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Paleozoic Life History


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Title: Paleozoic Life History

Chapter 13
Paleozoic Life History Vertebrates and Plants
Tetrapod Footprint Discovery
  • Tetrapod trackway
  • at Valentia Island Ireland
  • These fossilized fooprints
  • which are more than 365 million years old
  • are evidence of one of the earliest four-legged
    animals on land
  • Photo courtesy of Ken Higgs, U. College Cork,

Tetrapod Footprint Discovery
  • The discovery in 1992 of fossilized Devonian
    tetrapod footprints
  • more than 365 million years old
  • has forced paleontologists to rethink
  • how and when animals emerged onto land
  • The newly discovered trackway
  • has helped shed light on the early evolution of
  • the name is from the Greek tetra, meaning four
    and podos, meaning foot
  • Based on the footprints, it is estimated
  • that the creature was longer than 3 ft
  • and had fairly large back legs

Tetrapod Wader
  • Furthermore, instead of walking on dry land
  • this animal was probably walking or wading around
    in a shallow, tropical stream,
  • filled with aquatic vegetation and predatory fish
  • This hypothesis is based on the fact that
  • the trackway showed no evidence of a tail being
    dragged behind it
  • Unfortunately, there are no bones associated with
    the tracks
  • to help in reconstructing what this primitive
    tetrapod looked like

Why Limbs?
  • One of the intriguing questions paleontologists
    ask is
  • why did limbs evolve in the first place?
  • It probably wasn't for walking on land
  • In fact, many scientists think
  • aquatic limbs made it easier to move around
  • in streams, lakes, or swamps
  • that were choked with water plants or other
  • The scant fossil evidence also seems to support
    this hypothesis

Unable to Walk on Land
  • Fossils of Acanthostega,
  • a tetrapod found in 360 million year old rocks
    from Greenland,
  • reveals an animal with limbs,
  • but one clearly unable to walk on land
  • Paleontologist Jenny Clack,
  • who recovered hundreds of specimens of
  • points out that Acanthostega's limbs were not
    strong enough to support its weight on land,
  • and its ribcage was too small for the necessary
    muscles needed to hold its body off the ground

Acanthostega Had Gills and Lungs
  • In addition, Acanthostega had gills and lungs,
  • meaning it could survive on land, but was more
    suited for the water
  • Clack believes that Acanthostega
  • used its limbs to maneuver around
  • in swampy, plant-filled waters,
  • where swimming would be difficult
  • and limbs would be an advantage

Unanswered Questions
  • Fragmentary fossils
  • from other tetrapods living at about the same
    time as Acanthostega
  • suggest that some of these early tetrapods
  • may have spent more time on dry land than in the
  • At this time, there are many more unanswered
  • about the evolution of the earliest tetrapods
  • than there are answers
  • However, this is what makes the study of
    prehistoric life so interesting and exciting

Vertebrates and Plants
  • Previously, we examined the Paleozoic history of
  • beginning with the acquisition of hard parts
  • and concluding with the massive Permian
  • that claimed about 90 of all invertebrates
  • and more than 65 of all amphibians and reptiles
  • In this section, we examine
  • the Paleozoic evolutionary history of vertebrates
    and plants

Transition from Water to Land
  • One of the striking parallels between plants and
  • is the fact that in passing from water to land,
  • both plants and animals had to solve the same
    basic problems
  • For both groups,
  • the method of reproduction was the major barrier
  • to expansion into the various terrestrial
  • With the evolution of the seed in plants and the
    amniote egg in animals,
  • this limitation was removed, and both groups were
    able to expand into all the terrestrial habitats

Vertebrate Evolution
  • A chordate (Phylum Chordata) is an animal that
  • 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

Characteristics of Chordates
  • The structure of the lancelet Amphioxus
    illustrates the three characteristics of a
  • a notochord, a dorsal hollow nerve cord, and gill

Phylum Chordata
  • The ancestors and early members of the phylum
  • were soft-bodied organisms that left few fossils
  • so little is known of the early evolutionary
    history of the chordates or vertebrates
  • Surprisingly, a close relationship exists between
    echinoderms and chordates
  • They may even have shared a common ancestor,
  • because the development of the embryo is the same
    in both groups
  • and differs completely from other invertebrates

A Very Old Chordate
  • Yunnanozoon lividum is one of the oldest known
  • Found in 525 Myr old rocks in Yunnan province,
  • 5 cm-longanimal

Spiral Versus Radial Cleavage
  • Echinoderms and chordates
  • have similar
  • embryonic development
  • In the arrangement of cells resulting from
    spiral cleavage, (a) at the left,
  • cells in successive rows are nested between each
  • In the arrangement of cells resulting from radial
    cleavage, (b) at the right,
  • cells in successive rows are directly above each
  • This arrangement exists in both chordates and

