The Mesozoic (Age Of Cycads And Dinosaurs): Fact Sheet
References:
http://seaborg.nmu.edu/earth.
http://www.carleton.ca/~tpatters/teaching/intro/cambrian/cambrianex20.html.
ACTIVITY 1
- Scan the fact sheets below. Set up a chart and find the most
dominant form of plant and of animal in the key time periods
given. Check the web sites for images.
- Look at other references to find out what these dominant
forms looked like.
- Compare this chart with the information in the previous section:
- Extinctions, the Geological Time Scale and the Glaciers sections
beginning 600 million years ago.
- State your findings. Defend your theory.
Mesozoic seas saw a rise and spread of another iteration of
the cephalopods, the ceratites, eventually to be replaced by
the ammonites. Coral reefs, which had been decimated by the Carboniferous
extinction returned to prominence with the evolution of new groups
of reef-building animals and algae. Modern bony fish evolved
and began to compete with the ammonites as swimming predators.
Terrestrial animals saw the rise of major groups of tetrapods:
dinosaurs and mammals and the later advent of birds. Tetrapods
took to new environments in the air as well as a secondary return
to the waters. Forests took on an increasingly modern look, with
the evolution of cycads and cycadeoids, conifers and ginkgoes.
Flowering plants finally appeared about 140 million years ago
and have risen to become the dominant floral component of many
areas of the planet.
The Cretaceous
The Cretaceous period, spanning the time interval from 144 to
65 million years ago, saw the final phases of the opening of
the Atlantic Ocean, as well as the northward migration of India
toward its collision with Asia during the Tertiary period of
the Cenozoic era. The last of the major forms of plant life,
the angiosperms, appeared near the beginning of the period. Dinosaurs
continued their diversification, with the increasing dominance
of the herbivorous ornithischians.
Life in Water
The bivalves recovered from the Jurassic extinctions and again
became major reef-formers in the numerous shallow marginal seas
that encroached onto the continents during the Cretaceous.
Baculites, a genus of straight-shelled cephalopods, was
particularly abundant in the Cretaceous seas. Note the elaborate
suture patterns in the fossil specimen below.
The plesiosaurs from the Jurassic continued their diversification.
Ichthyosaurs, however, declined during the early Cretaceous before
becoming extinct well before the end-of-the-Cretaceous extinction.
Both the long-necked plesiosaurs, and the short-necked pliosaurs
were present during the Cretaceous. Despite inevitable competition
and changes in their diets as new prey evolved, plesiosaurs were
doing well up until the extinction at the close of the Cretaceous.
To the list of swimming reptiles is added the mosasaurs, essentially
swimming "lizards". These ferocious predators reached lengths
to 30 feet (9 meters). Mosasaurs are usually considered relatives
of the monitor lizards, although some recent analyses suggest
a closer link to snakes is a distinct possibility. Mosasaurs
were predators of the Cretaceous seas. Mosasaurs apparently gave
birth to live young and perhaps had some level of parental care
for their young.
Life on Land
Plants
Perhaps the greatest change in the terrestrial life of the Cretaceous
was the rise and diversification of the flowering plants, the
angiosperms. The angiosperms, the last of the seed plant groups
to evolve, appeared over 140 million years ago during the the
beginning of the Cretaceous. All members of this group produce
flowers.
The flowering plants most likely evolved from a Mesozoic gymnosperm
group. The classical view of flowering plant evolution suggests
early angiosperms were evergreen trees that produced large Magnolia-like
flowers. However, this view has recently been contradicted.
The angiosperms underwent an adaptive radiation during the Cretaceous,
and for the most part escaped the major extinctions at the end
of the Cretaceous.
Animals
Crocodiles, which had been around since the Triassic, produced
a massive form, Deinosuchus, during the Cretaceous. This
marine crocodile was over 20 feet long, and dwarfs the largest
modern crocodile.
The ornithischian dinosaurs during the Cretaceous evolved into
a number of interesting groups. From bipedal ornithischians during
the Jurassic, like Camptosaurus, the larger and more nimble
ornithopods of the Cretaceous evolved. One of the first dinosaurs
to be scientifically described was Iguanodon, a bipedal
herbivore thought to have moved about in herds. Iguanodon was
between 6 and 10 meters long and weighed around 5 tons.
