BIOLOGICAL EVOLUTION

CHAPTER SIX

The earliest photosynthetic organisms were the photo-bacteria. However, when the algae developed [eucaryotic protista] the diversity of life forms that obtained energy by radiation increased dramatically. Many phyla of protista evolved such as the brown algae (Phaeophyta), red algae (Rhodophyta) and green algae (Chlorophyta) amongst others. These algal groups are particularly distinctive in the types of pigments they contain. In the Chlorophyta the major pigment is chlorophyll which is used in the photosynthetic process. The general relationships between the chlorophyta and the plants indicates that the chlorophyta gave raise to the true land plants (Kingdom Plantae). It is probable that the early chlorophyta developed large aquatic communities living in fresh water - just as they do today. However, in order for them to evolve into true plants important modifications had to take place.

The first change was from saline to brackish to fresh water. The final stage was adjusting to using rain water and the use of roots. The change to utilizing fresh water probably took place in the chlorophyta and thus the earliest land plants did not have this problem.

The initial problem was one of water storage. Probably the first plants developed immersed in water. Photosynthesis using direct sunlight was possibly the driving force causing plants to become sub-aqueous i.e. with part of their structure above the water level. In order to do this they had to develop a supporting structure of stiff tissue. At the same time the development of a primitive vascular system [an internal tubular system for carrying nutrients and water] allowed efficient metabolism.

The earliest true fossil plants belong to the Division Psilophyta. They consist of horizontal stalks (rhizomes) which grow in damp soil. Coming off from the rhizomes are short vertical leafless stems which have branches and sporangia. The rhizome later developed into a rooting system but initially it was a simple stalk that absorbed nutrients and water. There is some evidence that in these earliest plants a trachea existed (find fossilized woody [lignin] tubes with cellulose in the center of the stalks when looked at under a microscope). Thus rather than diffusion an actual transport of nutrients took place via xylem tissue (upward movement of nutrients and water) and phloem tissue (downward movement of manufactured food) that were formed of elongated hollow cells.

The development of a method of overcoming dehydration was another major problem. This was done by developing cell walls. These cell walls do not occur in animal cells. In plants they consist of layers of lipids (waxes) that form a protective coating around the cell. The outer surface of the plant developed a thick covering of cells that were full of lipids and this formed an epidermis which prevented excessive evaporation. This epidermis along with the lignin and cellulose, acted as a support structure allowing the plants to grow taller. The rhizome developed not only into a root system for gathering nutrients but also as an anchoring system.

The Division Bryophyta is a group of leafless plants that do not have true roots (e.g. liverworts, and mosses) but have rhizoids, epidermis and reproduction spores that have a thick cell coating (called an exine).

During the Late Silurian and Lower Devonian a root system and primitive leaves developed. The earliest forms with true root, stems and leaves, were the Division Lycopodophyta. From the early Devonian on plants truly began to conquer the land. Initially the Lycopodophyta, then the Division Arthrophyta [e.g the present day Horsetail plant which was very abundant and huge in the Pennsylvanian and Mississippian forests], and then the Division Pterophyta (Ferns) which also were extremely abundant in the Middle Paleozoic. From the Division Pterophyta the Division Coniferophyta arose. These are the evergreen conifers and have dominated the colder regions of earth since the late Paleozoic.

During the Mesozoic Era a number of new divisions arose such as the Division Ginkgophyta, and Division Cycadophyta. However, the most important development was the development of the Division Magnoliophyta [the flowering plants or Angiosperms]. From the early Cretaceous until today this Division increased and the flowering plants dominate our landscape today.

After the origin of the flowering plants the only major evolution development was the evolution of grass at the very end of the Cretaceous Period. Grass was rare during the Paleocene Epoch but took over by the Miocene Epoch.

Once the hurdle of dehydration and adequate nutrition was overcome the plants were able to dominate the terrestrial landscape. The real major changes from an evolutionary viewpoint were related to better reproductive methods.

The simple Bryophyta have reproductive cells [spores] that were all of one and the same appearance (called Homospores). These spores develop into small gametophytic haploid phase plants with antheridia [male] and archegonia [female]. Flagellated sperms are produced by the antheridia and swim to the archegonia causing fertilization and the formation of a zygote. In the archegonia the small plant originates (sporophytic diploid phase) which then produce sporongia with homospores. The Psilophyta, Anthrophyta and Pterophyta similarly produce homospores. In the more advanced plants such as the Lycopodophyta some primitive forms only produce homospores but in general two kinds of spores are produced: microspores and megaspores. These spores are borne on the leaves. In the Anthrophyta the homospores are borne on cone-like structures separated from the leaves.

The next stage was the development of the seed producing plants that did not have flowers. Out of the Pteridophyta came a group called the seed ferns, which produced protected seeds on their leaves [Glossopteris, Pecopteris, Neuropteris]. The Coniferophyta are the naked seed bearers with their seeds on cones.

The Magnoliophyta carry enclosed seeds.

The Phylum Hemichordata is related to the Phylum Chordata compared with which it shows some transitional features. In the Hemichordata a short elastic rod represents a notochord-like structure occurring in the anterior region [head end]. Gill slits similar to those found in true Chordates are present. Today the Hemichordates are represented by some worm-like marine organisms e.g. Balanoglossus which has a 6' long alimentary canal, a solid dorsal nerve and a chitinous skeleton. However, during the lower part of the Paleozoic Era the Hemichordata were very important because they included two very diverse, abundant and biostratigraphically important groups called the graptolites and chitinozoa.

The Phylum Chordata has a tremendously large evolutionary potential. This potential for genetic variability is clearly seen in it's geological record which extends from the Cambrian Period to the present day. The underlying unifying characteristic of the Chordata is the presence of a notochord, as an unsegmented elastic rod situated dorsally to the alimentary canal and ventrally to a dorsal nerve chord. The notochord is the basis of the complex endoskeleton found in the higher Chordates. In addition to the notochord, gill slits and a hollow nerve chord are present.

Three subphyla of the Chordata are commonly recognized.

These include the Tunicates and Ascidians. Tunicates are marine organisms that in their larval stages are motile with a well developed notochord and nervous system. However, in their adult stage they usually become sessile and the notochord disappears. The sessile forms are regularly colonial and feed by passing currents through their gill slits using cilia.

These include the include the Lancelets e.g. Amphioxus which is a fish-like marine animal with a notochord, gill slits and nerve chord.

In the Vertebrates the notochord is chemically replaced by bone or cartilage to form a vertebral column. A skull surrounds a well developed brain and a tail is usually developed.

The endoskeleton comprising the axial skeleton (vertebral column and cranium) and the appendicular skeleton gave the vertebrates an important evolutionary advantage over competition. The endoskeleton is internal and is living tissue. Because the endoskeleton is living tissue the organism does not grow in stages (like the terrestrial arthropods which must shed its skeleton occasionally. Instead the skeleton of the vertebrates continuously grows with the animal. In the larval form the endoskeleton is made of cartilage but as the organism matures the endoskeleton tends to become ossified (turned into bone) except in some specialized groups such as the Sharks and Rays.

The vertebrae of the axial skeleton contain the notochord in the centrum and the nervous and blood systems in the dorsal neural arch and the ventral haemal arch. Ribs are merely an expression of the haemal arch and in the higher vertebrates are united by the sternum. The skull consists of two parts: the cranium and the jaws.

The cranium originates as a pair of cartilaginous rods lying on either side of the anterior end of the notochord. From these the cartilaginous material grows upwards to surround the brain: this is ossified in the higher vertebrates. The jaws originate below the cranium from a series of arches surrounding the front of the alimentary canal. The front arch forms the jaws. Associated with all jawed vertebrates except the birds are teeth. Teeth are important to the paleontologist because the dentation (tooth pattern) is useful in classifying vertebrates and also can be used to infer eating habits for teeth are readily adapted to food type.

In primitive mammals the maximum number of teeth is 44, in man it is 32. The mammals show a variety of teeth forms according to their function. These are incisors, canines, pre-molars and molars.

The dentation is referred to by a dental formula e.g. in Homo (1/2 jaw) the dental formulae is:

The teeth in mammals may allow us to distinguish between forest dwellers, browsers and plain dwellers.

The appendicular skeleton shows adaptations to aquatic, terrestrial, arboreal and aerial life modes. e.g. shark (fish), ichthyosaurus (reptile), dolphin (mammal), or wings of bird, bat and flying squirrel. The exoskeleton in the vertebrates consists of scales or bony plates in the primitive forms, modified to feathers in birds, and fur (hair) in mammals.

