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PREHISTORIC CORAL SEAS To better understand the variety of creatures that dominated ancient seas, one must step back in to a point when life began, then follow it forward as life developed and adapted to the changing aquatic environment. By the mid-nineteenth century scientists understood the events of early earth well enough to divide time based on fossil rock characteristics. For example, fossils that were found in coal-bearing strata came to be referred to as the Carboniferous Period (carbon, being Latin for charcoal). Younger rock (more current in history) abundant with chalk became known as the Cretaceous Period (creta, being Latin for chalk). Many decades later a complete time log was formalized with 16 different periods, 11 of which are listed below. These periods were further divided into three recognized eras; Paleozoic (palaios, being Latin for ancient), Mesozoic (meso, being Latin for middle) and Cenozoic, formerly Cainozoic (caino, being Latin for recent).
The formation of oceans and seas Many theories about the origin of the ocean have been proposed by scientists. The most widely accepted one is that the earth at some time in its very early history was hot enough to melt the materials from which it was formed. While in this molten state, lighter rock-forming materials floated on the surface of the heavier ones, in the same manner oil floats on top of water. Then, between four and a half and four billion years ago, the molten earth cooled sufficiently to form a crust of rock that was many miles thick. Surrounding the earth was an unbroken canopy of clouds miles thick and made up mostly of water vapor. Rain falling toward the still-hot earth was heated to steam and rose to the clouds again. But many millions of years later, as the earth continued to cool, its surface temperature fell below the boiling point of water. Rainwater could now remain on the earth, covering its whole surface except for the higher places on earth that had been formed from the lighter rock materials. In 1970, scientists had pieced together evidence that the lighter rock materials had formed one huge continent surrounded by a vast ocean. Then, about 200 million years ago, the great continent began to break up, the pieces moving slowly apart. The onrushing waters of the single huge ocean now entered and filled the spaces between the separating continents--and became the several oceans and seas we know today. How the oceans and seas became salty When water comes in contact with soil, it dissolves some minerals. You can prove this by taking a clean saucer and filling it up with tap water. Let the saucer stand in a warm place a few days until all the water has evaporated. The film you see on the bottom of the saucer is made by the small amount of mineral matter that is ordinarily dissolved in tap water. The early world ocean was only faintly salty. But, for countless centuries, rain and melted snow have been running over the land, dissolving various minerals, and carrying them down to the ocean. During all this time, water has been passing through the successive stages of evaporation and condensation that make up the water cycle. Pure water evaporates from the surface of the ocean and eventually returns to it, carrying various dissolved materials. Thus, the mineral content of ocean water has been increasing ever since the first rainfall. When we say that the ocean is salty, we mean that its content of dissolved minerals is high. Sodium chloride (common table salt) makes up approximately three-quarters of the dissolved material in ocean water. The remainder is made up of varying quantities of chemical compounds, containing almost every known element. Some of these elements, manly magnesium and bromine, are now taken from the ocean water commercially. A great deal of the magnesium used in the manufacture of lightweight alloys for airplanes and satellites, for example, comes from the sea. The amounts of the many other minerals in ocean water are so small that it is not yet commercially profitable to claim them. Scientists may yet find ways to make it worthwhile. The total dissolved salts or salinity of seawater varies a great deal in different parts of the ocean. On an average, however, there are 35 parts of salt in every 1,000 parts of seawater or 3.5 per cent. But nowhere does the ocean approach the salinity of the Great Salt Lake in Utah (average salt content is about 28 per cent). This landlocked lake is believed to be the last remains of an ancient sea that once covered much of western North America. But, the world ocean in itself, contains enough salt to cover all the continents with a layer 500 feet thick. At one point, after millions of years of chemicals being dissolved into the oceans, the conditions to form primitive life was created. Protozoa--the "simplest animals" The animals with bodies made up of a single cell represent a separate level of organization from all the rest, for their body processes are performed by the one cell. These single-celled animals are called Protozoa (sometimes referred to as acellular, i.e., without distinct cells) are assumed to be low on the scale of evolutionary sophistication. In reality their single cell