EARLY TO MID MESOZOIC ERA
TRIASSIC PERIOD: 251 to 200 million years ago
JURASSIC PERIOD: 200 to 146 million years ago
In the early to middel Mesozoic Era, Pangea
began to divide. The Northern Continents began to separate from Gondwana
by the Late Triassic at a rate of several centimeters per year. A series
of basins began to form between the east coast of North America and Northwestern
Africa in the Late Triassic. These basins represented the beginning of
the Atlantic Ocean. During the Jurassic Period, the Atlantic Ocean widened
and fused to the Pacific as North and South America separated. By the
Early Jurassic, Gondwana had begun to form separate continents. Initially,
a block containing Antarctica, Australia, and India separated from that
composing Africa and South America. Between the Late Jurassic and Early
Cretaceous, Africa and South America began to separate and India separated
from the landmass composed of Antarctica and Australia. By the Late Jurassic,
North America had begun to separate from the continent which contained
Eurasia and Greenland. During the Jurassic, Madagascar separated from
the east coast of Africa. In the mid-Jurassic, much of western North America
was covered by a shallow sea (Seyfert, 1979).
Following the mass extinction at the end of the Permian Period, new marine groups evolved including the ancestors of the corals, sharks, and bony fish which exist today. Diverse groups of reptiles adapted to marine environments to become dominant predators.
Conifers and ferns were the dominant plants on earth but, by the end of the Jurassic, the first flowering plants had evolved.
In the Triassic Period, reptile lineages had evolved into diverse groups including crocodiles, flying pterosaurs, and the ancestors of dinosaurs.
By the middle of the Triassic, the earliest dinosaurs had evolved which subsequently diverged into basal theropods, prosauropods, and basal ornithischians.
In the Jurassic, dinosaurs produced a great diversity of carnivorous theropods, enormous sauropods, armored stegosaurs, and iguanodonts.
From feathered, birdlike dinosaurs evolved the first birds.
In the Triassic, mammal-like reptiles evolved into the first mammals.
A mass extinction event occurred at the end of the Triassic. Many archosauromorphs became extinct as did some predatory archosaurs. Synapsids were the dominant tetrapods during the Triassic and achieved considerable diversity but they virtually disappeared after this extinction except for the ancestors of mammals. Many plants became extinct or virtually extinct such as seed ferns, many tree ferns, ginkgoes, cycadophytes, horsetails, and many conifers. These trees were replaced by conifers and cycad-like bennetitaleans.
The end-Triassic extinction was the third worst in the Phanerozoic Eon which caused the extinction of 30% marine organisms (genera), half the tetrapods, and 95% of plant species. Changes in the carbon cycle resulting in a 3-4 degree increase seems to have been a factor (McElwain, 1999). At the end of the Triassic, the supercontinent Pangaea began to fragment and this was probably also a factor. In the Triassic Period, North America began to separate from Pangaea. A watery rift separated northern Greenland from northern Norway and much of the East Coast was covered with a shallow sea. The western edge of North America had not yet been formed and was composed of a series of islands in the modern Rocky Mountain region (Russell, 1989).
It also seems that a large meteorite impacted the earth at this time given an increase in the amount of iridium in rocks, the presence of shocked quartz, and faunal changes (Olsen, 2002). The Manicouagan Crater in Quebec measures 70 km in diameter (Russell, 1989).
Why did dinosaurs thrive after this extinction? Maybe there was an intrinsic reasonperhaps their speed or bipedal posture made them to superior to the other reptiles. Maybe the reasons were extrinsic--they simply took advantage of the new plant life or the niches left vacant by the extinctions of other reptiles.