Perhaps no other dinosaur has inspired as many plush toys, tattoos,
and awe as Tyrannosaurus rex. Sporting six inch
long serrated teeth T. rex was a meat-eating machine.
Tyrannosaurs ranged from 10-14 meters in length and weighed in
at an estimated 5-7 tons, and were restricted to North America
during the latest part of the Cretaceous period.
A smaller carnovore that precedes T. rex in time is Albertosaurus.
This smaller version of T. rex reached lengths of between 7 and
8.5 meters and weighed in at a mere 2 tons. It ranged North America
in the time interval just prior to the rise of T. rex.
The flying reptiles of the Cretaceous were the pterodactyls,
short tailed descendants of the long tailed Jurassic forms. The
largest of the pterodactyls was Quetzalcoatlus, named
after the Aztec feathered serpent god. Quetzalcoatlus northropii had
a wingspan of 11 meters and may have weighed over 200 km.
Cretaceous birds continued to expand their environmental range. Hesperornis was
a flightless swimming bird that lived in the shallow seas of
the Cretaceous. There were other Cretaceous birds that flew.
Large, flightless birds, like the ostrich, would not appear until
the Tertiary period.
Cretaceous mammals were still quite small and minor components
of the fauna. Their significance would change with the Cretaceous
mass extinction, from which they would emerge as co-rulers of
the land (with the avian dinosaurs).
The Jurassic
The Jurassic period spanned the time interval from 208 to 146
million years ago. The major events of the Jurassic were the
explosive adaptive radiation of dinosaurs and the evolution of
birds (sometimes referred to as the avian dinosaurs), the diversification
of the cycads, seed ferns and the "cycadeoids", and the continued
breakup of Pangaea.
The opening of the Atlantic Ocean between Africa and North America
created new shallow-water environments, as did the global rise
of sea-levels that occurred during the Jurassic. As you might
expect, marine life responded with a burst of adaptation and
the appearance of new forms.
Life in Water
The separation of Pangaea caused the formation of new oceans,
such as the Atlantic. It also led to a global rise in sea-level.
Consequently there were many more environmental spaces open for
exploitation. Since the Triassic extinction was not nearly as
calamitous as the Permian one, life rebounded much quicker.
Dinoflagellates and coccoliths became major elements of the
oceanic phytoplankton. The bivalves, specifically the rudistids,
began to play a larger role in reef formation.
Coral reefs expanded into these new ocean environments.
The cephalopods again produced a new coiled, chambered form,
the ammonites. Suture patterns of these forms were even more
elaborate than those found in the Triassic ceratites. The belemnites
were straight-shelled cephalopods with elaborate suture patterns.
The ichthyosaurs had been virtually unaffected by the Triassic
extinction. During the Jurassic they reached their peak of diversity
and prevalence ecologically. Whereas the triassic forms had resembled
swimming lizards, Jurassic and later forms took on increasngly
fish-like shapes.
In addition to ichthyosaurs, Jurassic seas also contained long-necked
plesiosaurs. Plesiosaurs evolved at the end of the Triassic,
most likely from nothosaurs, and diversified during the Jurassic,
persisting until the terminal Cretaceous event that closed the
Mesozoic era.
While most of us think of plesiosaurs as having long necks,
there are also short-necked forms. Unlike the ichthyosaurs, which
swam in a fishlike manner, plesiosaurs swam by using their paddle-like
limbs in a manner similar to that of a sea lion or seal. Plesiosaurs
did not give birth to live young, but instead crawled ashore
to lay eggs as turtles do.
Life on Land
The Jurassic terrestrial environments saw an increase in gymnosperm
groups such as the cycads, ginkgoes, and cycadeoids. Reptilian
groups also became more adept at flying, while the dinosaurs
grew in size with such giants as Apatosaurus and Brachiosaurus,
and ferocious predators like Allosaurus. The first birds
appeared, as did the most famous fossil bird, Archaeopteryx.
Plants
The Jurassic was a time of increasing diversity for the cycads
and a similar-looking group, the cycadeoids. The cycadeoids,
sometimes known as the Bennettitales, produced leaves that superficially
resembled cycad leaves. However, details of the reproductive
structures indicate the two were not closely related. During
the Mesozoic, leaves of these groups were so common that paleobotanists
refer to the era as the age of cycads.
