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Chapter 13
Paleozoic Life History:
Vertebrates and Plants
Tetrapod Footprint Discovery
• Tetrapod trackway
– at Valentia Island,
Ireland
• These fossilized
footprints
– which are more
than 365 million
years old
– are evidence of
one of the earliest
four-legged
animals on land
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 Late Devonian trackway
– has helped shed light on the early evolution of
tetrapods
• the name is from the Greek tetra, meaning four and
podos, meaning foot
– Based on the footprints, it is estimated
• that the creature was longer than 3 ft
• and had fairly large back legs
Tetrapod Wader
• Furthermore, instead of walking on dry land
– this animal was probably walking or wading
around in a shallow, tropical stream,
• filled with aquatic vegetation and predatory fish
• This hypothesis is based on the fact that
– the trackway showed no evidence of a tail being
dragged behind it
• Unfortunately, there are no bones associated
with the tracks
– to help in reconstructing what this primitive
tetrapod looked like
Why Limbs?
• One of the intriguing questions paleontologists
ask is
– Why did limbs evolve in the first place?
• It probably was not 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
Unanswered Questions
• Presently, there are many more unanswered
questions
– about the evolution of the earliest tetrapods
– than there are answers
• During the 1990s, only a few Devonian
tetrapods were known
• Today, paleontologists have a more detailed
knowledge
– and are able to fill the gaps between the fish and
amphibians
– leading to more complete fish-amphibian phylogeny
New Information from Fossils
• As more paleoenvironmental and paleoecologic
data and analyses
– from a variety of sites
• are made available,
• A better understanding of the linkage
– between morphological changes and the
environment
– is fast emerging.
• New technologies now provide the means
– to extract more and more detailed information
– from the fossils
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
• Now we examine
– the Paleozoic evolutionary history of vertebrates
and plants
Transition from Water to Land
• One of the striking parallels between plants and
animals
– is the fact that making the transition from water to land,
– both plants and animals had to solve the same basic
problems
• For both groups,
– the method of reproduction was the major barrier
– to expansion into the various terrestrial environments
• With the evolution of the seed in plants and the
amniote egg in animals,
– this limitation was removed,
– and both groups were able to expand into all the terrestrial
habitats
Vertebrate Evolution
• A chordate (Phylum Chordata) is an animal that
has,
• at least during part of its life cycle,
– a notochord,
– a dorsal hollow nerve cord,
– and gill slits
• Vertebrates, which are animals with backbones,
are simply a subphylum of chordates
Characteristics of Chordates
• The structure of the lancelet Amphioxus
illustrates the three characteristics of a chordate:
– a notochord, a dorsal hollow nerve cord, and gill
slits
Phylum Chordata
• The ancestors and early members of the phylum
Chordata
– were soft-bodied organisms that left few fossils
– so little is known of the early evolutionary history
of the chordates or vertebrates
• Surprisingly, a close relationship exists between
echinoderms and chordates
– They may even have shared a common ancestor,
– because the development of the embryo is the same
in both groups
– and differs completely from other invertebrates
A Very Old Chordate
• Yunnanozoon lividum is one of the oldest known
chordates
– Found in 525 MY old rocks in Yunnan province,
China
– 5 cm-long
animal
Spiral Versus Radial Cleavage
• Echinoderms
and chordates
– have similar
– embryonic
development
• In the arrangement
of cells resulting
from spiral
cleavage,
– cells in successive rows are nested between each other
• In the arrangement of cells resulting from radial
cleavage,
– cells in successive rows are directly above each other
– This arrangement exists in both chordates and echinoderms
Echinoderms and Chordates
• Both echinoderms and chordates have similar
– biochemistry of muscle activity
– blood proteins,
– and larval stages
• The evolutionary pathway to vertebrates
– thus appears to have taken place much earlier and
more rapidly
– than many scientists have long thought
Hypothesis for Chordate Origin
• Based on fossil evidence and recent advances
in molecular biology,
– vertebrates may have evolved shortly after an
ancestral chordate acquired a second set of genes
• the ancestor probably resembled Yunnanozoon
• According to this hypothesis,
– a random mutation produced a duplicate set of
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
Fish
• The most primitive vertebrates are fish
– and some of the oldest fish remains are found
– in Upper Cambrian Deadwood Formation,
• northeastern WY
– Here, phosphatic scales and plates of Anatolepis,
• a primitive member of the class Agnatha
– have been recovered from marine sediments.
