Extinctions

Following the Permian mass extinction, life was abundant but there was a low diversity of species. However, through the Triassic, Jurassic, and Cretaceous, major faunal radiations resulted in a large number of new species and forms. New terrestrial fauna that made their first appearance in the Triassic included the dinosaurs, mammals, pterosaurs (flying reptiles), amphibians (including frogs and turtles). In addition, the first birds appeared in the Jurassic. Among the terrestrial flora, the gymnosperms of the Permian remained dominant until the evolution of the angiosperms (flowering plants) in the Cretaceous. In the Cretaceous there was also major radiations occurring in several esablished grounps including the the marine reptiles, rudist bivalves, ammonoids, belemnoids, and scleractinian corals. Bivalves, and brachiopods. Marine groups that were present but did not undergo major evolutionary expansion in the period included the gastropods,bryozoans, crinoids, sea urchins, and sponges.

During the End-Cretaceous (K-T) extinction (65 million years ago) eighty-five percent of all species disappeared, making it the second largest mass extinction event in geological history. This mass mass extinction, extinction event has generated considerable public interest, primarily because of its role in the demise of the dinosaurs. Although dinosaurs were among the unfortunate victims to perish in the K-T extinction, several other terrestrial and marine biotic groups were also severely affected or eliminated in the crisis. Among those that perished were the pterosaurs, belemnoids, many species of plants (except amongst the ferns and seed-producing plants), ammonoids, marine reptiles, and rudist bivalves. Organisms which were severly affected included planktic foraminifera, calcareous nannoplankton, diatoms, dinoflagellates, brachiopods, molluscs, echinoids, and fish. Remarkably, most mammals, birds, turtles, crocodiles, lizards, snakes, and amphibians were primarily unaffected by the End-Cretaceous mass extinction.

The End-Cretaceous mass extinction has generated considerable public interest in recent years, in response to the controversial debates in the scientific community over its cause. The more prominent of these new hypoteses invoke extra-terrestrial forces, such as meteorite impacts or comet showers as the causative extinction agent. Older hypotheses cite earthly mechanisms such as volcanism or glaciation as the primary agent behind this mass extinction. Evidence for catastrophism at the Cretaceous-Tertiary boundary is found in a layer of sediment which was deposited at the same time that the extinction occurred. This layer contains unusually high concentrations of Iridium, found only in the earth's mantle, and in extra-terrestrial meteors and comets. This layer has been found found in both marine and terrestrial sediments, at numerous boundary sites around the world.

Some paleontologists believe that the widespread distribution of this Iridium layer could have only been caused by meteorite impact. Further, these researchers cite the abundance of small droplets of basalt, called spherules, in the boundary layer as evidence that basalt from the earth's crust that were melted and flung into the air upon impact. The presence of shocked quartz - tiny grains of quartz that show features diagnostic of the high pressure of impact - found in the boundary layer provides additional evidence of an extra- terrestrial impact at the Cretaceous-Tertiary boundary layer. Recent research suggests that the impact site may have been in the Yucatan Peninsula of Mexico.

The high concentrations of Iridium in the boundary layer has also been attributed to another source, the mantle of the earth. It has been speculated by some scientists that the Iridium layer may be the result of a massive volcanic eruption, as evidenced by the Deccan Traps - extensive volcanic deposits laid down at the Cretaceous-Tertiary boundary - of India and Pakistan. These lava flows came about when India moved over a "hot spot" in the Indian Ocean, producing flows that exceeded one hundred thousand square kilometers in area and one hundred and fifty meters in thickness. Such flows would have produced enormous amounts of ash, altering global climatic conditions and changing ocean chemistry. Evidence that volcanism was a primary extinction agent at this boundary is also relatively strong. In addition, and the presence of spherules and shocked quartz worldwide in the boundary layer may also have been the result of such explosive volcanism. Thus at present, both the volcanic and meteorite impact hypotheses are both viable mechanisms for producing the Cretaceous mass extinction, although the latter is more popular.