Precambrian: Archean, Lecture Materials + ch 8
Proterozoic: Ediacara Fauna, Lecture Materials + ch 9
alligator gar: Lepisosteus osseus, Cretaceous to Recent
Reading Assignment: Chapters 9, and 10
(things you must do)
(what you learned)
The Paleozoic took up over half of the Phanerozoic, approximately 300 million years. During the Paleozoic there were six major continental land masses; each of these consisted of different parts of the modern continents. For instance, at the beginning of the Paleozoic, today's western coast of North America ran east-west along the equator, while Africa was at the South Pole. These Paleozoic continents experienced tremendous mountain building along their margins, and numerous incursions and retreats of shallow seas across their interiors. Large limestone outcrops, like the one shown above, are evidence of these periodic incursions of continental seas.
Many Paleozoic rocks are economically important. For example, much of the limestone quarried for building and industrial purposes, as well as the coal deposits of Western Europe and the eastern United States, were formed during the Paleozoic.
The Silurian was a time when the Earth underwent considerable changes that had important repercussions for the environment and life within it. The Silurian witnessed a relative stabilization of the earth's general climate, ending the previous pattern of erratic climatic fluctuations. One result of these changes was the melting of large glacial formations. This contributed to a substantial rise in the levels of the major seas.
Coral reefs made their first appearance during this time, and the Silurian was also a remarkable time in the evolution of fishes. Not only does this time period mark the wide and rapid spread of jawless fish, but also the highly significant appearances of both the first known freshwater fish as well as the first fish with jaws. It is also at this time that our first good evidence of life on land is preserved, including relatives of spiders and centipedes, and also the earliest fossils of vascular plants.
The Silurian is a time when many biologically significant events occurred. In the oceans, there was a widespread radiation of crinoids, a continued proliferation and expansion of the brachiopods, and the oldest known fossils of coral reefs.
Horn and honeycomb corals were common. By mid-period, these same corals were building reefs in the warm seas.
The time period also marks the wide and rapid spread of jawless fish, along with the important appearances of both the first known freshwater fish and early jawed fish. Other marine fossils commonly found throughout the Silurian record include trilobites, graptolites, conodonts, corals, stromatoporoids, and mollusks.
Numerous brachiopods, worms, and shrimp lived on the muddy bottoms. The larger scorpions (eusarus) and sea scorpions (ptertogus) were less numerous but dominated the scene because of their larger size.
It is also in the Silurian that we find the first clear evidence of life on land. While it is possible that plants and animals first moved onto the land in the Ordovician, fossils of terrestrial life from that period are fragmentary and difficult to interpret. Silurian strata have provided likely ascomycete fossils (a group of fungi), as well as remains of the first arachnids and centipedes.
striking of all biological events in the Silurian was the evolution of
vascular plants, which have been the basis of terrestrial ecology since
their appearance. Most Silurian plant fossils have been assigned to the
genus Cooksonia, a collection of branching-stemmed plants which produced
sporangia at their tips. None of these plants had leaves, and some appear
to have lacked vascular tissue. The Silurian was a time for important events
in the history of evolution, including many "firsts," that would prove
highly consequential for the future of life on earth.
The vegetation of the early Devonian consisted primarily of small plants, the tallest being only a meter tall. By the end of the Devonian, ferns, horsetails and seed plants had also appeared, producing the first trees and the first forests in swampy areas.
By the beginning of the Devonian early terrestrial vegetation had begun to spread. These plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall. These plants included the now extinct zosterophylls and trimerophytes. The early fauna living among these plants were primarily arthropods: mites, trigonotarbids, wingless insects, and myriapods, though these early faunas are not well known.
Insects appeared during the middle of the system and were common by the end of the system. Most were small, but a few grew to sizes larger than the largest insects known today. Fossil remains include cockroaches up to 10 centimeters (4 inches) long and dragonflies with wingspans about 74 centimeters (29 inches) wide. Scorpions, spiders, and centipedes also were among the more than 400 species found as fossil remains.
Plants: By the Upper Devonian, lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many are rather tall plants. The progymnosperm Archaeopteris was a large tree with true wood. In fact it is the oldest such tree known, and produced some of the world's first forests. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". Along with this diversification in terrestrial vegetation structure came a diversification of the arthropods.
Also during the Devonian, two major animal groups colonized the land. The first tetrapods, or land-living vertebrates, appeared during the Devonian, as did the first terrestrial arthropods, including wingless insects and the earliest arachnids. In the oceans, brachiopods flourished. Crinoids and other echinoderms, tabulate and rugose corals, and ammonites were also common. Many new kinds of fish appeared.
In warm seas, corals had become the dominant form of live. At least 700 different species of brachiopods lived on the ocean floor with lily-like crinoids which were scattered everywhere.
The Devonian seas were dominated by brachiopods, such as the spiriferids, and by tabulate and rugose corals, which built large bioherms, or reefs, in shallow waters. Encrusting red algae also contributed to reef building. In the Lower Devonian, ammonoids appeared, leaving us large limestone deposits from their shells. Bivalves, crinoid and blastoid echinoderms, graptolites, and trilobites were all present, though most groups of trilobites disappeared by the close of the Devonian.
is also notable for the rapid diversification in jawed and bony fish. Benthic
armored fish were common by the early Devonian. These early fish are collectively
called "ostracoderms", and include a number of different groups. By mid-Devonian,
placoderms, the first jawed fish, appear. Many of these grew to large sizes
and were fearsome predators. Of the greatest interest to us is the rise
of the lobe-finned fish, which eventually produced the first tetrapods
just before the end of the Devonian. The lungfish developed the ability
to breathe air at the surface. Some lungfish still survive today in Africa.
The first amphibians developed from this group of animals by the end of
the period. The footprint of a primitive salamander-like amphibian was
found in Pennsylvania.
Schizocoels: A group of animal phyla, including Bryozoa, Brachiopoda, Phoronida, Sipunculoidea, Echiuroidea, Priapuloidea, Mollusca, Annelida, and Arthropoda, all characterized by the appearance of the coelom as a space in the embryonic mesoderm.
Lophophore: A horseshoe-shaped ciliated organ located near the mouth of brachiopods, bryozoans, and phoronids that is used to gather food.
Class Agnatha: Jawless fish, includes the living lampreys and hagfish, and the extinct Ostracoderms. Cambrian-Recent.
1. Myxini [Hagfishes]
2. Petromyzontida [Lampreys]
Class Acanthodii: Primitive jawed fish with numerous spiny fish (Silurian-Permian)
Class Placodermii: Primitive armored jawed fish (Silurian-Permian)
Class Chondrichthyes: Cartilaginous fish such as sharks and rays. Devonian-Recent
Class Osteichthyes: Bony fish
Subclass Actinopterygi: Ray-finned fish. Devonian to Recent. eg, SALMON, PERCH, FLATFISHES,
Subclass Sarcopterygi: Lobe-finned, air-breathing fish. Devonian-Recent. eg, coelacanths and lungfishes, and also including the tetrapods,
i.e, amphibians to mammals, since tetrapods are thought to have evolved from sarcopterygian fishes.
Most bony fish belong to the “ray-finned” group.There are approximately 50 species of jawless fish, 600 species of cartilaginous fish and more than 30,000 species of bony fish. There are approximately 70 fish orders known to biologists.