The modern cycads are a pale remnant of the dominant plants
of the Mesozoic, with today only 160 species in 11 genera, confined
to the tropics. They retain some fern-like features, notably
pinnate leaves and circinate vernation. However, they (usually)
produce cones of non-photosynthetic reproductive structures.
The cycadeoids superficially resemble cycads in having recurrent
leaf bases on soft-wood stems. Unlike the cycads, however, the
cycadeoids have bisexual reproductive structures and structure
of their stomata is more like those found in angiosperms and
gnetophytes.
The ginkgoes, like the cycads, originated during the late Paleozoic
and diversified during the Mesozoic. Beginning during the Jurassic
the number of species increased to a maximum of 11 during the
Cretaceous.
The Mesozoic saw the spread of other gymnosperms - true conifers,
including forms similar to Metasequioia and Sequoia,
as well as numerous types of pines. A wide variety of extinct
gymnosperms were present, including Caytonia and Pentoxylon.
Animals
The flying reptiles of the Triassic underwent diversification
during the Jurassic. The pterosaurs (literally winged-lizard)
consisted of the earlier forms with long tails, like Rhamphorhynchus shown
below, and the later forms that ruled the Cretaceous period,
the pterodactyls. Pterosaurs had hollow bones, large brains and
possibly some had endothermy.
The true giants of the dinosaur world were the sauropods. These
quadrupedal, herbivores included Apatosaurus, Brachiosaurus,
and Diplodocus. They ate numerous types of plants, often
specializing in plants of a certain height. Since the teeth of
sauropods are not adapted for grinding, this mechanical part
of the digestive process was accomplished in their stomachs.
Sauropod skeletons are often found in association with curiously
rounded stones in the approximate position of the stomach. These "stomach
stones" are known as gastroliths, and served a similar function
in sauropods as bird gravel does in birds: to aid digestion of
plant material not ground up enough in the mouth. Gastroliths
are not unique to sauropods, as many other herbivorous dinosaurs
and plesiosaurs have been found with stomach stones.
Apatosaurus, once much better known as "Brontosaurus",
was a herbivore that ranged from 21 to 26 meters long, and had
an estimated weight of between 30 and 35 tons. In modern reconstructions
of the the Apatosaurus environment the vegetation is not
swampy but almost desert-like, and Apatosaurus is considered
to have been a fully terrestrial animal.
Barchiosaurus (22 to 30 meters long; 30 to 80 tons weight)
was a larger dinosaur than Apatosaurus, and for many years
was considered the heaviest of the sauropods. Brachiosaurs were
different from other sauropods in having their forelegs longer
than their hind legs, as well as having their nostrils high on
the forehead. The largest known doinosaur, Ultrasaurus,
either belongs to the brachiosaurs or is in fact a large specimen
of Brachiosaurus, according to some paleontologists.
The remaining saurishian dinosaurs were theropods, the bipedal
carnivores exemplified by Allosaurus. Current thought
is that there were several groups of theropods, and that one
group, the carnosaurids contained the largest of the carnivores
such as Allosaurus. Another theropod group was the coelurosaurids,
which contained the birds and smaller carnivores such as ornithomimids, "raptors",
and the tyrannosaurs.
The carnosaurs, such as the Jurassic-aged Allosaurus,
had long narrow skulls, strong forelimbs, and a variety of horns
or knobs projecting from the skull.
The ornithischian dinosaurs became more significant elements
of the dinosaurian faunas during the Jurassic, and especially
so during the Cretaceous period. The ornoithischians are the
so-called bird-hipped dinosaurs. However, the resemblance of
hip structure between birds and this group is no longer considered
evidence for the rise of birds from ornithischians, but as a
result of convergent evolution.
A major group during the Jurassic was the stegosaourids. These
quadrupedal animals sported spiked tails and large flat armor
plates arrayed along the spine. These plates functioned for regulation
of body heat. Stegosaurus was about 9 meters long and weighed
about 2 tons. Even among dinosaurs it is noteworthy for its incredibly
small brain.
The ankylosaurs were ornithischians that bore armor and spikes
on their backs and may have defended themselves by using their
tails (which often had armor balls at the tips) and by their
dense back armor.
Another group of ornithischians was the ornithopods. Among the
earliest ornithopods was Camptosaurus, a 17 foot long
Jurassic dinosaur from Europe and North America. Camptosaurus reached
Australia by the early Cretaceous. During the Cretaceous the
ornithopods became much more diverse.
