Lower Mesozoic: ch13, ch15
 Week 8, , Chapters 13 and 15
Phanerozoic Eonothem The Lower Mesozoic

Ammonite from the Lower Mesozoic of NE Mexico, note its large size, 1.6 m in maximum diameter

Unit (Chapter) Plan:
Biological Features
Geological Features
Paleontological Features
Back to Course Syllabus || Back to Course Plan (weekly schedule)
Visit this web site: UCMP   to review the History of Life of  this interval of time
 Biological Features:
  • The Evolutionary Processes of Phanerozoic Biota; From Eukaryotes to Animalia
  • Paleoecology: || Types of marine environments
  • Life of Land:
  • The K/T Mass extinction: Crisis in the history of life
  • Animals diversified on land and invaded freshwater habitats
    Click here for a summary of Mesozoic Bio-events:  ||  Dinosauria
    Important Facts to Remember
    Geological Features:
    Read Chapters nine and ten  of your textbook
    Review the story of plate tectonics
    For a more simplistic model visit this site <http://www.pbs.org/wgbh/aso/tryit/tectonics>
    Paleogeography: The  Fragmentation of Pangaea begins in the upper Triassic
    Paleontological Features:
    • Divisions of time
    • Chronstratigraphic Divisions: The Triassic System || The Jurassic System ||The Cretaceous System
    • The Invertebrates: Protista: Foraminifera || Mollusks: gastropods, bivalves, cephalopods ||
    • The Plants:  The angiosperms: Flowering plants

    Phanerozoic Eonothem

     See Chronostratigraphic  Division of the Mesozoic  Eratherm

    Phanerozoic Eonothem:

    This Eonothem is often referred to as the tine of "Visible Life". Organisms with skeletons or hard shells appeared by the first time in the geological record. The Phanerozoic Eonothem spans from 543 mya  through today. The Phanerozoic is divided into three Erathems, from older to younger: Paleozoic, Mesozoic, and Cenozoic.
    The Mesozoic Erathem:  248  to 65 mya
    The Mesozoic is divided into three time systems: the Triassic (245-208 Million Years Ago), the Jurassic (208-146 Million Years Ago), and the Cretaceous (146-65 Million Years Ago). 

    Mesozoic means "middle life", and is the time during which the world fauna changed drastically from that which had been seen in the Paleozoic. Dinosaurs, which are perhaps the most popular organisms of the Mesozoic, evolved in the Triassic, but were not very diverse until the Jurassic. Except for birds, dinosaurs became extinct at the end of the Cretaceous. Some of the last dinosaurs to have lived are found in the late Cretaceous deposits of Montana in the United States

    The Mesozoic was also a time of great change in the terrestrial vegetation. The Lower Mesozoic was dominated by ferns, cycads, ginkgophytes,bennettitaleans, and other unusual plants. Modern gymnosperms, such as conifers, first appeared in their current recognizable forms in the early Triassic. By the middle part of the Cretaceous System, the earliest angiosperms had appeared and began to diversify, largely taking over from the other plant groups. 

    The breakup of Pangaea can be divided into four stages: a. The first stage involved the separation of North America from Africa during the Late Triassic, followed by the sepa­ration of North America from South America. b. The second stage involved the separation of Antarctica, India, and Australia from South America and Africa during the Jurassic. During this stage, India broke away from the still-united Antarctica and Australia landmass. c. During the third stage, South America separated from Africa, while Europe and Africa began converging. d. In the last stage, Greenland separated from North America and Europe.