The development of a structure that could bite, grasp, and manipulate led to a whole new active way of life. Pisces [fish] became browsers on algae and predators on animals. The original structures were gill supports which in primitive forms extended on either side of the throat region to the front. As the mouth enlarged the first and/or second anterior gill supports were eliminated. However, the next pair enlarged and the lower part became hinged to the upper part. At this time the upper jaw articulated with base of the brain case. Later development saw the fusion of the upper jaw to the cranium and the articulation of the lower jaw. The gill slit posterior to this structure became the spiracle openings which eventually developed into the auditory apparatus and the gill support behind it became a support for the moveable tongue. Within the Subphylum Vertebrata two superclasses can be recognized.

Pisces is the general term for all of the primary groups of aquatic vertebrates of which there are four classes. The fish are the truly dominant group in the marine environment and have shown great evolutionary development. The four (4) classes are:

Agnatha. Jawless fishes e.g. the hagfish and lamprey

Placoderma. Bone supported hinged jawed fishes

Chondrichthyes. Cartilaginous fishes

Osteichthyes. Boney fishes

The ancient jawless vertebrates are grouped together as the Agnatha. They are mainly bottom dwellers and are known from the Ordovician Period on. Pre-Ordovician vertebrates are believed to have been essentially soft bodied and probably fresh water organisms. These early organisms already had eyes, ears, and a single nostril on the top of the head. They were powered by a strong tail, which is usually turned up (heterocercal) suggesting a bottom scavenging mode of existence. Some forms have a hypoceral tail (turned down). The larvae of the modern sea-squirt (tunicates) is thought to be somewhat similar to the earliest fishes. Extant g[living today] groups are the cyclostomes (hagfish, and lampreys), with amphioxus as an advanced form (4" long) that has degenerated (Subphylum Cephalochordata).

The agnatha are a diverse group and include four main subgroups.

Osteastaraci which originated in the Silurian Period and became extinct at the end of the Devonian Period. They had a head shield of bone, which had two openings for the eyes, a pineal opening and a single nasal opening. There also are groove-like depressions down the side of the head shield. The whole body and head was flattened and it is presumed to be a bottom dwelling filter feeder. e.g. Boreapsis living in brackish or fresh water.

Anapsida which are very similar to the Osteostraci except they had a more streamline body and were adapted to active swimming.

Heterostraci which was a successful group that adapted to the marine environment. They had a row of scales at the edged of the mouth which are like the teeth of the shark in origin and were presumably used for nibbling at algae rather than filtering. The head shield is not solid bone but is made up of a series of plates. The eyes are more lateral (this increases survival rate when laterally attacked while stationary feeding). The earliest forms are known from the Ordovician Period.

Coelolepida which are very similar to the Heterostraci but are brackish water dwellers. They are covered with small spiny scales. They lived during the Silurian and Devonian periods.

The jawless fishes and the jawed fishes coexisted for a while but the jawed fishes rapidly took over and dominated the environment. The Placoderma were the earliest jawed fish and they occurred in the Silurian Period. By the Devonian Period the jaw was well developed. It was from the Placoderma that the modern fishes developed. There are two groups of Placoderma.

Acanthodians (spiny sharks) originated in the Silurian Period. Some of these were armored but most had scales.

Arthrodires (true placoderma) originated in the Devonian Period and had an armored head region e.g. the Antiarchs. The evolution of the Arthrodires during the Devonian Period represent a period of experimentation. Many varieties developed at this time. Some became very streamlined and developed appendages, fins, and teeth. Some of these developed a truly hinged jaw. An air bladder [hydrostatic body] which is present in the higher fishes may have already developed in the Placoderms. The air bladder is a very primitive form of lung. The more advanced fishes [the osteichthyes and chondrichthyes] probably arose from a somewhat shark-like placoderms possessing numerous gill slits and which had discarded the boney armor [this allowed a more active speedy existence]. Those groups that lost their armor did so in one of two ways. One group, which led to the boney fish [osteichthyes], gradually reduced the armor until all that remained of it were thin scales covering the body and a slightly thicker boney operculum protecting the gill slits. The boney fish developed as specialized speedsters! Their bodies became flattened laterally, scales reduced, tails shortened and symmetrical, and the pelvic fins moved forward. The swim bladder was perfected in this group and by inflating and deflating it they are able to remain at rest in the water at any depth (this improve stability). A second group, leading to the chondrichthyes, lost the ability to produce bone entirely and all armor disappeared and the entire skeleton became cartilaginous. These cartilaginous fish developed very powerful jaws and also became fairly swift swimmers. They did not perfect the swim bladder and must thus continue to move in the water in order to maintain any particular level. They developed a method of stabilizing their movement by fin and appendicular skeleton development. To control pitch the front of the bodies became slightly flattened underneath and the upper lobe of the tail enlarged. To prevent roll the pectoral and pelvic fins became enlarged.

The original types of chondrichthyes were well adapted to life in the oceans. Powerful jaws and swift movement. Two of the earliest forms found in the fossil record still survive today (these are the so-called 6-7 gilled sharks, and the Port Jackson shark: Heterodontus). Heterodontus is interesting because it has two kinds of teeth: pointed front and flat rear teeth. These sharks are similar to the modern sharks in having only five gill slits (the spiracle is the remains of the sixth). The Port Jackson shark is adapted to benthic feeding and eats molluscans and crustaceans. The anterior flattening became pronounced and the pectoral and pelvic fins became broader and flattened out (similar in appearance to the sand shark Rhinobatis). One branch of the Cartilaginous fish retained the bottom habit and gave rise to the skates and rays. These evolved further and lost there pointed teeth and in the extreme form the lower fins broadened and flattened and became attached to the front of the head. The rays (Raja) exemplify the first stage. In the electric ray (Torpedo) the whole thing coalesces as a disk and the tail is reduced in size. In the extreme case such as the eagle ray the tail is whip-like and thin (Aetobatis). The only fin on the tail is a small dorsal fin. One further interesting adaptation is that some forms have improved their method of respiration. In most fish the water is taken in through the mouth and then by a swallowing process is forced out through the gills. However, in some rays both the mouth and gills are on the lower source next to the mud and the spiracle is on the upper surface. The spiracle becomes enlarged and then is used to intake water which is forced out through the gills.

The sawfish (Pristis) shows another interesting adaptation in which its smooth snout is greatly elongated and armed with teeth. It feeds by moving into a pack of fish and slashing them so that they can be more easily caught.

The second main branch of cartilaginous fish [the sharks] lost the back flattened teeth as they became adapted to a life of prey. The earliest of the modern sharks were around during the late Mesozoic Era and were somewhat ray-like e.g. Angel sharks [Squatina] which are somewhat flattened. The majority of true sharks belong to the Order Galea and developed from Angel shark-like ancestors e.g. Carcharias or the ragged tooth shark. Some sharks adapted to eating plankton and gave up the predatory way of life e.g. Whale shark [Rhincodon which growth to a length of up to 60 feet] and the Basking shark [Cetorhinus which grows to a length of 50 feet.

Another important shark group is that which includes the Hammerhead. This fish lives close to the water - air interface and the shape of the head gives it additional lift, and counteracts pitching (however, it cannot see directing in front of it and must move its head from side to side to see ahead e.g. the Great Hammerhead [Sphyrna tudes].

The remaining branch of modern sharks is one of the largest and it is omnivorous e.g. the Tiger shark [Galeocerdo]. How omnivorous is demonstrated that the stomach of one example contained tin cans, a gold watch, a purse contain three english shillings, the arm of a murdered man, roast potatoes, and a parcel of papers thrown overboard by modern pirates being pursued by the British Navy. The arm led to a murder hunt and the papers convicted the pirates. One group of these modern sharks adapted to the fresh water. These belong to Grey sharks and include the Zambezi River shark and the Lake Nicaragua shark.

Under-water organisms are supported by the medium with which they live. This water also keeps them moist and supplies them with dissolved oxygen. Adaptations to land must overcome these problems associated with skeletal support, dehydration, and oxygen supply.

Several Devonian fishes had lungs which evolved as an extension of the gut and probably were an adaptive advantage in waters with low oxygen. Fishes with lungs could gulp in air and as the fish dived, air bubbled into lungs. With advance onto the land the animal could not dive to force air into the blood stream and had to develop a pump to force air into lungs. This was the development of the rib cage and muscles.

Only the Salamanders and Newts (Caudata), the Frogs and Toads (Order Salientia) and some legless amphibians (Order Apoda) survive today. They developed during the early Mesozoic Era. The Upper Palaeozoic primitive amphibians belong to the Order Labyrinthodonta. These were quite reptile like, more so than today's amphibians and some species grew to at least 10 feet long.