Fish
• All known Cambrian and Ordovician fossil fish
– have been found in shallow nearshore marine
deposits,
• while the earliest nonmarine (freshwater) fish
remains
– have been found in Silurian strata
• This does not prove that fish originated in the
oceans,
– but it does lend strong support to the idea
Fragment of Primitive Fish
• 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
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
Geologic Ranges of
Major Fish Groups
Bottom-Dwelling Ostracoderms
• The majority of ostracoderms lived on 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
Devonian Seafloor
• Recreation of a Devonian seafloor showing:
an acanthodian
(Parexus)
a ray-finned fish
(Cheirolepis)
– a placoderm (Bothriolepis)
an ostracoderm (Hemicyclaspis)
Swimming Ostracoderm
• Another type of ostracoderm,
• represented by Pteraspis
– was more elongated and probably an active swimmer
– although it also seemingly fed on small pieces of food it
could suck up
Evolution of Jaws
• The evolution of jaws
– was a major evolutionary advantage
– among primitive vertebrates
• While their jawless ancestors
– could only feed on detritus
• jawed fish
– could chew food and become active predators
– thus opening many new ecological niches
• The vertebrate jaw is an excellent example of
evolutionary opportunism
– The jaw probably evolved from the first two or
three anterior gill arches of jawless fish
Evolutionary Opportunism
• Because the gills are soft
– they are supported by gill arches composed of bone or
cartilage
• The evolution of the jaw may thus have been
related to respiration rather than feeding
–
–
–
–
–
By evolving joints in the forward gill arches,
jawless fish could open their mouths wider
Every time a fish opened and closed its mouth
it would pump more water past the gills,
thereby increasing the oxygen intake
• The hinged forward gill arches enabled fish to
also increase their food consumption
– The evolution of the jaw for feeding followed rapidly
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
Acanthodians
• The fossil remains of the first jawed fish are
found in Lower Silurian rocks
– and belong to the acanthodians,
• a group of small, enigmatic fish
– characterized by
•
•
•
•
•
large spines,
scales covering much of the body,
jaws,
teeth,
and reduced body armor
Acanthodians: Most Abundant
during Devonian
• Although their relationship to other fish has not
been well established,
– many scientists think the acanthodians
– included the probable ancestors of the present-day
• bony and cartilaginous fish groups
• The acanthodians were most abundant during
the Devonian,
– declined in importance through the Carboniferous,
– and became extinct during the Permian
Other Jawed Fish
• The other jawed fish, the placoderms
• whose name means “plate-skinned”
– evolved during the Late Silurian
• 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
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 midcontinental North American epeiric
seas
– It was by far the largest fish of the time
•
•
•
•
•
attaining a length of more than 12 m
It had a heavily armored head and shoulder region
a huge jaw lined with razor-sharp bony teeth
and a flexible tail
all features consistent with its status as a ferocious
predator
Late Devonian Marine Scene
• A Late Devonian marine scene from the
midcontinent of North America featuring the
giant placoderm, Dunkleosteus
Age of Fish
• Many fish evolved during the Devonian Period
including
–
–
–
–
the abundant acanthodians
placoderms,
ostracoderms,
and other fish groups,
• such as the cartilaginous and bony fish
• It is small wonder, then, that the Devonian is
informally called the “Age of Fish”
– because all major fish groups were present during
this time period
Cartilaginous Fish
• Cartilaginous fish,
– class Chrondrichthyes,
– represented today by
• sharks, rays, and skates,
– first evolved during the Early Devonian
– and by the Late Devonian,
– primitive marine sharks
• such as Cladoselache
• were quite abundant
Cartilaginous Fish Not Numerous
• Cartilaginous fish have never been
– as numerous nor as diverse
– as their cousins,
• the bony fish,
– but they were, and still are,
– important members of the marine vertebrate fauna
• Along with cartilaginous fish,
– the bony fish, class Osteichthyes,
– also first evolved during the Devonian
Ray-Finned Fish
• Because bony fish are the most varied and
numerous of all the fishes
– and because the amphibians evolved from them,
– their evolutionary history is particularly important
• There are two groups of bony fish
– the common ray-finned fish (subclass Actinopterygii)
– and the less familiar lobe-fined fish (subclass
Sarcopterygii)
• The term ray-finned refers to
– the way the fins are supported by thin bones that
spread away from the body
Ray-Finned and Lobe-Finned Fish
• Arrangement of fin
bones for
(a) a typical
ray-finned fish
(b) a lobe-finned fish
– Muscles extend
into the fin
– allowing greater
flexibility
Ray-Finned Fish Rapidly
Diversify
• From a modest freshwater beginning during the
Devonian,
– ray-finned fish,
• which include most of the familiar fish
• such as trout, bass, perch, salmon, and tuna,
– rapidly diversified to dominate the Mesozoic and
Cenozoic seas
Lobe-Finned Fish
• 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
allowing a powerful stroke and
making the fish a powerful swimmer
• Three orders of lobe-finned fish are recognized:
– coelacanths
– lungfish
– and crossopterygians
Coelacanths
• Coelacanths are marine lobe-finned fish
– that evolved during the Middle Devonian
– and were thought to have gone extinct
– at the end of the Cretaceous.
