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Historical Geology Exam III Review
The Early Paleozoic:
Cambrian, Ordovician, and Silurian
Physical Characteristics:
 Six major continents existed at the beginning of the
Paleozoic Era; four were located near the paleo-equator.
 During the Early Paleozoic (Cambrian to Silurian), Laurentia
(North America) was moving northward and Gondwana was
moving to a south polar position as indicated by the tillite
deposits.
 Most continents consisted of two components: 1.) a relatively
stable craton over which epieric seas transgressed and
regressed, surrounded by 2.) mobile belts (platform
materials) in which mountains were built.
 Collisions between continental masses deformed the
platform materials creating Arches and Basins. The arches
forming elongated island masses and the basins forming
elongated seas (epieric).
 The geologic history of North America can be divided into
cratonic sequences that reflect eustatic (worldwide) sea
level changes. (see cratonic sequence chart)
 The Sauk Sea was the first major transgression onto the
craton. At its maximum, it covered the craton except for
parts of the Canadian Shield and the Transcontinental Arch,
forming a series of large, northeast-southwest trending
islands.
 The Tippecanoe sequence began with the deposition of an
extensive sandstone over the exposed and eroded Sauk
landscape. During Tippecanoe time extensive carbonate
deposition took place. In addition, large barrier reef
complexes of Rugosa and Tabulate corals enclosed basins,
resulting in evaporite deposition within these basins.
 The eastern edge of North America was a stable carbonate
platform during Sauk time. During Tippecanoe time, an
oceanic-continental convergent plate boundary formed,
resulting in the Taconic orogeny, the first of several
orogenies to create the Appalachian Mountains.
 The newly formed Taconic Highlands shed sediments into
the western epieric sea, producing the Queenstone Delta, a
clastic wedge.
 Early Paleozoic-age rocks contain a variety of mineral
resources including building stone, limestone for cement,
silica sands, hydrocarbons, evaporates, and iron ores.
Paleozoic Orogenies of Laurentia
1.) Taconic – Orovician – east coast - First uplift of the Appalachian
Mountains
2.) Acadian – Devonian – upper east coast - Uplift of the northern
Appalachians (Maine, New York)
3.) Antler - Devonian – west coast – First beginnings of the uplift of
the subsequent Rocky Mountians
4.) Quachita – Mississippian/Pennsylvanian – southern North America
(Llano, Texas northeast through Arkansas area) – resulted in the Llano
Uplift of central Texas, the Quachita Mountains of Oklahoma and
Arkansas.
5.) Alleghenian – Permian – east coast – further uplift of
Appalachians.
The Late Paleozoic:
Devonian, Mississippian, Pennsylvanian, and
Permian
 During the Late Paleozoic, Baltica and Laurentia collided, forming
Laurasia. Siberia and Kazakhstania collided and were finally
sutured to Laurasia. Gondwana moved over the South Pole and
experienced several glacial-interglacial periods, resulting in
eustatic sea level changes and transgressions and regressions
along low lying craton margins.
 Laurasia and Gondwana underwent a series of collisions
beginning in the Mississippian/Pennsylvanian. During the
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Permian, the formation of Pangea was completed.
Surrounding the supercontinent was a global ocean –
Panthalassa.
The Late Paleozoic history of the North American craton can be
deciphered from the rocks of the Kaskaskia and Absaroka
cratonic sequences.
A common Late Paleozoic rock series is the Cyclothem. This is
a series of non-marine sediments overlain by a clay (subsoil),
overlain by a coal layer, overlain by younger marine lithofacies
sediments. This is interpreted at the bottom as terrestrial
“swampy” conditions of the Carboniferous, followed by an eustatic
rise in the sea level inundating the swampy lowlands. This
changing sea level was caused by the adancing and retreating of
the Gondwanan ice sheets in the polar regions.
The basal beds of the Kaskaskia sequence that were deposited
on the exposed Tippecanoe surface consisted of either
sandstones derived from the eroding Taconic highlands, or
carbonate rocks.
Most of the Kaskaskia sequence is dominated by carbonates and
associated evaporates.
The Devonian Period was a time of major reef building in western
Canada, southern England, Belgium, Australia, and Russia.
A persistent and widespread black shale was deposited over a
large area of the craton during the Late Devonian and Early
Mississippian, with the Mississippian being dominated for the
most part by carbonate deposition.
Cratonic mountain building, specifically the Ancestral Rockies,
occurred during the Pennsylvanian Period, resulting in thick nonmarine detrital rocks and evaporates in the intervening basins.
