Geological Changes and Biological Evolution From the Triassic Period on

This was a time [250 Million Years Ago] when an abundance of varied animal phyla first developed in conditions varying from sea to land, the latter including freshwater lakes and lagoons. From Triassic reptiles evolved a great range of dinosaur families, some up to one hundred feet in length and others small and more delicate. They included creatures still walking on their hind legs, four-legged types, the flying Pterodactyl, and certain fishlike forms. Some were carnivores, others—often heavily armoured—fed on estuarine plants. Small primitive mammals are found locally in the Jurassic [which succeeded the Triassic 199 MYA], as well as a true fossil bird. Among marine fauna, large flat echinoids, lamellibranchs, and oysters are plentiful in fossil remains, while the presence of coral colonies suggests warm-water conditions. Great coiled ammonites, which had almost died out, suddenly flourish again in great variety— in evolution a rare phenomenon!

In the succeeding Cretaceous period, beginning perhaps 135 million years ago, much of today's Europe was at first covered by a complex pattern of uplands, drained by rivers and interspersed with shallow-water and true marine areas.

Later the seas encroached over much of the region, including the former USSR, in part from the southern Tethys ocean, leaving islands composed of much earlier, more resistant rock formations, such as the ancient Scandinavian 'shield' of Precambrian age.

This was a period of abundant life. Dinosaurs reached their peak in North America and elsewhere. Mammals, though still small, had by now developed into both pouched and placental types. Among plants, modern, flowering dicotyledons appeared. Away from the land and its swamps, the seas were mostly clear and of moderate depth, recalling conditions not unlike those in today's North Atlantic, with its sediments of radiolarian ooze. Large coiled and uncoiled ammonites are frequent in the chalk, as well as fossil shark's teeth and, in the upper chalk, the well-known heart-shaped and domed echinoids, or sea urchins. Clear-cut seams of hard black flints often contain remains of sponges and microscopic life.

With the end of this great Cretaceous marine formation of clear sediments, we reach too the end of that era of "middle life"—the Mesozoic—and enter, biologically, a different world—that of the Tertiary or Cenozoic era, opening around 70 million years ago.

When the Tertiary era opens—with the Eocene period, the "dawn of the recent"—we have already traversed, geologically, a great blank interval—a question mark in earth history. In Europe, Eocene deposits rest on the uplifted chalk in a great unconformity, marking a major time-lapse. But, more important, many animal forms, including all the dinosaur types and in fact most reptiles except crocodiles, turtles, lizards, and snakes, have been wiped out. Various explanations, including, perhaps, the possible effects of a large meteorite, have so far failed to clear up this mystery.

The 45 million years of the Palaeogene—better known geologically as the Eocene and Oligocene periods—enjoyed in what is now northwest Europe a tropical or subtropical climate, which during the main Tertiary era gradually became cooler. Sands, limestones, and clays such as the famous "London Clay" denote marine and shallow-water conditions, often containing an abundance of shells, as well as shark's teeth and giant foraminifera (nummulites). But on the northeastern Atlantic coasts—and associated with long-term continental drift— there was widespread eruption of volcanic rocks, producing some of today's well-known beauty spots on Britain's western coasts.

An appearance of grasses and fruit-bearing trees allowed small mammals to evolve in land areas and proliferate widely—into herbivorous types, carnivores, marine, and flying species—just as the reptiles had dominated the world in Mesozoic times. In this period, the present-day horse began its evolution, as did the smaller mammals that, much later, were to lead on to humankind itself.

We come next to the Miocene and Pliocene periods—often grouped nowadays as the Neogene. Just as the earth's slow cooling, continental drifting, volcanic, geosynclinal, and thrusting movements had produced two great periods of mountain-building in Palaeozoic times, so the early Tertiary or Palaeogene saw the beginning of a third major period of mountain-building which, in its later stages, produced the Alps and the Pyrenees. Slow uplift of southern parts of the British Isles then caused the forming of a great Wealden anticline or dome, comprising chalk and earlier strata—reduced now by long erosion to the North and South Downs and a central high sandy ridge. It also produced the geological synclinc or depression known as the London Basin, with chalk reappearing from under north London as the Chiltern Hills. Enormous pressures from Africa were producing great overfolds of strata seen now as the Alps and neighbouring ranges, as well as today's Jura ridge running through Switzerland and France. Further north, surface formations were pushed forward against ancient shieldlike resistant masses in Britanny, central France, southern Germany, and Bohemia. As a result, marine or freshwater sediments formed in Europe during the Miocene and Pliocene periods became localized in basin areas, with eroded platforms of dry land remaining elsewhere. Oils formed under pressure from organic remains deposited at this time have produced some of today's important non-Palaeozoic sources of petroleum.

Certain main lines of evolution, too, were producing not just the slow extinction of many life-forms, but a great flowering of mammalian species. Thus the first antlered deer and horned rhinoceros, an early elephant, and the first apes all emerged in Miocene times, some 20 million years ago.

From Cold War to Chaos? James Dilloway, Chapter 4

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