• In 1938, a fisherman caught a coelacanth
– in the deep waters off Madagascar,
– and several dozen more have been caught since then
• in Madagascar and in Indonesia
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
• The “lung” is actually a modified swim bladder
– that most fish use for buoyancy in swimming
• In lungfish, this structure absorbs oxygen,
– allowing them to breathe air
– when the lakes or streams in which they live become
stagnant or dry up.
Lungfish Respiration
• When the lakes become stagnant and dry up,
– the lungfish burrow into the sediment to prevent
dehydration
– and breathe through their swim bladder
– until the stream begins flowing or the lake fills with
water
• When the water is well oxygenated,
– however, lungfish rely upon gill respiration
Amphibians Evolved from
Crossopterygians
• The crossopterygians are an important group of lobefinned fish
– because amphibians probably evolved from them
• However, the transition to amphibians
– is not as simple as once portrayed
• Among the crossopterygians
– the rhipidistians appear to be the ancestral group
• These fish, reaching over 2 m in length,
– were the dominant freshwater predators
– during the Late Paleozoic.
Amphibian Ancestor
• Eusthenopteron,
–
–
–
–
a good example of a rhipidistian crossopterygian,
had an elongated body
that enabled it to move swiftly in the water,
as well as paired muscular fins that may have been
used for moving on land
• The structural similarity between
crossopterygian fish
– and the earliest amphibians is striking
– and one of the most widely cited transitions
– from one major group to another
Rhipidistian Crossopterygian and
Eusthenopteron
Fish/Amphibian Comparison
• Similarities between the crossopterygian lobefinned fish and the labyrinthodont amphibians
• Their
skeletons
were similar
Comparison of Limbs
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
Comparison of Teeth
• Comparison of tooth cross sections show
– the complex and distinctive structure found in
– both crossopterygians (left) and amphibians (right)
Paleozoic Evolutionary Events
• Before discussing this transition
– and the evolution of amphibians,
– we should place the evolutionary history of
Paleozoic fish
– in the larger context of Paleozoic evolutionary
events
• Certainly, the evolution and diversification of
jawed fish
– as well as eurypterids and ammonoids
– had a profound effect on the marine ecosystem
Defenseless Organisms
• Previously defenseless organisms either
– evolved defensive mechanisms
– or suffered great losses, possibly even extinction
• Recall that trilobites
– experienced extinctions at the end of the Cambrian,
– recovered slightly during the Ordovician,
– then declined greatly from the end of the
Ordovician
– to final extinction at the end of the Permian
Extinction by Predation
• Perhaps their lightly calcified external covering
– made them easy prey
– for the rapidly evolving jawed fish and
cephalopods
• Ostracoderms,
– although armored,
– would also have been easy prey
– for the swifter jawed fishes
• Ostracoderms became extinct by the end of the
Devonian,
– a time that coincides with the rapid evolution of
jawed fish
Late Paleozoic Changes
• 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
• In contrast, cartilaginous and ray-finned bony
fish
– expanded during the Late Paleozoic,
– as did the ammonoid cephalopods,
– the other major predator of the Late Paleozoic seas
Amphibians—
Vertebrates Invade the Land
• Although amphibians were the first vertebrates
to live on land,
– they were not the first land-living organisms
• Land plants, which probably evolved from
green algae,
– first evolved during the Ordovician
• Furthermore, insects, millipedes, spiders,
– and even snails invaded the land before amphibians
Land-Dwelling Arthropods
Evolved by the Devonian
• Fossil evidence indicates
– that such land-dwelling arthropods as scorpions
and flightless insects
– had evolved by at least the Devonian
Water to Land Barriers
• The transition from water to land required that
several barriers be surmounted
• The most critical for animals were
–
–
–
–
desiccation,
reproduction,
the effects of gravity,
and the extraction of oxygen
• from the atmosphere
– by lungs rather than from water by gills
Problems Partly Solved
• Until the 1900s, the traditional evolutionary sequence had
a rhipidistian crossopterygian,
– like Eusthenopteron
• evolving into a primitive amphibian
– such as Ichthyostega.