By Early Permian, the Absaroka Sea occupied a narrow zone of
the south-central craton. Here, several large reefs and
associated evaporates developed. By the end of the Permian
Period, the Absaroka Sea had retreated from the craton.
This Permian regression of the Absaroka Sea left behind massive
Permian Red-Beds consisting of oxidized marine sediments.
Since Pangea also had formed, this caused a drying out of the
central craton creating very arid, and desert-like conditions on the
middles of the cratons.
 With much of the coastlines destroyed at this time, the reduction
of the “coastal effects” of precipitation had ceased.
Environmental pressures were HIGH resulting in the Permian
Mass Extinction. Approximately 95% of life on earth perished
during the Permo-Triassic Extinction!
 The Cordilleran Mobile Belt – was the site of the Antler orogeny,
a minor Devonian orogeny during which deep water sediments
were thrust eastward over shallow water sediments.
 During the Pennsylvanian and Permian Periods, mountain
building occurred in the Quachita mobile belt. This tectonic
activity was partly responsible for the cratonic uplift that took
place in the southwest, resulting in the Ancestral Rockies.
 During the Paleozoic Era, numerous terranes and microplates
such as Avalonia, existed and played an important role in the
formation of Pangea.
 Late Paleozoic –aged rocks contain a variety of mineral resources
including petroleum, coal, evaporates, silica sand, lead, zinc, and
other metal deposits.
Life of the Lower Paleozoic Era
Epieric Seas: broad, shallow, warm, nutrient seas that were formed by continental fringe
collisions that created "Arches and Basins". These basins were filled with seawater as they
formed creating almost an "incubator" effect on marine life forms. As the tilt of the axis of the
earth wobbled throughout geologic time causing growth of the ice caps, followed by their
melting, these Epieric seas would fill and drain accordingly. Whenever there was times of ice
cap buildup, there was a simultaneous "Eustatic" (= "worldwide") sea level drop draining the
Epieric seas causing extinctions and intense environmental pressures on the life forms.
Whenever the ice caps melted, there was an eustatic rise in sea level resulting in the epieric
seas to again fill and the incubator effect would cause life forms to flourish. The subsequent
rise and fall of the sea level is recorded in the geologic record as series of changing marine
lithofacies upon the continents called Cratonic Sequences. These changes (i.e. the rise and
fall of the Sauk Cratonic) were the driving forces of life evolution, molding and establishing
ecosystems worldwide. (see Cratonic Sequence Chart)
The Cambrian Explosion: the adaptive radiation of the marine life of the Cambrian due to not
only the development of the epieric seas, but also on the rise of gene mutations seen in
Cambrian marine life that utilized calcium carbonate and calcium phosphate for the first
time as body parts: bone, shells, & teeth. Hence, the beginning of the Phanerozoic Eon
("visible life" meaning that there began to be abundant fossils in the geologic record).
This increased the competition of the species worldwide resulting in the so-called "Cambrian
Explosion of Life". This also is what separates the Edicarian (Late Proterozoic 700MYA - 570
MYA) "soft bodied metazoan" fossil assemblages from the "hard shelled" Cambrian
assemblages.
Life forms beginning in the Cambrian:
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50% of all fossils of the Cambrian are trilobites - Phylum Arthropoda, Class Trilobita. All
Periods of the Paleozoic have trilobite Index Fossils.
Archeocyathids - little known Coral-like? Sponge like? Organisms that flourished for a
while but became extinct in the Early Paleozoic.
Brachiopods - Phylum Brachiopoda, Class Articulata & Class Inarticulata became
extremely abundant, even throughout the Paleozoic.
Most Cambrian life forms fall into the preceding three groups: Trilobites, Brachiopods, and
Archeocyathids.
Phylum Mollusca - Classes Gastropoda (snails), Bivalvia (clams), Cephalopoda (nautilus,
octopus, squid) also were abundant starting in the Cambrian throughout most of the
Paleozoic.
Early Echinoderms- Phylum Echinodermata Class Crinoidea (et al) were established.
Many other bizarre forms of life evolved but became extinct in the Early Paleozoic
The Burgess Shale: Discovered in the Canadian Rockies of British Colombia by Charles
Doolittle Walcott in 1910 is world renown for excellent fossil evidence of the “Cambrian
Explosion”.