As animals migrated onto the land the appendicular skeleton [as limbs] strengthened to bear the weight of the body. The elevation of body was an adaptive advantage because the organism could move with less friction. Associated with this the dermal shoulder shield separated from the back of the skull which was an adaptive advantage in seeing the enemy! In addition, because the fish-like construction is weak, this separation of the shoulder shield and skull allowed bigger amphibians to develop.

The amphibians are a group of transitional chordata representing the first step in the lineage of terrestrial Chordata that gave rise to the reptiles, the birds and mammals. Arising out of the specialized fish group called the Crossopterygians. The true Amphibians retain some characteristics of the fish particularly in their young forms. However, the general differentiating characteristics of the amphibians include the following.

The reptiles are the first truly terrestrial vertebrates in that they do not have a stage in their life cycle that requires a return to the aquatic environment. The key to this success is the development of the amniotic egg. The young form develops in an aqueous media within the egg and leaves the egg in its adult form (although it is not yet itself sexually mature). In addition the egg is fertilized internally. The egg is either lain as soon as the shell is formed or it is kept within the body until the young hatches (internally giving life birth).

The reptiles are characterized by the following.

These features are associated with a improved efficiency for dwelling on land. However, the earliest reptiles e.g. Seymouria of the Carboniferous Period were very amphibian-like. [the Cotylosaurs] and because they gave rise to all the other reptiles they are called the stem reptiles. Two main groups arose from the stem reptiles. The mammal-like reptiles which dominated the Permian and Early Triassic periods and eventually gave rise to the mammals. The dinosaurs which dominated during the rest of the Mesozoic Era and became extinct at the beginning of the Cenozoic Era.

Reptiles are classified according to the position and number of openings in the skull occurring in the temple area. The reptile skull consists of two containers. A small structure encloses the brain and this is contained posteriorly within the larger skull.

The early Cotylosauria and the Chelonia [turtles] are orders in which the outer skull is solidly roofed i.e. no holes. These are called the Anapsida.

The mammal-like reptiles are called Synapsida because they have a single opening in the skull occurring relatively low [below the squamosal and the post-orbital bones]. The earliest forms occurred during the late Pennsylvanian Period and are called Pelycosaurians [e.g. Dimetrodon] and gave raise to the other more advanced group of mammal-like reptiles called the Therapsids. The therapsids show a number of mammal-like characteristics including enlargement of the dental bone at the expense of the other lower-jaw bones and the development of firmly rooted teeth divided into incisors, canines, and cheek teeth. These teeth allow food to be sliced and chewed to small particles (and thus a greater total surface area for digestive processes to act upon). In addition, the limbs in the Therapsids evolved so that they were more or less directly under the body, and the toe bones were reduced to the characteristic mammalian formulae of 2-3-3-3-3. Most of the therapsids became extinct during the Triassic Period, due to competition from the Dinosaurs. Only a few survived but these eventually give rise to the mammals N.B. if the dinosaurs had not evolved true mammals may have developed some 100,000,000 years earlier.

The Diapsida have two temporal openings and were the group that gave rise to the dinosaurs. During the Triassic Period a group called the Thecodonta arose. These were small light constructed dinosaurs which had a tendency to be bi-pedal and were probably quite agile. Bi-pedalism necessitated a strengthening and modification of the hind legs and the re-arrangement of the bones in the hip region and this provides the means whereby dinosaurs are classified into two major groups.

The Pterosauria are the flying reptiles with a wing span of from a few inches to a few feet. Two general groups are known. The long tailed pterosauria e.g. Rhamphorhynchus and the short tailed pterosauria e.g. Pterandon [with a 25 foot wing span but a weight of only 40 lbs.]

The Eurypsida have one temporal opening on each side, occurring in the upper part of the skull. It consists of those reptilian forms which back-evolved into a marine habitat. Paddle-like limbs and stream-lined bodies again developed and they evolved increasingly more efficient lungs e.g. Pleisiosaurus [some had a skull 9' long], Macroplata, Ichthyosaurus.

N.B. Jesus Cristo Laganto [clocked at 33 feet in 4 secs]

The birds are a kind of glorified reptile. Even today they retain scales on their feet. The first birds probably appeared in the Jurassic Period but is was not until the late Cretaceous and the Cenozoic Era that the birds gained a dominant place. The traditional Jurassic transitional fossil is the bird-reptile called Archaeopteryx [this definitely had feathers and was probably warm blooded]. Cretaceous birds still had teeth but they gradually evolved a beak. The finger bones gradually grew closer together forming stronger wings.

The main characteristics of the birds are as follows.

In the early stages of evolution the main differences between Mammals and Reptiles are physiological and reproductive rather than skeletal. The important mammalian adaptation is care of the young and warm blood. The warm blood permitted mammals to survive in cold regions, they could search for food in all seasons and during the cool of the night. Insulating hair helps regulate body heat and a more efficient heart and lungs evolved. The tooth and jaw adaptation allowed mammals to eat and digest food more efficiently. The lower jaw is a single bone which is more efficient for chewing. The secondary brain increased and sense of smell and hearing became more accurate. The internal embryo and the mammary glands are both adaptations that improved the care of the young.

The mammals are grouped into three categories:

The primitive mammalian skull is basically synapsid but with a greatly expanded brain case. Eyes, ears and especially the nose are important sense organs and the cerebral hemispheres originally dedicated to the olfactory function are greatly enlarged and from them arises the higher brain centers of the higher mammals.

Although the Cenozoic Era is the age of the mammals the Mesozoic Era was their time for experimentation. The way in which they met the competition of the reptiles was to develop more efficient nervous and reproductive systems, greater speed and agility, and a more reliable system of bodily temperature control.

There are only three Monotreme species still live today.

They represent an evolutionary level of the mammal-like reptiles but they are not considered ancestral to the other mammals.

Australia and South America are the continents where larger numbers of marsupials evolved. In Australia the number began to decrease with the coming of european immigration. In South America it was the connection between the Americas that initiated the reduction in number. The initial separation of the north and south American continents was at the beginning of the Tertiary Period. At this time some herbivorous placental mammals were present which continued to live alongside the marsupials [which were both herbivorous and carnivorous]. However, when the continents were reconnected the marsupials did not fare well against the placental carnivores.

There are at least 28 orders of Placental mammals that evolved principally during the Cenozoic Era. These can be grouped in 4 types.

Among the clawed types are included the orders:

The Insectivora is presumed to have given rise to all the other groups, including the primates and is thus a direct ancestral type of man.

The earliest known placental mammals are unspecialized Insectivora [insect eaters] found in the rocks of the Cretaceous System. From this group the mammalian hoard arose. Today's Insectivora include the shrew and the mole. They are primitive and have a skull somewhat similar to the marsupials. The aerial mammals such as the bats and gliding Colugo are closely related to the Insectivora. Bats probably evolved from arboreal insectivora and their earliest fossils are found in rocks of the Eocene Epoch. They are the only mammals to achieve true flight. The ant eaters [Edentata] include the armadillos, tree sloth and South American Anteater and originated during the early Tertiary Period. The Rodentia is probably the most diverse group and is geologically of great importance. These are the gnawers and nibblers and a very successful group of mammals. They are the most abundant mammals and have adapted into numerous environments e.g. arboreal such as the squirrel; partially aquatic such as the beavers; deserts such as the kangaroo rat. The teeth of the Rodentia are adapted to gnawing i.e. lack canines but have two continuously growing prominent opposing pairs of incisors, one set in each jaw. The front of the tooth has hard enamel so that as it is worn down it returns a sharp edge. The jaw muscles are strong and the jaws have flat articulation so they can grind better with their cheek teeth. The earliest true rodents are from the Paleocene Epoch. The Carnivores include many familiar types and are the flesh eaters. The flesh eater has to track, attack and overcome his food and this necessitated the evolution of speed and a crafty nature! Keen senses are characteristic of the carnivores and weapons are in the form of clawed feet, sharp teeth and a muscular jaw. The carnivores originated in the Cretaceous Period. Dogs [Canids] are an old group of carnivores and originated at least as far back as the Oligocene Epoch. They show some fascinating adaptations such as the following.

Whereas dogs are a social group the independent, stealthy and cunning cats [Felids] are not. They appear in the late Eocene Epoch and have remained more or less the same ever since. There are two groups of cats.

The Cetacea are of interest in their back-evolution to the marine environment.

Hoofed animals [Ungulates] evolved from one of the earliest explosive developments from the Insectivora diverging from a carnivorous stock near the end of the Cretaceous Period. They are clearly present during the Paleogene Period. They are a very large and varied herbivores mammalian group. They include forms with interesting evolutionary histories such as the Horse and Rhinoceros. The Rhinoceroses, for example, were once part of a highly flourishing group, although they are now confined to only two species in Africa and three in Asia (e.g. the evolution of Indricotherium, a hornless Rhinoceros that lived in Asia during the Oligocene and early Miocene epochs. This creature was about 17 feet tall and built somewhat like a heavy giraffe. In their evolution both one and two horned Rhinoceros' are known: the horn being unique in that it is formed from coalescing hair rather than bone. The horse became extinct in North America during the Pleistocene Epoch and was reintroduced here by man.