The Triassic
The Triassic period lasted from 245 to 208 million years ago.
Following the massive extinction at the end of the Permian, the
survivors underwent adaptive radiatives as they diversified and
began to reoccupy many of the now-vacated environmental roles.
Dinosaurs arose in the Triassic. Plateosaurus (larger)
spies two Yaleosaurus (left) and a smaller Coelophysus behind.
Cycads were a dominant vegetative type. The taller trees belong
to the genus Bjuvia. Check images at http://seaborg.nmu.edu/earth.
Life in Water
Coral, decimated by Paleozoic extinctions, rebounded during
the Triassic with the evolution of the scleractinian corals.
Dinoflaellates made their first appearance as part of the phytoplankton.
A phytoplankton group that appeared at the end of the Triassic
was the coccolithophorids, usually shortened to coccoliths. These
microscopic creatures collect calcium carbonate from the seawater
and use it to build their small "skeletons".
The cephalopods recovered from the extinction of the goniatites
and developed a remarkably similar group, the ceratites. These
coiled, chambered animals had slightly more complex sutures than
did the goniatites.
Bivalves (specifically pelecypods) became more abundant and
important parts of reefs as the Triassic progressed, and would
remain important parts of the marine fauna throughout the Mesozoic.
Life on Land
Plants
The Triassic was a period of transition for the world's floras,
as Paleozoic plants such as the calamites and lycopsids declined
in favor of newer gymnosperm groups such as the "cycads". During
the Triassic up to 20% of the world's flora consisted of cycads.
During the Mesozoic ginkgoes were worldwide in their distribution
and important elements bin the gymnosperm forests that dominated
the land.
Animals
Terrestrial faunas of the early Triassic were dominated by therapsids,
evolutionary offshoots of the by-then extinct "sail-backs". These
therapsids divided into two groups: herbivores and carnivores.
The therapsids of this time may have had some sort of "hair" as
a body covering as well as some degree of endothermy. By the
close of the Triassic therapsids had given rise to early mammals.
The ancestors of modern turtles appeared during the Triassic.
Whereas modern turtles have lost their teeth, these Triassic
turtles retained teeth on their jaws.
Another reptile group, the euryapsids, made the transition from
land to sea during the Triassic. These swimming reptiles diverged
into three groups during the Triassic: the ichthyosaurs (dolphin-shaped
swimming reptiles), the placodonts (bulky, paddle-limbed reptiles
that went extinct by the end of the Triassic), and the nothosaurs
(streamlined, long-necked swimming predators).
The diapsids, the "reptiles" except for turtles and a few extinct
forms, rose to prominence during the latter Triassic times. Diapsids,
you recall, have two openings in the skull behind the eye socket.
While the diapsids had originated possibly as far back as the
late Carboniferous, they did not become major elements of the
fauna until the Triassic. The major group of diapsids, the archosaurs
(ruling reptiles) evolved in the later part of the Permian, and
underwent a major adaptive radiation in the late Triassic.
The first true crocodiles also appeared during the Triassic.
Several other reptile groups, such as the Triassic phytosaurs,
adapted to the environment that crocodiles lived in. Due to evolutionary
convergence, these animals took on an outer semblance of crocodiles,
even though there were numerous anatomical differences.
One early archosaur was Euparkeria, a small, Triassic-aged,
quadrupedal reptile that some scientists think may have been
able to run short distances on its hind legs. Euparkeria specimens
reach about 0.5 meters in length. The genus appears restricted
to the Triassic of South Africa.
Another archosaur, at one time considered a dinosaur, was Herrerasaurus,
from the Triassic of Argentina. This animal marked the transition
between the archosaur stem group and the derived dinosaurs. In
all but a few characteristics Herrerasaurus is a
dinosaur, although a smallish one of 3-4 meters in length and
a body weight estimated at around 300 kg.. Eoraptor, from
the same age and area, was another archosaur with a mosaic of
dinosaurian and nondinosaurian characteristics.
The Triassic saw the evolution of the first true dinosaurs,
the ceratosaurians, or horned dinosaurs. These bipedal, carnivorous
dinosaurs have skeletons with a number of bird-like features.
Famous members of this group include Dilophosaurus and Coelophysis.