Neotony: The retention of larval characteristics into the sexually reproductive adult form. This
is a major driving force of evolutionary change as in:
Echinoderm Bipinnaria Larvae: This is one of the best examples of neotony known. The
bilaterally symmetrical echinoderm larvae is the stock group of the Phylum Chordata. By
remaining nektonic, some larvae developed a notochord for support and locomotion. This
created the Phylum Chordata. By the addition of mutations within this group, namely the
absorption of calcium salts from the sea, allowed for the formation of internal bony supports
protecting the spinal cord. This created the first member of the Subphylum Vertebrata.
First Vertebrate Organism: The Agnathans – These were the first vertebrate fishes of the
Cambrian. They in turn became the stock group that gave rise to all subsequent vertebrate
groups.
Ontogeny Recapitulates Phylogeny – Change within an organism’s lifetime (ontogeny)
retells (recapitulates) its evolutionary history (phylogeny).
The Rise of the Chordates and Vertebrates: The Cambrian seas were the site of another
adaptive radiation of the vertebrate fish organisms. From the “jawless” Agnathan
(Ostracoderm) stock, there arose the following:
 Placoderms – “armored fishes” - Silurian to Mississippian
 Acanthodii – Silurian to Permian
 Chondrichthyes – “sharks, skates, and rays” - Devonian to Recent
 Osteichthyes – “bony fishes” - Devonian to Recent
The Ordovician
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This is marked by a major extinction due to the regression of the Sauk.
Bryozoans and Graptolites began to inhabit the seas.
But, with the rise of the Tippecanoe Cratonic Sequence, life again began to rebound.
Evidence shows that the climate worldwide began to stabilize in the Ordovician.
Corals (Rugosa and Tabulata) began to inhabit the warm shallow seas.
The Silurian
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First coral reefs appeared with their lagoons mimicking epieric seas. This production of
calm, warm, nutrient-rich lagoons, along with the production of numerous niches and
habitats was their major importance.
Know the cross-section of a coral reef: land, lagoon, reef core, reef tallus, fore-reef, and
back-reef
The first land plants (modified Chlorophyte Algae w/ waxy cuticle and vascular tissue)
altering the climate and creating a more oxygen rich atmosphere. Ozone levels began to
rise as lowland forests spread across the continents. Eumeric flora in Laurasia and
Glossopteris sp. seed fern forests in Gondwana. (see evolution of Plants page).
The rise of terrestrial plants provided a food supply that was soon exploited.
Eurypterus sp. "sea scorpions" were the first animal life to crawl onto land and it is thought
that they were exploiting the terrestrial plant diversity as a food supply.
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This lead to Eurypterus sp. giving rise to terrestrial arthropods (insects, et al.) creating yet
another food supply to become exploited.
The Taconic Orogeny (convergence of N. America and Africa) was first uplift of the
Appalachians, diverting the jet stream and altering the weather patterns resulting in a very
long "swampy" climate of most of the equatorial continents.
Evolutionary Relationships of Terrestrial Plants to Algal Ancestors
The fossil evidence for the evolution of modern plants supports the idea that all terrestrial
plants evolved from a common ancestor, namely marine algae related to members of the
Phylum Chlorophyta (the Green Algae). If one compares the physiology of Chlorophytan cells
to modern terrestrial plants, one is able to see many similarities: a cellulose cell wall, storing
excess food as starch, similar pigments, etc. The two main differences between Green Algae
and Modern Plants are the absence of a “Cuticle” and “Vascular Tissue” in the Green Algae.
It is thought that the “Cuticle” (which is a waxy, water-proofing coating found on the outside of
plant epidermis) evolved through natural selection from a group of inter-tidal Chlorophytan-type
seaweeds that were thrown into intermittent tidal pools and subjected to desiccation. Having
the ability to form a protective coat, this group of algae was able to exploit a new habitat in the
semi-dry environment of the beach- fronts. This stock group was also subjected to natural
selection by the competition to survive in this new “frontier”. From this group arose individuals
that had series of internal tissues (“Vascular Tissues”) that were able to transport water and
nutrients throughout the “plant”. The specialization of these tissues gave rise to the vascular
tissues of modern terrestrial plants.
