The Permian Mass Extinction- 90-95% of marine species became extinct in the Permian
Life in the Permian: With the formation of the super-continent Pangea in the Permian, continental area exceeded that of oceanic area for the first time in geological history. The result of this new global configuration was the extensive development and diversification of Permian terrestrial vertebrate fauna and accompanying reduction of Permian marine communities. Among terrestrial fauna affected included insects, amphibians, reptiles (which evolved during the Carboniferous), as well as the dominant terrestrial group, the therapsids (mammal-like reptiles). The terrestrial flora was predominantly composed of gymnosperms, including the conifers. Life in the seas was similar to that found in middle Devonian communities following the late Devonian crisis. Common groups included the brachiopods, ammonoids, gastropods, crinoids, bony fish, sharks, and fusulinid foraminifera. Corals and trilobites were also present, but were exceedingly rare.

Organisms affected: The Permian mass extinction occurred about 248 million years ago and was the greatest mass extinction ever recorded in earth history; even larger than the previously discussed Ordovician and Devonian crises and the better known End Cretaceous extinction that felled the dinosaurs. Ninety to ninety-five percent of marine species were eliminated as a result of this Permian event. The primary marine and terrestrial victims included the fusulinid foraminifera, trilobites,rugose and tabulate corals, blastoids, acanthodians, placoderms, and pelycosaurs, which did not survive beyond the Permian boundary. Other groups that were substantially reduced included the bryozoans, brachiopods, ammonoids, sharks, bony fish, crinoids, eurypterids, ostracodes, and echinoderms.

Causes of the Permian Extinction:
Glaciation: Although the cause of the Permian mass extinction remains a debate, numerous theories have been formulated to explain the events of the extinction. One of the most current theories for the mass extinction of the Permian is an agent that has been also held responsible for the Ordovician and Devonian crises, glaciation on Gondwana. A similar glaciation event in the Permian would likely produce mass extinction in the same manner as previous, that is, by a global widespread cooling and/or worldwide lowering of sea level.

Plate Tectonics: The Formation of Pangea. Another theory which explains the mass extinctions of the Permian is the reduction of shallow continental shelves due to the formation of the super-continent Pangea. Such a reduction in oceanic continental shelves would result in ecological competition for space, perhaps acting as an agent for extinction. However, although this is a viable theory, the formation of Pangea and the ensuing destruction of the continental shelves occurred in the early and middle Permian, and mass extinction did not occur until the late Permian.

Climatic Fluctuations: A third possible mechanism for the Permian extinction is rapid warming and severe climatic fluctuations produced by concurrent glaciation events on the north and south poles. In temperate zones, there is evidence of significant cooling and drying in the sedimentological record, shown by thick sequences of dune sands and evaporites, while in the polar zones, glaciation was prominent. This caused severe climatic fluctuations around the globe, and is found by sediment record to be representative of when the Permian mass extinction occurred.

Volcanic Eruptions: The fourth and final suggestion that paleontologists have formulated credits the Permian mass extinction as a result of basaltic lava eruptions in Siberia. These volcanic eruptions were large and sent a quantity of sulphates into the atmosphere. Evidence in China supports that these volcanic eruptions may have been silica-rich, and thus explosive, a factor that would have produced large ash clouds around the world. The combination of sulphates in the atmosphere and the ejection of ash clouds may have lowered global climatic conditions. The age of the lava flows has also been dated to the interval in which the Permian mass extinction occurred. Click here to read more about the latest data found by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, that supports the view that extensive volcanic activity over the course of hundreds of thousands of years released large amounts of carbon dioxide and sulphur dioxide into the air, gradually warming up the planet.