The horse and zebra belong to the odd toed ungulates [the Perissodactyla]. The commonest and most widespread of living mammals belong to the even toed ungulates [Artiodactyla]. Basically these can be divided into two groups: the Pigs and the Ruminants. The pig group includes the following.

The Ruminants include the Camels, Deer, Giraffes, Bucks, and Cattle. The Camel is one of the more ancient ruminants and like the horse underwent most of its evolution in North America. The present distribution of the Camel is only a relatively recent phenomena. The hump of the modern camel also is a recent development.

Photo-illustration: primitive camels in a Miocene landscape of North America.

Photo-illustration: a typical scene during the Pliocene Epoch, with ancestral elephants, rhinoceros, and a peculiar ruminant with a horn that bifurcated and originated from the nasal region. The Elephants [Probiscans], which today are represented by but two species, were once a large and diverse group of nearly world-wide distribution. The earliest Elephants were about the size of a modern pig.

The Order Primates belong to the Chordata and evolved through the Cenozoic Era starting during the Paleocene Epoch. Like so many of the mammals the Primates arose from the Insectivores during the early Cenozoic. They were an adaptation that became omnivorous and arboreal [instead of ground dwelling insect eaters]. This new habit led to changes in the skeletal structure, particularly the development of the grasping inner digit, and stereoscopic vision. The development of the stereoscopic vision led to the forward movement of the eye sockets and the flattening of the face. These two developments allowed man to develop excellent coordination of hand with vision: it led to man the weapon maker and hunter. The characteristics of the Primates are most generally the same as those for the Chordata and Mammals. Chordata characteristics include an embryonic notochord replaced by individual bony vertebrae. Mammalia characteristics include mammary glands for nourishment of the young, hair on the bodies, and young that are retained within the uterus of the mother during early development.

The Prosimians

These are tree dwelling, small and represented today by the Tasier, Loris, and Lemur. The grasping hand and stereoscopic vision developed in these early Prosimians at least as early as the Eocene Epoch, from fossil finds in Eurasia and North America. These are not true monkeys: Prosimian means Pre-monkey. They were widespread in the Paleocene, Eocene and Oligocene epochs but decreased drastically when the Anthropoidea evolved in the Oligocene Epoch. The most primitive surviving form is the Lemur. This is distinctly quadrupedal. It has a long bushy tail and a small brain behind a slender pointed muzzle. The eyes are fairly far apart and it very much resembles an insectivore. It does not have stereoscopic vision. They are found today only on the island of Malagasy [Madagascar], where they apparently survived because the island separated from the rest of Africa during the early Cenozoic Era and very few mammalian predators ever developed on the island. Two other groups of Prosimians have survived: the Tasiers are prosimian and also monkey-like. They have a shorter muzzle than the Lemur, and eyes closer together with stereoscopic vision. Tasiers occur in Borneo, Sumatra and the Philippines. The other group is the Loris which today lives in Africa, India and Southeast Asia. The modern Prosimians are well adapted to mild, moist climate and were widespread during the Lower part of Cenozoic Era, becoming more restricted in the Oligocene Epoch (to tropical, subtropical climates).

The Suborder Anthropoidea is divided into two Infra-orders.

It seems that the Monkeys evolved from a Tarsier-like ancestor. Their progress is evidenced by their forward facing eyes, more complex molar teeth, larger brain case, improved hands, and a bony bar protecting the eye orbit. The New World monkeys evolved at the same time as the Old World monkeys during the Oligocene Epoch. The two groups apparently originated separately from different Prosimians and evolved in a parallel manner into similar environmental niches. the New World monkeys evolved in the New World although it is now only found in South America. They have a prehensile tail and an extra premolar. The Old World monkeys evolved in Africa and Asia. At the same time as the two monkey groups were evolving during the Oligocene Epoch the apes evolved. Already by the Miocene Epoch fossil finds indicate differentiation between the apes and monkeys; with the new world monkey living in isolation but the old world monkey and apes competing together. The old world Rhesus monkey was the first Primate ever to fly in space!

The Hominoidea is divided into two groups.

1. Family Pongidae which includes the Orangutan, Chimpanzee, and Apes such as the Gorilla.

2. Family Hominidae which includes Australopithecus and Homo.

There are a number of separating characteristics.

9. The Hominidae have a big toe that is not opposable. The Pongidae have big toes opposable

From weathering to mass movement the next logical step is erosion and transportation of the debris via rivers, glaciers, and wind. This erosional stage gives raise to the sediment, which hardens into the sedimentary rocks: products of the erosion-transportation-depositional cycle. Sedimentary rock formations make-up about 75 percent of the rocks occurring at the surface of the earth. We can most easily classify the sediments and their lithified products [sedimentary rocks] by essentially recalling what happens to the igneous rock when it weathers. These produce essentially resistant minerals e.g. quartz survives mainly unchanged and is easily incorporated in sediments. This can be termed an inherited mineral. Non-inherited minerals are new minerals. These include clays, amorphous aluminum, and the aluminum and iron oxides amongst others. In addition, the weathered material taken into solution may be, and usually is, deposited somewhere by precipitation. Finally, dead organic matter may pile up on a weathered surface (i.e. on and in the soil) and this will be transported along with the normal mineral products by the erosional and transporting agent and deposited along with the sediment e.g. coal, peat and petroleum are essentially formed in this way. The products of weathering thus are sorted into fairly distinctive groups. This is termed sedimentary differentiation. The initial sedimentary differentiation provides us with sediments of two types. Detrital sediments are made up of the accumulation of fragments or clastic particles of minerals or rocks. We conventionally divide the detrital or clastic rocks according to size classes of their modal grains. The pebble sized grains form the Rudaceous detrital sediments. The sand sized particles form the arenaceous detrital sediments. The clay sized particles form the Argillaceous detrital sediements. The precipitation of material from solution or by organic processes give the chemical and organic sediments, represented mainly by limestones, evaporites, and coals.

The lithification process changes sediments into sedimentary rocks. Lithification may involve cementation by the precipitation of material as cement [particularly iron, quartz, and the carbonates calcite and dolomite]. This cement is precipitated around and between the mineral grains and precipitates from the percolating ground water. Compaction takes place by the squeezing out of the water [dessication].

The two processes of cementation and compaction cause the sediment to lithify. This process of lithification is regarded as the first stage in diagenesis.

In order to be able to determine the depositional environment of a sedimentary rock it is often necessary to examine a large number of rock charcteristics. One basic idea behind classifying sedimentary rocks is to provide an easy descriptive method of linking a knowledge of depositional environment and sediemntary process with the name of the rock. Characteristics of important in describing sedimentary rocks from the viewpoint of interpreting the depositional environment include the following.

The textures of the detrital sediments are determined mainly by the size, shape and arrangement of the particles. Size may vary from a boulder down to a very fine rock flour. The shape of the particles can be generalized as angular, subangular, or rounded. The degree of sorting of particles is controlled mainly by the way in which the sediment was transported and deposited. The well sorted sediments are those in which the particles are all of roughly the same size. They are formed where the wind, waves, or current have time to work over the material and to separate finer and coarser material. The poorly sorted sediments in which the fine and coarse material is jumbled together, result from rapid deposition, from deposition by turbidity currents and from deposition by glaciers. Phanoclastic rocks are equigranular and Phenoclastic rocks are not equigranular.

The ways in which the constituent mineral grains fit together is termed the fabric of the rock. In some rocks the grains seem to be arranged quite haphazardly; in others we can see a definite pattern about the arrangement of their longer and shorter axes. This is termed preferred orientation e.g. pebbles transported by strong streams may come to rest with their longest axes pointing in the direction of the stream flow.

The textures of the chemico-organic rocks are very complicated. The chemical rocks are frequently strongly altered by recrystallization during diagenesis and the rock becomes a crystalline mass (in many ways similar to an igneous rock). This recrystallization takes place at normal temperatures and pressures existing at the earth's surface.

Two further important properties of sedimentary rocks that are related to particles size, shape and sorting are porosity and permeability. Porosity is the ability of a material to hold fluids. Permeability is the ability of a material to pass fluids. Porosity and permeability of surface rocks control the movement and storage of liquids in the earth's crust, particularly ground-water and oil. Porosity is really the ratio of the spaces (voids) not occupied by solid rock, to the bulk volume of the rock. The average porosity in rocks is probably around 10% Permeability depends on the size, shape and packing of the particles to a very high degree. The void spaces may be so minute that no continuity of voids exist and water cannot pass through. Poor sorting, cementation and recrystallization all can reduce permeability to almost nil.