Through natural selection, because of changing environmental pressures, the stage was set
for the exploitation of the terrestrial niches and habitats by these early primitive vascular plants
such as Cooksonia sp. and Asteroxylon sp. It is thought that these groups evolved into
other primitive vascular plants such as the Pterophytes (“True Ferns”), Sphenophytes
“Horsetails”), Psilophytes (“Whisk Ferns”), and Lycophytes (“Club Mosses”). These were
Spore-producing plants that did not produce seeds for reproduction. A spore must first
germinate into a “Gametophyte”, which produces the sperm and eggs necessary for
producing the “Sporophyte” (sexual phase plant) after fertilization. One group, the Phylum
Pteridospermophyta (the “Seed Ferns”), did develop the “seed” as a means of reproduction,
but they became extinct at the beginning of the Mesozoic because of the changing climate.
At the end of the Paleozoic, Selaginella sp . (a member of the Phylum Lycophyta), developed
the property of producing both male and female spores on a structure called a “Strobilus”.
This resembled a “pinecone” and allowed for the retention of the female spores and the
releasing the male spores creating a process similar to Pollination. Also, after the strobilus
containing the female spore received a male spore, fertilization occurred in the strobilus (rather
than on the ground, separate from the mother plant such as in the other primitive vascular
plants). Furthermore, the developing sporophyte was nourished and protected by the mother
plant’s strobilus, producing for at least the second time during the evolution of life on Earth, the
“SEED”. From this stock group arose the “Seed Plants” called the “Gymnosperms” (“Naked
Seed” because no fruit is produced).
Gymnosperms flourished during the Mesozoic to the point of taking over many niches and
habitats of the Primitive Vascular plants that had given rise to them. One sees a reduction in
the number of Primitive Vascular plants during the Mesozoic because of the rapid spread of
Gymnosperms. Included in this Gymnosperm group are the Phyla Coniferophyta (“Conifers”),
Ginkgophyta (“Ginkgos”), Cycadophyta (“Cycads”), and Gnetophyta (“Gnetophytes”). All of
these members produce their seeds in “Cone-like“structures, releasing their seeds to the wind.
Because of changing environmental pressures along with intense competition of all extant
plant groups of the time, natural selection caused the development of a certain group of
Gymnosperms, namely the Gnetophytes, to undergo an important evolutionary change.
This change is seen in the fossil evidence in plants related to the Gnetophyte group containing
a plant called Gnetum sp. This tropical, vine-like Gymnosperm had a reproductive habit of
clustering its strobili at the ends of their branches and protecting them with modified leaves
and other plant organs. This was the birth
of flowering and fruiting plants: the “Angiosperms” (Phylum Anthophyta). One of the earliest
Angiosperm fossils found dates back to the Late Jurassic/Early Cretaceous (approximately 150
million years ago). These fossil flowers resemble the modern Magnolia flower.
Since the seeds of Angiosperms are kept to developmental term in the ovary of the flower (the
“Fruit”), they were more successful at reproductive fecundity (creating offspring) than the
Gymnosperms. So, once again we see a more successful product of natural selection, the
flower and fruit, take over the niches and habitats of a previously successful Gymnosperm
group. Near the end of the Mesozoic, we see from the fossil evidence a rapid decline in the
number and variety of Gymnosperms and the rapid adaptive radiation and proliferation of the
Angiosperms throughout the Cenozoic through today.
Botanically speaking, the Paleozoic is known as the “Age of Primitive Vascular Plants”, the
Mesozoic as the “Age of Gymnosperms (particularly the Cycads)”, and the Cenozoic as the
“Age of Angiosperms”. It has even been speculated, and in many cases substantiated by
fossil evidence, that the rise of the Angiosperms contributed to the demise of the Dinosaurs.
By diminishing the Gymnosperm food supply for the herbivorous dinosaurs, upsetting
predator/prey ratios of the populations, and by possibly producing pollen that was foreign to
some dinosaur’s immune systems, the Angiosperms are thought to have played a major role in
the extinction of the dinosaurs. What possibilities lie ahead for the next successor in the
evolution of the Plant Kingdom?