Both occur in Triassic deposits in the North American Painted
Desert. Coelophysis and similar dinosaurs are known from other
Triassic rocks throughout Pangaea. Dilophosaurus was a slender
6-7 meters long dinosaur that weighed an estimated 300-400 kg.
Coelophysis was less than 3 meters long and weighed a slight
15-30 kg.
PALEOZOIC
The Permian
The Permian period spanned the time interval from 286 to 245
million years ago. During the Permian the assembly of Pangaea
was completed and a whole host of new groups of organisms evolved.
Life in Water
The fusulinid foraminiferans that had appeared during the Carboniferous
continued their diversification. Trilobites were rarely encountered,
although brachiopods and crinoids had some recovery of species
diversity after the Carboniferous extinction. Marine environments
were much reconstructed due to the Pangea-related tectonism and
resulting uplift of the supercontinent.
Life on Land
At first glance it might appear that the only evolutionary changes
of note were occurring on the land during the Permian. Part of
this illusion results from the relative scarcity of Permian-aged
marine sediments as well as the tendency of land-dwellers to
view themselves as the pinnacle of evolution.
Plant life of the Permian took on an increasingly modern "look" with
the rise of a number of gymnosperm (naked seeded) plants during
the late Carboniferous and their diversification during the Permian.
The amniote lineage (the first truly terrestrial vertebrates
that did not need to return to water to lay eggs), especially
the fossils, often is classified based on the number of openings
in the skulls. These openings allow for muscle attachment and
have traditionally been used to separate the "reptiles" into
several groups, including the anapsids, synapsids, and diapsids.
Recent studies suggest that many animals in this group were not
so strongly reptilian (such as ectothermy) as the obsolete term "mammal-like
reptiles" implied.
Dimetrodon, shown above, was a member of the pelycosaurs,
or non-mammalian therapsids. By the end of the Permian the therapsids
had developed, a group possibly characterized by some degree
of endothermy (warm-bloodedness). The early mammals of the Triassic
period are possibly an offshoot or descendant group of the therapsids.
The Carboniferous
The Carboniferous, at least as seen in North American deposits,
is really two distinct sets of conditions: marine dominated early
Carboniferous environments characterized by shallow seas, followed
in time by terrestrial "coal-swamp" environments of the late
Carboniferous.
Coal Swamps and Glaciers
The Carboniferous period (360 to 286 million years ago) in Europe
is better known in North America as the Mississippian period
and the Pennsylvanian period. The regression of the Mississippian
seas from North America provided a natural break in deposition
that was not echoed in Europe.
The Carboniferous takes its name from the widespread occurrence
of coal deposits formed during this timespan in Europe and North
America. Coal is a sedimentary rock composed of plant debris
that was deposited in a bog or swamp with little biological activity
at its bottom.
Life in Water
The marine environments, following the rebound from the late
Devonian mass extinction underwent changes, with crinoids becoming
more dominant and diverse. The early Carboniferous is sometimes
known as the "age of crinoids". The primitive, armored Devonian
fish were replaced by more "modern" ray-finned fish.
Invertebrates
Fenestrate bryozoans were particularly common in the early Carboniferous
seas. Archimedes, a corkscrew-shaped fossil, represents
the secreted support of a colony of bryozoans that are usually
no longer present. The so-called lacy bryozoans were among the
first invertebrate groups to recover after the Devonian extinction.
Brachiopods became increasingly important animals. The spiriferids
resumed or increased their ecological dominance during the early
Carboniferous.
Corals were much restricted after the Devonian crisis and the
large reefs of the Devonian were replaced with smaller reefs
known as patch reefs. The role of corals in these new reefs was
much reduced from what it had been in earlier times. Crinoids
were also important contributors to the building of these small
reefs, as were blastoids, a similar group of echinoderms.
Forminiferans a group of unicellular protozoans that date from
the Cambrian, developed a new series of forms with foram "shells".
The fusulinids were a large group of foraminiferans that reached
sizes of several centimeters in length.
Vertebrates
Sharks and bony fish continued to diversify during the good
times of the early Carboniferous, before the dropping sea levels
of the late Carboniferous caused loss of habitat. The earliest
bony fish to dominate the aquatic environment were the palaeoniscoids,
a group that raged in time from the Carboniferous to the Triassic
wjich somewhat resembled modern bony fish but had significant
anatomical differences.