The color of a sedimentary rock is an immediate characteristic. The color is dependent upon the particles and cement. It also may be a reflection of a coating of some material over the particles. Alther color is quantified using a special method [ the Muntzel Scale] two colors are particularly important at a very general level. The blackness of a sedimentary rock is mainly due to the presence of organic matter. Black shales are formed only if organic material is preserved and for this to be so the organic material must not have decayed (when the organic particles are buried rapidly so that decay does not set in ). Red sedimentary rocks on the other hand are usually due to the presence of the iron oxide mineral haematite and they are indicative of an oxidizing province and environment.

The mineral composition of the sedimentary rocks can indicate the kind of rock that was being eroded and thus provide information on the area which was the source of the sediments [the provenance]. The weathering of granite gives typically quartz and the clay minerals. On the other hand the weathering of metamorphic rock also gives minerals produced by metamorphism e.g. Corundum.

The depositional structure of a sedimentary rock is determined by the arrangement of particles on a large scale. The normal sequence of sedimentary rocks has the various layers separated by stratification of bedding planes. Each layer is known as a stratum or bed. A bed normally represents the product of a single act of sedimentation. No two beds are ever totally identical but a succession of beds may all have a common general component and are then grouped under the term formation. Bedding is of four main types:

Additional sedimentary features that may be observed in the rocks which tell us something about the nature of the original environment include the following.

The major types of sedimentary rocks are Argillaceous clays, which form 45 percent of sediments; Arenaceous quartz sandstones, which form 32 percent; and, the Precipiated and biological calcareous limestones, which form 22 percent. All of the other sediments together only form about 1 percent of sedimentary rocks.

A rock or sediment normally has three main components: particles, cement, matrix. The matrix is the very fine grained detritus that lies between the obvious particles. Matrix may actually act as the binder between grains instead of cement.

GRAYWACKE: These have a high content of matrix, particularly of clay minerals, chlorite, and sericite (new minerals). The arenaceous particles consist of quartz, rock fragments, feldspars and a few minor constituents. The heterogenous nature and poor sorting indicates instability of both the source and depositional environment. Beds of graywacke usually vary from a few inches to a few feet in thickness and normally do not show any laminations. However, one may get thousands of feet of such beds in a formation. Graded bedding is common. Because we know that graded bedding is only readily achieved by settling out from a liquid, we believe that turbidity currents are largely responsible for graywacke. The fact that graywackes is associated with radiolarian cherts, submarine volcanic deposits, and other rocks (shales) containing deep water or bathyal faunas suggests that they were deposited i an unstable environment i.e. at the foot of the continental slope.

ARKOSE: These are the sandstones containing 15 percent or less of detrital matrix. Feldspar is greater than rock fragments in particles. Quartz and feldspar are the main minerals and kaolinite and mica are also important. Arkoses are generally well sorted and the particles vary in roundness. The beds generally occur i either thin sheet-like units overlying a granitic terrain, or as a wedge-like unit deposited in basins adjacent to a granitic mass.

ORTHOQUARTZITE: These have a very high quartz content and therefore are often called pure quartz sandstones. There is little or no clay matrix and they may or may not have cement. Associated with the quartz are small amounts of other stable minerals such as zircon and tourmaline. The particles are normally well sorted and well rounded. Cross bedding and ripple marks are particularly common in these rocks. They are often associated with or grade into, limestones and dolomites. They occur most often as blanket deposits only a few hundreds of feet thick but of wide areal extent. The indications are that they were deposited in a relatively stable environment, and there was either time for weathering and transportation to take out all of the unstable minerals and clays and leave only the stable residues, or alternatively these may be sandstones that have been derived from earlier sandstones (second generation sandstones).

C. Argillaceous rocks. These are formed from the fine grained quartz particles and clay minerals. They are classified according to content of organic material or content of precipitate material (calcium, iron etc.).

A. Precipitated rocks. The typical chemical type of sediment is an evaporite, although precipitate deposits also include the biochemical rocks. The principal precipitates are mono-mineralic rocks, and are composed either of calcite (CaCO₃), dolomite (CaMg (C0₃)2), gypsum (CaS0₄.2H₂0), halite (NaCl), or chert (Si0₂). Evaporites are normally formed in salt lakes (playas). A salt lake is the result of interior drainage and the absence of, or limited outflow. Excess evaporation causes the precipitation of salts from saturated solution in the reverse order of their solubilities. The order of precipitation is:

In addition one gets dripstone and travertine in caves which are evaporite rocks.

The non-evaporites. These include the biochemical rocks and non-evaporite chemical precipitates. The essential rocks formed are the carbonate rocks limestone (CaCO₃) and dolomite (MgCO₃CaCO₃). There are two principle types of sedimentaru rocks. The biostromes are simply layers or beds of calcareous shells which form shell limestones; and biohermes [reefs] which are mounds of colonial organisms having calcareous shells. The coral reef is the most typical example.

The bio-clastic rocks. These are the rocks formed by the breakup of shelly material and therefore are detrital.

The coals are the bedded carbonaceous debris. The effects of chemical and bacterial action at the earth's surface, followed by burial diagenesis convert the layer of loose vegetable matter into a compact coal seam. There are various recognizable stages in the conversion from plant to coal and each is a different looking rock. Simply this is from peat to lignite to bitumenous coal to anthracite. This is the coal series. The change of rank from peat to anthracite is marked by an increase in density, hardness, and carbon content of the rock and also by a decrease in content of moisture and volatile substances. In general, the higher the rank of the coal in the series, the greater its value as a fuel.

In addition to the coal series there is the bitumenous series (composed of hydrocarbons of which parafins, napthenes, and aromatics are the chief ones). These vary in form from the liquid oils to the solid substances such as asphalts, and they are the parent materials from which we get petroleum. Bitumens are produced by bacterial and thermal decay from original organic material entrapped in the sediments. Most plant tissues decompose under atmospheric conditions because they are mainly formed of cellulose which is oxidized.

However, when cellulose accumulates under water, the oxidation does not completely take place, and although there may be partial oxidation of the plant debris. CO₂, H₂O and CH₄ (Marsh gas) are formed in the simplest case. In addition to the cellulose lipids proteins and lignins are derived from the decaying vegetation. These, and particularly the lipids are the main components that produce the bulk of the hydrocarbons found in the sedimentary rocks. This is done principally by thermal decay [cracking] during burial.

The continental environments also are classed as the non-marine i.e. normally lying above sea level, although in some enclosed basins e.g. Death Valley, continental deposits may actually lie below sea level. Within the continental environments the distinction between terrestrial and aqueous depends upon whether the deposit is formed in water or formed on land, for example by wind or ice. The continental aqueous environments need not be fresh water e.g. the Great Salt Lakes.

The transitional environments including deltaic, lagoonal and littoral also includes the paludal [swamps and marshes] which are simply bodies of shallow standing water or low wet areas occupied by relatively abundant plant life. The water itself may actually be marine, brackish or fresh.

The marine environments show the development of large numbers of rock sub-environments of importance to the geologist.

Life forms have a major problem adapting to the desert and frigid environemnts.

B. Physical aspects of inland waters.

The truly marine environment can be looked at in very simple terms according to depth and temperature. The simplest depth classification is into:

In addition, special types of ocean basins are recognizable such as the geosyncline and continental seas.

Temperature naturally modifies many processes of sedimentation. For example in the neritic environments high temperature and can lead to the development of coral reefs and carbonates shelfs, especially where the depositional rate is slow.

Water depth and distance between depositional site and point of origin often interact to determine the natur of the sediments being deposited. For example, the type of rock occurring at the foot of the continental slope in a location near to shore (i.e. with a narrow continental shelf) is different from the type of rock in a similar depth position off a broad continental margin under normal conditions of sedimentation.

Chemical constraints are particularly active in the shallower seas and land-locked seas such as the Black Sea which may vary to a marked degree in their chemical environment and thus radically influence the sedimentation of the sea floor N.B. some of these inland seas are large and deep (e.g. Black Sea is over 2,000 meters deep). The abyssal parts of the ocean are not subject to such frequent variation.

Recent marine basins are not uniform in their hydrochemical regime but are basically of three types: normal, desalinated, salinated.

Normal basins are part of the open oceans or freely communicate with the open oceans and are characterized by fairly constant salinity and ionic composition of the salts.