Life of the Lower Paleozoic Era
Epieric Seas: broad, shallow, warm, nutrient seas that were formed by
continental fringe collisions that created "Arches and Basins". These
basins were filled with seawater as they formed creating almost an
"incubator" effect on marine life forms. As the tilt of the axis of the
earth wobbled throughout geologic time causing growth of the ice
caps, followed by their melting, these Epieric seas would fill and drain
accordingly. Whenever there was times of ice cap buildup, there was
a simultaneous "Eustatic" (= "worldwide") sea level drop draining the
Epieric seas causing extinctions and intense environmental pressures
on the life forms. Whenever the ice caps melted, there was an eustatic
rise in sea level resulting in the epieric seas to again fill and the
incubator effect would cause life forms to flourish. The subsequent
rise and fall of the sea level is recorded in the geologic record as series
of changing marine lithofacies upon the continents called Cratonic
Sequences. These changes (i.e. the rise and fall of the Sauk
Cratonic) were the driving forces of life evolution, molding and
establishing ecosystems worldwide. (see Cratonic Sequence Chart)
The Cambrian Explosion: the adaptive radiation of the marine life of
the Cambrian due to not only the development of the epieric seas, but
also on the rise of gene mutations seen in Cambrian marine life that
utilized calcium carbonate and calcium phosphate for the first
time as body parts: bone, shells, & teeth. Hence, the beginning of
the Phanerozoic Eon ("visible life" meaning that there began to be
abundant fossils in the geologic record). This increased the
competition of the species worldwide resulting in the so-called
"Cambrian Explosion of Life". This also is what separates the
Edicarian (Late Proterozoic 700MYA - 570 MYA) "soft bodied
metazoan" fossil assemblages from the "hard shelled" Cambrian
assemblages.
Life forms beginning in the Cambrian:
 50% of all fossils of the Cambrian are trilobites - Phylum Arthropoda,
Class Trilobita. All Periods of the Paleozoic have trilobite Index
Fossils.
 Archeocyathids - little known Coral-like? Sponge like? Organisms
that flourished for a while but became extinct in the Early Paleozoic.
 Brachiopods - Phylum Brachiopoda, Class Articulata & Class
Inarticulata became extremely abundant, even throughout the
Paleozoic.
 Most Cambrian life forms fall into the preceding three groups:
Trilobites, Brachiopods, and Archeocyathids.
 Phylum Mollusca - Classes Gastropoda (snails), Bivalvia (clams),
Cephalopoda (nautilus, octopus, squid) also were abundant starting
in the Cambrian throughout most of the Paleozoic.
 Early Echinoderms- Phylum Echinodermata Class Crinoidea (et al)
were established.
 Many other bizarre forms of life evolved but became extinct in the
Early Paleozoic
The Burgess Shale: Discovered in the Canadian Rockies of British
Colombia by Charles Doolittle Walcott in 1910 is world renown for
excellent fossil evidence of the “Cambrian Explosion”.
Neotony: The retention of larval characteristics into the sexually
reproductive adult form. This is a major driving force of evolutionary
change as in:
Echinoderm Bipinnaria Larvae: This is one of the best examples of
neotony known. The bilaterally symmetrical echinoderm larvae is the
stock group of the Phylum Chordata. By remaining nektonic, some
larvae developed a notochord for support and locomotion. This
created the Phylum Chordata. By the addition of mutations within this
group, namely the absorption of calcium salts from the sea, allowed for
the formation of internal bony supports protecting the spinal cord. This
created the first member of the Subphylum Vertebrata.
First Vertebrate Organism: The Agnathans – These were the first
vertebrate fishes of the Cambrian. They in turn became the stock
group that gave rise to all subsequent vertebrate groups.
Ontogeny Recapitulates Phylogeny – Change within an organism’s
lifetime (ontogeny) retells (recapitulates) its evolutionary history
(phylogeny).
The Rise of the Chordates and Vertebrates: The Cambrian seas
were the site of another adaptive radiation of the vertebrate fish
organisms. From the “jawless” Agnathan (Ostracoderm) stock, there arose the
following:
 Placoderms – “armored fishes” - Silurian to Mississippian
 Acanthodii – Silurian to Permian
 Chondrichthyes – “sharks, skates, and rays” - Devonian to
Recent
 Osteichthyes – “bony fishes” - Devonian to Recent
The Ordovician
 This is marked by a major extinction due to the regression of the
Sauk.
 Bryozoans and Graptolites began to inhabit the seas.
 But, with the rise of the Tippecanoe Cratonic Sequence, life again
began to rebound.
 Evidence shows that the climate worldwide began to stabilize in the
Ordovician.
 Corals (Rugosa and Tabulata) began to inhabit the warm shallow
seas.
The Silurian
 First coral reefs appeared with their lagoons mimicking epieric seas.
This production of calm, warm, nutrient-rich lagoons, along with the
production of numerous niches and habitats was their major
importance.
