THE PALEOZOIC ERAPart 3

Clues from the Paleozoic Era help us understand how the diversity of life developed. The “story” of this era explains how early life-forms moved from water to land, why the Appalachian Mountains exist, and why they contain coal deposits that fueled the industrial revolution.

THE EARLY PALEOZOIC

CONTINENTAL SETTING The ancient geographic setting of an area is referred to as paleogeography.

The paleogeography of the Earth at the end of the Proterozoic Eon resulted in a supercontinent (Rodinia).

At the start of the Cambrian Period (the earliest period within the Paleozoic Era) the ancient North American continent of Laurentia had again split off and was located near the equator - surrounded by ocean.

Throughout the Cambrian Period, there was not plate tectonic activity on Laurentia and it was completely surrounded by passive margins (continental margins that have not underwent tectonic activity).

CHANGES IN SEA LEVEL

Laurentia was experiencing a regression during the Cambrian period. A regression occurs when sea level falls

and causes the shoreline to move seaward. Results in shallow water deposits overlying

deeper water deposits. A transgression occurs when sea level rises

and the shoreline moves further inland. Results in deeper water deposits overlying

shallow water deposits.

EARLY PALEOZOIC LIFE

THE BURGESS SHALE, IN THE CANADIAN ROCKY MOUNTAINS, CONTAIN SOME OF THE BEST FOSSILIZED CAMBRIAN ORGANISMS

The beginning of the Cambrian Period is referred to as the “Cambrian Explosion,” as all but one major marine group appeared during this time.

The Cambrian Explosion is also marked by the development of mineralized skeletons, or hard parts.

The passive margins of Laurentia continued into the Early Ordovician Period, with paleography being equatorial (with the paleo-equator running from modern-day New Mexico through Minnesota).

THE MIDDLE PALEOZOIC

SEA LEVEL CHANGES… AGAIN!

Sea level rose during the Early Ordovician and a beach environment once again covered much of Laurentia’s margins (Transgression)

The base of rock layers is marked throughout much of North America by pure quartz sand of the St. Peter Sandstone, which is overlain by limestone deposits.

These deposits contain diverse organisms, including the first corals that build organic reefs

ORGANIC REEFS

Organic Reefs are composed of carbonate skeletons made by living organisms. They require warm, clear, shallow water to thrive. They are confined to latitudes between 30 degrees North and South of the equator.

During the middle Ordovician, corals and a group of sponges (stromatoporoids) became common and began to build reefs in the shallow seas covering Laurentia.

MIDDLE PALEOZOIC TECTONICS During the Middle

Ordovician, an ocean-continental collision boundary developed in modern day eastern North America.

This collision boundary resulted in a mountain building event, Taconic Orogeny, which accounts for formation of the Appalachian Mountains.

How do we know Taconic Orogeny occurred? Angular Unconformities Volcanic Activity Igneous Intrusions Clastic Wedging

OROGENIES AND DEFORMATION

Tectonism continued into the Devonian when Laurentia collided with Baltica (modern day Europe and Russia) and formed a larger Laurasia. This tectonic event is

referred to as the Caledonian Orogeny and closed the ocean that had separated the two continents.

Acadian Orogeny and Antler Orogeny also occurred, adding igneous intrusions and island arcs respectively.

MIDDLE PALEOZOIC LIFE

Middle Paleozoic seas were dominated by animals collectively referred to as Paleozoic fauna.

Life moved to land during the Middle Paleozoic, as plants and spores existed during the late Ordovician. Vascular plants (having tissues for water and nutrient circulation) were small, leafless plants that grew along the ground and were reproduced by spores.

NEW PLANTS EMERGE By the late

Devonian, three groups of plants were living on land: Ferns Sphenopsids (joint

stemmed plants) Lycopods

MASS EXTINCTIONS

Two of the greatest extinction events in Earth’s history occurred during the Middle Paleozoic. 1. Marine Life – nearly 57% extinction rate at end of

Ordovician Period. Species that did survive suffered large (but not complete) losses in numbers

2. Marine Life – nearly 50% extinction rate during the late Devonian Period. This included many species of coral and stromatoporoids

Why?1. Evidence of global cooling 2. Rapid decrease in sea level accompanied by glaciation

During the late Paleozoic, a collision similar to that of which occurred forming Laurasia formed the southern continent… Gondwana. Gondwana was made of up modern day South America, Africa, India, and Antarctica.

