245 million to 65
million BP : Triassic to Cretaceous
The Mesozoic Era begins with the Triassic Period around 245 million
years ago. As part
of Pangaea, this area now endured a
warm, arid climate and was mostly nondepositional,
erosive, and near the equator. Around
230 million BP, the dinosaurs appear. By 228 million
BP, Eoraptor was stalking Argentina.
225 million years ago, the Petrified Forest of Arizona
grew beneath a night sky that looked
much the same as today, that being the length of time
it takes the sun to orbit the Milky
Way. Waterbugs, the caddis fly, crayfish, shrimps,
crocodiles, and early mammals appear
during the Triassic. Around 214 million years ago,
toward the end of the Triassic Period,
a large asteroid entered the atmosphere, split into
at least three large pieces, and left
craters in France, eastern Canada, and Manitoba, Can.,
and around 210 million BP, another
global mass extinction occurred removing 23% of living
families. Among others, the mammal-like
reptiles and most crocodile-like reptiles died off.
After that, the dinosaur population and
variety exploded.
208 million BP marks the beginning of the Jurassic Period. 200 million
years ago
Pangaea splits as North and South
America begin to separate from Africa, North
America from Europe, and Antarctica
from Africa. The early Jurassic was a period of
low relief, called the Gondwana
planation, it is now seen only as the highest plateaus.
Around 180 million years BP, a large
asteroid hit Mars and the impact may have thrown
bits of the planet beyond its gravity
well possibly becoming some of the Martian meteorites
found on earth. Early to middle
Jurassic global temperatures remained high as this area
experienced an arid and hot climate
near the Tropic of Cancer. The middle Jurassic was a
geologically active period which
produced the Sierra Nevada Mountains and was accompanied
by a surge in gigantism in the
dinosaurs. Brontosaurs, megalosaurs, and Seismosaurus
roamed. Around 150 million BP,
Archaeopteryx, the earliest bird-like form, appeared. Linear
magnetic anomalies have been used to
determine the past positions of the continents back
to 150 million BP. Late Jurassic global
temperatures were a little less warm, and this
area had moved above the Tropic of
Cancer. By the end of the Jurassic wide ocean channels
existed at the earlier supercontinental
splits and the huge sauropods (brontosaurus type)
dwindled in North America as the
duckbills and ceratops rose in prominence. During the
Jurassic Period the seed ferns died
out, the Sequoias were born, the Nemertea order was
created, and frogs, squid, cuttlefish,
sting rays, lobsters, sea turtles, plesiosaurs,
stegosaurs, pterodactyls, and the
triconodont mammals appeared.
The Cretaceous Period began around 146 million years ago. At the very
beginning,
Diplodocus, a 65-88 ft. sauropod still
roamed western North America. By 140 million
years ago, at least some spiders were
spinning radial webs. Worldwide temperatures
rose again during the early Cretaceous
and this area reached near its' current latitude.
From early to middle it was in another
period of low relief called the Kretacic planation,
seen now as high plateaus. Late in the
early Cretaceous deposits are found the earliest
angiosperm pollen. By 130 million years
ago the flowering plants had spread throughout
the world. 120 million years ago a
superplume occurred in the Pacific Ocean, while in China,
Sinosauropteryx, a bird-like dinosaur
existed. 115 million years ago a shrew-sized mammal
lived in Australia and by 110 million
BP, Brazil had sea turtles and Australia had
Leaellynasaura, a turkey-sized
dinosaur.
The middle Cretaceous held an active period with widespread vertical
uplift. From
100 million year old deposits we have
well preserved fish in Brazil, dinosaurs in the
Patagonia of Argentina, tiny birds at
Spain, a 36 ft. spinosaur (crocodile-like) in the
Sahara, and the oldest marsupial at
central Utah. 95 million years ago an early water
snake which still possessed one inch
long rear legs lived on the West Bank of the
Jordan River.