Echinoderms and Chordates
  • Both echinoderms and chordates have similar
  • biochemistry of muscle activity
  • blood proteins,
  • and larval stages
  • The evolutionary pathway to vertebrates
  • thus appears to have taken place much earlier and
    more rapidly
  • than many scientists have long thought

Hypothesis for Chordate Origin
  • Based on fossil evidence and recent advances in
    molecular biology,
  • vertebrates may have evolved shortly after an
    ancestral chordate acquired a second set of genes
  • the ancestor probably resembled Yunnanozoon
  • According to this hypothesis,
  • a random mutation produced a duplicate set of
  • allowing the ancestral vertebrate animal to
    evolve entirely new body structures
  • that proved to be evolutionarily advantageous
  • Not all scientists accept this hypothesis and the
    evolution of vertebrates is still hotly debated

  • The most primitive vertebrates are fish
  • and some of the oldest fish remains are found in
    Upper Cambrian rocks
  • All known Cambrian and Ordovician fossil fish
  • have been found in shallow nearshore marine
  • while the earliest nonmarine fish remains have
    been found in Silurian strata
  • This does not prove that fish originated in the
  • but it does lend strong support to the idea

Fragment of Primitive Fish
  • A fragment of a plate from Anatolepis cf. A.
    Heintzi from the Upper Cambrian marine Deadwood
    Formation of Wyoming
  • Anatolepis is one of the oldest known fish
  • a primitive member of the class Agnatha (jawless

Ostracoderms Bony Skinned Fish
  • As a group, fish range from the Late Cambrian to
    the present
  • The oldest and most primitive of the class
    Agnatha are the ostracoderms
  • whose name means bony skin
  • These are armored jawless fish that first evolved
    during the Late Cambrian
  • reached their zenith during the Silurian and
  • and then became extinct

Geologic Ranges of Major Fish Groups
Bottom-Dwelling Ostracoderms
  • The majority of ostracoderms lived on the
  • Hemicyclaspis is a good example of a
    bottom-dwelling ostracoderm
  • Vertical scales allowed Hemicyclaspis to wiggle
  • propelling itself along the seafloor
  • while the eyes on the top of its head allowed it
    to see predators approaching from above
  • such as cephalopods and jawed fish
  • While moving along the sea bottom,
  • it probably sucked up small bits of food and
    sediments through its jawless mouth

Devonian Seafloor
  • Recreation of a Devonian seafloor showing

an acanthodian (Parexus)
a ray-finned fish (Cheirolepis)
  • a placoderm (Bothriolepis)

an ostracoderm (Hemicyclaspis)
Swimming Ostracoderm
  • Another type of ostracoderm,
  • represented by Pteraspis
  • was more elongated and probably an active
  • although it also seemingly fed on small pieces of
    food it could suck up

Evolution of Jaws
  • The evolution of jaws
  • was a major evolutionary advantage
  • among primitive vertebrates
  • While their jawless ancestors
  • could only feed on detritus
  • jawed fish
  • could chew food and become active predators
  • thus opening many new ecological niches
  • The vertebrate jaw is an excellent example of
    evolutionary opportunism
  • The jaw probably evolved from the first three
    gill arches of jawless fish

Evolutionary Opportunism
  • Because the gills are soft
  • they are supported by gill arches composed of
    bone or cartilage
  • The evolution of the jaw may thus have been
    related to respiration rather than 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
  • Hinged forward gill arches enabled fish to also
    increase their food consumption
  • the evolution of the jaw for feeding followed

Evolution of Jaws
  • The evolution of the vertebrate jaw
  • is thought to have occurred
  • from the modification of the first two or three
    anterior gill arches
  • This theory is based on the comparative anatomy
    of living vertebrates

  • The fossil remains of the first jawed fish are
    found in Lower Silurian rocks
  • and belong to the acanthodians,
  • a group of enigmatic fish
  • characterized by
  • large spines,
  • scales covering much of the body,
  • jaws,
  • teeth,
  • and reduced body armor

Acanthodians most abundant during Devonian
  • Although their relationship to other fish has not
    been well established,
  • many scientists think the acanthodians
  • included the probable ancestors of the
  • bony and cartilaginous fish groups
  • The acanthodians were most abundant during the
  • declined in importance through the Carboniferous,
  • and became extinct during the Permian

Other Jawed Fish
  • The other jawed fish
  • that evolved during the Late Silurian were the
  • whose name means plate-skinned
  • 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 exhibited considerable variety,
  • including small bottom dwellers
  • as well as large major predators such as
  • a late Devonian fish
  • that lived in the mid-continental North American
    epeiric seas
  • It was by far the largest fish of the time
  • attaining 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

Late Devonian Marine Scene
  • A Late Devonian marine scene from the
    midcontinent of North America

