Chapter 12 Late Paleozoic World

Chapter 12 – Late Paleozoic World

12.1 Introduction to the Late Paleozoic Era

· During this time the collision and suturing of continents that began with earlier orogenies continued

o By the end of the Paleozoic, further collisions produced the supercontinent Pangaea

· Most of the second half of the Paleozoic is marked by widespread glaciation over the South Pole

· Plants spread to continental interiors, resulting in decreased CO2 and increased O2 in the atmosphere

o Extensive terrestrial forests opened new habitats and niches for insects, amphibians and reptiles

o As coal-forming swamps declined during the Permian, they gave way to the gymnosperms

· The largest and most complex of all mass extinctions occurred at the end of the Permian

12.2 Tectonic Cycle: Impacts on the Hydrosphere, Atmosphere, and Rock Cycle

· Sea Level, CO2, and Sedimentary Facies

o As Panagea was assembled, a number of smaller ocean basins were destroyed and replaced by one large ocean, Panthalassa

o The large number of continental collisions destroyed spreading centers and subduction zones, while uplifting continental crust

o Even though sea level remained relatively high during the Late Paleozoic, sea level reached its lowest point of the entire Phanerozoic

· Ocean Circulation and Chemistry

o Pangaea blocked the east-to-west flow of tropical waters around the planet

o The resulting circulation patterns established a great range of climatic extremes along the margins of Pangaea

o Southern hemisphere glaciers may have increased ocean circulation and oxygenation of deep water masses during the Late Paleozoic

o Nutrient runoff to the oceans may have increased as a result of orogeny and the spread of land plants

o The mineralogy of Late Paleozoic calcareous hard parts secreted by benthic invertebrates tended to consist of aragonite, which crystallizes more easily when the concentration of dissolved magnesium ions in seawater is high

12.3 Tectonic Cycle and Orogeny

· Alleghenian and Related Orogenies

o The supercontinent of Laurasia began to form as a result of the collisions of North America, Europe, Greenland, and parts of Asia

o Gondwana collided with Laurasia to produce the Hercynian, Alleghanian and Ouachita orogenies

o The mountains formed by these orogenies would eventually stretch as a continuous belt across southern Europe to southwestern Laurentia

· Ancestral Rockies

o Unlike the other Paleozoic orogenies, the episodes of uplift that produced the Ancestral Rockies occurred well within the craton

o The Ancestral Rockies are thought to have been formed by high-angle faulting in response to compressive stresses generated during the collision of Gondwana with Laurasia

· Sonoma Orogeny

o The western margin of North America had already shifted from a passive to an active margin with the Late Devonian Antler Orogeny

o During the Late Permian (and continuing into the Triassic), a volcanic island arc accreted in the area of what is now California in the Sonoma Orogeny

o The orogenic activity during the Sonoma Orogeny was similar to that of the Antler Orogeny, but accreted a much larger portion of crust to North America

12.4 Impact of Orogeny on Earth Systems

· Great climatic extremes developed on Earth due to the assembly of Pangaea

o Megamonsoons must have alternately dumped tremendous amounts of rain and carries dry air from the continental interior back out over the oceans

· Distinctine sedimentary sequences called cyclothems developed in lush coal forming swamps during the Permo-Carboniferous of eastern North America

· Later in the Permian, the area of interior drainage increased and the coal swamps gave way to drier conditions

o The coal swamps were replaced by forests dominated by gymnosperms (seed plants)

12.5 Diversification of the Marine Biosphere

· Plankton and Other Microfossils

o The Paleozoic Fauna (which had replaced the Cambrian Fauna duringthe Early Paleozoic) continued to dominate the fossil record until the end of the Permian

o However, the dominant members of both the benthos and the plankton were much changed by ecologic replacement in the Late Paleozoic

o Notable developments in the benthic microfossil record include the diversification of foraminifera, particularly the fusilinids

· Benthic Ecosystems

o A number of prominent Early-to-Middle Paleozoic taxa (such as trilobites and tabulate and rugose corals) persisted to the end of the Paleozoic, but with much diminished diversity

o Changes in the types of bryozoans occurred; ammonoids (which had decreased during Late Devonian extinctions) rediversified, while nautiloids declined

o Articulate brachiopods also rediversified, especially the spiny productids

o The Late Devonian extinctions had also decimated the stromatoporoid reefs; these were placed in the Mississippian by loose mounds of dead skeletal debris from organisms such as crinoids

o Well developed reefs did not reappear until the Permian, when the Permian Reef Complex of west Texas and New Mexico formed

12.6 Diversification of the Terrestrial Biosphere

· Terrestrial Floras

o Late Paleozoic coal-forming swamps were dominated by several types of spore-bearing plants, including the lycopods and sphenoisids

o These ancestral lineages gave rise to the progymnosperms during the Middle and Late Devonian

o Seed ferns (which resembled spore-bearing ferns but were actually primitive gymnosperms) appeared in the Late Devonian; in the Late Paleozoic they diversified to produce the Glossopteris flora on Gondwana

o The vast, dry continental interiors of Pangaea caused the gymnosperms (including conifers) to dominate the terrestrial floras during the Middle-to-Late Permian

· Terrestrial Floras and Oxygen

o Because of the enormous amount of carbon sequestered in living forests and dead plant debris, the oxygen content of the atmosphere is calculated to have increased to about 30% during the Pennsylvanian (compared to less than 20% before and after this time)

· Invertebrate Life on Land

o The fossil record of insects from the Pennsylvanian indicates that wings had appeared, possibly having evolved from temperature regulatory organs

o High oxygen levels during the Late Paleozoic may account for an increase in insect forms, especially among flying forms

· Vertebrates

o Amphibians continued to diversify into both herbivores and carnivores

o A major evolutionary innovation leading to the success of reptiles was the amniote egg; like the seed, it freed terrestrial vertebrates from water, allowing them to exploit drier habitats

o By the Pennsylvanian, two amniote lineages had diverged: the reptiles and the synapsids

o The fin-backed synapsids were the prominent terrestrial vertebrate taxon in the Late Paleozoic; they later declined because an advanced synapsid group, the therapsids, arose from the cynodonts

o Like other reptiles, many synapsids were ectothermic, although the sailbacks may have used their sail to aid in regulating body temperature

o In addition to other advanced (mammal-like) traits in jaw structure, tooth form, and possible body covering of hair, therapsids may also have been endothermic

12.7 Multiple Causes of Extinction?

· By the end of the Permian the largest of all mass extinctions in Earth’s history occurred on land and in the sea

o Somewhere between 80 and 95% of marine species disappeared, many of them tropical

o Many groups that had survived other extinctions perished (such as trilobites, rugose and tabulate coral)

· The exact cause or causes of the Late Permian extinctions is unknown

o Evidence points to ocean anoxia and rapid warming that resulted from massive volcanism; sea level fall, extraterrestrial impact, and methane release from gas hydrates may also have played a role

o The Modern Marine Fauna, which had lurked in the background during most of the Paleozoic, rediversified in the Mesozoic

o The reptiles would come to rule in the gymnosperm forests of the Mesozoic on land