Paleozoic Life History—
Vertebrates and Plants main points…
1. Vertebrates first appeared in Cambrian,
‘Age of Fish’= Devonian
2. amphibians first appear in Devonian; very abundant during Pennsylvanian
3. Late Mississippian- evolution of amniote egg allows reptiles to colonize land…
4. pelycosaurs (fin-backed reptiles) dominate the Permian; ancestors to mammals
5. earliest land plants occur in Ordovician; oldest vascular plants appear in Mid Silurian
6. seedless vascular plants very abundant during Pennsylvanian
7. onset of arid conditions in Permian, gymnosperms dominate flora

• 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

• 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

• 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

• 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

• 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

• 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

• The structure of the lancelet
Amphioxus illustrates the three characteristics of a chordate:
– a notochord, a dorsal hollow nerve cord, and gill slits

• Yunnanozoon lividum is one of the oldest known chordates
– Found in 525 Myr old rocks in Yunnan province, China
– 5 cm-long animal

• 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

• 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

• The majority of ostracoderms lived along the seafloor
•
Hemicyclaspis is a good example of a bottomdwelling 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

• Recreation of a Devonian seafloor showing: an acanthodian
( Parexus ) a ray-finned fish
( Cheirolepis )
– a placoderm ( Bothriolepis ) an ostracoderm ( Hemicyclaspis )

• 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

• 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

• 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

• 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

• 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

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

• 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
,
– 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 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

• 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

• Arrangement of fin bones for
(a) a typical ray-finned fish
(b) a lobe-finned fish
– Muscles extend into the fin
– allowing greater flexibility

• From a modest freshwater beginning during the
Devonian,
– ray-finned fish ,
• which include most of the familiar fish
• such as trout, bass, perch, salmon, and tuna ,
– rapidly diversified to dominate the Mesozoic and
Cenozoic Seas

• Present-day lobe-finned fish are characterized by muscular fins
• The fins do not have radiating bones
– but rather 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 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

• 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

•
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

• Similarities between the crossopterygian lobefinned fish and the labyrinthodont amphibians
• Their skeletons were similar

ulna radius humerus
• Comparison of the limb bones
– of a crossopterygian (left) and an amphibian (right)
• Color identifies the bones that the two groups have in common

http://tiktaalik.uchicago.edu/

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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• Reconstruction of a Carboniferous coal swamp
The serpentlike Dolichosoma
Larval Branchiosaurus
Large labyrinthodont amphibian Eryops

• 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

• Amphibians were limited in colonizing the land
– because they had to return to water to lay their gelatinous eggs
• The evolution of the amniote egg freed reptiles from this constraint
• In such an egg, the developing embryo
– is surrounded by a liquid-filled sac,
• called the amnion
– and provided with both a yolk, or food sac,
– and an allantois, or waste sac

• In 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

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

• Many of the differences between amphibians and reptiles are 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

• Evolutionary relationship among the
Paleozoic 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

• Most pelycosaurs have a characteristic sail on their back
The herbivore Edaphosaurus
The carnivore Dimetrodon

• An interesting feature of the pelycosaurs is their sail
– It was formed by vertebral spines that,
– in life, were covered with skin
• The sail has been variously explained as
– a type of sexual display,
– a means of protection
– and a display to look more ferocious
– but...

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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)

• 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

• Reconstruction of an Early Devonian landscape
– showing some of the earliest land plants
Protolepidodendron
\
Dawsonites /
- Bucheria

• 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

• 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

• 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

• 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

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

• This heterosporous plant is reconstructed here
Chaleuria cirrosa

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

• 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

• While the seedless vascular plants
– dominated the flora of the Carboniferous coalforming swamps,
• the gymnosperms
– made up an important element
– of the Late Paleozoic flora,
• particularly in the non-swampy areas

• 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

• Reconstruction of a Pennsylvanian coal swamp
– with its characteristic vegetation
Amphibian Eogyrinus

• 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

• Not all plants were restricted to the coalforming 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 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

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

• Reconstruction of a Carboniferous coal swamp
Larval Branchiosaurus

• Reconstruction of a Carboniferous coal swamp
The serpentlike Dolichosoma

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• The gametophyte plants produce sperm and eggs
• The fertilized eggs grow into
• the spore-producing mature plant
• and the sporophytegametophyte life cycle begins again

• 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

• 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

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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

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

• 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

• 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

• 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

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

• Generalized life history of a gymnosperm plant
• The mature plant bears both
– male cones that produce spermbearing pollen grains
– and female cones that contain embryonic seeds

• 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

• 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

• 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)

• 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