Age of Fish
  • Many fish evolved during the Devonian Period
  • the abundant acanthodians
  • placoderms,
  • ostracoderms,
  • and other fish groups,
  • such as the cartilaginous and bony fish
  • It is small wonder, then, that the Devonian is
    informally called the Age of Fish
  • because all major fish groups were present during
    this time period

Cartilaginous Fish
  • Cartilaginous fish,
  • class Chrondrichthyes,
  • represented today by
  • sharks, rays, and skates,
  • first evolved during the Middle Devonian
  • and by the Late Devonian,
  • primitive marine sharks
  • such as Cladoselache were quite abundant

Cartilaginous Fish Not Numerous
  • Cartilaginous fish have never been
  • as numerous nor as diverse
  • as their cousins,
  • the bony fish,
  • but they were, and still are,
  • important members of the marine vertebrate fauna
  • Along with cartilaginous fish,
  • the bony fish, class Osteichthyes,
  • also first evolved during the Devonian

Ray-Finned Fish
  • 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
  • There are two groups of bony fish
  • the common ray-finned fish
  • and the less familiar lobe-fined fish
  • The term ray-finned refers to
  • the way the fins are supported by thin bones that
    spread away from the body

Ray-Finned and Lobe-Finned Fish
  • Arrangement of fin bones for
  • (a) a typical ray-finned fish
  • (b) a lobe-finned fish
  • Muscles extend into the fin
  • allowing greater flexibility

Ray-Finned Fish Rapidly Diversified
  • From a modest freshwater beginning during the
  • 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

Lobe-Finned Fish
  • Present-day lobe-finned fish are characterized by
    muscular fins
  • The fins do not have radiating bones
  • but rather articulating bones
  • with the fin attached to the body by a fleshy
  • Two major groups of lobe-finned fish are
  • lungfish
  • and crossopterygians

Lungfish Fish
  • Lungfish were fairly abundant during the
  • 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
  • Studies of present-day lung fish indicate that
    lungs evolved
  • from saclike bodies on the ventral side of the

Lungfish Respiration
  • These saclike bodies enlarged
  • and improved their capacity for oxygen
  • eventually evolving into lungs
  • When the lakes or streams in which lungfish live
  • become stagnant and dry up,
  • they breathe at the surface
  • or burrow into the sediment to prevent
  • When the water is well oxygenated,
  • however, lungfish rely upon gill respiration

Amphibians Evolved from Crossopterygians
  • The crossopterygians are an important group of
    lobe-finned fish
  • because amphibians evolved from them
  • During the Devonian, two separate branches of
    crossopterygians evolved
  • One led to the amphibians,
  • while the other invaded the sea

  • The crossopterygians that invaded the sea,
  • called the coelacanths,
  • were thought to have become extinct at the end of
    the Cretaceous
  • In 1938, however,
  • fisherman caught a coelacanth in the deep waters
    of Madagascar,
  • and since then several dozen more have been
  • both there and in Indonesia

Rhipidistians Ancestors of Amphibians
  • The group of crossopterygians
  • that is ancestral to amphibians
  • are rhipidistians
  • These fish, attaining lengths of over 2 m,
  • were the dominant freshwater predators
  • during the Late Paleozoic

Amphibian Ancestor
  • Eusthenopteron,
  • a good example of a rhipidistian crossopterygian,
  • had an elongate body
  • that enabled it to move swiftly in the water,
  • as well as paired muscular fins that could be
    used for locomotion on land
  • The structural similarity between crossopterygian
  • and the earliest amphibians is striking
  • and one of the better documented transitions
  • from one major group to another

  • Eusthenopteron,
  • a member of the rhipidistian crossopterygians
  • had an elongate body
  • and paired fins
  • that it could use to move about on land
  • The crossopterygians are thought to be amphibian

Fish/Amphibian Comparison
  • Similarities between the crossopterygian
    lobe-finned fish and the labyrinthodont amphibians
  • Their skeletons were similar

Comparison of Limbs
  • Comparison of the limb bones
  • of a crossopterygian (left) and an amphibian
  • Color identifies the bones that the two groups
    have in common

Comparison of Teeth
  • Comparison of tooth cross sections show
  • the complex and distinctive structure found in
  • both crossopterygians (left) and amphibians

Paleozoic Evolutionary Events
  • Before discussing this transition
  • and the evolution of amphibians,
  • we should place the evolutionary history of
    Paleozoic fish
  • in the larger context of Paleozoic evolutionary
  • Certainly, the evolution and diversification of
    jawed fish
  • as well as eurypterids and ammonoids
  • had a profound effect on the marine ecosystem

Defenseless Organisms
  • Previously, defenseless organisms either
  • evolved defensive mechanisms
  • or suffered great losses, possibly even
  • Recall that trilobites
  • experienced major extinctions at the end of the
  • recovered slightly during the Ordovician,
  • then declined greatly from the end of the
  • to their ultimate demise at the end of the Permian