 Know the cross-section of a coral reef: land, lagoon, reef core, reef
tallus, fore-reef, and back-reef
 The first land plants (modified Chlorophyte Algae w/ waxy cuticle
and vascular tissue) altering the climate and creating a more oxygen
rich atmosphere. Ozone levels began to rise as lowland forests
spread across the continents. Eumeric flora in Laurasia and
Glossopteris sp. seed fern forests in Gondwana. (see evolution of
Plants page).
 The rise of terrestrial plants provided a food supply that was soon
exploited.
 Eurypterus sp. "sea scorpions" were the first animal life to crawl
onto land and it is thought that they were exploiting the terrestrial
plant diversity as a food supply.
 This lead to Eurypterus sp. giving rise to terrestrial arthropods
(insects, et al.) creating yet another food supply to become
exploited.
 The Taconic Orogeny (convergence of N. America and Africa) was
first uplift of the Appalachians, diverting the jet stream and altering
the weather patterns resulting in a very long "swampy" climate of
most of the equatorial continents.
Life in the Upper Paleozoic:
The Devonian "Age of Fishes"
Agnathan/Ostracoderm Stock Group – These Cambrian jawless
fishes became the stock group for the rise of fishes during the
Devonian.
 Placoderms – “armored fishes” - Silurian to Mississippian
 Acanthodii – Silurian to Permian
 Chondrichthyes – “sharks, skates, and rays” - Devonian to
Recent
 Osteichthyes – “bony fishes” - Devonian to Recent
Rise of Placoderms - the formation of the "Basic Vertebrate
Skeleton" – This group genetically established the “tetrapod” (four
appendage) body form, possessing an anterior “pectoral girdle” with
two appendages, and a posterior pelvic girdle, also possessing two
appendages. These Placoderms are named so because of the heavy
platy armored coverings found on the surfaces of these fishes. These
armors were probably in response to increased environmental
pressures and competition between groups.
Development of the "Jaw" and Teeth" – Embryonically, the jaws
(maxilla and mandible) arise from the calcification and forward rotation
of the first two pharangeal branchial arches associated with the gill slits
of the Agnathan stock group. In later groups, (chondrichthyes)
developed acrodont teeth (set on top of the jaw and held in place with
ligaments) from the Dermal Placoid Scales imbedded in its skin.
Class Chondrichthyes – the sharks, skates, and rays. These
possess cartilaginous skeletons with the only ossified portion being
the teeth. These immediately became very successful and diverse,
with many forms (i.e. sharks) becoming top carnivore in the sea. The
sharks achieved an almost perfect body adaptation to their
environment
Class Osteichthyes – the “bony” fishes. These possess calcified,
internal bony skeletons and diversified greatly in the Devonian and
Late Paleozoic.
Lobe Fin vs. Ray Fin & Peduncle comparisons – Some groups
adapted for fast swimming by developing “ray” fins (thin and
maneuverable for speed), and remained in open waters. The
peripheral isolates of these coastal groups that could not compete
moved into deltaic or swamp environments. These isolate groups
developed characteristics or adaptations to anerobic reducing
environments:
 Lung for air breathing
 Lobe fins covering the pentadactyl appendage
 Girdle positioning allowing for limbs to be used for support
on the bottom
 These are now called Rhipidistrian Fishes
The Pentadactyl Appendage - The 5-fingered vertebrate hand that
was chosen for its support superiority: a middle phalange for support,
two smaller pair on each side for lateral support. Any more phalanges
and they interfere with movement; any less and support is lost. This is
an evolutionary archetype common in terrestrial vertebrate organisms.
The Labrynthodonts - the First "Amphibian" of the Devonian –
were direct descendants the Rhipidistrian stock groups. An
“amphibian” means “both life” indicating that these organisms must
return to water for reproduction due to their possessing a shell-less
egg: the Anamniotic Egg. Modern forms include the frogs, toads,
salamanders, newts, etc.
Amphibian Characteristics:
 3-chambered heart to cope with higher energy requirements
having become terrestrial (i.e. coping with gravity)
 eyelids for prevention of desiccation of the eyes by exposure
to the air.
 Further girdle rotation for limb’s position in a more under
body mode.
 Development of the primal terrestrial ear for hearing in the
air. (inner ear bones evolving from pieces of the mandible)
 Anamniotic shell-less eggs: must be lain in water
 Example: Semouria sp.
The Cotylosaurs - the First "reptile" of the Devonian – beginnings
of the formation of Pangea began to dry out some portions of the earth
causing reptilian characteristics to develop from the Labrynthodons
stock groups.