    The  Lower Mesozoic = Triassic + Jurassic Sytems
     The Lower Mesozoic is an informal division of the Mesozoic Erathem which includes the lower two systems: the Triassic and the Jurassic. See Chronostratigraphic Chart
    Paleogeography: Triassic <http://www.scotese.com/newpage8.htm>
                              Jurassic <http://www.scotese.com/jurassic.htm>
                              Cretaceous <http://www.scotese.com/cretaceo.htm>
    Summary of Lower Mesozoic Events: Important  facts in the history of Mesozoic life
    The Mesozoic is divided into three time systems: the Triassic (245-208 Million Years Ago), the Jurassic (208-146 Million Years Ago), and the Cretaceous (146-65 Million Years Ago).
    Mesozoic means "middle life", and is the time during which the world fauna changed drastically from that which had been seen in the Paleozoic. Dinosaurs, which are perhaps the most popular organisms of the Mesozoic, evolved in the Triassic, but were not very diverse until the Jurassic. Except for birds, dinosaurs became extinct at the end of the Cretaceous. Some of the last dinosaurs to have lived are found in the late Cretaceous deposits of Montana in the United States.
    The Mesozoic was also a time of great change in the terrestrial vegetation. The Lower Mesozoic was dominated by ferns, cycads, ginkgophytes, bennettitaleans, and other unusual plants. Modern gymnosperms, such as conifers, first appeared in their current recognizable forms in the early Triassic. By the middle part of the Cretaceous System, the earliest angiosperms had appeared and began to diversify, largely taking over from the other plant groups.
    The breakup of Pangaea can be divided into four stages: a. The first stage involved the separation of North America from Africa during the Late Triassic, followed by the sepa¬ration of North America from South America. b. The second stage involved the separation of Antarctica, India, and Australia from South America and Africa during the Jurassic. During this stage, India broke away from the still-united Antarctica and Australia landmass. c. During the third stage, South America separated from Africa, while Europe and Africa began converging. d. In the last stage, Greenland separated from North America and Europe.
    The breakup of Pangaea influenced global climatic and at¬mospheric circulation patterns. While the temperature gradi¬ent from the tropics to the poles gradually increased during the Mesozoic, overall global temperatures remained equable.
    Among the marine invertebrates, survivors of the Permian ex¬tinction diversified and gave rise to increasingly complex Mesozoic marine invertebrate communities.
    Triassic and Jurassic land-plant communities were composed of seedless plants and gymnosperms. Angiosperms, or flower¬ing plants, evolved during the Early Cretaceous, diversified rapidly, and soon became the dominant land plants.
    Dinosaurs evolved during the Upper Triassic, but were most abundant and diverse during the Jurassic and Cretaceous. Based on pelvic structure, two distinct orders of dinosaurs are recognized-Saurischia (lizard-hipped) and Ornithischia (bird¬hipped).
    Pterosaurs were the first flying vertebrate animals. Small pterosaurs were probably active, wing-flapping fliers, while large ones may have depended more on thermal updrafts and soaring to stay aloft. At least one pterosaur species had hair or feathers, so it was likely endothermic.
    The fish-eating, porpoise-like ichthyosaurs were thoroughly adapted to an aquatic life. Female ichthyosaurs probably re¬tained eggs within their bodies and gave birth to live young. Plesiosaurs were heavy-bodied marine reptiles that probably came ashore to lay eggs.
    Crocodiles become the dominant freshwater predators during the Jurassic. Turtles and lizards were present during most of the Mesozoic, but by the Cretaceous snakes evolved from lizards.
    Birds probably evolved from small carnivorous dinosaurs. The oldest known bird, Archaeopteryx) appeared during the Jurassic, but few other Mesozoic birds are known. Protoavis in Triassic rocks may represent a bird older than Archaeopteryx.
    The earliest mammals evolved during the Upper Triassic, but they are difficult to distinguish from advanced cynodonts. Details of the teeth, the middle ear, and lower jaw are used to distinguish the two.
    Several types of Mesozoic mammals existed, but all were small, and their diversity was low. A group of Mesozoic mammals known as eupantotheres gave rise to both marsupials and pla¬centals during the Cretaceous.
    Because the continents were close together during much of the Mesozoic and climates were mild even at high latitudes, animals and plants dispersed widely.
    Mesozoic mass extinctions account for the disappearance of dinosaurs, several other groups of reptiles, and a number of marine invertebrates. One hypothesis holds that the extinc¬tions were caused by the impact of a large meteorite. Many pa¬leontologists reject the meteorite proposal and claim that withdrawal of epeiric seas and climatic changes can account for these extinctions.


    Introduction to the Tetrapoda: The Four-Legged Vertebrates

    The word "Tetrapoda" means "four legs" in Greek. Amphibians, reptiles (including dinosaurs and birds) and mammals are the major groups of the Tetrapoda.

