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Fossilization in accordance to generations

ammonite
Passing the prior Cambrian period and into the Paleozoic Era, invertebrates including trilobites increased in numbers. Forms of life which possessed a firm shell or skeleton appeared with an explosion. Trilobites are the most representative form of living organisms and belong to arthropods. The trilobites which existed during the Paleozoic Era were especially abundant during the Cambrian-Ordovician period. The Cambrian sea was a habitat for foraminifera, which included trilobites, porifera, archaeocyatha, jellyfish, pelecypoda, gastropoda and graptolite. Also, at the end of the Cambrian period primitive fish appeared. The living organisms in the Ordovician period were similar to those in the Cambrian period and coral and sea lilies were abundant. In the Silurian period graptolite gradually deteriorated and continuing from the Ordovician period brachiopoda and sea lilies were extremely abundant. Upon entering the Devonian period, plants on the land’s water edge succeeded in living in a mass and fish became abundant. During the Carboniferous period large plants formed forests. Due to such abundant forests even more insects became abundant. Upon entering the Permian period the gingko and cycad and such gymnosperm started to appear and reptile were also extremely abundant.
During the Mesozoic era with dinosaurs at the center the abundance of reptilia was most impressionable. Although the mammalian made an appearance in this period the influence was but insufficient. In the sea ammonites were abundant. The large archosaurian which represented the Mesozoic Era became extinct in the Permian period and did not reappear until millions of years later. This reappearance includes ichthyosaurs, plesiosaurs, mosasaurs and sea reptilian, dinosaurs and pterosauria. In the Jurassic period the most peculiar incident was the appearance of the ancestor of birds, the archaeopteryx. The archaeopteryx is a reptilian which developed into a bird type, and is a considered good specimen sign of the evolution stage. Through the Mesozoic era the peak of abundance was reached and dinosaurs or ammonites started rapidly diminishing and then completely vanished. From the middle of the Mesozoic Era gymnosperm became largely abundant. Towards the end of the Cretaceous period flower-blooming angiosperm appeared and became even more abundant.

The Cenozoic Era which started from 65 million years ago could be called the mammalian period. Invertebrates from this period, fish, reptilians, and similar living organisms were already similar in appearance to those of today. However, the mammalian, birds, protozoa and angiosperm still newly appeared or evolved. The sea was habitat to round nummulites similar to the size of coins. Out of the primates humans’ ancestors gradually started to evolve and reached today’s appearance.