Asteroid Impact:: Finally, a fifth theory to explain the mass extinction of Permian biota is the impact of an asteroid o coment at the end of the Permian, about 250 mya. Accoridng to this theory the Earth's most severe mass extinction - an event 250 million years ago that wiped out 90 percent of all marine species and 70 percent of land vertebrates - was triggered by a collision with a comet or asteroid, this theory is supported by  new findings by a team led by a University of Washington scientist. Click here to read more about this theory

More on the Buckyballs: Cosmic Stowaways as 'New Evidence Ties Mass Extinction to Massive Collision ' <http://news.nationalgeographic.com/news/2001/02/0222_buckyballs.html>
What are Buckyballs? - a much higher level of complex carbon molecules called buckminsterfullerenes, or Buckyballs, with the noble (or chemically nonreactive) gases helium and argon trapped inside their cage structures. Fullerenes, which contain at least 60 carbon atoms and have a structure resembling a soccer ball or a geodesic dome, are named for Buckminster Fuller, who invented the geodesic dome. See picture above

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The Origin of Vertebrates
A chordate (phylum Chordata) is an animal that has, at least during part of its life cycle, a notochord, a dorsal hollow nerve cord, and gill slits.  Vertebrates, which are animals with backbones, are simply a subphylum of chordates. The ancestors and early members of the phylum Chordata were soft-bodied organisms that left few fossils. As a result, we know little about the early evolutionary history of the chordates or vertebrates. Surprisingly, a close relationship exists between echinoderms and chordates, and they may even have shared a common ancestor (see phylogeny tree above). This is because in the developing embryo of echinoderms and chordates, cells divide by radial cleavage so that the cells are aligned directly above each other.  In all other invertebrates, cells undergo spiral cleavage, which results in having cells nested between each other in successive rows.

The Fishes: The most primitive vertebrates are fish, and some of the oldest fish remains are found in the Upper Cambrian Deadwood Formation in northeastern Wyoming. .Here phosphatic scales and plates of Anatolepis, a primitive member of the class Agnatha (jawless fish), have been recovered from marine sediments. All known Cambrian and Ordovician fossil fish have been found in shallow, nearshore marine deposits, whereas the earliest nonmarine (freshwater) fish remains have been found in Silurian strata. This does not prove that fish originated in the oceans, but it does lend strong support to the idea. As a group, fish range from the Late Cambrian to the present.

The Agnatha:  are the oldest and most primitive of class of fish and typified by  the ostracoderms, whose name means «bony skin" are . These are armored, jawless fish that first evolved during the Late Cambrian, reached their zenith during the Silurian and Devonian, and then became extinct. The majority of ostracoderms lived on the seafloor. A typical examples of ostracoderm are the genera Hemicyclaspis and Pteraspis

Hemicyclaspis which is a bottom-dwelling ostracoderm. Vertical scales allowed Hemicyclaspis to wiggle sideways, propelling itself along the seafloor, and the eyes on the top of its head allowed it to see such predators as cephalopods and jawed fish approaching from above. While moving along the sea bottom, it probably sucked up small bits of food and sediments through its jawless mouth.

Pteraspis, was more elongated and probably an activeswimmer, although it also seemingly fed on small pieces of food that it was able to suck up.

Primitive jawed fish: The evolution of jaws was a major evolutionary advance among primitive vertebrates. Although their jawless ancestors could only feed on detritus, jawed fish could chew food and become active predators, thus opening many new ecologic niches. The vertebrate jaw is an excellent example of evolutionary opportunism. Various studies suggest that the jaw originally evolved from the first three gill arches of jawless fish. Because the gills are soft, they are supported by gill arches of bone or cartilage. The evolution of the jaw may thus have been related to respiration rather than to feeding. By evolving joints in the forward gill arches, jawless fish could open their mouths wider. Every time a fish opened and closed its mouth, it would pump more water past the gills, thereby increasing the oxygen intake. The modification from rigid to hinged forward gill arches let fish increase both their food consumption and oxygen intake, and the evolution of the jaw as a feeding structure rapidly followed. The fossil remains of the first jawed fish are found in Lower Silurian rocks and belong to the acanthodians (Class Acanthodii), a group of small, enigmatic fish characterized by large spines, paired fins, scales covering much of the body, jaws, teeth, and greatly reduced body armor. Although their relationship to other fish is not well established, many scientists think the acanthodians included the probable ancestors of the present-day bony and cartilaginous fish groups. The acanthodians were most abundant during the Devonian, declined in importance through the Carboniferous, and became extinct during the Permian.

