This image depicts a 'buckyball' fullerene with noble gas trapped inside (University of Washington)
Reading Assignment: Chapters 9, and 10
Phanerozoic EonothemThe Upper Paleozoic
Click here for a summary of Paleozoic Bio-events Important Facts to Remember
The Evolutionary Processes of Phanerozoic Biota; From Eukaryotes to Animalia Paleoecology: || Types of marine environments Life of Land: Trees grew in swamps: Lycopods Biological Features of the Carboniferous System Biological Features of the Permian System The Permian Mass extinction involving: Fusulinids (Foraminifera), Rugose corals, Tabulate corals, Bryozoans, Ammonoids DVD: BBC-The Day The Earth Nearly Died. Click here to view a summary of this TV program Additional reference on the Permian mass extinction: Hoffmann, J.H., When life nearly came to an end, National Geographic 198(3):100113,2000. Animals diversified on land and invaded freshwater habitats
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.
Great coal-forming forests developed as a result of rare freezing temperatures and a warm, humid climate. In the closed swamps, accumulating layers of decaying plant matter produced numerous layers of coal.
One of the greatest evolutionary innovations of the Carboniferous was the amniote egg, which allowed for the further exploitation of the land by certain tetrapods. The amniote egg allowed the ancestors of birds, mammals, and reptiles to reproduce on land by preventing the desiccation of the embryo inside. There was also a trend towards mild temperatures during the Carboniferous, as evidenced by the decrease in lycopods and large insects and an increase in the number of tree ferns.
Scale trees (lepidodendrons) grew to 35 meters (115 feet) forming dense forests. The forerunners of conifers formed and there was a large variety of ferns at ground level.
The beginning of the Carboniferous generally had a more uniform, tropical, and humid climate throughout the year than exists today. Seasons if any were indistinct. These observations are based on comparing the morphology of the plants that exist in the fossil record with plants that are present today. The morphology of the Carboniferous plants resembles the plants that live in tropical and mildly temperate areas today. Many of them lack growth rings, suggesting a uniform climate. This uniformity in climate may have been the result of the large expanse of ocean that covered the entire surface of the globe except for a small, localized section where Pangea, the massive supercontinent that existed during the late Paleozoic and early Triassic, was forming during the Carboniferous.
Shallow, warm, marine waters often flooded the continents. Attached filter feeders such as bryozoans, particularly fenestellids, were abundant in this environment, and the sea floor was dominated by brachiopods. Trilobites were increasingly scarce while foraminifers were abundant. The heavily armored fish from the Devonian became extinct, being replaced with fish fauna that look more modern.
Coelacanths (see' la kanth) swam in streams along with other lung-finned and lobe-finned fish that developed during the period.
Foraminifera (tiny, mostly marine animals that were microscopic to near-microscopic with shell-like chambers) became so abundant that numerous limestone structures were found in Indiana.
Amphibians increased in numbers but remained rather small. Most were less than 20 centimeters (8 inches) long, with the largest growing to about 2 meters (6.5 feet) long.
Near the end of the Mississippian, uplift and erosion of the continents occurred, causing an increase in the number of floodplains and deltas present. The deltaic environment supports fewer corals, crinoids, blastoids, cryozoans, and bryzoans, which were abundant earlier in the Carboniferous. Bryozoa built lacy, moss-like structures and were the most successful shellfish of the period.
Freshwater clams first appear along with an increase in gastropod, bony fish, and shark diversity. At first glance, it may seem that the marine habitat has grown allowing the diversity of marine life to increase, but in actuality, the movement of the continents to form one large continental mass decreased the sea coast area.
The amount of space available for marine life declined, and the sea levels all over the world fluctuated because of the presence of two large ice sheets at the southern pole which suck up large amounts of water and lock it away from the water cycle as ice. Because so much water is taken out of the water cycle, the sea levels drop leading to the mass extinction of shallow marine invertebrates, the gradual decline of swamps, and the increase in terrestrial habitat. These effects are reversed when the glaciers start to recede, releasing the water that they had stored as ice back into the oceans, flooding the swamps again and the floodplains. Carboniferous rock formations often occur in patterns of stripes with shale and coal seams alternating, indicating the cyclic flooding and drying of an area.
The uplift of the continents caused a transition to a more terrestrial environment during the Pennsylvanian period. Swamp forests as well as terrestrial habitats became common and widespread. In the swamp forests, the vegetation was marked by the numerous different groups that were present. Seedless plants such as lycopsids were extremely important in this community and are the primary source of carbon for the coal that is characteristic of the period. The lycopods underwent a major extinction event after a drying trend, most likely caused by the advance of glaciers.
Ferns and sphenopsids became more important later during the Carboniferous, and the earliest relatives of the conifers appeared. The first land snails appeared, and insects with wings that can't fold back such as dragonflies and mayflies flourished and radiated. These insects, as well as millipedes, scorpions, and spiders became important in the ecosystem.
The trend towards
aridity and an increase in terrestrial habitat lead to the increasing importance
of the amniotic egg for reproduction. The earliest amniote fossil was the
lizard-like Hylonomus, which was lightly built with deep, strong jaws and
slender limbs. The basal tetrapods became more diverse during the Carboniferous.
Fish-like bodies were replaced with large predators with long snouts, short
sprawling limbs and flattened heads such as temnospondyls. Anthracosaurs
(basal tetrapods and amniotes with deep skulls and a less sprawling body
plan which led to increasing agility) appeared during the Carboniferous
and were quickly followed by diapsids which divided into two groups: the
marine reptiles, lizards, and snakes versus the archosaurs (crocodiles,
dinosaurs, and birds). The synapsids also made their first appearance.
The cooler climate allowed conifer trees to become the dominate woody plant. The insect population expanded in variety and numbers, but their overall size decreased. The amphibians continued to increase in variety, but their size too remained small.
Reptiles began to increase in variety on land along two lines: Long bodies with short legs (ex: pelycosaur with large fins which might have assisted in regulating temperature); and therapsids which showed changes in jaw structure with teeth that later appeared in mammals. In addition, this second group of reptiles carried their bodies off the ground and chewed their food using canines, incisors, and molars.
The distinction between the Paleozoic and the Mesozoic is made at the end of the Permian in recognition of the largest mass extinction recorded in the history of life on Earth. It affected many groups of organisms in many different environments, but it affected marine communities the most by far, causing the extinction of most of the marine invertebrates of the time.
The extinction marked a time in which about 75% of the existing amphibian and 80% of the reptile families died out. A possible reason for the extinction was a combination of overspecialization and climate changes caused by marine mountain building.
Some groups survived the Permian mass extinction in greatly diminished numbers, but they never again reached the ecological dominance they once had, clearing the way for another group of sea life.
On land, a relatively smaller extinction of diapsids and synapsids cleared the way for other forms to dominate, and led to what has been called the "Age of Dinosaurs". Also, the great forests of fern-like plants shifted to gymnosperms, plants with their offspring enclosed within seeds. Modern conifers, the most familiar gymnosperms of today, first appear in the fossil record of the Permian. In all, the Permian was the last of the time for some organisms and a pivotal point for others, and life on earth was never the same again.
that have been used as index fossils include brachiopods, ammonoids, fusilinids,
conodonts, and other marine invertebrates. Some genera occur within such
specific time frames that strata are named for them and permit stratigraphic
identification through the presence or absence of specified fossils.
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.
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.
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
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.
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.
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.