Paleozoic Life History

1
Chapter 13
Paleozoic Life History Vertebrates and
Plantsmain points1. Vertebrates first appeared
in Cambrian, Age of Fish Devonian2.
amphibians first appear in Devonian very
abundant during Pennsylvanian3. Late
Mississippian- evolution of amniote egg allows
reptiles to colonize land4. pelycosaurs
(fin-backed reptiles) dominate the Permian
ancestors to mammals5. earliest land plants
occur in Ordovician oldest vascular plants
appear in Mid Silurian6. seedless vascular
plants very abundant during Pennsylvanian7.
onset of arid conditions in Permian, gymnosperms
dominate flora
2
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

3
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

4
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

5
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

6
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

7
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

8
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

9
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-long animal

10
Fish

• 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

11
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

12
Geologic Ranges of Major Fish Groups
13
Bottom-Dwelling Ostracoderms

• The majority of ostracoderms lived along 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

14
Devonian Seafloor

• Recreation of a Devonian seafloor showing

an acanthodian (Parexus)
a ray-finned fish (Cheirolepis)

• a placoderm (Bothriolepis)

an ostracoderm (Hemicyclaspis)
15
Swimming Ostracoderm

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

16
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

17
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

18
Acanthodians

• 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

19
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

20
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

21
Placoderms

• 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

22
Late Devonian Marine Scene

• A Late Devonian marine scene from the
  mid-continent of North America

23
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

24
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

25
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

26
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

27
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

28
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

29
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

30
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

31
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

32
Coelacanths

• 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

33
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

34
Eusthenopteron

• 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

35
Fish/Amphibian Comparison

• Similarities between the crossopterygian
  lobe-finned fish and the labyrinthodont amphibians

• Their skeletons were similar

36
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

37
Tiktaalik rosea- transitional fossil
http//tiktaalik.uchicago.edu/
38
Comparison of Teeth

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

39
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

40
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

41
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

42
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
  algae,
• first evolved during the Ordovician
• Furthermore, insects, millipedes, spiders,
• and even snails invaded the land before amphibians

43
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

44
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

45
A Late Devonian Landscape

• A Late Devonian Landscape in the eastern part of
  Greenland

• 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

46
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

47
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

48
Carboniferous Coal Swamp

• Reconstruction of a Carboniferous coal swamp

The serpentlike Dolichosoma
Larval Branchiosaurus
Large labyrinthodont amphibian Eryops
49
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

50
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

51
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

52
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

53
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

54
Paleozoic Reptile Evolution

• Evolutionary relationship among the Paleozoic
  reptiles

55
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

56
Pelycosaurs (Finback Reptiles)

• Most pelycosaurs have a characteristic sail on
  their back

The herbivore Edaphosaurus
The carnivore Dimetrodon
57
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...

58
Pelycosaurs Sail Function

• The current consensus seems to be
• that the sail served as some type of
  thermo-regulatory 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

59
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
  lineage
• and rapidly diversified into
• herbivorous
• and carnivorous lineages

60
Therapsids

• A Late Permian scene in southern Africa showing
  various therapsids

• Many paleontologists think therapsids were
  endothermic
• and may have had a covering of fur

• as shown here

Moschops
Dicynodon
61
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

62
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

63
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

64
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

65
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 emergence of seeds

• secondary growth

• Heterospory
• the appearance of leaves

66
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

67
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

68
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

69
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

70
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

71
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

72
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

73
Roots

• 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

74
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)

75
Earliest Land Plant

• 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

• 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

76
Early Devonian Plants

• Reconstruction of an Early Devonian landscape

• showing some of the earliest land plants

Protolepidodendron\
Dawsonites /
- Bucheria
77
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

78
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

79
Gymnosperms

• 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

80
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

81
An Early Devonian Plant

• Chaleuria cirrosa
• from New Brunswick, Canada
• was heterosporous, producing two spore sizes

82
An Early Devonian Plant

• This heterosporous plant is reconstructed here
• Chaleuria cirrosa

83
Spores of Chaleuria cirrosa

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

84
Evolution of Conifer Seed Plants

• The appearance of heterospory
• was followed several million years later
• by the emergence of pro-gymnosperms
• 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

85
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

86
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

87
Pennsylvanian Coal Swamp

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

Amphibian Eogyrinus
88
Sphenopsids

• 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

89
Horsetail

• Living sphenopsids include the horsetail Equisetum

90
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

91
A Cordaite Forest

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

92
Glossopteris

• 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

93
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

94
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

95
Summary

• 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

96
Summary

• 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

97
Summary

• 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

98
Summary

• 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

99
Summary

• 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

100
Summary

• 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

101
Summary

• 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

102
Summary

• 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

103
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

104
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

105
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

106
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

107
Labyrinthodonts

• 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

108
Labyrinthodont Teeth

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

109
Carboniferous Coal Swamp

• Reconstruction of a Carboniferous coal swamp

Larval Branchiosaurus
110
Carboniferous Coal Swamp

• Reconstruction of a Carboniferous coal swamp

The serpentlike Dolichosoma
111
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

112
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

113
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

114
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

115
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

116
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

117
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

118
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

119
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

120
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

121
Gymnosperm Plants

• producing a fertile seed

• which then grows into a cone-bearing mature plant

122
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

123
Lycopsids

• 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

124
Lycopsids

• 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

125
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

126
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

127
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

128
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

129
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

130
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

131
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

132
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

133
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

134
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

135
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

136
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

137
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

138
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

139
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

140
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

141
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)

142
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