• At the time, fossils of these two genera
– were all that paleontologists had to work with
• Although there were gaps in morphology
– the link between crossopterygians and these earliest amphibians
– was easy to see.
Problems Partly Solved
• Crossopterygians already had a backbone and
limbs
– that could be used for walking
– and lungs that could extract oxygen
• The oldest amphibian fossils are found
– in the Upper Devonian Old Red Sandstone of eastern
Greenland
– and belong to the genus Ichthyostega
Oldest Amphibians
• These amphibians,
– had streamlined bodies, long tails, and fins
• In addition, they had
–
–
–
–
four legs
a strong backbone
a rib cage
and pelvic and pectoral girdles,
• all of which were structural adaptations for
walking on land
A Late Devonian Landscape
• A Late Devonian Landscape
• Ichthyostega was
an amphibian that
grew to a length
of about 1 m
• The flora was
diverse,
– consisting of a
variety of small
and large seedless
vascular plants
Evolution of Amphibians
• The earliest amphibians
– appear to have inherited many characteristics
– from the crossopterygians
– with little modification
• The question still remained as to when animals made the
transition
– from water to land
– and perhaps more intriguing,
– why limbs evolved in the first place
Evolution of Amphibians
• Limbs probably were not for walking on land
• Current thinking of many scientists
– is that aquatic limbs made it easier
– for animals to move around in streams, lakes, or
swamps
– that were choked with water plants or other debris
• The fossil evidence that began to emerge in the
1990s
– seems to support this hypothesis
Transition to Amphibians
• Panderichthys,
– a large Late Devonian lobe-finned fish from Latvia
– was essentially a contemporary of Eusthenopteron.
• It had a large tetrapod-like head
– with a pointed snout
– dorsally located eyes
– and modifications to the part of the skull
• related to the ear region.
• It lived in shallow tidal flats or estuaries
Transition to Amphibians
• Fossils of Acanthostega,
– a tetrapod found in 360-million-year-old rocks from
Greenland
• reveal an animals that had limbs,
– but was unable to walk to land
Transition to Amphibians
• Paleontologist Jennifer Clack points out
– that Acanthostega’s limbs were not strong enough
– to support its weight
– and its rib cage was too small
– for necessary muscles 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 water
• Clack thinks Acanthostega used its limbs to
maneuver around in swampy, plant-filled waters
Transition to Amphibians
• Fragmentary fossils from other tetrapods
– living at about the same time as Acanthostega
– suggest that these early tetrapods
– may have spent more time on dry land than in water
• The discovery of such fossils shows
– that the transition between fish and amphibians
– involved a number of new genera that are intermediate
between the two groups
– and fills in some of the gaps between the earlier
postulated rhipidistian-crossopterygian-amphibian
phylogeny
Transition to Amphibians
• In 2006, an exciting discovery
– of a 1.2-2.8 m long
– 375-million-year-old (Late Devonian) “fishapod”
– was announced.
• Tiktaalik roseae (“large fish in a stream”)
– was hailed as an intermediary
– between lobe-finned fish like Panderichthys
– and the earliest tetrapod, Acanthostega.