Extinction by Predation
  • Perhaps their lightly calcified external covering
  • made them easy prey
  • for the rapidly evolving jawed fish and
  • Ostracoderms,
  • although armored,
  • would also have been easy prey
  • for the swifter jawed fishes
  • Ostracoderms became extinct by the end of the
  • a time that coincides with the rapid evolution of
    jawed fish

Late Paleozoic Changes
  • Placoderms also became extinct by the end of the
  • while acanthodians decreased in abundance after
    the Devonian
  • and became extinct by the end of the Paleozoic
  • On the other hand, cartilaginous and ray-finned
    bony fish
  • expanded during the Late Paleozoic,
  • as did the ammonoid cephalopods,
  • the other major predator of the Late Paleozoic

AmphibiansVertebrates 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
  • first evolved during the Ordovician
  • Furthermore, insects, millipedes, spiders,
  • and even snails invaded the land before amphibians

Land-Dwelling Arthropods Evolved by the Devonian
  • Fossil evidence indicates
  • that such land-dwelling arthropods as scorpions
    and flightless insects
  • had evolved by at least the Devonian

Water to Land Barriers
  • The transition from water to land required that
    several barriers be surmounted
  • The most critical for animals were
  • desiccation,
  • reproduction,
  • the effects of gravity,
  • and the extraction of oxygen
  • from the atmosphere
  • by lungs rather than from water by gills

Problems Partly Solved
  • These problems were partly solved by the
  • they already had a backbone and limbs
  • that could be used for walking
  • and lungs that could extract oxygen

Oldest Amphibians
  • The oldest amphibian fossils are found
  • in the Upper Devonian Old Red Sandstone of
    eastern Greenland
  • These amphibians,
  • which belong to genera like Ichthyostega,
  • had streamlined bodies, long tails, and fins
  • In addition, they had
  • four legs, a strong backbone, a rib cage, and
    pelvic and pectoral girdles,
  • all of which were structural adaptations for
    walking on land

A Late Devonian Landscape
  • A Late Devonian Landscape in the eastern part of
  • Ichthyostega was an amphibian that grew to a
    length of about 1 m
  • The flora was diverse,
  • consisting of a variety of small and large
    seedless vascular plants

Amphibians Minor Element of the Devonian
  • The earliest amphibians
  • appear to have had many characteristics
  • that were inherited from the crossopterygians
  • with little modification
  • Because amphibians did not evolve until the Late
  • they were a minor element of the Devonian
    terrestrial ecosystem

Rapid Adaptive Radiation
  • Like other groups that moved into new and
    previously unoccupied niches,
  • amphibians underwent rapid adaptive radiation
  • and became abundant during the Carboniferous and
    Early Permian
  • The Late Paleozoic amphibians
  • did not 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 was 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 in swamps and streams,
  • eating fish, vegetation, insects, and other small

Labyrinthodont Teeth
  • Labyrinthodonts are named for the labyrinthine
    wrinkling and folding of the chewing surface of
    their teeth

Carboniferous Coal Swamp
  • Reconstruction of a Carboniferous coal swamp

Large labyrinthodont amphibian Eryops
Carboniferous Coal Swamp
  • Reconstruction of a Carboniferous coal swamp

Larval Branchiosaurus
Carboniferous Coal Swamp
  • Reconstruction of a Carboniferous coal swamp

The serpentlike Dolichosoma
Labyrinthodont Decline
  • Labyrinthodonts were abundant during the
  • when swampy conditions were widespread,
  • but soon declined in abundance
  • during the Permian,
  • perhaps in response to changing climactic
  • Only a few species survived into the Triassic

Evolution of the Reptiles the Land is Conquered
  • 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

Amniote Egg
  • In an amniote egg,
  • the embryo is
  • surrounded by a liquid sac
  • the amnion cavity
  • and provided with a food source
  • yolk sac
  • and waste sac
  • allantois
  • Its evolution freed reptiles
  • to inhabit all parts of the land

Able to Colonize All Parts of the Land
  • In this way the emerging reptile is
  • in essence a miniature adult,
  • bypassing the need for a larval stage in the
  • The evolution of the amniote egg allowed
  • to colonize all parts of the land
  • because they no longer had to return
  • to the water as part of their reproductive cycle

Amphibian/Reptile Differences
  • Many of the differences between amphibians and
    reptiles are physiological
  • 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
  • based on the discovery of a well-preserved
  • of the oldest known reptile, Westlothiana, from
    Late Mississippian-age rocks in Scotland

Earliest Reptiles
  • Some of the oldest known reptiles are from
  • 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 were small and agile
  • and fed largely on grubs and insects