is often large and can be complex. Another fundamental question may be asked about single-celled animals. Are they, in fact, all animals? True animals derive their energy from relatively complex organic material that is plant or animal in origin--they are heterotrophic (obtaining their nourishment by a process of breaking down organic materials). However, some protozoa's have developed structures characteristic of plants, called chloroplasts, which are incorporated into their cells. The chloroplast enables the cell to synthesize organic materials, such as sugars, from mineral salts, carbon dioxide and water in the presence of sunlight (photosynthesis). Like green plants, these Protozoa can subsist on inorganic food from which they synthesize organic material. Today botanists consider such Protozoa to be single-celled plants. Protozoa are most important because, as the "simplest animals" they are likely to provide important keys to two fundamental questions. These concern the origin of life, and the origin of multi-cellular animals called Metazoan. It is thought that the earth is not quite 5,000 million years old, and realistic estimates suggest that life began in its simplest form 4,000 million years ago. The first sedimentary rocks, not quite 4,000 million years old, contain fossils of simple cells which resemble those of present-day bacteria, that is, they lacked a distinct nucleus (i.e., were prokaryotes). These lived in a primeval atmosphere devoid of oxygen. The appearance of oxygen on the earth, 1,800 million years ago, brought with it many new evolutionary possibilities. Scientists believe that for three-quarters of the period for which life has existed on earth, the only cells were prokaryotes, i.e., resembling bacteria. It was not until the Cambrian period 600-500 million years ago that invertebrates, such as mollusks, trilobites, lampshells and echinoderms became established. Unfortunately, invertebrates with soft delicate bodies, such as flatworm and sea squirts, have not left any fossil record. For life to have appeared, many conditions had to be fulfilled. It seems quite possible that the physical conditions prevailing on the surface of the early earth could have generated simple organic molecules, such as amino acids, and then proteins, from inorganic molecules. The big unanswered question is how such substances could form themselves into organized living systems capable of reproducing their own kind. The origins of multi-cellular animals are a bit speculative, but rather more can usefully be said about their possible early history. Because so many of the early animals had soft bodies, they left very little fossil record. All theories about the early evolution of animals therefore rely mainly on the study of similarities between developing embryos and the adults animals in that groups. A widely accepted theory is that protozoan animals gave rise to multi-cellular ones by colony formation. A number of types of colonial protozoan are known to exist, such as Volvox (a loose colony of protozoan). The famous 19th-century German biologist Haeckel proposed that a hollow Volvox-like ancestor could have developed into a two-layered organism. Views differ as to whether or not this was a planktonic (floating) or a bottom-dwelling organism, but it may have somewhat resembled the planula larvae of the current day sea anemones and jellyfishes and could have given rise to bottom-dwelling animals. While it is not certain how the multi-cellular animals evolved from protozoan ancestors, it is possible to distinguish groups of metazoans on the basis of relative simplicity or complexity of structure. In metazoans, there are three categories that can be used to determine level of complexity: how the cells are organized; how many layers of cells are to be found within the body; and whether or not a body cavity is present. There are few types of cell in the most lowly metazoans and sponges, that have cells that are never arranged into groups of similar cells (tissues). Such animals are said to have a cellular grade of organization. The next step, as found in jellyfishes and allies and comb jellies, cells with similar functions are arranged together into tissues, each tissue having its own function or series of functions--these animals have a tissue grade of organization. In all animals apart from those just mentioned, requirements for functional specialization increase such that specific organs (often comprising a series of tissues) have evolved. Thus, all animals from flatworm to man are said to have an organ grade of organization. The second way to divide up multi-cellular animals is to look at the number of layers of cells that make up the animal's body. Mesozoans and sponges consist of just one layer of cells, but in the jellyfishes and comb jellies, two layers appear (ectoderm outside and endoderm inside). This condition contrasts with the single layer of cells seen in the protozoan colony Volvox. The two layers develop from the egg and remain throughout adult life, separated from each other by a sheet of jelly-like membrane. The first known fossil animal to show the beginnings of vertebrate characteristics is Pikaia, which dates from mid-Cambrian times, around 535 million years ago.