Life on Land
Plants
The Lycophytes became even more significant elements of the
world's flora during the Carboniferous. These non-seed plants
evolved into trees in the fossil genera Lepidodendron and Sigillaria, with
heights reaching up to 40 meters and 20-30 meters respectively. Lepidodendron trunks
can be over 1 meter in diameter. However, the stems are composed
of less wood (secondary xylem) that usually is found in gymnosperm
and angiosperm trees.
Lepidodendrids had a diamond-shaped "snakeskin" pattern produced
by the leaf cushions arranged in a helix.. On the left is a lower
magnification view of this type of pattern, showing the general
features of many of these trees.
Pecopteris is the foliage of a tree fern, Psaronius, one
of the most common fossil tree ferns. It reached a height of
about ten meters (32 feet) and resembled a modern palm tree. Neuropteris the
foliage of a seed fern, Pteridospermales, an extinct group of
gymnosperms. Although their foliage resembled that of modern
ferns, they reproduced by means of seeds. Modern ferns reproduce
by means of spores.
During the late Carboniferous the Cycads, considered
by paleobotanists to be closest to the gymnosperms, appear.
Cordaites, the name-bearer for the cordaitaleans, were
large trees with strap-shaped, leathery leaves that often had
parallel leaf veins. The cordaites had their reproductive structures
and seeds arranged in cones.
Animals
The major animals on land during the Carboniferous were the
amphibians (and their descendants such as the stem reptiles)
and insects. Dragonfly fossils have been found with wingspans
up to 75 cm.
With climate changes occurring during the late Carboniferous,
the plants changed, as did the terrestrial animals. Glaciation
in Gondwana has been blamed for this chanmge. One response that
tetrapods made was to develop the amniotic egg. With the egg
and resulting freedom from water for reproduction (to which amphibians
to this day must return to lay their eggs) reptiles (sometimes
known as stem-reptiles, could exploit drier environments further
from water.
Hylonomus was one of the earliest reptiles. This quadrupedal
(four-legged) stem reptile ate insects (which were prolific in
the coal swamps). From this group of early reptiles evolved the
pelycosaurs, the great reptiles of the Carboniferous-Permian
times, and posibly the bipedal thecodonts of the permian-Triassic.
Pelycosaurs included both herbivorous and carnovorous species.
The group was characterized by an elaborate fin or sail on their
back.
The Devonian
The Devonian world was one of major changes, both in the biological
realm and the geological one. The two large landmasses, Gondwana
to the south and Laurasia to the north began moving toward the
formation (again) of a world supercontinent, Pangaea.
Mountain building events were occurring on the edges of each
of these continents. On Laurasia, the collision of Eurpoe with
Laurentia produced the beginnings of the Appalachian Mountains.
Changes at the end of the Devonian brought an end to the time
of extensive reef building.
The oldest jawed fish, the placoderms, the ostracoderms and
acanthodian fish experienced an explosion of their fish groups.
The Silurian
Life in Water
The Silurian period (440 to 410 million years ago) saw a return
to the moderate climates that had existed before the Cambrian
and Ordovician glaciations. Eukaryotic life colonized terrestrial
environments. Fish underwent an adaptive radiation with the jawed
fish appearing and spreading into freshwater.
Corals diversified into a number of groups during the Silurian.
Tabulate corals and rugose corals were major components of the
new, larger reefs built during the Silurian through Permian.
Rugose corals included the horn corals, while tabulate corals
were colonial.
Crinoids had been present since the Ordovician (and possibly
the Cambrian). With the flooding of continents during the Silurian,
crinoids underwent an adaptive radiation and produced a number
of interesting forms.
Icthyocrinus illustrates the three regions of the body
that can be often found as isolated fossils. Caryocrinites
ornatus shows details of the stalk and head region.
One arthropod group became more diverse during the Silurian
and Devonian times, the Eurypterids, or sea scorpions became
more diverse. They were among the major swimming predators of
the Silurian-Devonian seas.
Fish continued to divesify, with the oldest jawed fish, the
placoderms, joining the ostracoderms and acanthodian fish. themselves
in both marine and freshwater environments.
Life on Land
Perhaps the most significant advance of life during the Silurian
was the colonization of the land, first by plants and insects,
and later by certain "fish" and their offshoots, the amphibians.
