THE CHORDATES
CHAPTER SEVEN
CHAPTER SUMMARY
I. Hemichordata
II. Chordata
1. Tunicata, Cephalochordata, Vertebrata
2. Agnatha
3. Placoderma
IV. The Reptiles
V. The Birds
VI. The Mammals
VII. The Primates
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:
Upper 2i - 1c - 2pm - 3m
Lower 2i - 1c - 2pm - 3m
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 Hinged Jaw
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.
Superclass Pisces (fishes)
Superclass Tetrapoda (amphibians, reptiles, birds, mammals).
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 [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 Placoderms.
Acanthodians (spiny sharks) originated in the Silurian Period. Some of these were armored but most had scales.
Arthrodires (true Placoderms) 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 Placoderma 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, moreso 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.
1. Smooth, thin and usually moist skin. [i.e. no scales, feathers, or fur].
2. Feet are usually webbed and the toes are soft and lack claws.
3. Immature forms are vegetarian; adults are usually carnivorous.
4. Eggs laid in moist places and fertilized externally as soon as lain.
5. Respiration is by gills, lung and through the skin.
6. Immature forms have a two chambered heart; adults have a three chambered heart (2 auricles in parallel and one ventricle).
7. Pair of well developed nasal passages leading to the throat (to improve breathing).
8. Strengthened pectoral and pelvic structures to support the body weight. The bones were enlarged and improved for the attachment of powerful link muscles.
9. Pectoral girdle is free from an attachment to the skull thus permitting movement of the head independently of the body.
10. Changes in spinal column to a flexible yet sturdy series of interlocking bones with a series of modifications for muscle attachments. The earliest amphibians (the Ichthyostegans) had a spinal column similar to the Crossopterygians. An important aspect in understanding the evolution of the amphibians are the changes in the spinal column.
11. Changes in the sense organs. Fish sense sound in liquid media because vibrations are readily transmitted in water and received by the lateral line of sensor receptors. In amphibians the media is a gas [air] and there was a adaptive need to transmit sound to the inner ear. A bone (the stapes) which originally was part of a gill arch and subsequently became a connector of the jaw to the cranium was modified as a sound transmitter.
12. Eyes required modification because they were no longer continually immersed in water. Thus eyelids developed and also a mechanism of lubricating the eyes.
13. A sense of smell developed. The parts of the brain connected with association moved forward toward the olfactory organs (smell). Associated with this was an increase in the size of the bones in the front part of the skull and a decrease in those in the rear of the skull.
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.
1. Scales
2. Dry skin, thickened to prevent dehydration
3. If limbs are present they have claws.
4. Well developed lungs
5. Partial division of ventricle which separates oxygenated blood from de-oxygenated blood - this is complete in the crocodiles [as it is in the birds and mammals].
6. Some of the bone structures are different from the amphibians e.g. shape of ribs, vertebrae, pelvic region and the skull.
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, cannines, 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 bipedal 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.
1. The Ornithischian dinosaurs have a pelvic structure similar to the birds. They were plant eaters, the forward teeth were lost and a beak developed to chop vegetation. With the group quadra-pedal forms developed e.g. Stegosaurus as did form with severe bulk e.g. Ankylosaurus [designed like a tank] and Triceratops.
2. The Saurian dinosaurs which enveloped along two main lines. The carnivorous bipeds [Theropods e.g. Tyrannosaurus, Megalosaurus. The herbivorous quadrupeds that appeared at beginning of Jurassic Period. These were long necked, long tailed, large reptiles. They reverted to quadripedalism to support their weight e.g. Brontosaurus (60' long and weighed more than 30 tons). Brachiosaurus weighed up to 100 tons and probably consumed 500 lbs. of food per day! Apatosaurus, Diplodocus and Cetiosaurus were similar forms.
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 pterosaurs e.g. Rhamphorhynchus and the short tailed pterosaurs e.g. Pterandon [with a 25 foot wing span but a weight of only 40 lbs.]
The Euripsida 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. Plesiosarus [some had a skull 9' long], Macroplata, Ichthyosaurus.
N.B. Jesus Cristo Laganto [clocked at 33 feet in 4 seconds]
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.
1. Feathers
2. Light, porous bones
3. Forelimbs specialized as wings.
4. Body supported by two hind limbs
5. Beak
6. 4-chambered heart
7. Warm-blooded
8. Amniotic egg encased in a lime-shell.
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:
1. Monotremes
2. Marsupials
3. Placentals
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.