The body of this small eel like creature was stiffened and elongated by the presence of a stiff but flexible rod called a notochord. It is believed that from notochord organisms (called chordates) all vertebrates evolved. The notochord developing into a backbone from which a skeleton of shoulder and hip girdles (pelvic bone-like) could be hung. Eventually, paired limbs were slung from the girdles for improved steering and locomotion. The development of the front-back body axis led to the concentration of the sense organs at the front, where they encountered the environment "head-on." Intriguingly the fossil record reveals another group of primitive chordates, the extinct conodont animals such as the Promissum, which had the ability to use bonelike material in their bodies. The mineral bonelike tissue is found in their tiny arrays of teeth which intermesh as a very effective prey-catching apparatus.
Fossil records also reveal a great diversity of bizarre-looking marine fish like animals that had neither teeth nor jaws. The jawless fishes (agnathans), who had bony scales and plates embedded in their skin, fed by sucking and filtering organic debris and microorganism from seawater and seabed deposits, Agnathans, the stratigraphic rock record shows, were soon joined by jawed fishes with teeth. The jaws that revolutionized life The development of jawed fishes (called gnathostomes) meant that fishes no longer had to rely for sustenance on microscopic organisms. Gnathostomes could actively pursue and seize prey. They could grow larger and specialize in particular lifestyles and diets. Jaws were an evolutionary innovation that led to an explosive radiation and diversification. The chondrichthyans (sharks, skates rays, and chimaeras), kept the cartilaginous skeleton of their ancestors. Bone replaced cartilage in the others to produce the bony fishes. Two types of bony fishes evolved from a common ancestor--the ray-finned fishes and the lobe-finned fishes (so named because of a fleshy, scale-covered lobe at the base of many fins).
The most successful of these fishes are the ray-finned with more than 21,000 current living species. In comparison in terms of vertebrate groups abundance, there are about 4,000 different species of mammal (humans being one), 8,600 species of birds, 4,000 species of reptile and 2,500 species of amphibian. Only seven species of lobe-finned fishes exist today; six species of lung fish and the Coelacanth.
The Paleozoic Era In the early part of the Paleozoic Era life was mostly confined to the sea with only traces of life on the shores. During the early years animals started to make shells from limestone dissolved in the water, shells that are identical to shells we find today. Corals appeared in the mid-Ordovician period and became common by the Devonian period. Within the coral realm the first fishes developed. During this period is when the first shark appeared, Cladoselache, as well as several other species that developed later such as the Stethacanthus and the Tristychius. For the most part, sharks got it right from the beginning with a body structures that resemble current day sharks.
During mid to late-Paleozoic came land plants that covered the bare rocks of the earth for the first time. There soon followed amphibians, animals that lived part of their lives in the water and part of it on shore, as frogs do today. A few land animals and insects also appeared in this era. This Era came to an end with much activity on land and in the sea. A long, sediment-filled trough had existed along the eastern edge of the North American continent. The floor of the trough kept sinking and allowing more sediment to pile on top. At one time the trough was five miles deep. At the end of the Paleozoic the sediment was violently pushed sideways (to the west), and was driven upwards by gigantic earth forces. The result was the making of the Appalachian Mountains. This is how early sea life fossils can be found on mountain ranges, as they were once part of the sea floor. Land changes were also happening elsewhere in the world, and as they did the oceans began making pathways into the landscape to create vast inland seas. It was during this time that a variety of different fish species developed in accordance to an increasingly diverse ocean world. The Permo-Triassic Extinction Event The Permo-Triassic extinction, the most drastic event in terms of extinction of life marked the end of the Paleozoic Era. In fact, it is considered to be the greatest disaster in the history of the world. At least 57% of all families of marine organisms died, and as much as 95% of all ocean species. The Brachiopods (lamp shells), bryozoans (moss animals) and Crinoids (sea lilies) were the hardest hit. Many corals also vanished during this period of extinction. The cause of this extinction is believed to be environmental, and was drawn out over some 10 million years. There is evidence that the earth began a global cooling that soon led to the formations of glaciers and the polar ice caps. These polar ice caps consumed much of the seawater and as a consequence sea levels fell drastically around the world. The receding oceans and seas exposed seabed’s and coral reefs, quickly altering the life pattern of many marine creatures. It is believed that many of them did not have time to adapt and perished. The Mesozoic Era The Mesozoic Era began with great upheavals of the earths crust. It also retained much of the surviving life forms from the Paleozoic Era that developed important and distinct elements that led to modern life forms. For the first time, the major events in development of living things occurred on land. It was an era in which reptiles flourished, among them the dinosaurs, which dominated the land. The first birds had also appeared and began to fill the skies. It was during this time that many animals returned to the sea. These animals developed into distinct reptiles, such as the Elasmosaurus and the Kronosaurus and the Placocheleys.