Tiktaalik roseae
• This “fishapod” has
characteristics of both fish
and tetrapods
– It has gills and fish scales
– but also a broad skull, eyes
on top of its head, flexible
neck and large ribcage
– that could support its body
on land or shallow water,
– and lungs
• It has the beginning of a
true tetapod forelimb
– with functional wrist bones
and five digits
– and a modified ear region
Tiktaalik roseae
• Diagram illustrating
how Tiktaalik roseae
is a transitional
species between
lobe-finned fish and
tetrapods
Rapid Adaptive Radiation
• 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
Rapid Adaptive Radiation
• 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
Carboniferous Coal Swamp
• The amphibian fauna was varied
Large labyrinthodont amphibian Eryops
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
Labyrinthodont Teeth
• Labyrinthodonts are named for the
labyrinthine wrinkling and folding of the
chewing surface of their teeth
Carboniferous Coal Swamp
• Reconstruction of a Carboniferous coal swamp
The serpentlike Dolichosoma
Labyrinthodont Decline
• Labyrinthodonts were abundant during the
Carboniferous
– when swampy conditions were widespread,
– but soon declined in abundance
– during the Permian,
– perhaps in response to changing climactic
conditions
• Only a few species survived into the Triassic
Evolution of the Reptiles —
the Land is Conquered
• Amphibians were limited in colonizing the land
– because they had to return to water to lay their
gelatinous eggs
• The evolution of the amniote egg freed reptiles
from this constraint
• In such an egg, the developing embryo
– is surrounded by a liquid-filled sac,
• called the amnion
– and provided with both a yolk, or food sac,
– and an allantois, or waste sac
Amniote Egg
• The amnion cavity
– surrounds the embryo.
• The yolk sac
– provides the food source
• while the allantois
– serves as a waste sac
• The evolution of the
amniote egg freed
reptiles
– to inhabit all parts of the
land
Colonization of
All Parts of the Land
• In this way the emerging reptile is
– in essence a miniature adult,
– bypassing the need for a larval stage in the water
• The evolution of the amniote egg allowed
vertebrates
– to colonize all parts of the land
– because they no longer had to return
– to the water as part of their reproductive cycle
Amphibian/Reptile Differences
• Many of the differences between amphibians
and reptiles are physiologic
– and are not preserved in the fossil record
• Nevertheless, amphibians and reptiles
– differ sufficiently in
• skull structure, jawbones, ear location, and limb and
vertebral construction
– to suggest that reptiles evolved from labyrinthodont
ancestors by the Late Mississippian
• based on the discovery of a well-preserved skeleton
• of the oldest known reptile, Westlothiana, from Late
Mississippian-age rocks in Scotland
Earliest Reptiles
• Some of the oldest known reptiles are from
– the Lower Pennsylvanian Joggins Formation in
Nova Scotia, Canada
– Here, remains of Hylonomus are found
• in the sediments filling in tree trunks
• These earliest reptiles were small and agile
– and fed largely on grubs and insects
One of the Oldest Known Reptiles
• Reconstruction and skeleton of Hylonomus
lyelli from the Pennsylvanian Period
– Fossils of this
animal have
been collected
from
sediments that
filled tree
stumps
– Hylonomus
lyelli was
about 30 cm
long
Permian Diversification
• The earliest reptiles are loosely grouped
together as protorothyrids,
– whose members include the earliest reptiles
• During the Permian Period, reptiles diversified
– and began displacing many amphibians
• The success of the reptiles is partly because
– of their advanced method of reproduction
– and their more advanced jaws and teeth,
• their tough skin and scales to prevent dessication,
• and their ability to move rapidly on land
Paleozoic Reptile Evolution
• Evolutionary
relationship
among the
Paleozoic
reptiles
Pelycosaurs—Finback Reptiles
• The pelycosaurs,
• or finback reptiles,
– evolved from the protorothyrids
• during the Pennsylvanian
– and were the dominant reptile group
• by the Early Permian
• They evolved into a diverse assemblage
– of herbivores,
• exemplified by Edaphosaurus,
– and carnivores
• such as Dimetrodon
Pelycosaurs (Finback Reptiles)
• Most pelycosaurs have a characteristic sail on
their back
The herbivore Edaphosaurus
The carnivore Dimetrodon
Pelycosaurs Sails
• An interesting feature of the pelycosaurs is
their sail
– It was formed by vertebral spines that,
– in life, were covered with skin
• The sail has been variously explained as
–
–
–
–
a type of sexual display,
a means of protection
and a display to look more ferocious
but...
