Major changes continued during the rest of the Paleozoic era

Geologists divide the remainder of the Paleozoic era into five periods: the Ordovician, Silurian, Devonian, Carboniferous, and Permian (see Table 22.1). Each period is characterized by the diversification of specific groups of organisms. Mass extinctions marked the ends of three periods: the Ordovician, Devonian, and Permian.

the ordovician (490-443 mya). During the Ordovician period, the continents, which were located primarily in the Southern Hemisphere, were still devoid of multicellular plants. Evolutionary radiation of marine organisms was spectacular during the early Ordovician, especially among animals that filter small prey from the water, such as bra-chiopods and mollusks. All animals lived on the seafloor or burrowed in its sediments. At the end of the Ordovician, as massive glaciers formed over Gondwana, sea levels dropped about 50 meters and ocean temperatures dropped. About 75 percent of the animal species became extinct, probably because of these major environmental changes.

the silurian (443-417 mya). During the Silurian period, the northern continents coalesced, but the general positions of the continents did not change much. Marine life rebounded from the mass extinction at the end of the Ordovician. Animals able to swim and feed above the ocean bottom appeared for the first time, but no new phyla of marine organisms evolved. The tropical sea was uninterrupted by land barriers, and most marine genera were widely distributed. On land, the first known tracheophytes (plants with true vascular tissue; see Chapter 29) appeared late in the Silurian period, about 420 mya. These plants, in the genus Cooksonia, were less than 50 cm tall and lacked roots and leaves (Figure 22.10). The first terrestrial arthropods—scorpions and millipedes—appeared at about the same time.

the devonian (417-354 mya). Rates of evolutionary change accelerated in many groups of organisms during the Devonian period. Both northern and southern land masses slowly moved northward (Figure 22.11a). There were great evolutionary radiations of corals and shelled squidlike cephalopods (Figure 22.11fr). Fishes diversified as jawed forms replaced jawless ones, and heavy armor gave way to the less rigid outer coverings of modern fishes. All current major groups of fishes were present by the end of the period.

Terrestrial communities also changed dramatically during the Devonian. Club mosses, horsetails, and tree ferns became common toward the end of the Devonian; some attained the size of trees. Their deep roots accelerated the weathering of rocks, resulting in the development of the first forest soils. Distinct floras evolved on the two major land masses toward the end of the period, and the ancestors of gymnosperms, the

Early Tracheophytes

22.10 Cooksonia,the Earliest Known Tracheophyte These plants were small and very simple in structure. However, they were true vascular plants (tracheophytes) with internal water-conducting cells (tra-cheids), well equipped to make the move from the aquatic to the terrestrial environment.This fossil of Cooksonia pertoni is from the Silurian (415 mya).

22.10 Cooksonia,the Earliest Known Tracheophyte These plants were small and very simple in structure. However, they were true vascular plants (tracheophytes) with internal water-conducting cells (tra-cheids), well equipped to make the move from the aquatic to the terrestrial environment.This fossil of Cooksonia pertoni is from the Silurian (415 mya).

first plants to produce seeds, appeared in the fossil record. The first known fossils of centipedes, spiders, mites, and insects date to this period. Fishlike amphibians began to occupy the land.

An extinction of about 75 percent of all marine species marked the end of the Devonian. Paleontologists are uncertain about the cause of this mass extinction, but two large meteorites that collided with Earth at that time, one in present-day Nevada and the other in Western Australia, may have been responsible.

the carboniferous (354-290 mya). Large glaciers formed over high-latitude Gondwana during the Carboniferous period, but extensive swamp forests grew on the tropical continents. These forests were not made up of the kinds of trees we know today, but were dominated by giant tree ferns and horsetails (see Figure 29.11). Fossilized remains of those trees formed the coal we now mine for energy.

The diversity of terrestrial animals increased greatly. Snails, scorpions, centipedes, and insects were abundant and diverse. Insects evolved wings, becoming the first animals to fly. Flight gave them access to tall plants; plant fossils from this period show evidence of chewing by insects. Amphibians became larger and better adapted to terrestrial existence. From one amphibian stock, the first reptiles evolved late in the period. In the seas, crinoids (sea lilies and feather stars) reached their greatest diversity, forming "meadows" on the seafloor (Figure 22.12)

Cambrian

Ordovician

Silurian

Devonian

Carboniferous

Permian

Triassic

Jurassic

Cretaceous

22.11 Devonian Continents and Marine Communities

(a) Positions of the continents during the Devonian period (417-354

mya). (b) This museum reconstruction depicts a Devonian coral reef.

Devonian Continents

22.11 Devonian Continents and Marine Communities

(a) Positions of the continents during the Devonian period (417-354

mya). (b) This museum reconstruction depicts a Devonian coral reef.

Cambrian

Ordovician

Silurian

Devonian

Carboniferous

Permian

Triassic

Jurassic

Cretaceous

543 490 443 417 354 290 248 206 144 Millions of years ago (mya)

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  • Semolina
    What animals lived in the paleozoic era?
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