The salinity is about 3.5% ± 0.2% with chlorides predominating and constituting about 87% of salts (NaCl, KCl, and MgCl₂); then the sulphates (CaSO₄,MgSO₄); and the carbonates (CaCO₃). The evenness throughout the oceans is easily explained by the mixing processes of the ocean. Another characteristic of normal seas is the abundance of dissolved oxygen at the surface as well as at the bottom; the absence of toxic material such as H₂S, and NH₃, which are harmful to organisms, and a very low content of CO₂. This is again the result of the mixing processes of the ocean which ensures that the oxygen released by photosynthesis by the phytoplankton in the surface waters is well distributed. The normal marine basins are represented by our normal continental shelf and continental slope. The depth of deposition in the normal marine basin can often be determined by the type of organic community, type of authigenic mineral, the structure and texture of sediment, and the sedimentary rock type.

The shallow water neritic zone is characterized by a richness of species and the occurrence of large thick-shelled types which are often found on the beach damaged by wave action. The lower part of the shelf (100-200m) is not as richly populated but has mainly bryozoans, echinoids, gastropods, and bivalves. The bathyal zone is inhabited mainly by porifera, crinoids, brachiopods and bryozoa with a few echinoids. With increasing depth one gets an increase in the number of shells of planktonic organisms which have sunk to the bottom on death. The ocean floor at depth, particularly in the abyssal zone, is comparatively but not completely barren. Life is very depth sensitive.

There are a number of authigenic mineral types of importance for indicating depth. Calcite oolites (small spheres of calcite growing to the size of a wheat grain by concentric layering) are confined to the shallow waters (0-10m), and may thus occur very near to the coast. Iron oolites are found at depths of 35-40m. Phosphates are found at depths of 50-150m. Glauconite occurs at depths down to about 400 meters, but is at a maximum at around 200m, or at the depth where clayey sediments normally become dominant.

The deposits of the extreme shallows are typically rudaceous sediments beyond these are the arenaceous deposits and at a depth of about 200m or more, typically move into the clay-zone. Below 200m: one can tell very little regarding depth of sediment deposition. The texture of the sediment depends very much on the type of material being brought into the basin; and also on the the amount of energy of the environment of deposition.

The normal sedimentary pattern is particularly broken down where large rivers such as the Amazon and Mississippi move into the sea and deposit large areas of coastal mud. The abundance of sediment supplied may be such that the currents cannot move it away quickly enough and it accumulates around the shore line which causes the shore-line to build-out resulting in a facies shift seawards. This can take place when the level of the sea is lowering [regressive succession] or rising [transgressive succession].

Desalinated seas occur at the present under a humid environment or in those areas that drain a humid environment. Such seas include the Black Sea and Baltic Sea. The water surplus (due to being a humid environment) raises the water level relative to the open ocean and thus currents flow essentially outwards. The salinity of the water is drastically reduced. If the outlet is very shallow then one gets only a one way flow of water. If it is deep one gets a counter current set up at depth: the denser oceanic sea water moving into the basin. If the depth of the inlet changes with time obviously the hydrochemical regime of the basin will change and this known to have happened in the past, for example, in the Black Sea. The important factor is that these hydrochemical conditions control the type of sediment deposited in the sea to an important extent. In the desalinated basin which has a one-way circulation the waters become definitely layered. The fresher, lighter water occurs at the top and the heavy water below, and between them a transitional zone exists. This type of dstructure strongly curbs vertical circulation and thus the oxygen from the surface water does not reach the bottom: the bottom thus gradually changes into an anaerobic environmentd. Sulfates get reduced by bacteria and one gets toxic conditions occurring so that benthic organisms die off. In the Black Sea today about 85-90% of the water mass suffers from H₂S poisoning and aerobic life ceases at a depth of about 150m (the Black Sea is over 2,000m deep). The sediment at the bottom is essentially a black mud with iron pyrites and very little life over most of its bottom.

Salinated seas occur in the arid environment. The mechanism is very simple: due to the scantiness of precipitation and the high evaporated the level of the basin is lowered relative to the oceans and water flows inwards, bringing in new salts. Evaporation takes place at the surface and a heavy brine results which due to it's density sinks. In some cases obvious circulation stops and one can get condition similar to a desalinated basin with toxic conditions at depth. However, most salinated basins are very shallow and because there is little water coming into them from the land salts are precipitated and form the main rock type. Thus the typical rock is an evaporite.

We know that normal seas are characterized by their abundant organic content: both plants and animals. In abnormal seas the organic remains are often profoundly different. In desalinated seas one gets an impoverishment of the fauna and flora: characterized particularly by fewer species. Some groups of marine types such as starfish, brachiopods, and cephalopods may be entirely absent. In addition, those organisms that are present may be stunted e.g Mytilus edulis the edible mussel, grow to 110mm in the Western Balti, 50mm in Central Baltic; 27mm in the gulf of Finland and 21mm in the Gulf of Bothnia.

In salinated seas, such as part of the Caspian Sea, a very similar effect occurs. The number of species is reduced and at about a salinity of 5.0 to 5.5% all the larger organisms die off, and only a few Crustacea, bacteria remain. At a salinity of 6-7% precipitation of salts commence. At 15% Calcium Sulphate [CaSO₄.2HSO = gypsum); and Sodium Sulphate (Na₂SO₄10H2O = Mirabilite). At 24% one gets Sodium Chloride precipitated (NaCl).

The process-response models using the adaptive characteristics of organisms and the characteristic lithic components provides information that allows the facies characteristics of sedimentary environemnts to be ascertained. In general the post-depositional effects such as diagenesis and metamorphism do not negate against such determinations unless they are extreme.

Environmental determinations derive and evaluate evidence from a number of clearly defined sources.

Walthers Law plays a very important role in understanding the facies architecture of a rock sequence.

Sequence stratigraphy is an outgrowth of seismic stratigraphy and came about as a result of geologists attempting to derive stratigraphic information from a seismic record. Although Peter Vail is generally credited with the initial development of the discipline it came about as a result of the efforts of a number of scientists in the Exxon Production Research Company in Houston, Texas. Of particular note is the initial and original research work of Macombe Jervey, an LSU geology graduate, who developed the idea upon which later work was based.

The sequence stratigraphers believe that spatially distributed sets of seismic reflections can be grouped into depositional packets that represent chronostratigraphic units. These units are called depositional sequences and system tracts, and contain predictable lithofacies. Depositional sequences are thought to correlate throughout a basin and often globally, because they are controlled by short-term eustatic sea level changes superimposed on long-term tectonic effects [Vail,198?]. They are bounded by unconformities and correlative conformable strata. The depositional sequences are divided into system tracts which occupy definite positions in the sequence and have characteristic stacking patterns of parasequence subsets and parasequences bounded by marine flooding surfaces [Van Wagoner et al., 1987]. The fundamental point is that if the depositional sequences and system tracts can be recognized on seismic lines then important palaeogeographic information can be derived of relevance to understanding the historical geology of an area. In petroleum exploration this relates to the search for source rocks, conduit beds, seals, and reservoirs; and, to the understanding of migrational history. From the viewpoint of classical historical geology sequence stratigraphy is the extension of WALTHERS LAW to three-dimensions. The sequences form in response to the interaction among eustasy, subsidence, sediment supply and accommodation.

DEFINITIONS. Van Wagoner et al., 1987 provide a glossary of sequence stratigraphy terminology.

UNCONFORMITY. "A surface separating younger from older strata, along which there is evidence of subaerial erosional truncation ...or subaerial exposure, with a significant hiatus indicated."

CONFORMITY. "A surface separating younger from older strata, along which there is no evidence of erosion (either subaerial or submarine), or non-deposition, and along which no significant hiatus is indicated."

MARINE-FLOODING SURFACE. "A surface separating younger from older strata, across which there is evidence of an abrupt increase in water depth."

PARASEQUENCE. This is "a relatively conformable succession of genetically related beds or bedsets bounded by marine-flooding surfaces and their correlative surfaces (Van Wagoner, 1985). Parasequences are progradational and therefore the beds within parasequences shoal upward."

PARASEQUENCE SET. These are "a succession of genetically related parasequences which form a distinctive stacking pattern that is bounded, in many cases, by major marine-flooding surfaces and their correlative surfaces (Van Wagoner, 1985)." The stacking patterns may be progradational, retrogradational, or aggradational. They my be coincidental with sequence or system tract boundaries.

SEQUENCE. "Relatively conformable succession of genetically related strata bounded by unconformities and their correlative conformities [Mitchum, 1977]. Two type of conformable sequences are recognized in the rock record. They are both bounded by regional subareally exposed surfaces.