    The Dinosauria

    Dinosaurs, one of the most successful groups of animals (in terms of longevity) that have ever lived, evolved into many diverse sizes and shapes, with many equally diverse modes of living. The term "Dinosauria" was invented by Sir Richard Owen in 1842 to describe these "fearfully great reptiles," specifically Megalosaurus, Iguanodon, and Hylaeosaurus, the only three dinosaurs known at the time. The creatures that we normally think of as dinosaurs lived during the Mesozoic Era, from late in the Triassic period (about 225 million years ago) until the end of the Cretaceous (about 65 million years ago). But we now know that they actually live on today as the birds.
    Some things to keep in mind about dinosaurs:
    Not everything big and dead is a dinosaur. All too often, books written (or movies made) for a popular audience include animals such as mammoths, mastodons, pterosaurs, plesiosaurs, ichthyosaurs, and the sail-backed Dimetrodon.
    Dinosaurs are a specific subgroup of the archosaurs, a group that also includes crocodiles, pterosaurs, and birds. Although pterosaurs are closely related to dinosaurs, they are not true dinosaurs. Even more distantly related to dinosaurs are the marine reptiles, which include the plesiosaurs and ichthyosaurs.
    Mammoths and mastodons are mammals and did not appear until many millions of years after the close of the Cretaceous period.
    Dimetrodon is neither a reptile nor a mammal, but a basal synapsid, i.e., an early relative of the ancestors of mammals.
    Not all dinosaurs lived at the same time. Different dinosaurs lived at different times. Despite the portrayals in movies like King Kong and Jurassic Park, no Stegosaurus ever saw a Tyrannosaurus, because Tyrannosaurus didn't appear on the scene until 80 or so million years following the extinction of stegosaurs. The same goes for Apatosaurus ("Brontosaurus") — it was already well-fossilized by the time T. rex came along.
    The Dinosaurs are not extinct: Technically, based on features of the skeleton, most people studying dinosaurs consider birds to be dinosaurs.
    This shocking realization makes even the smallest hummingbird a legitimate dinosaur. So rather than refer to "dinosaurs" and birds as discrete, separate groups, it is best to refer to the traditional, extinct animals as "non-avian dinosaurs" and birds as well as birds, or "avian dinosaurs." It is incorrect to say that dinosaurs are extinct, because they have left living descendants in the form of cockatoos, cassowaries, and their pals — just like modern vertebrates are still vertebrates even though their Cambrian ancestors are long extinct. So at thanksgiving what you eat is a dinosaur we called a turkey


    The Evolutionary Process of Phanerozoic Life
    Major groups of animals are already present in the Phanerozoic, at the beginning  of the Cambrian System, thier phylogenetic development is shown in the following diagrams as shown below:

    The opisthokonts (Greek: ??????- (opisth?-) = "rear, posterior" + ?????? (kontos) = "pole" i.e. flagellum) are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the phylum Choanozoa of the protist kingdom. Both genetic and ultrastructural studies strongly support that opisthokonts form a monophyletic group. One common characteristic is that flagellate cells, such as most animal sperm and chytrid spores, propel themselves with a single posterior flagellum. This gives the groups its name. In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagella.

    See Biological Principles for a review of these concepts
    Anapsida: amniote group whose skull does not have openings near the temples. The clade Anapsida includes turtles and all their extinct relatives. The anapsids once consisted of many groups, many of which could be considered to have been quite successful until their extinction. Today, only one group of anapsids remains ( Chelonia -- the turtles), which truly could be called an evolutionary success story.

    Diapsids ("two arches"):  are a group of tetrapod animals that developed two holes (temporal fenestra) in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all birds, crocodiles, lizards, snakes, and tuataras.  All members of the group called the Reptilia, except for the anapsids (turtles and their ilk), and a few extinct groups, are diapsids. The main diagnostic physical character for a diapsid is the presence of two openings on each side of the skull; the upper and lower temporal openings. Even the birds are considered diapsids (and hence reptiles), because they are descended from certain dinosaurs (which are also diapsids), and ancestrally have the paired skull openings along with other physical characteristics that unite them with diapsids. Thus, they are considered diapsids by their ancestry, which is illuminated by shared derived traits.

    Synapsids ('fused arch') also known as Theropsids ('beast face'), and traditionally described as 'mammal-like reptiles' , are a group of amniotes (the other being the sauropsids) that developed one opening in their skull (temporal fenestra) behind each eye, about 320 million years ago (mya) during the late Carboniferous Period. The mammals of today are but one branch of the Synapsida, a great vertebrate group with a 300 million year history. Pre-mammalian synapsids -- including the famous "finback" Dimetrodon dominated the land vertebrate fauna of the Permian and early Triassic before losing ground to the diversifying dinosaurs and other archosaurs. These pre-mammalian groups of synapsids have at times been called "mammal-like reptiles". This term is now discouraged because although many had characteristics in common with mammals, none of them were actually reptiles.

    Euryapsida are a group of tetrapod animals that are distinguished by a single opening behind the orbit (temporal fenestra). They are different from Synapsida by the precise placement of the opening below. It is now commonly believed that euryapsids are in fact diapsids (having two fenestrae) that lost the upper temporal fenestra.
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    The K/T  Extinction- 90-95% of marine species became extinct:

    Paleogeography: Cretaceous <http://www.scotese.com/cretaceo.htm>
                              The K/T <http://www.scotese.com/K/t.htm>

    Organisms affected:

    Causes of the K/T Extinction:

    Plate Tectonics:

    Climatic Fluctuations:

    Volcanic Eruptions:

    Asteroid Impact:


    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.

    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 pelycosaurs 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 fossil 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.


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