The other jawed fish, the placoderms (Class Placodermii), whose name means "plate-skinned," evolved during the Late Silurian. Placoderms were heavily armored, jawed fish that lived in both freshwater and the ocean, and, like the acanthodians, reached their peak of abundance and diversity during the Devonian.
The placoderms showed considerable variety, including small bottom dwellers, as well as large major predators such as Dunkleosteus, a Late Devonian fish that lived in the midcontinental North American epeiric seas. It was by far the largest fish of the time, reaching a length of more than 12 m. It had a heavily armored head and shoulder region, a huge jaw lined with razor-sharp bony teeth, and a flexible tail, all features consistent with its status as a ferocious predator.

Ages of Fish: Besides the abundant acanthodians, placoderms, and ostracoderms, other fish groups, such as the cartilaginous and bony fish, also evolved during the Devonian Pe¬riod. Small wonder, then, that the Devonian is informally called the "Age of Fish," because all major fish groups were present during this time period.

The cartilaginous fish: (Class Chrondrichthyes), represented today by sharks, rays, and skates, first evolved during the Early Devonian, and by the Late Devonian, primitive marine sharks such as Cladoselache were quite abundant. Cartilaginous fish have never been as numerous or as diverse as their cousins, the bony fish, but they were, and still are, important members of the marine vertebrate fauna.

The bony fish: (Class Oste¬ichthyes) also first evolved during the Devonian. Because bony fish are the most varied and numerous of all the fishes, and because the amphibians evolved from them, their evolutionary history is particularly important. There are two groups of bony fish: the common ray-finned fish (subclass Actinopterygii)  and the less familiar lobe-finned fish (subclass Sarcopterygii). The term ray-finned refers to the way the fins are supported by thin bones that spread away from the body. From a modest freshwater beginning during the Devonian, ray-finned fish, which include most of the familiar fish such as trout, bass, perch, salmon, and tuna, rapidly diversified to dominate the Mesozoic and Cenozoic seas. Present-day lobe-finned fish are characterized by muscular fins. The fins do not have radiating bones but rather have articulating bones with the fin attached to the body by a fleshy shaft. Such an arrangement allows for a powerful stroke of the fin, making the fish an effective swimmer. Three orders of lobe-finned fish are recognized: coelacanths, lungfish, and crossopterygians.

Coelacanths: (order Coelacanthimorpha) are marine lobe-finned fish that evolved during the Middle Devonian and were thought to have gone ex¬tinct at the end of the Cretaceous. In 1938, however, a fisherman caught a coelacanth in the deep waters off Madagascar and since then, several dozen more have been caught, both there and in Indonesia.

Lungfish: (order Dipnoi) were fairly abundant during the Devonian, but today only three freshwater genera exist, one each in South America, Africa, and Australia. Their present day distribution presumably reflects the Mesozoic breakup of Gondwana. The «lung" of a modern-day lungfish is actually a modified swim bladder that most fish use for buoyancy in swimming. In lungfish, this structure absorbs oxygen, allowing them to breath air when the lakes or streams in which they live become stagnant and dry up. During such times, they burrow into the sediment to prevent dehydration and breath through their swim bladder until the stream begins flowing or the lake they were living in fills with water. When they are back in the water, lungfish then rely on gill respiration.

The crossopterygians: (order Crossopterygii) are an important group of lobe-finned fish, because it is probably from them that amphibians evolved. However, the transition between crossopterygians and true amphibians is not as simple as it was once portrayed. The group of crossopterygians that appears to be ancestral to amphibians are rhipidistians. These fish, reaching lengths of over 2 m, were the dominant freshwater predators during the Late Paleozoic. Eusthenopteron, a good example of a rhipidistian crossopterygian and the classic example of the transitional form between fish and amphibians, had an elongated body that helped it move swiftly through the water and paired, muscular fins that many scientists thought could be used for moving on land. The structural similarity between crossopterygian fish and the earliest amphibians is striking and one of the most widely cited examples of a transition from one major group to another.. However, recent discoveries of older lobe-finned fish and tetrapods like Acanthostega, and newly published findings of tetrapod-like fish, are filling in the gaps in the time of the evolution between fish and tetrapods. Before discussing this transition and the evolution of amphibians, it is useful to place the evolutionary history of Paleozoic fish in the larger context of Paleozoic evolutionary events. Certainly, the evolution and diversification of jawed fish as well as eurypterids and ammonoids had a profound effect on the marine ecosystem. Previously defenseless organisms either evolved defensive mechanisms or suffered great losses, possibly even extinction.

