early reptiles could then pursue insects inland.
Land Plants
The seedless lycopsids (scale trees) and sphenopsids (horsetail-like) appeared and, along with ferns, formed vast forests. Swamps were common. Gymnosperms inhabited drier ground.
Pennsylvanian Period
Marine Invertebrates
Pennyslvanian invertebrates generally resembled Mississippian ones.
Fusilinid foraminifera (single-celled planktonic animals) occurred in large numbers with many species and make good guide fossils for the Pennsylvanian and Permian. Fusilinids were about the size of a grain of wheat. Whole limestones are made up largely of their shells.
Land Vertebrates
Pelycosaurs - sail-backed reptiles , evolved in the latter part of the Pennsylvanian. They included both large and small reptiles, and both herbivores and carnivores.
The sail was likely a means to regulate body temperature.
Land Plants
In tropical areas there were vast coal swamps made up of lycopsids and sphenopsids. In temperate locations seed ferns were abundant. Hence there were two major floras: the northern tropical type and the southern temperate (cool) type.
Permian Period
Marine Invertebrates
Fusilinids continued to be important marine invertebrates and make good guide fossils.
Vast reefs were built in the Texas region on the edge of the Kaskakia epeiric sea.
Shallow-sea marine environments were restricted due to the regression of epeiric seas.
The greatest recorded mass extinction in geological history occurred at the end of the Permian, wiping out 90% of marine invertebrate species. The cause is unknown, but evidence increasingly points to a geologically short event. Particularly suspicious is the fact that the Siberian Trap basalts were erupted at about that time. These eruptions are the largest in geological history, producing 2.5 million cubic kilometers of basalt in a few million years. This is enough basalt to cover the state of Texas 2 kilometers deep. The idea is that the gases emitted by these eruptions could have altered the atmosphere and thus the climate.
The important marine invertebrates that didn't survive the mass extinction were the corals, the trilobites, the fusilinids, and the blastoids.
Land Invertebrates
The mass extinction at the end of the Permian wiped out many land invertebrates, including about one-third of the insects.
Land Vertebrates
The sail-backed reptiles (pelycosaurs) were the dominant reptiles during the first part of the Permian but became extinct later in the Permian.
The pelycosaurs were replaced by the therapsids: mammal-like reptiles with advanced features such as specialized teeth and vertical legs. They were probably warm-blooded (endothermic) and may have had a body covering like fur.
The gorgonopsids were extremely mammal-like therapsid "sabertooth tigers", ranging in size up to that of a bear. Lycaenops, a gorgonopsid, was the premier carnivore of the late Permian. The size of a panther, it may have hunted in packs.
The end-of-Permian mass extinction destroyed many therapsid species, including all of the gorgonopsids. Two-thirds of reptile and amphibian species died out.
Land Plants
The drier climate of the Permian favored the gymnosperms, which flourished and came to dominate the landscape.
Plants did not suffer as badly from the mass extinction as animals.