In late Cretaceous deposits of Patagonia, sauropod eggs have been found
with the
embryos and skin clearly seen. 80
million year old deposits in central Asia have revealed
flightless birds, and have left a site
in the Gobi Desert in Mongolia with oviraptor
remains with its eggs, dromeosaurs,
early mammals, lizards, turtles, and crocodiles.
75 million years ago, Tyrannosaurus Rex
lived near Alberta, Canada, Maiasaura lived in
Montana, the Rocky Mountains began to
rise which continued at intervals into the early
Cenozoic, and sea levels rose, flooding
low-lying areas and creating an inland sea on North
America.
The last Pangean planation occurred from late Cretaceous into
mid-Cenozoic
times - called the Moorland Planation
or Schooley peneplain, it is now seen as plained
uplands. Concurrent with this planation
was widespread volcanism. During the late
Cretaceous, diatoms evolved, duckbills
were the most common dinosaur in North America,
and Pteranodon ingens evolved. 70
million years ago the placenta was introduced among
mammals, and 67 million years ago a T.
Rex, later named Sue, lived in South Dakota while
the Grand Tetons rose further west.
Late in the Cretaceous Period, the Ozarks were uplifted as the Great
Plains and
Mississippi Valley regions were
inundated by the sea. Isolated by hundreds of miles from
other flora, some species split from
ancestral stock or became localized to this area,
possibly including Castanea ozarkensis
and Hamamelis vernalis. Other flora probably existing
here at that time include the Acer
(maple f.), Celtis (elm f.), Populus (willow f.), Juglans
(walnut f.), Fagus (beech f.),
Sassafras (laurel f.), Rhamnus (buckthorn f.), Bumelia
(buckthorn f.), Quercus (beech f.),
Magnolia (magnolia f.), Liriodendron (magnolia f.), Cornus
(dogwood f.), Diospyros (persimmon f.),
Liquidambar (witch-hazel f.), Sapindus (soapberry f.),
and Gymnocladus (pea f.). Mosasaurs,
fish, and toothed swimming birds inhabited the
seas to the west of us, and batlike
flying reptiles appeared in the skys.
The Cretaceous Period also produced the chrysophyceae and
silicoflagellata protozoans,
the archeomonads, the Phylactolaemata
Class of bryozoa, the octopods, tarpons, salmon,
trout, Triceratops, mosasaurs, and
Giganotosaurus - a T. Rex type and the largest of
predatory dinosaurs to exist,
placentals and opossums, and poplar, sassafras, sycamores,
magnolias, oaks, maples, elms, willows,
palms, and eucalyptus. The Cretaceous Period saw
North America moving west, and the
Pacific plate moving north.
The Cretaceous Period and the Mesozoic Era ended with a literal 'bang'.
65 million
years ago the Yucatan Peninsula in
Mexico was apparently hit with a large asteroid or
comet, superplumes are theorized, there
was extensive volcanism occurring, and a
deposit rich in iridium appeared.
Apparently these or other occurrences produced an
uninhabitable climate which culminated
in the extinction of 17% of living families and
66% of all living species, notably the
ammonites and the dinosaurs, whose final leavings
are 65 million year old bipedal
footprints in Argentina and T. Rex coprolites in southwest
Saskatchewan.
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REFERENCES
Doyle, P., 1996, Understanding Fossils, An Introduction to Invertebrate
Paleontology. John Wiley and Sons
Fenton, C.L., Fenton, M.A., 1958, The Fossil Book. Doubleday
Kansas
Geological Survey, The Geologic History of Kansas
King, L.C., 1983, Wandering Continents and Spreading Sea Floors on an
Expanding
Earth. John Wiley and Sons
National Geographic Magazine
Stearn, C.W., Carroll, R.L., Clark, T.H., 1979, Geological Evolution of
North America.
John Wiley and Sons
Steyermark, J.A., 1959, Vegetational History of the Ozark Forest.
University of
Missouri Studies, vol.31
Tschudy, R.H., Scott, R.A., 1969, Aspects of Palynology. John Wiley and
Sons