Ostracoderms, although armored, would also have been easy prey for the swifter jawed fishes. Ostracoderms became extinct by the end of the Devonian, a time that coincides with the rapid evolution of jawed fish. Placoderms, like acanthodians, greatly decreased in abundance after the Devonian and became extinct by the end of the Paleozoic. In contrast, cartilaginous and ray-finned bony fish expanded during the Late Paleozoic, as did the ammonoid cephalopods, the other major predators of the Late Paleozoic seas.

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Amphibians- Vertebrates Invade the land
Although amphibians were the first vertebrates to live on land, they were not the first land-living organisms. Land plants, which probably evolved from green algae,
first evolved during the Ordovician. Furthermore, insects, millipedes, spiders, and even snails invaded the land before amphibians. Fossil evidence indicates that such land-dwelling arthropods as scorpions and flightless insects had evolved by at least the Devonian. The transition from water to land required animals to surmount several barriers. The most critical were desiccation, reproduction, the effects of gravity, and the extraction of oxygen from the atmosphere by lungs rather than from water by gills. Up until the 1990s, the traditional evolutionary sequence had a Rhipidistian crossopterygian, like Eusthenopteron, evolving into a primitive
amphibian like Ichthyostega. At that time, fossils of those two genera were about all paleontologists had to work with, and although there were gaps in morphology, the link between crossopterygians and these earliest amphibians was easy to see. Crossopterygians already had a backbone and limbs that could be used for walking and lungs that could extract oxygen. The oldest amphibian fossils, on the other hand, found in the Upper Devonian Old Red Sandstone of eastern Greenland and belonging to such genera as Ichthyostega, had streamlined bodies, long tails, and fins along their backs, in addition to four legs, a strong backbone, a ribcage, and pelvic and pectoral girdles, all of which were structural adaptations for walking on land. These earliest amphibians thus appear to have inherited many characteristics from the crossopterygians with little modification. However, with the discovery of such fossils as Acanthostega and others like it, the transition between fish and amphibians involves a number of new genera that are intermediary between the two groups. Panderichthys, a large (up to 1.3 m long), Late Devonian (~380 million years ago) lobe-finned fish from Latvia, was essentially a contemporary of Eusthenopteron. It had a large tetrapod-like head with a pointed snout, dorsally lo¬cated eyes, and modifications to that part of the skull related to the ear region. From paleoenvironmental evidence, Panderichthys lived in shallow tidal flats or estuaries, using its lobe fins to maneuver around in the shallow waters. Acanthostega, a Late Devonian (365 million years ago) tetrapod  seemed to be the perfect intermediary between fish and true land-dwelling tetrapods. However, its limbs could not support its weight on land, and thus it was an aquatic animal, using its limbs to navigate in water, rather than walking on land. In 2006, an exciting discovery of a 1.2-2.8 m long, 375-million-year-old (Late Devonian) "fishapod" was announced. Discovered on Ellesmere Island, Canada, Tiktaalik roseae, from the Inuktitut meaning "large fish in a stream," was hailed as an intermediary between the lobe-finned fish like Panderichthys and the earliest tetrapod, Acanthostega

Tiktaalik roseae is truly a "fishapod" in that it has a mixture of both fish and tetrapod characteristics. For example, it has gills and fish scales but also a broad skull, eyes on top of its head, a flexible neck and large rib cage that could support its body on land or in shal¬low water, and lungs, all of which are tetrapod features. What really excited scientists, however, was that Tiktaalik roseae has the beginnings of a true tetrapod forelimb, com¬plete with functional wrist bones and five digits, as well as a modified ear region. Sedimentological evidence suggests Tiktaalik roseae lived in a shallow water habitat associated with Late Devonian floodplains of Laurasia.

the oldest known amphib¬ian, Ichthyostega, had skeletal features that allowed it to spend its life on land. Because amphibians did not evolve until the Late Devonian, they were a minor element of the Devonian terrestrial ecosystem. Like other groups that moved into new and previously unoccupied niches, am¬phibians underwent rapid adaptive radiation and became abundant during the Carboniferous and Early Permian.

