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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

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Chapter 13

Paleozoic Life History— Vertebrates and Plants

tetrapod footprint discovery
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, Ireland
tetrapod footprint discovery3
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 tetrapods
      • 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
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
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 debris
  • The scant fossil evidence also seems to support this hypothesis
unable to walk on land
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 Acanthostega,
    • 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
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
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 water
  • At this time, there are many more unanswered questions
    • 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
Vertebrates and Plants
  • Previously, we examined the Paleozoic history of invertebrates,
    • beginning with the acquisition of hard parts
    • and concluding with the massive Permian extinctions
    • 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
Transition from Water to Land
  • One of the striking parallels between plants and animals
    • 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 environments
  • 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
Vertebrate Evolution
  • 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
characteristics of chordates
Characteristics of Chordates
  • The structure of the lancelet Amphioxus illustrates the three characteristics of a chordate:
    • a notochord, a dorsal hollow nerve cord, and gill slits
phylum chordata
Phylum Chordata
  • The ancestors and early members of the phylum Chordata
    • 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
A Very Old Chordate
  • Yunnanozoon lividum is one of the oldest known chordates
    • Found in 525 Myr old rocks in Yunnan province, China
    • 5 cm-longanimal
spiral versus radial cleavage
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 other
  • In the arrangement of cells resulting from radial cleavage, (b) at the right,
    • cells in successive rows are directly above each other
    • This arrangement exists in both chordates and echinoderms
echinoderms and chordates
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
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 genes
    • 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 deposits,
    • while the earliest nonmarine 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
fragment of primitive fish
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 fish)
ostracoderms bony skinned fish
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 Devonian
    • and then became extinct
bottom dwelling ostracoderms
Bottom-Dwelling Ostracoderms
  • The majority of ostracoderms lived on the seafloor
  • Hemicyclaspis is a good example of a bottom-dwelling ostracoderm
    • Vertical scales allowed Hemicyclaspis to wiggle sideways
      • 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
Devonian Seafloor
  • Recreation of a Devonian seafloor showing:

an acanthodian (Parexus)

a ray-finned fish (Cheirolepis)

  • a placoderm (Bothriolepis)

an ostracoderm (Hemicyclaspis)

swimming ostracoderm
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
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
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 rapidly
evolution of jaws27
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
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 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
other jawed fish
Other Jawed Fish
  • The other jawed fish
    • that evolved during the Late Silurian were the placoderms,
      • 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 Dunkleosteus,
      • 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
Late Devonian Marine Scene
  • A Late Devonian marine scene from the midcontinent of North America
age of fish
Age of Fish
  • Many fish evolved during the Devonian Period including
    • 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
  • 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 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
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 important
  • 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
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
Ray-Finned Fish Rapidly Diversified
  • 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
lobe finned fish
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 shaft
  • Two major groups of lobe-finned fish are recognized:
    • lungfish
    • and crossopterygians
lungfish fish
Lungfish Fish
  • Lungfish 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
  • Studies of present-day lung fish indicate that lungs evolved
    • from saclike bodies on the ventral side of the esophagus
lungfish respiration
Lungfish Respiration
  • These saclike bodies enlarged
    • and improved their capacity for oxygen extraction,
    • 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 dehydration
  • When the water is well oxygenated,
    • however, lungfish rely upon gill respiration
amphibians evolved from crossopterygians
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 caught,
    • both there and in Indonesia
rhipidistians ancestors of amphibians
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
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 fish
    • 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 ancestors
fish amphibian comparison
Fish/Amphibian Comparison
  • Similarities between the crossopterygian lobe-finned fish and the labyrinthodont amphibians
  • Their skeletons were similar
comparison of limbs




Comparison of Limbs
  • Comparison of the limb bones
    • of a crossopterygian (left) and an amphibian (right)
  • Color identifies the bones that the two groups have in common
comparison of teeth
Comparison of Teeth
  • Comparison of tooth cross sections show
    • the complex and distinctive structure found in
    • both crossopterygians (left) and amphibians (right)
paleozoic evolutionary events
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 events
  • Certainly, the evolution and diversification of jawed fish
    • as well as eurypterids and ammonoids
    • had a profound effect on the marine ecosystem
defenseless organisms
Defenseless Organisms
  • Previously, defenseless organisms either
    • evolved defensive mechanisms
    • or suffered great losses, possibly even extinction
  • Recall that trilobites
    • experienced major extinctions at the end of the Cambrian,
    • recovered slightly during the Ordovician,
    • then declined greatly from the end of the Ordovician
    • to their ultimate demise at the end of the Permian
extinction by predation
Extinction by Predation
  • Perhaps their lightly calcified external covering
    • made them easy prey
    • for the rapidly evolving jawed fish and cephalopods
  • 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
late paleozoic changes
Late Paleozoic Changes
  • Placoderms also became extinct by the end of the Devonian,
    • while acanthodians decreased in abundance after the Devonian
    • and became extinct by the end of the Paleozoic Era
  • 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 seas
amphibians vertebrates invade the land
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
land dwelling arthropods evolved by the devonian
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
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
Problems Partly Solved
  • These problems were partly solved by the crossopterygians
    • they already had a backbone and limbs
    • that could be used for walking
    • and lungs that could extract oxygen
oldest amphibians
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
  • 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
  • A Late Devonian Landscape in the eastern part of Greenland
amphibians minor element of the devonian
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 Devonian,
    • they were a minor element of the Devonian terrestrial ecosystem
rapid adaptive radiation
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 amphibians
labyrinthodont teeth
Labyrinthodont Teeth
  • Labyrinthodonts are named for the labyrinthine wrinkling and folding of the chewing surface of their teeth
carboniferous coal swamp
Carboniferous Coal Swamp
  • Reconstruction of a Carboniferous coal swamp