One of the Oldest Known Reptiles
  • Reconstruction and skeleton of Hylonomus lyelli
    from the Pennsylvanian Period
  • Fossils of this animal have been collected from
    sediments that filled tree stumps
  • Hylonomus lyelli was about 30 cm long

Permian Diversification
  • The earliest reptiles are loosely grouped
    together as protorothyrids,
  • whose members include the earliest reptiles
  • During the Permian Period, reptiles diversified
  • and began displacing many amphibians
  • The success of the reptiles is due partly
  • to their advanced method of reproduction
  • and their more advanced jaws and teeth,
  • as well as their ability to move rapidly on land

Paleozoic Reptile Evolution
  • Evolutionary relationship among the Paleozoic

PelycosaursFinback Reptiles
  • 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 Edaphosaurus,
  • and carnivores
  • such as Dimetrodon

Pelycosaurs (Finback Reptiles)
  • Most pelycosaurs have a characteristic sail on
    their back

The herbivore Edaphosaurus
The carnivore Dimetrodon
Pelycosaurs Sails
  • 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
  • but...

Pelycosaurs Sail Function
  • 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 to be the
  • from which therapsids (mammal-like reptiles)
  • it is interesting that they may have had some
    sort of body-temperature control

TherapsidsMammal-like Reptiles
  • The pelycosaurs became extinct during the Permian
  • and were succeeded by the therapsids,
  • mammal-like reptiles
  • that evolved from the carnivorous pelycosaur
  • and rapidly diversified into
  • herbivorous
  • and carnivorous lineages

  • A Late Permian scene in southern Africa showing
    various therapsids
  • Many paleontologists think therapsids were
  • and may have had a covering of fur
  • as shown here

Therapsid Characteristics
  • Therapsids were small- to medium-sized animals
  • displaying the beginnings of many mammalian
  • fewer bones in the skull due to fusion of many of
    the small skull bones
  • enlargement of the lower jawbone
  • differentiation of the teeth for various
    functions such as nipping, tearing, and chewing
  • and a more vertical position of the legs for
    greater flexibility,
  • as opposed to the sideways sprawling legs in
    primitive reptiles

Endothermic Therapsids
  • Many paleontologists think therapsids were
  • or warm-blooded,
  • enabling them to maintain a constant internal
    body temperature
  • This characteristic would have allowed them
  • to expand into a variety of habitats,
  • and indeed the Permian rocks
  • in which their fossil remains are found
  • have a wide latitudinal distribution

Permian Mass Extinction
  • As the Paleozoic Era came to an end,
  • the therapsids constituted about 90 of the known
    reptile genera
  • and occupied a wide range of ecological niches
  • The mass extinctions
  • that decimated the marine fauna
  • at the close of the Paleozoic
  • had an equally great effect on the terrestrial

Losses Fewer for Plants
  • By the end of the Permian,
  • about 90 of all marine invertebrate species were
  • compared with more than two-thirds of all
    amphibians and reptiles
  • Plants, on the other hand,
  • apparently did not experience
  • as great a turnover as animals did

Plant Evolution
  • When plants made the transition from water to
  • they had to solve most of the same problems that
    animals did
  • desiccation,
  • support,
  • and the effects of gravity
  • Plants did so by evolving a variety of structural
  • that were fundamental to the subsequent
  • and diversification that occurred
  • during the Silurian, Devonian, and later periods

Plant Evolution
  • Major events in the Evolution of Land Plants
  • The Devonian Period was a time of rapid evolution
    for the land plants
  • Major events were
  • the appearance of leaves
  • heterospory
  • secondary growth
  • and emergence of seeds

Marine, then Fresh, then Land
  • Most experts agree
  • that the ancestors of land plants
  • first evolved in a marine environment,
  • then moved into a freshwater environment
  • and finally onto land
  • In this way the differences in osmotic pressures
  • between salt and freshwater
  • were overcome while the plant was still in the
  • The higher land plants are composed of two major
  • the nonvascular
  • and vascular plants

Vascular Versus Nonvascular
  • Most land plants are vascular,
  • meaning they have a tissue system
  • of specialized cells
  • for the movement of water and nutrients
  • The nonvascular plants,
  • such as bryophytes
  • liverworts, hornwarts, and mosses
  • and fungi,
  • do not have these specialized cells
  • and are typically small
  • and usually live in low moist areas

Earliest Land Plants
  • The earliest land plants
  • from the Middle to Late Ordovician
  • were probably small and bryophyte-like in their
    overall organization
  • but not necessarily related to bryophytes
  • The evolution of vascular tissue in plants was an
    important step
  • as it allowed for the transport of food and water
  • Probable vascular plant megafossils
  • and characteristic spores indicate
  • to many paleontologists
  • that the evolution of vascular plants
  • occurred well before the Middle Silurian