THE LATE PALEOZOIC

SEA LEVEL AND DEPOSITION

The late Paleozoic began with Laurasia still covered by a shallow, tropical sea. At the end of the Mississippian Period, a major regression occurred… followed by a slow transgression to mark the beginning of the Pennsylvanian Period.

The Pennsylvanian Period is known for the coal deposits that accumulated in heavily vegetated swamps. Not enough plant material had developed on land to form significant coal deposits until the Mississippian Period. Since a shallow sea covered Laurasia during this time period, there are few Mississippian-aged coal deposits in North America.

CYCLOTHEMS When rocks display a

repeating cyclic pattern of sediments stacked one on top of another, it is referred to as a cyclothem.

Cyclothems record two changes in sea level (Regression and Transgression)

REEFS AND EVAPORITES

During the Permian, a shallow sea covered most of what is now western North America. A barrier reef complex formed, built by sponges and algae. Because barrier reefs restrict the flow of water, large evaporite deposits formed behind the reefs during the Middle and Late Permian.

CONTINENTAL COLLISIONS AND MOUNTAIN BUILDING

Three major mountain building events occurred during the late Paleozoic:1. Ouachita Orogeny – As Gondwana collided with

Laurasia, the mountains of present day Arkansas and Oklahoma were formed. This orogeny also resulted in the mountains in the Marathon area of Texas.

2. Ancestral Rockies – the collision between these two supercontinents was so intense that faults lifted Paleozoic rocks more than 2km as far “inland” as modern day Colorado.

3. Alleghenian Orogeny - As the collision between the two continents continued, the ocean separating the two supercontinents was closed. Colliding crusts formed the Appalachian Mountains.

LATE PALEOZOIC LIFE Invertebrate marine organisms that flourished during the

middle paleozoic continued to dominate the marine environment.

The Crinoids become dominant and were so abundant that some Mississippian limestones are most almost entirely of fossilized fragments.

Sharks and other ray-finned fishes became more abundant.

Lobe-finned fishes first appeared and were able to walk on land due to their club-shaped fins and thick bones. There were three major groups of lobe-finned fish, two of which are still living today (lungfishes and coelacanths). The third group gave rise to the amphibians.

LIFE ON LAND

On land life present during the late Devonian Period flourished, invading the low, swampy land of the Pennsylvanian. These plant environments were a breeding ground for insects. In fact, the largest insects that ever lived were preserved in coal swamps.

Amphibians evolved and lived in the swampy regions as well, most being slugglish and ate only fish, insects, vegetation, and smaller amphibians.

Amphibians, at this point, were the top carnivores on land.

FROM AMPHIBIANS TO REPTILES Although amphibians were the top carnivore, they had a major

limitation in that they had to colonize close to water sources because their eggs had no protective coating to prevent them from drying out on land.

Sometime during the late Mississippian, their dependence on water was overcome with the evolution of reptiles.

Reptiles developed a new type of egg, the amniote egg, which had a shell protecting the embryo. In addition, the embryo was surrounded by a liquid-filled sac containing a food sac and a waste sac.

Early reptiles were small, agile, and had more advanced teeth and jaws than amphibians.

During the Permian Period, more rapid and strong reptiles replaced the amphibians as the top carnivore on land

MASS EXTINCTION

Permo-Triassic Extinction Event - The largest mass extinction in the history of life on Earth. Nearly 95% of all species, including trilobites,

were wiped out. More than 65% of the amphibians and reptiles

did not survive. Almost 1/3 of all insects died, marking the only

major extinction in their 390 million year history.

WHAT HAPPENED???

Many believe the mass extinction is a result of a fatal combination of changes in sea level, reduced oxygen levels, massive volcanic eruptions, and global warming.