京都大学21世紀COEプログラム 活地球圏の変動解明 アジア・オセアニアから世界への発信
Seminar Series of Active Geosphere #21: July 27, 2005
by
Takeshi Setoguchi
Graduate School of Science, Kyoto University
"Evolution of Terrestrial Ecosystem during the Mesozoic, especially during the Cretaceous"
ABSTRACT
The Mesozoic is an era of the major changes in the composition of terrestrial floras, and in turn, of terrestrial ecosystem. The first land plants have been appeared in the early phase of the Paleozoic, during the Silurian.They were represented by the fern allies like Psilophyta, Lycopodophyta, Equisetophyta and Pteridophyta. They distributed along major rivers and lakes, or ponds. In the areas far from the water resources, these plants could not survived. The forested areas covered by these ferns were limited only in wet marsh or swamp areas. The first herbivorous terrestrial vertebrates have evoluved during the Carboniferous, represented by Diadectes which fed upon these ferns. During the early phase of the Permian, the primitive mammal-like reptiles, the Pelycosauria became flourished. Dimetrodon is the representative member of the group. A few herbivorous types like Edaphosaurus and Casea have evoluved fromthe original stocks of carnivorous type of the Pelycosauria. Thus, the major types of vertebrates were carnivorous, and herbivorous types have been evolved independently from the different carnivorous stocks.
During the Permian, new types of land plants, the gymnosperms have appeared. They represented by Pteridospermophyta, Cordaitales, Cycadeoidales, Ccycadales, Ginkgophyta, Coniferales, and Caytoniales. These new types of seed plants contained much higher degree of nutrition than in ferns. During the early phase of the Triassic, many types of herbivorous reptiles have been evolved. Among them, the advanced types of the mammal-like reptiles, the Therapsida evolved from the more primitive Pelycosauria, include many different lines of herbivorous groups. These herbivorous types have evolved independently from the carfnivorous stocks. Most these herbivorous Therapsida became extinct by the end of the Triassic; thus, they were very short-lived reptiles. The only two types of herbivorous Therapsida could survive into the Jurassic; one is the Ictidosauria represented by Diarthrognathus, and the other is tritylodontids which survived into the early Cretaceous. The latest occurrence of the tritylodontids have been recorded in the early Cretaceous rocks found in Japan recently.
During the Jurassic, several types of dinosaurs became flourished. Among them, large bodied dinosaurs are very famous; sauaropods and stegosaurids which are typically herbivorous. They fed primarily on gymnosperms. Allosaurus is a representative dinosaur of the Jurassic, which fed on herviborous dinosaurs of that age.
Some time during the early Cretaceous, advanced land plants, angiosperms have been appeared and by the end of the early Cretaceous, the Albian, angiosperms have become the major land plants. Leaves, fruits, nuts and nectors are very nutritious. In turn, many types of herbivorous dinosaurs have been evolved in relation to the evolution of angiosperms: hadrosaurs, ceratopsians and ankylosaurs are typical herbivorous dinosaurs during the late Cretaceous. And mor powerful carnivorous dinosaurs like Tyrannosaurus have evolved to feed upon the variety of herbivorous dinosaurs.
The evolution of angisperms situmulated the world of insects. The angiosperms possessed flowers and nectors on which variety of insects fed. New types of insects have been evolved in relation to the origin of the angiosperms. The larvae of insects, caterpillars, groveled around the trunks of angiosperms on which variety of insectivorous mammals fed. And new types of insectivorous mammals evolved in relation to the origin of the new types of insects.
These phenomena are called as "co-evolution". The co-evolution can be observed typically in the terrestrial world of the Cretaceous.