Features Resembling Present Land Plants
  • Sheets of cuticlelike cells
  • that is, the cells
  • that cover the surface
  • of present-day land plants
  • and tetrahedral clusters
  • that closely resemble the spore tetrahedrals of
    primitive land plants
  • have been reported from Middle to Upper
    Ordovician rocks
  • from western Libya and elsewhere

Ancestor of Terrestrial Vascular Plants
  • The ancestor of terrestrial vascular plants
  • was probably some type of green algae
  • While no fossil record of the transition
  • from green algae to terrestrial vascular plants
  • comparison of their physiology reveals a strong
  • Primitive seedless vascular plants
  • such as ferns
  • resemble green algae in their pigmentation,
  • important metabolic enzymes,
  • and type of reproductive cycle

Transitions from Salt to Freshwater to Land
  • Furthermore, the green algae are one of the few
    plant groups
  • to have made the transition from salt water to
  • The evolution of terrestrial vascular plants from
    an aquatic plant,
  • probably of green algal ancestry
  • was accompanied by various modifications
  • that allowed them to occupy
  • this new an harsh environment

Vascular Tissue Also Gives Strength
  • Besides the primary function
  • of transporting water and nutrients throughout a
  • vascular tissue also provides
  • some support for the plant body
  • Additional strength that acts to counteract
    gravity is derived
  • from the organic compounds lignin and cellulose,
  • which are found throughout a plant's walls

Problems of Desiccation and Oxidation
  • The problem of desiccation
  • was circumvented by the evolution of cutin,
  • an organic compound
  • found in the outer-wall layers of plants
  • Cutin also provides additional resistance
  • to oxidation,
  • the effects of ultraviolet light,
  • and the entry of parasites

  • Roots evolved in response to
  • the need to collect water and nutrients from the
  • and to help anchor the plant in the ground
  • The evolution of leaves
  • from tiny outgrowths on the stem
  • or from branch systems
  • provided plants with
  • an efficient light-gathering system for

Silurian and Devonian Floras
  • The earliest known vascular land plants
  • are small Y-shaped stems
  • assigned to the genus Cooksonia
  • from the Middle Silurian of Wales and Ireland
  • Upper Silurian and Lower Devonian species are
    known from
  • Scotland, New York State and the Czech Republic,
  • These earliest plants were
  • small, simple, leafless stalks
  • with a spore-producing structure at the tip

Earliest Land Plant
  • The earliest known fertile land plant was
  • seen in this fossil from the Upper Silurian of
    South Wales
  • Cooksonia consisted of
  • upright, branched stems
  • terminating in sporangia
  • It also had a resistant cuticle
  • and produced spores typical of vascular plants
  • These plants probably lived in moist environments
    such as mud flats
  • This specimen is 1.49 cm long

Earliest Land Plant
  • The earliest plants
  • are known as seedless vascular plants
  • because they do not produce seeds
  • They also did not have a true root system
  • A rhizome,
  • the underground part of the stem,
  • transferred water from the soil to the plant
  • and anchored the plant to the ground
  • The sedimentary rocks in which these plant
    fossils are found
  • indicate that they lived in low, wet, marshy,
    freshwater environments

Parallel between Seedless Vascular Plants and
  • An interesting parallel can be seen between
    seedless vascular plants and amphibians
  • When they made the transition from water to land,
  • they had to overcome the problems such a
    transition involved
  • Both groups,
  • while successful,
  • nevertheless required a source of water in order
    to reproduce

Plants and Amphibians
  • In the case of amphibians,
  • their gelatinous egg had to remain moist
  • while the seedless vascular plants
  • required water for the sperm to travel through
  • to reach the egg

Seedless Vascular Plants Evolved
  • From this simple beginning,
  • the seedless vascular plants
  • evolved many of the major structural features
  • characteristic of modern plants such as
  • leaves,
  • roots,
  • and secondary growth
  • These features did not all evolve simultaneously
  • but rather at different times,
  • a pattern known as mosaic evolution

Adaptive Radiation
  • This diversification and adaptive radiation
  • took place during the Late Silurian and Early
  • and resulted in a tremendous increase in
  • During the Devonian,
  • the number of plant genera remained about the
  • yet the composition of the flora changed

Early Devonian Plants
  • Reconstruction of an Early Devonian landscape
  • showing some of the earliest land plants

Dawsonites /
- Bucheria
Early and Late Devonian Plants
  • Whereas the Early Devonian landscape
  • was dominated by relatively small,
  • low-growing,
  • bog-dwelling types of plants,
  • the Late Devonian
  • witnessed forests of large tree-size plants up to
    10 m tall

Evolution of Seeds
  • In addition to the diverse seedless vascular
    plant flora of the Late Devonian,
  • another significant floral event took place
  • The evolution of the seed at this time
  • liberated land plants
  • from their dependence on moist conditions
  • and allowed them
  • to spread over all parts of the land