The Silurian land was populated by early land plants as well
as a variety of insects. Both plants and animals had a number
of challenges when they moved from the water to land.
Plants
Plants divide into two large groups: vascular plants that contain
lignified conducting cells, and the nonvascular plants, which
do not. The earliest vascular plants had no roots, leaves, fruits,
or flowers. Cooksonia is a typical early vascular plant.
It was less than 15 cm tall, with stems that dichotomously branched
(stem divides into two ewqual branches). Many scientists now
consider "Cooksonia" an evolutionary grade rather than a true
monophyletic taxon.
The Ordovician
The Ordovician period (505 to 440 million years ago). Diversification
occurred among the survivors of extinction. Corals become dominant
reef-building animals during the Ordovician, bryozoans and algae
were also dominant elements of the reef building biota. Trilobites
had survived the end of the Cambrian. The Ordovician is noteworthy
because of the moves some green algae made toward the shoreline,
and possibly onto land, becoming the first plants. Interesting
animals of the period include the conodonts (thought to represent
early vertebrates) and graptolites. The first fish also evolved.
Bryozoans are a group of organisms sometimes referred to as "moss
animals". They form colonies, often consisting of millions of
individuals.
Brachiopods were present but after the Cambrian extinction,
they underwent an adaptive radiation in the Ordovician. Brachiopods
have bilaterally symmetrical shells, which make them among the
most common marine fossils in Paleozoic rocks.
Mollusca increased in significance in the Ordovician faunas.
A type is a cephalopod. Nautiloids resemble, somewhat, their
living distant relatives the chambered nautilus and squids.
Gastropods, another class of the phylum Mollusca, also become
more prevalent in the Ordovician seas. Ordovician deposits yield
snails, as well as large, sedentary gastropods such as Maclurites.
Perhaps the most prfound advance during the Ordovician was the
development of plants from an ancestral group of green algae.
Fossil and biochemical evidence indicates plants are descended
from multicellular green algae. Various green algal groups have
been proposed for this ancestral type, with the Charophytes often
being prominently mentioned.
Cladistic studies support the inclusion of the Charophytes
(including the Coleochaetales) as sister taxa to the land plants.
Algae dominated the oceans of the precambrian time over 700 million
years ago. Between 500 and 400 million years ago, some algae
made the transition to land, becoming plants by developing a
series of adaptations to help them survive out of the water.
The vertebrates evolved during the Cambrian period. The oldest
group of fish, the ostracodemrs, first appeared during the Cambrian.
The Ordovician. was a time of great transgression of the seas
onto the continents, producing a great many habitats in which
new forms of life, such as fish, could develop. The exact appearance
of the Ordovician fish in unclear. What we do have is fragments
of hard coverings of soft tissue.
The Cambrian Explosion
The Cambrian period, the first in the Paleozoic era, spans the
time between 544 and 505 million years ago. During what is termed
the Cambrian Explosion, living things developed hard structures.
The early Cambrian was a wild time for animal evolution. During
a span of time perhaps as short as 5 million years numerous body
plans developed, including the three animal body plans that exist
today. All major animal phyla, save the Bryozoans, have their
first appearance during the Cambrian. One of the dominant animal
groups that appeared was the trilobites. These now-extinct members
of the Phylum Arthroopoda became the dominant animals in many
Cambrian marine environments. Another major change was the evolution
and spread of the archaeocyathids. They were important reef-building
organisms, along with some calcareous algae.
The Burgess Shale Fauna (in British Columbia)
The organisms range from the prokaryotic cyanobacteria to eukaryotic
green and red algae, to sponges, brachiopods, priapulids, annelids,
and many different arthropod groups, as well as echinoderms and
possibly one of the first chordates.
Check images at http://www.carleton.ca/~tpatters/teaching/intro/cambrian/cambrianex20.html.
One of the most common Burgess animals is the arthropod Marella.
These animals were small, and exhibited the jointed appendages
characterizing the arthropods. These delicate looking arthropods
are referred to as "lace crabs". The quality of preservation
of these fossils is quite extraordinary. On many specimens dark
stains are interpreted as body fluids that have leaked after burial.
