Paleozoic Life History

1
Chapter 13
Paleozoic Life History Vertebrates and Plants
2
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

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
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

6
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

7
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

8
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

9
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

10
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

11
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

12
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

13
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

14
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

15
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

16
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

17
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

18
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

19
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)

20
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

21
Geologic Ranges of Major Fish Groups
22
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

23
Devonian Seafloor

• Recreation of a Devonian seafloor showing

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

• a placoderm (Bothriolepis)

an ostracoderm (Hemicyclaspis)
24
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

25
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

26
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

27
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

28
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

29
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

30
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

31
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

32
Late Devonian Marine Scene

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

33
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

34
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

35
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

36
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

37
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

38
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

39
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

40
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

41
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

42
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

43
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

44
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

45
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

46
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

47
Fish/Amphibian Comparison

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

• Their skeletons were similar

48
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

49
Comparison of Teeth

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

50
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

51
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

52
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

53
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

54
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

55
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

56
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

57
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

58
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

59
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

60
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

61
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

62
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

63
Labyrinthodont Teeth

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

64
Carboniferous Coal Swamp

• Reconstruction of a Carboniferous coal swamp

Large labyrinthodont amphibian Eryops
65
Carboniferous Coal Swamp

• Reconstruction of a Carboniferous coal swamp

Larval Branchiosaurus
66
Carboniferous Coal Swamp

• Reconstruction of a Carboniferous coal swamp

The serpentlike Dolichosoma
67
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

68
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

69
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

70
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

71
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

72
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

73
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

74
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

75
Paleozoic Reptile Evolution

• Evolutionary relationship among the Paleozoic
  reptiles

76
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

77
Pelycosaurs (Finback Reptiles)

• Most pelycosaurs have a characteristic sail on
  their back

The herbivore Edaphosaurus
The carnivore Dimetrodon
78
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...

79
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

80
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

81
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
82
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

83
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

84
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

85
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

86
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

87
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

88
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

89
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

90
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

91
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

92
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

93
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

94
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

95
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

96
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

97
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)

98
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

99
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

100
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

101
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

102
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

103
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

104
Early Devonian Plants

• Reconstruction of an Early Devonian landscape

• showing some of the earliest land plants

Protolepidodendron\
Dawsonites /
- Bucheria
105
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

106
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

107
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

108
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

109
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

110
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

111
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

112
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

113
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

114
Gymnosperm Plants

• producing a fertile seed

• which then grows into a cone-bearing mature plant

115
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

116
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

117
An Early Devonian Plant

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

118
An Early Devonian Plant

• This heterosporous plant is reconstruction here
• Chaleuria cirrosa

119
Spores of Chaleuria cirrosa

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

120
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

121
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

122
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

123
Pennsylvanian Coal Swamp

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

Amphibian Eogyrinus
124
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

125
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

126
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

127
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

128
Horsetail

• Living sphenopsids include the horsetail Equisetum

129
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

130
A Cordaite Forest

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

131
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

132
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

133
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

134
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

135
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

136
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

137
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

138
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

139
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

140
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

141
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