Seedless Vascular Plants Require Moisture
  • Seedless vascular plants require moisture
  • for successful fertilization
  • because the sperm must travel to the egg
  • on the surface of the gamete-bearing plant
  • gametophyte
  • to produce a successful spore-generating plant
  • sporophyte
  • Without moisture, the sperm would dry out before
    reaching the egg

Seedless Vascular Plant
  • Generalized life history of a seedless vascular
  • The mature sporophyte plant produces spores
  • which upon germination grow into small
    gametophyte plants

Seedless Vascular Plant
  • The gametophyte plants produce sperm and eggs
  • The fertilized eggs grow into
  • the spore-producing mature plant
  • and the sporophyte-gametophyte life cycle begins

Reproduction by Seed
  • In the seed method of reproduction,
  • the spores are not released to the environment
  • as they are in the seedless vascular plants
  • but are retained
  • on the spore-bearing plant,
  • where they grow
  • into the male and female forms
  • of the gamete-bearing generation

  • In the case of the gymnosperms,
  • or flowerless seed plants,
  • these are male and female cones
  • The male cone produces pollen,
  • which contains the sperm
  • and has a waxy coating to prevent desiccation,
  • while the egg,
  • or embryonic seed,
  • is contained in the female cone
  • After fertilization,
  • the seed then develops into a mature,
    cone-bearing plant

Gymnosperm Plants
  • Generalized life history of a gymnosperm plant
  • The mature plant bears both
  • male cones that produce sperm-bearing pollen
  • and female cones that contain embryonic seeds

Gymnosperm Plants
  • Pollen grains are transported to the female cones
    by the wind
  • Fertilization occurs when the sperm moves through
    a moist tube growing from the pollen grain
  • and unites with the embryonic seed

Gymnosperm Plants
  • producing a fertile seed
  • which then grows into a cone-bearing mature plant

Gymnosperms Free to Migrate
  • In this way the need for a moist environment
  • for the gametophyte generation is solved
  • The significance of this development
  • is that seed plants,
  • like reptiles,
  • were no longer restricted
  • to wet areas
  • but were free to migrate
  • into previously unoccupied dry environments

Heterospory, an Intermediate Step
  • Before seed plants evolved,
  • an intermediate evolutionary step was necessary
  • This was the development of heterospory,
  • whereby a species produces two types of spores
  • a large one (megaspore)
  • that gives rise to the female gamete-bearing
  • and a small one (microspore)
  • that produces the male gamete-bearing plant
  • The earliest evidence of heterospory
  • is found in the Early Devonian plant
  • Chaleuria cirrosa,
  • which produced spores of two distinct sizes

An Early Devonian Plant
  • Chaleuria cirrosa
  • from New Brunswick, Canada
  • was heterosporous, producing two spore sizes

An Early Devonian Plant
  • This heterosporous plant is reconstruction here
  • Chaleuria cirrosa

Spores of Chaleuria cirrosa
  • The two spore types of Chaleuria cirrosa
  • shown at about the same relative scale

Evolution of Conifer Seed Plants
  • The appearance of heterospory
  • was followed several million years later
  • by the emergence of progymnosperms
  • Middle and Late Devonian plants
  • with fernlike reproductive habit
  • and a gymnosperm anatomy
  • which gave rise in the Late Devonian
  • to such other gymnosperm groups as
  • the seed ferns
  • and conifer-type seed plants

Plants in Swamps Versus Drier Areas
  • While the seedless vascular plants
  • dominated the flora of the Carboniferous
    coal-forming swamps,
  • the gymnosperms
  • made up an important element
  • of the Late Paleozoic flora,
  • particularly in the non-swampy areas

Late Carboniferous and Permian Floras
  • The rocks of the Pennsylvanian Period
  • Late Carboniferous
  • are the major source of the world's coal
  • Coal results from
  • the alteration of plant remains
  • accumulating in low swampy areas
  • The geologic and geographic conditions of the
  • were ideal for the growth of seedless vascular
  • and consequently these coal swamps had a very
    diverse flora

Pennsylvanian Coal Swamp
  • Reconstruction of a Pennsylvanian coal swamp
  • with its characteristic vegetation

Amphibian Eogyrinus
Coal-Forming Pennsylvanian Flora
  • It is evident from the fossil record
  • that whereas the Early Carboniferous flora
  • was similar to its Late Devonian counterpart,
  • a great deal of evolutionary experimentation was
    taking place
  • that would lead to the highly successful Late
    Paleozoic flora
  • of the coal swamps and adjacent habitats
  • Among the seedless vascular plants,
  • the lycopsids and sphenopsids
  • were the most important coal-forming groups
  • of the Pennsylvanian Period