The Palaeozoic Era began with a new framework of continental seas and geosynclines but these were controlled essentially by the Pre-Cambrian framework. The continent was essentially centered on the Canadian Shield and the geosynclines were marginal to the continental plate and shield area. The framework of North America for the Lower Palaeozoic (Cambrian, Ordovician and Silurian) has as the main feature a vast platform lying between the exposed Pre-Cambrian rocks and the inner edge of the geosynclines. The tectonic framework consists of two main geosynclines the WESTERN CORDILLERAN and the EATERN APPALACHIAN, with a minor NORTHERN FRANKLINIAN GEOSYNCLINE. In each of these geosynclines the outer magmatic EUGEOSYNCLINE and the inner non-magmatic MIOGEOSYNCLINE can be recognized. The early Cambrian seas were confined to the peripheral geosynclines and most of the plate and shield was continent.

The earliest Cambrian seas were in great contrast to those of the present day. There was only a relatively small number of types of organisms around: these consisted of the primitive representatives of the Coelenterata (Corals), Brachiopoda, Porifera (Sponges), Gastropoda (Snails) and of primitive arthropoda (see slide of Middle Cambrian sea floor over Bohemia and Middle Cambrian sea floor for North America.) Paleontologically the most significant oganisms in the Cambrian were the trilobites: these were both abundant and diverse. They arose through an explosive burst at the beginning of the Cambrian: and thereafter diminished in number of types until by the end of the Palaeozoic hey became extinct. In the lower Palaeozoic they are extremely used for strata parallelization. Most trilobites were small but some grew to the size of about 1 foot: they inhabited many environments: burrowers, swimmers, vagrant benthos etc. By the Silurian Period the whole general fauna had changed and we have the incoming of many new types of corals, Brahciopoda, Gastropoda, trilobites, ancestral Octopi (Cephalopoda) and stalked echinoids. The corals in particular were important. Two of the major groups of corals the TABULATE CORALS and the RUGOSE CORALS actually arise in the Silurian. They built extensive coral reefs i much the same form as the present day. The major difference was that the Silurian reefs had far more algal remains associated with the corals than do the present day reefs. The Brachiopoda also had a period of high living during the Silurian: they increased greatly in number and in form. From the beginning of the Ordivician through the Silurian, they are prominent and often whole graveyards composed mainly of Brachiopoda are found. Cepholopoda were prominent and also primitive echinoids. All the organisms that we have so far mentioned lived in the near shore shelly carbonate environment. The graptolites are the main representative of the deeper parts of the geosynclines. These lived attached to floating seaweed or had air filled bladders which kept them afloat. A few lived attached to the sea bottom. The floating forms were fairly cosmopolitan and because of their fairly rapid evolution are often used in strata parallelization. Although the floating forms lived only during the periods of Early Ordovician to Late Silurian they are used wherever they occur to date the strata. Rare groups of lower Palaeozoic forms include the Eurypterids: relatives of the trilobites but which grew to 10 feet in length. They were the fierce predators of the lower Palaeozoic.

The Upper Palaeozoic includes the Devonian, Carboniferous (Mississippian and Pennsylvanian) and Permian: covering the period of approximately 400-225 million years B. P. The shrinking of the Late Silurian seas produced great unconformities over which the Devonian sea transgressed. The geosynclines were still very much the same at the beginning of the upper Palaeozoic: and eastern, southern, western and northern geosyncline.

ORGANIC REEFS ARE ALSO COMMON in the Devonian of Canada.

The sedimentary deposits of the Louisiana detrital plain have been of varying interest to scientists and engineers since the initial mapping of the deltaic region by Captain Talcott [1839] and intensely studied for over sixty years. The facies architecture and chronostratigraphic framework was established through a series of projects between 1930 and 1960 [Trowbridge, 1930; Russell & Howe, 1935; Howe et al, 1935; Russell, 1936; Russell & Russell, 1939; Fisk, 1944, 1947, 1948, 1952, 1960, 1961; Fisk et al, 1954; McIntire, 1958; Kolb & Van Lopik, 1958; Gould & McFarlan, 1959; Scruton, 1960]. Scruton presented the first conceptual framework for delta evolution from contruction to destruction. Subsequent work principally centered at Louisiana State University revealed details of the lithofacies and biofacies characteristics of the system and began with the work of Coleman and Gagliano who put the delta switching mechanism within a sedimentological framework [Coleman et al, 1964; Coleman & Gagliano, 1964; Frazier, 1967, 1974; Coleman & Wright, 1975; Coleman, 1966; Morgan & Shaver, 1970; Coleman et al, 1974; Hart, 1979; Coleman & Prior, 1980; Van Heerden & Roberts, 1980; Penland & Boyd, 1981; Wells & Kemp; 1981; Wells & Roberts, 1981; Bouma et al, 1985, 1986; Van Heerden & Roberts, 1988; Coleman, 1988; Coleman & Roberts, 1988 a, b]. The most recent phase of study, largely spear-headed by the Louisiana Geological Survey, attempted to understand the detrital plain in terms of sequence stratigraphy [Suter, Berryhill & Penland, 1987; Penland, Boyd, and Suter, 1988; Coleman, 1988; Van Heerden & Roberts, 1988; Boyd, Suter & Penland, 1989; Penland et al, 1991; McBride, Penland & Mestayer, 1990; Penland, 1991; Hart, 1991 a, b]. Building upon this historic record the Louisiana detrital plain now can be viewed within a chronostratigraphic framework.

The Gulf of Mexico Basin has received sediments from the Mississippi River drainage system since the late Jurassic Period [Worzel & Burke, 1978], producing a combined thickness of Mesozoic and Cenozoic sediments of over 15 km [Martin & Bouma, 1978, Bouma et al, 1978]. Sediment loading, salt and shale diapirism, and sea level fluctuations have all interacted with detrital progradation to produce vertical and lateral deltaic sequences the reflect not only the shifting depo-center but also the phase of sea level change [text figure 1]. Over Coastal Louisiana, despite the changing global and local regressions and transgressions, overall sediment progradation, rapid shoreline advancement, and relative sea-level drop has occurred. The average progradation during the Cenozoic was 5-6 km per million years [Coleman et al, 1989]. During the Quaternary alone some 3,600 meters of sediments accumulated on the shelf, and 3,000 meters in the deep basin [Mississippi Fan].

The sediments of the northern Gulf of Mexico deposited during the latest depositional sequence [since late Wisconsinian] are primarily influenced by the last sea level raise and fall. Thus the events of interest commenced some 18,000 years ago [during lowstand] when sea level was 60-120 meters below present amsl [Bloom, 1983]. During periods of rising sea level and highstand condensed sections were formed that provide excellent chronostratigraphic markers covering large areas of the continental shelf and upper slope. Calcareous-rich deposits including hemipelagics and shell hashes also were deposited. During lowstand sediment thicknesses vary, with sections of expanded section [rapid sedimentation], coarse grained detritals, and well defined depositional trends. The essential deposition framework is shown in figure 2.

Boyd et al [1989] outlined the present sequence stratigraphic framework for the Louisiana detrital plain. This view is a product of intense work during the last few years [Penland et al, 1988; Penland, 1991] and attempts to fit the sequences into the model illustrated in text figure 3.

Text figure 4 is a cross-section through the detrital plain showing the sequence stratigraphic relationships. The sequence starts with the Wisconsinian low stand systems tract consisting of the most recent lobe of the Mississippi Fan deposited during 10-12 ka and 22-25 ka by mass movement processes channelling through the incised Mississippi Canyon [Bouma et al, 1986; Mazzulo, 1986; Coleman et al 1983]. This sequence is over 400 meters thick and extend downslope over 500 km [Bouma et al., 1985]. Relative sea level was -130 meters [Berryhill, 1986] producing the shelf edge deltas off the exposed continental shelf [Berryhill & Suter, 1986]. An incised lowstand surface was formed at about 18 ka and is a Type 1 unconformity [Boyd et al, 1989] across the Pleistocene Prairie terrace. The characteristics of this unconformable surface were described by Fisk & McFarlan [1955].

Coastal onlap occurred as sea level rose between 18-9 ka to -20 meters when sedimentary infilling of the Mississippi Canyon with more than 600 meters of sediment took place [Coleman et al., 1983]. The river system expanded beyond the Canyon walls during the 9-3.5 ka period and developed shallow water, back-stepping, transgressive systems tract deltas [Outer Shoal, Maringouin, and Teche] on the middle and outer continental shelf. Accommodation space was occasionally filled as evidenced by the development of deltaic plains but in general the transgression was maintained and the back-stepping delta deposits were reworked [Frazier, 1967, 1974; Penland et al, 1988]. Boyd et al [1989] noted that this resulted in a "retrogradational parasequence set and defines a transgressive systems tract ..and ... the outer shelf region received little sediment supply after regression and constitutes a condensed section".