Reptilian Characteristics:
 3-chambered heart to cope with higher energy requirements
having become terrestrial (i.e. coping with gravity)
 Even further girdle rotation for limb’s position in an even
more “under body mode” for ground clearance when walking
on land.
 Body developed epidermal “scales” for protection from
desiccation
 A better developed ear
 The development of the amniotic, shelled egg cut the ties
with water creating the first truly terrestrial reptilian group –
the Cotylosaurs of the Devonian
The Mississippian & Pennsylvanian
"Age of Coal Forests”
During the Mississippian and Pennsylvanian, many of the continental
plates were equatorial whereby conditions were swampy. This
created the vast coal deposits of today found in the upper eastern
states of America. All over the northern continents of Pangea
(North America and Eurasia) there were also coal deposits
formed during this time from Eumeric Flora (primitive vascular
plants). Cyclothems are sequences of coal bearing strata with
marine sediments on the bottom, then clay layers (paleo-soils)
then the coal seam, then terrestrial layers on top. The presence
of these cyclothems in strata core samples tells the modern
geologist where coal can be found.
In Gondwana, the Glossopteran Flora were “Seed Ferns” of the
Phylum Pteridospermophyta, Genus Glossopterus sp. These
seed ferns became extinct, but provide evidence that the “seed”
has evolved at least twice on the earth. Many coal beds of
Gondwana are made of fossil seed ferns.
As the continents continued to collide towards the end of the Paleozoic
to eventually form Pangea, the coal swamps dried up and the
conditions became very arid at the end of the Paleozoic.
From the diversification of Cotylosaurs we get Pelycosaurs and
Therapsids:
The Permian – the forming of Pangea
 Pangea continues to form, creating arid condition inland dur to a
loss of the “coastal effect of weather” since coastlines of individual
continents were being fused with other coastlines of other
continents.
 Permian-Triassic “Red beds” were formed from the evaporation of
shallow seas and subsequent oxidation of the marine deposits
(i.e. Panhandle of Texas – Paladura Canyon)
 Marine environments were being crushed between the converging
continents. Land bridges opened between continents that had
been separated by seas.
 This caused very strong environmental pressures so that there
was another Mass Extinction of the Permian. Some estimates
place the loss of life from 50% to 80% worldwide.
 Reptilian characteristics (especially the amniotic, shelled egg)
were chosen for by those transitional amphibian groups for
survival.
The Pelycosaurs – of the Permian - The carnivorous reptilian stock
group (i.e. Dimetrodon – sail-back lizards) of the Permian became the
stock group of the Mesozoic modern reptiles, marine reptiles, flying
reptiles, and Dinosauria. These Pelycosaurs were “thecodonts” having
teeth set in sockets.
The Therapsids – of the Permian - The “Mammalian Stock Group”
of the Permian that gave rise to the first mammal of the Triassic
Period. These thecodont reptile-like ancestors of the mammal had a
moveable jaw that allowed for “chewing” of foodstuffs. This, along with
the abundant Haversian Canals in the bones, in an indication these
were “endothermic”, able to create their own body heat.
Metabolic Terms associated with body heat:
Endothermic - i.e. an organism capable of creating their own body
heat (i.e. deriving more energy from its food and having a higher rate
of metabolism).
Ectothermic – i.e. an organism that must rely upon an external heat
source for its metabolism such as basking in the sun.
Homeothermic – i.e. an organism that is capable of maintaining a
constant body temperature such as birds and mammals.
Poikilothermic – i.e. an organism that is not capable of maintaining a
constant body temperature such as amphibians and reptiles
Evolutionary Relationships of Terrestrial Plants to
their Algal Ancestors
The fossil evidence for the evolution of modern plants supports the
idea that all terrestrial plants evolved from a common ancestor, namely
marine algae related to members of the Phylum Chlorophyta (the
Green Algae). If one compares the physiology of Chlorophytan cells to
modern terrestrial plants, one is able to see many similarities: a
cellulose cell wall, storing excess food as starch, similar pigments, etc.
The two main differences between Green Algae and Modern Plants
are the absence of a “Cuticle” and “Vascular Tissue” in the Green
Algae.
It is thought that the “Cuticle” (which is a waxy, water-proofing coating
found on the outside of plant epidermis) evolved through natural
selection from a group of inter-tidal Chlorophytan-type seaweeds that
were thrown into intermittent tidal pools and subjected to desiccation.