THE PRECAMBRIAN
The Origins of Multicellularity
The oldest accepted prokaryote fossils date to 3.5 billion years;
Eukaryotic fossils to between 750 million years and possibly
as old as 1.2-1.5 billion years. Multicellular fossils, purportedly
of animals, have been recovered from 750Ma rocks in various parts
of the world. The first eukaryotes were undoubtedly Protistans,
a group that is thought to have given rise to the other eukaryotic
kingdoms. Multicellularity allows specialization of function,
for example muscle fibres are specialized for contraction, neuron
cells for transmission of nerve messages.
The Precambrian
The Archean Eon encompasses the time from the formation of the
earth until 2.5 billion years ago. The rocks formed during this
eon are the most ancient rocks known, up to 3.96 billion years
old. Perhaps the biggest development during the Archean was the
first appearance of life. The earliest forms of life were simple
prokaryotic cells. Ontario has parts of the Precambrian shield
exposed-eg along the St. Lawrence River at Brockville.
Layers in the stromatolites are alternating biogenic and sedimentologic
in origin. Stromatolites become more common in the Proterozoic
and decline during the Cambrian. Modern stromatolites are found
in marine environments where the presence of herbivorous :grazers" is
limited.
Simple, prokaryotic cells still dominated the world's environments
until the evolution of simple eukaryotes approximately 1.5 -
1.2 billion years ago. With the appearance of eukaryotes comes
the development of sexual reproduction, which greatly increased
the variation available for natural selection.
A major enbvironmental change, initiated by living things, was
the development of oxygenic photosynthesis. This led to increasing
oxygen levels during later Proterozoic. Geologists refer to the "great
iron crisis" when the rising levels of oxygen in the world's
oceans caused the formation or iron oxide (Fe2O3),
often preserved as the banded iron formation (an important commercial
source of iron).
Some problematic fossils, thought by some paleobotanists to
be multicellular algae, have been found in rocks approximately
one billion years old. However, the multicellular algae are usually
classified based on their pigments, which commonly are not preserved
in the fossils. During Vendian times the Ediacaran organisms
thrived on the sea floor.
The Major Mass Extinctions of the Phanerozoic
Mesozoic Era
The Mesozoic was the time of the beginning of the breakup of
Pangea about 225-200 million years ago. It eventually fragmented
into the modern continents. This breakup had profound consequences
for living things, as had the earlier formation of Pangaea during
the later Paleozoic. The breakup continues today as the continue
to drift!
Mesozoic seas saw a rise and spread of another cephalopods,
the ceratites, eventually to be replaced by the ammonites. Coral
reefs, which had been decimated by the Carboniferous extinction
returned to prominence with the evolution of new groups of reef-building
animals and algae. Modern bony fish evolved and began to compete
with the ammonites as swimming predators.
Terrestrial animals saw the rise of major groups of tetrapods:
dinosaurs and mammals and the later advent of birds. Tetrapods
took to new envirponments in the air as well as a secondary return
to the waters. Forests took on an increasingly modern look, with
the evolution of cycads and cycadeoids, conifers and ginkgoes.
Flowering plants finally appeared about 140 million years ago
and have risen to become the dominant floral component of many
areas of the planet.
The Mesozoic ended with the great mass extinction that eliminated
nearly 75% of all species, including dinosuars, swimming and
flying reptiles, and ammonites. The causes of this extinction
have long been speculated on and are not presently conclusively
settled.
Paleozoic Era
The Paleozoic Era means "time of ancient life" and spans between
544 and 245 million years ago. The Paleozoic is the first of
three eras within the Phanerozoic Eon (the time of visible life).
While life originated during the Archean and increased in complexity
during the earlier Proterozoic, the Paleozoic Era is marked by
the spread of animals with hard preservable parts such as shells
and exoskeletons.
This led to the Cambrian Explosion, the sudden appearance of
a stupendous array of animal life, much of which is not closely
related to modern forms. Despite extinctions at various times,
the Paleozoic is notable for the increasing modernization of
life. By the end of the Paleozoic, almost all major groups of
life had developed.
During the Paleozoic several major advances in life occurred.
The Cambrian Explosion is the first with evolution of plants
from some group of green algae. The Ordovician had plants move
from water onto land, paving the way for vertebrate animals to
follow. The first vertebrates, amphibians, were little
more than legged fish, although their future descendants would
rule the land as reptiles, the first truly terrestrial
vertebrates.
The Paleozoic ended in the greatest mass extinction event in
world history. During this massive die-off nearly 96% of all
marine species became extinct.
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