The Late Paleozoic amphibians did not at all resemble the familiar frogs, toads, newts, and salamanders that make up the modern amphibian fauna. Rather, they displayed a broad spectrum of sizes, shapes, and modes of life. One group of amphibians were the labyrinthodonts, so named for the labyrinthine wrinkling and folding of the chewing surface of their teeth. Most labyrinthodonts were large animals, as much as 2 m in length. These typically sluggish creatures lived inswamps and streams, eating fish, vegetation, insects, and other small amphibians. Labyrinthodonts were abundant during the Carboniferous when swampy conditions were widespread  but soon declined in abundance during the Permian, perhaps in response to changing climatic conditions. Only a few species survived into the Triassic.

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Evolution of Reptiles: The Land is Conquered - Early Reptiles
Amphibians were limited in colonizing the land because they had to return to water to lay their gelatinous eggs. The evolution of the amniote egg freed reptiles from this constraint. In such an egg, the developing embryo is surrounded by a liquid-filled sac called the amnion and provided with both a yolk, or food sac, and an allantois, or waste sac. In this way the emerging reptile is in essence a miniature adult, bypassing the need for a larval stage in the water. The evolution of the amniote egg allowed vertebrates to colonize all parts of the land, because they no longer had to return to the water as part of their reproductive cycle. Many of the differences between amphibians and reptiles are physiologic and are not preserved in the fossil record. Nevertheless, amphibians and reptiles differ sufficiently in skull structure, jawbones, ear location, and limb and vertebral construction to suggest that reptiles evolved from labyrinthodont ancestors by the Late Mississippian. This assessment is based on the discovery of a well-preserved fossil skeleton of the oldest known reptile, Westlothiana, and other fossil reptile skeletons from Late Mississippian-aged rocks in Scotland. Other early reptile fossils occur in the Lower Pennsylvanian Joggins Formation in Nova Scotia, Canada. Here remains of Hylonomus are found in the sediments filling in tree trunks. These earliest reptiles from Scotland and Canada were small and agile and fed largely on grubs and insects. They are loosely grouped together as protorothyrids, whose members include the earliest known reptiles. During the Permian Period, reptiles diversified and began displacing many amphibians. The reptiles succeeded partly because of their advanced method of reproduction and their more advanced jaws and teeth, as well as their ability to move rapidly on land.

The pelycosaurs, or finback reptiles, evolved from the protorothyrids during the Pennsylvanian and were the dominant reptile group by the Early Permian. They evolved into a diverse assemblage of herbivores, exemplified by the herbivore Edaphosaurus and carnivores such as Dimetrodon. An interesting feature of the pelycosaurs is their sail. It was formed by vertebral spines that, in life, were covered with skin. The sail has been variously explained as a type of sexual display, a means of protection, and a display to look more ferocious. The current consensus seems to be that the sail served as some type of thermoregulatory device, raising the reptile's temperature by catching the sun's rays or cooling it by facing the wind. Because pely¬cosaurs are considered the group from which therapsids evolved, it is interesting that they may have had some sort of body-temperature control. The pelycosaurs became extinct during the Permian and were succeeded by the therapsids, mammal-like reptiles that evolved from the carnivorous pelycosaur lineage and rapidly diversified into herbivorous and carnivorous lineages.

Therapsids were small- to medium-sized animals that displayed the beginnings of many mammalian features: fewer bones in the skull, because many of the small skull bones were fused; enlarged lower jawbone; differentiation of teeth for various functions such as nipping, tearing, and chewing food; and more vertically placed legs for greater flexibility, as opposed to the way the legs sprawled out to the side in primitive reptiles. In addition, many paleontologists think therapsids were endothermic, or warm-blooded, enabling them to maintain a constant internal body temperature. This characteristic would have let them expand into a variety of habitats, and indeed, the Permian rocks in which their fos¬sil remains are found are distributed not only in low latitudes but in middle and high latitudes as well.
As the Paleozoic Erathem came to an end, the therapsids constituted about 90% of the known reptile genera and occupied a wide range of ecologic niches. The mass extinctions that decimated the marine fauna at the close of the Paleozoic had an equally great effect on the terrestrial population. By the end of the Permian, about 90% of all marine invertebrate species were extinct, compared with more than two-thirds of all amphibians and reptiles. Plants, in contrast, apparently did not experience as great a turnover as animals.

Clich here to review the Evolution of Plants
 
 
 
 
 
 

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