Having the ability to form a protective coat, this group of algae was
able to exploit a new habitat in the semi-dry environment of the beachfronts. This stock group was also subjected to natural selection by the
competition to survive in this new “frontier”. From this group arose
individuals that had series of internal tissues (“Vascular Tissues”) that
were able to transport water and nutrients throughout the “plant”. The
specialization of these tissues gave rise to the vascular tissues of
modern terrestrial plants.
Through natural selection, because of changing environmental
pressures, the stage was set for the exploitation of the terrestrial
niches and habitats by these early primitive vascular plants such as
Cooksonia sp. and Asteroxylon sp. It is thought that these groups
evolved into other primitive vascular plants such as the Pterophytes
(“True Ferns”), Sphenophytes “Horsetails”), Psilophytes (“Whisk
Ferns”), and Lycophytes (“Club Mosses”). These were Sporeproducing plants that did not produce seeds for reproduction. A spore
must first germinate into a “Gametophyte”, which produces the sperm
and eggs necessary for producing the “Sporophyte” (sexual phase
plant) after fertilization. One group, the Phylum Pteridospermophyta
(the “Seed Ferns”), did develop the “seed” as a means of
reproduction, but they became extinct at the beginning of the Mesozoic
because of the changing climate.
At the end of the Paleozoic, Selaginella sp . (a member of the Phylum
Lycophyta), developed the property of producing both male and female
spores on a structure called a “Strobilus”. This resembled a
“pinecone” and allowed for the retention of the female spores and the
releasing the male spores creating a process similar to Pollination.
Also, after the strobilus containing the female spore received a male
spore, fertilization occurred in the strobilus (rather than on the ground,
separate from the mother plant such as in the other primitive vascular
plants). Furthermore, the developing sporophyte was nourished and
protected by the mother plant’s strobilus, producing for at least the
second time during the evolution of life on Earth, the “SEED”. From
this stock group arose the “Seed Plants” called the “Gymnosperms”
(“Naked Seed” because no fruit is produced).
Gymnosperms flourished during the Mesozoic to the point of taking
over many niches and habitats of the Primitive Vascular plants that had
given rise to them. One sees a reduction in the number of Primitive
Vascular plants during the Mesozoic because of the rapid spread of
Gymnosperms. Included in this Gymnosperm group are the Phyla
Coniferophyta (“Conifers”), Ginkgophyta (“Ginkgos”), Cycadophyta
(“Cycads”), and Gnetophyta (“Gnetophytes”). All of these members
produce their seeds in “Cone-like“structures, releasing their seeds to
the wind.
Because of changing environmental pressures along with intense
competition of all extant plant groups of the time, natural selection
caused the development of a certain group of Gymnosperms, namely
the Gnetophytes, to undergo an important evolutionary change.
This change is seen in the fossil evidence in plants related to the
Gnetophyte group containing a plant called Gnetum sp. This tropical,
vine-like Gymnosperm had a reproductive habit of clustering its strobili
at the ends of their branches and protecting them with modified leaves
and other plant organs. This was the birth
of flowering and fruiting plants: the “Angiosperms” (Phylum
Anthophyta). One of the earliest Angiosperm fossils found dates back
to the Late Jurassic/Early Cretaceous (approximately 150 million years
ago). These fossil flowers resemble the modern Magnolia flower.
Since the seeds of Angiosperms are kept to developmental term in the
ovary of the flower (the “Fruit”), they were more successful at
reproductive fecundity (creating offspring) than the Gymnosperms. So,
once again we see a more successful product of natural selection, the
flower and fruit, take over the niches and habitats of a previously
successful Gymnosperm group. Near the end of the Mesozoic, we see
from the fossil evidence a rapid decline in the number and variety of
Gymnosperms and the rapid adaptive radiation and proliferation of the
Angiosperms throughout the Cenozoic through today.
Botanically speaking, the Paleozoic is known as the “Age of Primitive
Vascular Plants”, the Mesozoic as the “Age of Gymnosperms
(particularly the Cycads)”, and the Cenozoic as the “Age of
Angiosperms”. It has even been speculated, and in many cases
substantiated by fossil evidence, that the rise of the Angiosperms
contributed to the demise of the Dinosaurs. By diminishing the
Gymnosperm food supply for the herbivorous dinosaurs, upsetting
predator/prey ratios of the populations, and by possibly producing
pollen that was foreign to some dinosaur’s immune systems, the
Angiosperms are thought to have played a major role in the extinction
of the dinosaurs. What possibilities lie ahead for the next successor in
the evolution of the Plant Kingdom?