  • The lycopsids were present during the Devonian,
  • chiefly as small plants,
  • but by the Pennsylvanian,
  • they were the dominant element of the coal
  • achieving heights up to 30 m in such genera as
    Lepidodendron and Sigillaria
  • The Pennsylvanian lycopsid trees are interesting
  • because they lacked branches except at their top

  • The leaves were elongate and similar to the
    individual palm leaf of today
  • As the trees grew,
  • the leaves were replaced from the top,
  • leaving prominent and characteristic rows or
    spirals of scars on the trunk
  • Today, the lycopsids are represented by small
    temperate-forest ground pines

  • The sphenopsids,
  • the other important coal-forming plant group,
  • are characterized by being jointed and having
    horizontal underground stem-bearing roots
  • many of these plants, such as Calamites, average
    5 to 6 m tall
  • Living sphenopsids include the horsetail
  • Equisetum
  • and scouring rushes
  • Small seedless vascular plants and seed ferns
  • formed a thick undergrowth or ground cover
    beneath these treelike plants

  • Living sphenopsids include the horsetail Equisetum

Plants on Higher and Drier Ground
  • Not all plants were restricted to the
    coal-forming swamps
  • Among those plants occupying higher and drier
    ground were some of the cordaites,
  • a group of tall gymnosperm trees
  • that grew up to 50 m
  • and probably formed vast forests

A Cordaite Forest
  • A cordaite forest from the Late Carboniferous
  • Cordaites were a group of gymnosperm trees that
    grew up to 50 m tall

  • Another important non-swamp dweller was
    Glossopteris, the famous plant so abundant in
  • whose distribution is cited as critical evidence
    that the continents have moved through time

Climatic and Geologic Changes
  • The floras that were abundant
  • during the Pennsylvanian
  • persisted into the Permian,
  • but due to climatic and
  • geologic changes resulting from tectonic events,
  • they declined in abundance and importance
  • By the end of the Permian,
  • the cordaites became extinct,
  • while the lycopsids and sphenopsids
  • were reduced to mostly small, creeping forms

Gymnosperms Diversified
  • Those gymnosperms
  • with lifestyles more suited to the warmer and
    drier Permian climates
  • diversified and came to dominate the Permian,
    Triassic, and Jurassic landscapes

  • Chordates are characterized by
  • a notochord,
  • dorsal hollow nerve cord,
  • and gill slits
  • The earliest chordates were soft-bodied organisms
  • that were rarely fossilized
  • Vertebrates are a subphylum of the chordates

  • Fish are the earliest known vertebrates
  • with their first fossil occurrence in Upper
    Cambrian rocks
  • They have had a long and varied history
  • including jawless and jawed armored forms
  • ostracoderms and placoderms
  • cartilaginous forms, and bony forms
  • Crossopterygians
  • a group of lobe-finned fish
  • gave rise to the amphibians

  • The link between
  • crossopterygians and the earliest amphibians
  • is convincing and includes a close similarity of
    bone and tooth structures
  • The transition from fish to amphibians occurred
    during the Devonian
  • During the Carboniferous,
  • the labyrinthodont amphibians
  • were dominant terrestrial vertebrate animals

  • The earliest fossil record of reptiles is from
    the Late Mississippian
  • The evolution of an amniote egg
  • was the critical factor in the reptiles' ability
  • to colonize all parts of the land
  • Pelycosaurs were the dominate reptile group
  • during the Early Permian,
  • whereas therapsids dominated the landscape
  • for the rest of the Permian Period

  • Plants had to overcome the same basic problems as
    animals, namely
  • desiccation,
  • reproduction,
  • and gravity
  • in making the transition from water to land
  • The earliest fossil record of land plants
  • is from Middle to Upper Ordovician rocks
  • These plants were probably small and
    bryophyte-like in their overall organization

  • The evolution of vascular tissue
  • was an important event in plant evolution
  • as it allowed food and water to be transported
  • throughout the plant
  • and provided the plant with additional support
  • The ancestor of terrestrial vascular plants
  • was probably some type of green algae
  • based on such similarities
  • as pigmentation,
  • metabolic enzymes,
  • and the same type of reproductive cycle

  • The earliest seedless vascular plants
  • were small, leafless stalks with spore-producing
    structures on their tips
  • From this simple beginning,
  • plants evolved many of the major structural
    features characteristic of today's plants
  • By the end of the Devonian Period,
  • forests with tree sized plants up to 10 m had

  • The Late Devonian also witnessed
  • the evolution of the flowerless seed plants
  • whose reproductive style freed them
  • from having to stay near water
  • The Carboniferous Period was a time
  • of vast coal swamps,
  • where conditions were ideal for the seedless
    vascular plants
  • With the onset of more arid conditions during the
  • the gymnosperms became the dominant element of
    the world's flora
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