Museum of Natural History, Tohoku University, Sendai, Japan
TOUR COURSE
"Evolution of Life on Earth"(5)
Mesozoic Era
Triassic Period
Triassic Period, 245-208 million years ago, is the first time unit in the
Mesozoic Era. The single large continent of Pangaea built at the end of
the Paleozoic began to split apart and the sea level began to rise. Following
the most drastic mass extinction at the end of the Permian, new types of
organisms started to evolve. Molluscs including ammonoid began to occupy
about 60 % of all marine invertebrates, instead of the earlier dominance
of brachiopods, cnidarians, bryozoans, and trilobites in the Paleozoic.
Among the verterates the age of dinosaurs began to emerge on land. Coelophysis
represents one of the first dinosaur. Dinosaurs evolved from a group of
reptiles called Thecodontia which thrived from late Paleozoic to early Triassic.
At the Triassic/Jurassic boundary another mass extinction occurred and many
groups of ammonoids became extinct. Among the plant species, lush forests
of conifers and cycads replaced ferns species which flourished in late Paleozoic.
Monotis salinaria
(Bivalve)
Europe
(23cm wide)
Ginkgoites sibirica
North Korea
(8cm long)
Chirotherium storetonense (Foot print of reptile)
England
(17.5cm long)
Utatsusaurus hataii
(Ichthyosaur)
Utatsu, Miyagi, Japan
(72cm long)
Jurassic Period
Jurassic Period, 208-144 million years ago, is the second time unit of the
Mesozoic. The Atlantic Ocean began to open, and a period of high sea level
and warm climates was established. In the age of dinosaurs, the Jurassic
represents its first half of development. Various species of huge dinosaurs
such as Brachiosaurus, Seismosaurus, and Apatosaurus prospered.
Dinosaurs are composed of two major orders, Saurischia and Ornithischia,
which possibly share a common thecodont ancestry in the Triassic. Pterosauria
flew in the sky and the oldest true bird of Archaeopteryx branched
out from a small reptilian ancestor. Ammonites and marine reptiles such
as Ichthyosaurus and Pleisiosaurus flourished in the sea.
In the surface layer of oceans, calcareous plankton species emerged. In
the plant kingdom, conifers and cycads continued to grow thickly. The horseshoe
crab has a shape similar to living species; this is the reason why it is
called a living fossil.
Aspidoceras faustum
(Ammonite)
France
(29cm across)
Mesolimulus
walchi (Limulina)
Eichstatt, Germany
(20cm long)
Lycoptera
sp. (Pisces, Osteichthyes)
Hopei, China
(several bodies: about 5cm long)
Stenopterygius quadriscissus (Ichthyosaur)
Holzmaden, Germany
(212cm long)
Stegosaurus stenops (Dinosaur)
Utah, U.S.A.
(5.7m long, Replica)
Cretaceous Period
Cretaceous Priod, spanning a time interval of 144-65 million years ago,
is the youngest time unit of the Mesozoic Era. The once unified continent
of Pangaea was fragmented and the sea level rose precipitously. In the mid-Cretaceous,
there was the most extensive development of shallow seas. Warm climates
prevaled over all areas of the Earth and plants and animals inhabited Antarctica.
True-flowering plants of Angiosperm appeared for the first time in the Cretaceous.
Large sauropodian dinosaurs became extinct and they were replaced by such
ornithischian dinosaurs as Triceratops and Hadrosaurus.
These plant-eating dinosaurs were preyed upon by big meat-eating dinosaurs
such as Tyrannosaurus. All species of dinosaurs, ammonites and most
calcareous plankton species such as planktonic foraminifera became extinct
at the end of Cretaceous. Such a mass extinction of organisms is thought
to have resulted from a catastrophic environmental change caused by an impact
of 10-km-diameter comet or asteroid.
Anapachydiscus karafutoensis (Ammonite)
South Sakhalin, Russia
(30cm across)
Inoceramus naumanni
(Bivalve)
North Sakhalin, Russia
(15.5cm long)
Claystone recording the Cretaceous / Tertiary Boudary
Urahoro, Hokkaido, Japan
(11cm wide)
Next Page
Monotis salinaria
(Bivalve)
Ginkgoites sibirica
Utatsusaurus hataii
(Ichthyosaur)