The Monotremes
There are only three Monotreme species still live today.
1. The highly specialized stream dwelling Duck Billed Platypus.
2. Two species of Spiny Anteaters.
They represent an evolutionary level of the mammal-like reptiles but they are not considered ancestral to the other mammals.
The Marsupials
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.
The Placentals
There are at least 28 orders of Placental mammals that evolved principally during the Cenozoic Era. These can be grouped in 4 types.
1. Clawed placental mammals. e.g. bear
2. Fish-like placental mammals. e.g. whales and porpoises
3. Hoofed placental mammals. e.g. elk
4. Nailed placental mammals. e.g. primates
Among the clawed types are included the orders:
1. Insectivore e.g. shrew
2. Chiroptera e.g. bat
3. Carnivora e.g. dog, cat
4. Edentate e.g. sloth
5. Rodentia e.g. rat, squirrel
6. Cetacea e.g. fish-like forms
The Insectivore 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 Insectivore [insect eaters] found in the rocks of the Cretaceous System. From this group the mammalian hoard arose. Today's Insectivore 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 insectivores. Bats probably evolved from arboreal insectivores and their earliest fossils are found in rocks of the Eocene Epoch. They are the only mammals to achieve true flight. The ant eaters [Edentates] 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.
1. Longer limbs for running down prey.
2. Large brain case to accommodate elaboration of the cerebral cortex.
3. Hunting tactics that use packs and strategies to exhaust their prey. Wolves for example station members along the route they intend to drive their prey.
4. They communicate - an adaptation necessary when one is in rapid pursuit of food.
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.
1. Stalking cats e.g. Saber tooth tigers
2. Biting cats e.g. true cats.
The Cetacea are of interest in their back-evolution to the marine environment.
The Plant Eaters
Hoofed animals [Ungulates] evolved from one of the earliest explosive developments from the Insectivore 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 Perissodactyl]. The commonest and most widespread of living mammals belong to the even toed ungulates [Artiodactyl]. Basically these can be divided into two groups: the Pigs and the Ruminants. The pig group includes the following.
1. Giant Hogs: straight backs and long slender legs adapted to running
2. Pigs Proper: forest dwellers
3. Hippopotamus: late comers in the evolutionary tree.
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.
Primate Characteristics
1. Placental mammals with enlarged mammary glands in female and a pendulous penis and scrotum in males.
2. Eye orbits encircled by bone.
3. Three kinds of teeth, at least during one period of growth.
4. Brain always possesses a posterior lobe.
5. Caecum is well developed (blind pouch).
6. Four limbs each bear five digits with flattened nails.
7. Innermost digits of at least one pair of extremities are opposable.
8. The only truly distinctive feature of the primates that differentiates them form all other mammals is the tendency of the growth, development and enlargement of the brain.
Suborders
1. Prosimii
2. Anthropoidea
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 Anthropoidea
The Suborder Anthropoidea is divided into two Infra-orders.
1. Platyrrhini which has one Super Family the Ceboidea or New World Monkey e.g. Vakari (cat sized animal from the Amazon), Marmoset, and the Squirrel Monkey.
2. Catarrhini which has two Super Families
A. Cercopithecoidea or Old World Monkey. This includes the Snow Monkey, Indian Langur, Mandrill, and Barbary Ape.
B. Hominoidea or Apes. These include the Orangutan, and Gorilla.
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.
1. Erect walking posture. The upright posture involves automatically the development of certain other skeletal features such as the basin-like pelvis in which the viscera are supported and the specialization of the hind limbs for bipedal locomotion.
2. Enlargement of brain. Generally brain size of the Hominoidea is 80-1475 cc and the APES are never greater than 650 cc. In humans a brain size of less than 900 cc is an idiot or imbecile.
3. Articulate speech. Speech as in the humans can only develop with the use of a large mouth cavity. Thus the jaw of man is quite different from that of apes.
4. Hominidae have the face shortened, and expansion of brain. Pongidae has the face longer and a smaller brain.
5. Hominidae have the rows of cheek teeth tending to diverge posteriorly. The Pongidae have the rows of cheek teeth parallel
6. Hominidae have the canine teeth shorter. The Pongidae have enlarged canines
7. The Hominidae are distinctly Bipedal. The Pongidae usually have a brachiating posture.
8. The Hominidae have legs longer than arms. The Pongidae have legs shorter than arms
9. The Hominidae have a big toe that is not opposable. The Pongidae have big toes opposable