The final phase was the development of the deltas formed during the highstand system tract [St Bernard, Lafourche, Plaquamines-Balize, and the Atchafalaya]. These progradational deltaic parasequences average 10-50 meters thick [Frazier, 1967; Penland et al., 1988] and extend some 200 km down slope. The highstand saw maximum flooding stage forming the coastline at the entrance to the Mississippi Alluvial Valley [the Teche Shoreline]. The progradational deltaic parasequences of the St Bernard [4.6-1.8 ka], Lafourche [3.5-0.4 ka], Balize [1.0 ka], and Atchafalaya [initiated circa 1952] expanded over 150 km to the south-east [Frazier, 1967]. The modern depositional environments are part of the transgressive and highstand systems tracts [text-figure 3]. Sea level is believed to have drop a couple of meters during this progradational phase.

Standstill occurred during the two phases of transgressive systems tract development resulting in a more extensive revinement surface than is normally present at the top of an abandoned delta lobe. This allowed Penland et al, 19XX, to identify an early and a late Holocene deltaic plain sequence existing over central coastal Louisiana. Effectively the depo-center was switching further to the east.

Coleman [1988] most recently summarized the overall pattern of deltaic plain development. Depo-center switching takes place about every 1,500 years. Each delta covers an average area of 30,000 sq km and has an average thickness of some 35 meters. The Mississippii River has formed six major delta complexes and at least 18 individual deltas during the last 7,000 years [Frazier, 1967]. In noting that the shifting sites of sedimentation result in overlapping and laterally displaced deltaic sequences extending 400 km along depositional strike and 200 km along depositional dip, with sediment thickness ranging from only a few tens of meters to a maximum of 200m Coleman made a very pertinent observation relevant to sequence stratigraphy. This is that the size, scale and temporal relationship of the various facies make it difficult, if not impossible, to use biostratigraphic or radiometric age dating techniques that can resolve such differences in the subsurface. Essentially, this helps to define the resolution of sequence stratigraphy at the basinal level. Stratigraphic resolution is 10,000 to 20,000 years for the late Pleistocene and 40,000 to 50,000 years for the early Pleistocene [Williams & Trainor, 1986]. Resolution within such limits must be based upon detailed facies correlations.

The Balize Delta has already finished it's major progradational stage and the Mississippi River has begun to switch to form the Atchafalaya Delta. The Corp of Engineers has attempted to control the switching. The extensive interdistributary bays that exist between the main distributary channels are important sites of deltaic deposition. Initially receiving fine grained argillaceous material by overbank flooding these regions will form extensive mud-flats if the process is uninterrupted. However, in most cases the levess are cut by crevasses and a splay deposit results. On a large scale this results in the formation of a subdelta [six of which exist for the present Balize Delta]. The smaller splay are rarely active for more than 15 years by which time they have filled-in the local bay area and the crevasse is finally choked-off. Cubits Gap subdelta, West Bay subdelta, and Garden Island Bay subdelta are well understood subdeltas.

The low density fresh water [1.0 g/cc] overrides the higher density sea water [1.028 g/cc] and forms a visible offshore plume [Wright & Coleman, 1974]. Gravity and hydraulic sorting results in the classical coarse to fine grain size further down-plume from the river mouths [figure ..]. Coleman [1988] notes the distributary mouth bar at South West Pass has prograded 17 km in 200 years producing a sand body some 8-10 km wide, 17 km long, and in excess of 80 m thick. The finer grained offshore deposits have high sedimentation rates and in parts of the continental shelf [from as shallow as 5 meters] and continental slope this has led to unstable conditions and gravity induced mass movement [on slopes less than 2^o. In recent years the whole of the delta front has been mapped from side-scan sonar and high resolution geophysical techniques and it is now apparent that the whole area is scarred by mass movement indicating that the most dynamic part of the delta is actually the subaqueous portion.

Currently relative sea level raise for the Balize Delta is more than 400 cm per century [Swanson & Thurlow, 1973]; and, for the highstand systems tract deltaic plain an average of 55 cm per century over the last 7,000 years [Penland & Boyd, 1986]. The highstand system tract consists of four progradational deltaic parasequences.

The present physiography of the detrital plain shows that each deltaic parasequence is in a different stage of development. In classical geomorphological terminology the Atchafalaya is in the stage of Youth, the Balize is in the stage of Maturity, the Lafourche in Old Age and the St. Bernard in Late Old Age. In litho-facies terms the Atchafalaya is forming argillaceous - arenaceous delta front - distributary mouth bar sediments, prograding onto bay deposits; the Plaquamine-Balize provide a full spectrum of main sequence fluvio-deltaic facies; the present-day Lafourche is a bay-marsh-swamp organic rich argillaceous facies; and, the St. Bernard is a deteriorating marsh dominated by reworking of delta front arenaceous deposits.

The Atchafalaya delta is a modern Bay Head Delta [Van Heerden & Roberts, 1988], which became a subaqueous delta in 1952 [Morgan et al., 1953; Shlemon, 1972] and developed subaerial expression in 1973. At it's present stage of development it is comparable with a subdelta of the Balize Delta. Current activity involves distributary channel elongation and bifurcation and channel abandonment with associated lobe fusion. Small crevassing in the form of narrow overbank channels supply sediment to the interior of the sediment lobes formed between second order channels. The growth sequence for the Atchafalaya Delta are shown in text figure X-449.

The present depositional environments are similar to those present in a subdelta and include distributary mouth bars, distal bars, distributary channels, levees, and algal flats. An oyster reef forms the delta front.

The distribution of lithofacies are shown in Text figures X-446 and X-447.

Shell hash layers and back-bar algal flats are obvious major biofacies. Detailed biofacies analysis is lacking on this delta.

Vibro-core studies from 5 lines are the basis of subsurface information on the delta. The basal sediments are bluish-gray clay deposited as bay sediments. The initial prodelta deposits are a brownish-gray clay [up to 1.5 meters thick] with oyster shells hash layers [3-8 cm thick]. The upper prodelta clays are parallel laminated similar to those occurring off the front of the modern Mississippi River.

The Bayou Petit Caillou subdelta was responsible for the sedimentation in the Isle Derniers region. The isles formed with the abandonment of this part of the Lafourche delta forming a barrier about 32 km long, and 0.5 - 2.0 km wide. The chain consists of four small islands separated by tidal inlets [text-figure X-210]. Distributary channels [4-5 meter thick] underlie the island are associated with interdistributary and beach facies overlain by the barrier [up to 5 meters thick] and lagoonal facies [1-2 meters thick].

The main Lafourche Delta was formed over the Caminada-Moreau coastal area. Since the abandonment of the delta shore-face erosion has moved sand to the flanking barrier islands [Grande Isle to the east and Timbalier Isle to the west. Large tidal inlets separate the islands at Barataria Pass, Caminada Pass, Little Pass Timbalier, and Cat Island Pass. Lagoons are formed by Barataria Bay, Caminada Bay and Timbalier Bay. Ebb-tidal delta off the passes a up to 6 km long and 8 km wide [Penland et al., 1988].

The largest barrier island system of the detrital plain is associated with the reworking of the distributary channel and mouth bar sands of the Saint Bernard deltaic complex. The chain is over 45 miles [75 km] long and the individual islands up to 1.5 miles wide. They have been moving landward for at least the last 100 years [Penland et al., 1985], retreating over a thick [up to 7 meters] sequence of lagoonal deposits. Because of the dominant south-east wave energy the sediments migrate northward. Smaller islands are to the south [12-15 feet of sand] and larger to the north [15-30 feet of sand]. Submergence is causing a diminishing sediment supply and, especially to the south, the destruction of the barrier islands to leave remnant inner-shelf shoals. The original source of the sand are buried distributary channels and river mouth bars and lie on the lower shoreface and inner shelf and extend seaward under the thin and discontinuous central and southern Chandeleur Islands. Distributary channels form a veneer 50 feet thick and 1,300 feet wide [Suter et al., 1988]. and extend up to 8 miles seaward of the central barrier islands. A broad sand sheet consisting of gently offshore dipping beds occurs under the ravinement surface.

Tidal channels, probably cut by hurricanes, are important associated environments. Offshore of the barrier islands are [assumed] submerged beach-ridge environments, about 20 feet thick and extending several miles offshore, identified as high angle clinoform reflectors.

The thickness of barrier island sediments relative to the location of the revinement surface determines whether or not the sands will be preserved. If the transgressed barrier shoreline sediment package lies above the advancing revinement surface, the entire sequence is truncated [Penland, Suter, and Boyd, 1985]. The mechanism of formation of the Barrier Island Chains from the deterioration of a delta was outlined by Penland and Boyd, 1981; and Suter et al, 1985 and is depicted in text-figures X-201 and X-206].