Large labyrinthodont amphibian Eryops

carboniferous coal swamp65
Carboniferous Coal Swamp
  • Reconstruction of a Carboniferous coal swamp

Larval Branchiosaurus

carboniferous coal swamp66
Carboniferous Coal Swamp
  • Reconstruction of a Carboniferous coal swamp

The serpentlike Dolichosoma

labyrinthodont decline
Labyrinthodont Decline
  • Labyrinthodonts were abundant during the Carboniferous
    • when swampy conditions were widespread,
    • but soon declined in abundance
    • during the Permian,
    • perhaps in response to changing climactic conditions
  • Only a few species survived into the Triassic
evolution of the reptiles the land is conquered
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
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
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 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
amphibian reptile differences
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 Mississippian
      • based on the discovery of a well-preserved skeleton
      • of the oldest known reptile, Westlothiana, from Late Mississippian-age rocks in Scotland
earliest reptiles
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
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
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
Paleozoic Reptile Evolution
  • Evolutionary relationship among the Paleozoic reptiles
pelycosaurs finback reptiles
Pelycosaurs—Finback 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 reptiles77
Pelycosaurs (Finback Reptiles)
  • Most pelycosaurs have a characteristic sail on their back

The herbivore Edaphosaurus

The carnivore Dimetrodon

pelycosaurs sails
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
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 group
    • from which therapsids (mammal-like reptiles) evolved,
    • it is interesting that they may have had some sort of body-temperature control
therapsids mammal like reptiles
Therapsids—Mammal-like Reptiles
  • 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
  • Many paleontologists think therapsids were endothermic
  • and may have had a covering of fur
  • A Late Permian scene in southern Africa showing various therapsids
  • as shown here



therapsid characteristics
Therapsid Characteristics
  • Therapsids were small- to medium-sized animals
    • displaying the beginnings of many mammalian features
      • 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 food
      • and a more vertical position of the legs for greater flexibility,
      • as opposed to the sideways sprawling legs in primitive reptiles
endothermic therapsids
Endothermic Therapsids
  • Many paleontologists think therapsids were endothermic,
    • 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
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 population
losses fewer for plants
Losses Fewer for Plants
  • 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, on the other hand,
    • apparently did not experience
    • as great a turnover as animals did
plant evolution
Plant Evolution
  • When plants made the transition from water to land,
    • 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 adaptations
    • that were fundamental to the subsequent radiations
    • and diversification that occurred
    • during the Silurian, Devonian, and later periods
plant evolution87
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
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 water
  • The higher land plants are composed of two major groups,
    • the nonvascular
    • and vascular plants
vascular versus nonvascular
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
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
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
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 exits,
    • comparison of their physiology reveals a strong link
  • 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
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 freshwater
  • 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
Vascular Tissue Also Gives Strength
  • Besides the primary function
    • of transporting water and nutrients throughout a plant,
    • 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
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 soil
    • 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 photosynthesis
silurian and devonian floras
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 (sporangia)
earliest land plant
Earliest Land Plant
  • 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
  • The earliest known fertile land plant was Cooksonia
    • seen in this fossil from the Upper Silurian of South Wales
  • Cooksonia consisted of
    • upright, branched stems
    • terminating in sporangia
earliest land plant99
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 amphibians
Parallel between Seedless Vascular Plants and Amphibians
  • 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
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
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
Adaptive Radiation
  • This diversification and adaptive radiation
    • took place during the Late Silurian and Early Devonian
    • and resulted in a tremendous increase in diversity
  • During the Devonian,
    • the number of plant genera remained about the same,
    • yet the composition of the flora changed
early devonian plants
Early Devonian Plants
  • showing some of the earliest land plants
  • Reconstruction of an Early Devonian landscape



- Bucheria

early and late devonian plants
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
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
  • 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
Seedless Vascular Plant
  • Generalized life history of a seedless vascular plant
  • The mature sporophyte plant produces spores
  • which upon germination grow into small gametophyte plants
seedless vascular plant109
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 again
reproduction by seed
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
Gymnosperm Plants
  • Generalized life history of a gymnosperm plant
  • The mature plant bears both
  • male cones that produce sperm-bearing pollen grains
  • and female cones that contain embryonic seeds
gymnosperm plants113
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 plants114
Gymnosperm Plants
  • producing a fertile seed
  • which then grows into a cone-bearing mature plant
gymnosperms free to migrate
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
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 plant
    • 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
An Early Devonian Plant
  • Chaleuria cirrosa
    • from New Brunswick, Canada
    • was heterosporous, producing two spore sizes
an early devonian plant118
An Early Devonian Plant
  • This heterosporous plant is reconstruction here

Chaleuria cirrosa

spores of 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
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
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
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 Pennsylvanian
    • were ideal for the growth of seedless vascular plants,
    • and consequently these coal swamps had a very diverse flora
pennsylvanian coal swamp
Pennsylvanian Coal Swamp
  • Reconstruction of a Pennsylvanian coal swamp
    • with its characteristic vegetation

Amphibian Eogyrinus

coal forming pennsylvanian flora
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 swamps,
    • 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
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
  • 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 Gondwana,
    • whose distribution is cited as critical evidence that the continents have moved through time
climatic and geologic changes
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
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 evolved
  • 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 Permian,
    • the gymnosperms became the dominant element of the world's flora