Pelycosaurs Sail Function
• The current consensus seems to be
– that the sail served as some type of
thermoregulatory device,
– raising the reptile's temperature by catching the
sun's rays or cooling it by facing the wind
• Because pelycosaurs are considered to be the
group
– from which therapsids evolved,
– it is interesting that they may have had some sort of
body-temperature control
Therapsids:
Mammal-like Reptiles
• The pelycosaurs became extinct during the
Permian
– and were succeeded by the therapsids,
• mammal-like reptiles
– that evolved from the carnivorous pelycosaur
ancestry
– and rapidly diversified into
• herbivorous
• and carnivorous lineages
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
Therapsid Characteristics
• Therapsids were small- to medium-sized
animals
– displaying the beginnings of many mammalian
features
• fewer bones in the skull because many of the small skull
bones were fused
• enlarged lower jawbone
• differentiation of the teeth for various functions such as
nipping, tearing, and chewing food
• and a more vertical placed legs for greater flexibility,
• as opposed to the sideways sprawling legs in primitive
reptiles
Endothermic Therapsids
• Many paleontologists think therapsids were
endothermic,
– or warm-blooded,
– enabling them to maintain a constant internal body
temperature
• This characteristic would have allowed them
– to expand into a variety of habitats,
– and indeed the Permian rocks
• in which their fossil remains are found
– have a wide latitudinal distribution
Permian Mass Extinction
• As the Paleozoic Era came to an end,
– the therapsids constituted about 90% of the known
reptile genera
– and occupied a wide range of ecological niches
• The mass extinctions
– that decimated the marine fauna
– at the close of the Paleozoic
– had an equally great effect on the terrestrial
population
Losses Fewer for Plants
• 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, in contrast,
– apparently did not experience
– as great a turnover as animals did
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
Major Events in the Evolution of
Land Plants
• The Devonian Period was a time of rapid evolution for
land plants
– Major events
were
– the
appearance
of leaves
– heterospory
– secondary
growth
– and
emergence of
seeds
Marine, then Fresh, then Land
• Most experts agree
–
–
–
–
that the ancestors of land plants
first evolved in a marine environment,
then moved into a freshwater environment
and finally onto land
• In this way, the differences in osmotic pressures
– between saltwater and freshwater
– were overcome while the plant was still in the water
• The higher land plants are divided into two major
groups,
– the nonvascular
– and vascular plants
Vascular Versus Nonvascular
• Most land plants are vascular,
– meaning they have a tissue system
– of specialized cells
– for the movement of water and nutrients
• The nonvascular plants,
– such as bryophytes
• liverworts, hornwarts, and mosses
– do not have these specialized cells
– and are typically small
– and usually live in low moist areas
Earliest Land Plants
• The earliest land plants
• from the Middle to Late Ordovician
– were probably small and bryophyte-like in their overall
organization
• but not necessarily related to bryophytes
• The evolution of vascular tissue in plants was an
important step
– Because it allowed for the transport of nutrients and water
• Probable vascular plant megafossils
– and characteristic spores indicate
• to many paleontologists
– that vascular plants evolved
– well before the Middle Silurian
Features Resembling
Present Land Plants
• Sheets of cuticlelike cells
• that is, the cells
• that cover the surface
• of present-day land plants
– and tetrahedral clusters
• that closely resemble the spore tetrahedrals of primitive
land plants
– have been reported from Middle to Upper
Ordovician rocks
– from western Libya and elsewhere
Upper Ordovician Plant Spores and
Cells
• These fossils closely resemble spore
tetrahedrals of primitive land plants
Ancestor of Terrestrial
Vascular Plants
• The ancestor of terrestrial vascular plants
– was probably some type of green alga
• Although no fossil record of the transition
– from green algae to terrestrial vascular plants has
been found,
– comparison of their physiology reveals a strong
link
• Primitive seedless vascular plants
• such as ferns
– resemble green algae in their pigmentation,
– important metabolic enzymes,
– and type of reproductive cycle
Transitions from
Salt to Freshwater to Land
• 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 alga ancestry
– was accompanied by various modifications
– that allowed them to occupy
– this new and harsh environment
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 is derived from
– the organic compounds lignin and cellulose,
– found throughout a plant's walls
Problems of Desiccation and