The K/T extinction (K for kreta, being German for chalk, and T for the Tertiary period) marked the end of the Mesozoic and the beginning of the Cenozoic Era. Although this event only killed off 15% of living species at the time, it is given much attention and study because of the swiftness of the event. In terms of natural extinction, environmental or adaptive inability, the disappearance of a species takes thousands and millions of years, whereas the K/T event only took a few years, marking the abrupt and global end of the dinosaurs 130 million year reign. So how could such an extinction of a successful species happen so suddenly? Especially when you consider the environment had not changed much prior to this event. The most likely answer to this question came recently with the aid of modern day satellites orbiting many miles above the earth. In 1995 a satellite photograph revealed what appeared to be the outline of a huge crater impact located in the Yucatan Peninsula of Mexico. This impact area (named the Chicxulub Crater) centered near the city of Chicxulub, Mexico, measures a huge 60 miles across and is buried deep beneath younger sediment. The crater was later confirmed by a team of scientists sent to investigate, and in doing so also discovered unusually high levels or rare metals around its rim that date back to the exact time of the dinosaurs extinction. These metals are consistent with meteorites or asteroids. In addition, a thin layer of clay containing high levels of these same metals can be found around the world that date back to the same time.
Although it is difficult to state precisely what happened after the meteor impact, scientists speculate the following:
During this darkness and cooling period, many more coral reefs died off than in the Permo-Triassic extinction. The reason for this can be found in the ‘Anatomy of Coral’ file. The Cenozoic Era During the Cenozoic Era living things were beginning to look more like they do today. Up to this time there had been very few flowers, but now they thrived worldwide. Forests of maples, camphor trees, fig trees and other trees covered Greenland. Insect species exploded and many new mammals appeared. The direct ancestors of tigers, horses, rats, wolves, dogs, monkeys, and other mammals were roaming the earth. A long chain of volcanoes appeared off the Pacific coast, now called the Cascade Range. Coral reefs thrived again and many species of tropical fish developed. Appearing for the first time were massive, toothed whale carnivores, looking more prehistoric than current day whales. The largest of these whales was the Basilosaurus that roamed the oceans in search of fish and giant squid, much like current day tooth whales do.
Also developing during this time was echolocation in porpoises and certain other species of toothed whales. A well-known example of this is the Eurhinodelphis.
The most recent extinction has been during the Ice Age. As a zone of ice spread south from the North Pole, the large mammals had to retreat southward. But it wasn’t during this time the extinction occurred, but rather the end of the Ice Age. We know that wooly mammoths and rhinoceros, bear, wolves, wolverines, giant deer and other mammals occupied these cold regions because of their fossils, bones and drawings of them on rocks by Indians long ago. These animals had adapted themselves to the cold climate, but when the climate rapidly changed back to a warmer one ending the Ice Age, bringing new vegetation, many of these animals died. Moreover, at this point man was hunting throughout North America and may have played a role in eliminating certain species. The conclusion of the ice age marked the start of the Holocene Period, our present day. Although the Holocene is more than 100,000 years old, it is considered in its early infancy on the grand scale of geological time. To learn about the conditions of today’s coral reefs, see the ‘Coral Reef Report’ file.
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