Oxidation
• The problem of desiccation
– was circumvented by the evolution of cutin,
• an organic compound
• found in the outer-wall layers of plants
• Cutin also provides additional resistance
– to oxidation,
– the effects of ultraviolet light,
– and the entry of parasites
Roots
• Roots evolved in response to
– the need to collect water and nutrients from the soil
– and to help anchor the plant in the ground
• The evolution of leaves
– from tiny outgrowths on the stem
– or from branch systems
• provided plants with
– an efficient light-gathering system for
photosynthesis
Silurian and Devonian Floras
• The earliest known vascular land plants
– are small Y-shaped stems
– assigned to the genus Cooksonia
– from the Middle Silurian of Wales and Ireland
• Upper Silurian and Lower Devonian species
are known from
• Scotland, New York State, and the Czech Republic,
• These earliest plants were
– small, simple, leafless stalks
– with a spore-producing structure at the tip
(sporangia)
Earliest Land Plant
• 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
Earliest Land Plant
• The earliest plants
– are known as seedless vascular plants
– because they do not produce seeds
• They also did not have a true root system
• A rhizome,
• the underground part of the stem,
– transferred water from the soil to the plant
– and anchored the plant to the ground
• The sedimentary rocks in which these plant
fossils are found
– indicate that they lived in low, wet, marshy,
freshwater environments
Parallel between Seedless
Vascular Plants and Amphibians
• An interesting parallel can be seen between
seedless vascular plants and amphibians
• When they made the transition from water to
land,
– Both plants and animals had to overcome the same
problems such a transition involved
• Both groups,
– while successful,
– nevertheless required a source of water in order to
reproduce
Plants and Amphibians
• In the case of amphibians,
– their gelatinous egg had to remain moist
• while the seedless vascular plants
– required water for the sperm to travel through
– to reach the egg
Seedless Vascular Plants Evolved
• From this simple beginning,
– the seedless vascular plants
– evolved many of the major structural features
– characteristic of modern plants such as
• leaves,
• roots,
• and secondary growth
• These features did not all evolve
simultaneously
– but rather at different times,
• a pattern known as mosaic evolution
Adaptive Radiation
• This diversification and adaptive radiation
– took place during the Late Silurian and Early
Devonian
– and resulted in a tremendous increase in diversity
• During the Devonian,
– the number of plant genera remained about the
same,
– yet the composition of the flora changed
Early Devonian Plants
• Reconstruction of an Early Devonian landscape
– showing some of the earliest land plants
Protolepidodendron\
Dawsonites /
- Bucheria
Early and Late Devonian Plants
• Whereas the Early Devonian landscape
– was dominated by relatively small,
– low-growing,
– bog-dwelling types of plants,
• the Late Devonian
– witnessed forests of large tree-size plants up to 10
m tall
Evolution of Seeds
• In addition to the diverse seedless vascular
plant flora of the Late Devonian,
– another significant floral event took place
• The evolution of the seed at this time
–
–
–
–
liberated land plants
from their dependence on moist conditions
and allowed them
to spread over all parts of the land
Seedless Vascular Plants
Require Moisture
• Seedless vascular plants require moisture
– for successful fertilization
– because the sperm must travel to the egg
– on the surface of the gamete-bearing plant
• gametophyte
– to produce a successful spore-generating plant
• sporophyte
• Without moisture, the sperm would dry out
before reaching the egg
Seedless Vascular Plant
• Generalized life
history of a
seedless vascular
plant
• The mature
sporophyte plant
produces spores
– which upon
germination
grow into small
gametophyte
plants
Seedless Vascular Plant
• The gametophyte
plants produce
sperm and eggs
• The fertilized eggs
grow into
• the spore-producing
mature plant
• and the sporophytegametophyte life
cycle begins again
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
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
Gymnosperm 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
Gymnosperm Plants
• Pollen grains
are transported
to the female
cones by the
wind
• Fertilization
occurs when the
sperm moves
through a moist
tube growing
from the pollen
grain
• and unites with
the embryonic
seed
Gymnosperm Plants
• producing a
fertile seed
• which then
grows into a
cone-bearing
mature plant
Gymnosperms Free to Migrate
• In this way, the need for a moist environment
– for the gametophyte generation is solved
• The significance of this development
• is that seed plants,
• like reptiles,
–
–
–
–
were no longer restricted
to wet areas
but were free to migrate
into previously unoccupied dry environments
Heterospory, an Intermediate Step
• Before seed plants evolved,
– an intermediate evolutionary step was necessary
• This was the development of heterospory,
– whereby a species produces two types of spores
– a large one (megaspore)
• that gives rise to the female gamete-bearing plant
– and a small one (microspore)
• that produces the male gamete-bearing plant
• The earliest evidence of heterospory
– is found in the Early Devonian plant
– Chaleuria cirrosa,
• which produced spores of two distinct sizes
An Early Devonian Plant
• Chaleuria cirrosa
– from New Brunswick, Canada
– was heterosporous, producing two spore sizes
Spores of Chaleuria cirrosa
• The two spore
types of
Chaleuria
cirrosa
– shown at
about the
same relative
scale
Evolution of Conifer Seed Plants
• The appearance of heterospory
– was followed several million years later
– by the emergence of progymnosperms
• Middle and Late Devonian plants
• with fernlike reproductive habit
• and a gymnosperm anatomy
– which gave rise in the Late Devonian
– to such other gymnosperm groups as
• the seed ferns
• and conifer-type seed plants
Plants in Swamps
Versus Drier Areas
• Although 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 nonswampy areas
Late Carboniferous and
Permian Floras
• The rocks of the Pennsylvanian Period
• Late Carboniferous
– are the major source of the world's coal
• Coal results from
– the alteration of plant remains
– accumulating in low swampy areas
• The geologic and geographic conditions of the
Pennsylvanian
– were ideal for the growth of seedless vascular
plants,
– and consequently these coal swamps had a very
diverse flora
Pennsylvanian Coal Swamp
• Reconstruction of a Pennsylvanian coal
swamp
– with its characteristic vegetation
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
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
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
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
– or scouring rushes
• Small seedless vascular plants and seed ferns
– formed a thick undergrowth or ground cover
beneath these treelike plants
Horsetail
• Living
sphenopsids
include the
horsetail
Equisetum
Plants on Higher and Drier Ground
• 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
• A cordaite
forest from
the Late
Carboniferous
• Cordaites
were a group
of
gymnosperm
trees that
grew up to
50 m tall
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
Climatic and Geologic Changes
• The floras that were abundant
–
–
–
–
–
during the Pennsylvanian
persisted into the Permian,
but because of climatic and
geologic changes resulting from tectonic events,
they declined in abundance and importance
• By the end of the Permian,
– the cordaites became extinct,
– while the lycopsids and sphenopsids
– were reduced to mostly small, creeping forms
Gymnosperms Diversified
• Gymnosperms
– with lifestyles more suited to the warmer and drier
Permian climates
– diversified and came to dominate the Permian,
Triassic, and Jurassic landscapes
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
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
• It is from the lobe-finned fish
– that amphibians evolved
Summary
• The link between
– crossopterygian lobe-finned fish
– and the earliest amphibians
– is convincing and includes a close similarity of
bone and tooth structures
• However, new fossil discoveries
– show that the transition between the two groups
– is more complicated than originally
hypothesized
– and includes several intermediate forms
Summary
• Amphibians evolved
– during the Late Devonian
• with labyrinthodont amphibians
– becoming the dominant terrestrial vertebrate animals
– during the Carboniferous
• The Late Mississippian
– marks the earliest fossil record of reptiles
• The evolution of an amniote egg
– was the critical factor that allowed reptiles
– to completely colonize the land
Summary
• Pelycosaurs were the dominate reptiles
– during the Early Permian,
• whereas therapsids dominated the landscape
• for the rest of the Permian Period
• In making the transition from water to land,
– plants had to overcome the same basic
problems as animals, namely
• desiccation,
• reproduction,
• and gravity
Summary
• 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
Summary
• The ancestor of terrestrial vascular plants
– was probably some type of green alga
– 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
Summary
• 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
Summary
• The Late Devonian also witnessed
– evolution of flowerless seed plants (gymnosperms)
– 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 seedless vascular plants
• With the onset of more arid conditions during the
Permian,
– the gymnosperms became the dominant element
– of the world's flora