Key features of Chordates
dorsal hollow nerve cord
pharyngeal gills slits
post anal tail
an elongate, rod-like, skeletal structure dorsal to the gut tube and ventral to the nerve cord. The notochord should not be confused with the backbone or vertebral column of most adult vertebrates. The notochord appears early in embryogeny and plays an important role in promoting or organizing the embryonic development of nearby structures. In most adult chordates, the notochord disappears or becomes highly modified. In some non-vertebrate chordates and fishes the notochord persists as a laterally flexible but incompressible skeletal rod that prevents telescopic collapse of the body during swimming.
Nerve cord
develops dorsally in the body as a hollow tube above the notochord. In most species, it differentiates in embryogeny into the brain anteriorly and spinal cord that runs through the trunk and tail. Together the brain and spinal cord are the central nervous system to which peripheral sensory and motor nerves connect.
Visceral clefts and arches/ pharyngeal or gill
located in the pharyngeal part of the digestive tract behind the oral cavity and anterior to the esophagus. The visceral clefts appear as several pairs of pouches that push outward from the lateral walls of the pharynx eventually to reach the surface to form the clefts. Thus the clefts are continuous, slit-like passages connecting the pharynx to the exterior.
The soft and skeletal tissues between adjacent clefts are the visceral arches. The embryonic fate of the clefts and slits varies greatly depending on the taxonomic subgroup. In many of the non-vertebrate chordates, such as tunicates and cephalochordates, the clefts and arches are elaborated as straining devices concerned with capture of small food particles from water.
In typical fish-like vertebrates and juvenile amphibians the walls of the pharyngeal clefts develop into gills that are organs of gas exchange between the water and blood.
In adult amphibians and the amniote tetrapods (= reptiles, birds and mammals) the anteriormost cleft transforms into the auditory (Eustachian) tube and middle ear chamber, whereas the other clefts disappear after making some important contributions to glands and lymphatic tissues in the throat region.
The skeleton and muscles of the visceral arches are the source of a great diversity of adult structures in the vertebrates. For example, in humans (and other mammals) visceral arch derivatives include the jaw and facial muscles, the embryonic cartilaginous skeleton of the lower jaw, the alisphenoid bone in the side wall of the braincase, the three middle ear ossicles (malleus, incus and stapes), the skeleton and some musculature of the tongue, the skeleton and muscles of the larynx, and the cartilaginous tracheal rings.
This is a small, very unusual subphylum of creatures commonly called lancelets or amphioxus. These animals are fish-like in appearance, but are invertebrates with a notochord, and a nerve cord right above it. They lack bones, a brain, eyes, and most other organs associated with the brain. There are 25 species.
This is a small, very unusual subphylum of creatures commonly called lancelets or amphioxus. These animals are fish-like in appearance, but are invertebrates with a notochord, and a nerve cord right above it. They lack bones, a brain, eyes, and most other organs associated with the brain. There are 25 species.
Cephalochordata (lancelets)
This is a large subphylum of unusual invertebrates that do not look like anything much more than a strange underwater worm or mushroom. They start off life as tadpole-like larvae with notochords and all the rest. This stage lasts only a short time, after which they anchor to the seabed and live a sedentary life. They completely change shape at this point, and it is hard to believe that they are in the same phylum as humans. The adults lack the notochord but do keep the pharyngeal slits. They have a highly-developed internal structure, with a heart and other organs. Tunicates are named for their protective covering, known as a tunic. This tunic is made up of cellulose, which is very rare in animals. There are 2000 species in 4 classes:
Appendicularia or Larvacea (free swimming tunicates) Ascidiaceae (sea squirts) Sorberacea (benthic tunicates) Thaliacea (salps)
This is the largest subphylum with the more well-known animals, including humans, reptiles, fish, etc. Every animal with a backbone is present in this subphylum. The notochord is developed at an early age, and is replaced with vertebrate. All vertebrates have a skeleton of either bone or cartilage. Their brain is protected by a boney cranium, and consists of three parts. They all have well-developed hearts with 2-4 chambers and have a closed circulatory system. There are 41700 species in 8 classes:
Amphibia (frogs, salamanders) Aves (birds) Cephalaspidomorphi (Lampreys) Chondrichthyes (cartilaginous fish) Mammalia (mammals) Myxini (Hagfish) Osteichthyes (bony fish) Reptilia (crocodiles, snakes, turtles)
Chephalachordata: Lancelet
Lancelets are usually about 1 in. (2.5 cm) long, with transparent bodies tapered at both ends. There is no distinct head and no paired fins. Lancelets are filter feeders and live in shallow marine waters; they can swim through water or wet sand, but are usually found buried in the sand with only the mouth end projecting. Small food particles enter the pharynx through the mouth and are filtered out as the water exits through the gill slits. Respiration probably occurs mostly through the skin. The use of the gill slits for feeding rather than respiration is characteristic of the lower chordates (see tunicate ). The lancelet has a dorsal notochord, or stiffening rod, extending from tip to tail, that gives it its characteristic pointed shape. It retains the notochord as the major skeletal support throughout life; in vertebrates the notochord is surrounded and usually replaced by a vertebral column during embryonic development. In the lancelet there is a nerve cord above the notochord, but no brain and no eyes. A ventral blood vessel carries the colorless blood; there is no heart. It is thought that vertebrates evolved from ancestors similar to lancelets.
Unochordata: Sea Squirts
The Ascidiacea are the Sea Squirts or Tunicates and they make up the bulk of the species found within the Urochordata.
They are all sessile (non-moving or staying in one place) as adults. Most species are common coastal animals occurring in rock pools and out into deeper water to about 400 meters depth, though there are species which have been found living ad depths up to 5,000 meters. They can be either solitary or colonial and the colonial species may share a common exhalent siphon.
Many species are translucent or whitish in color but some species are much more colorful and can be red, brown, yellow and even blue. The name Tunicates arises from the existence of the tunic.
The larvae resemble tadpoles and are far more obvious members of the phylum Chordata than the adults. The larvae swim towards light and thus the surface of the sea at first then after a short while they reverse direction and swim down towards the sea floor, often in less than one day.
Tunicate larvae do not feed and are essentially a dispersal form. Soon they find a suitable spot on the sea floor and the settle in a head down, tail up position. They attach themselves to the sea floor (substrate) using special adhesive glands in the front of their head. Once settled they undergo an amazing metamorphosis during which the symbols of the phylum Chordata, the post-anal tail and the notochord it contained are lost. The remainder of the body twists through 180 degrees in order to become a small tunicate. Most tunicates are thought to live about one year as adults.
Vertebral column replaces notochord
highly cephalized well developed sense organs and brain
usually paired appendages
closed circulatory system:heart, arteries, and veins
sexes seperate in most vertebrates
Evolution of Vertebrata
The most primitive of the vertebrates evident in the fossil record are the jawless Ostracoderms. From this ancestral form two lines of vertebrates evolved. A jawless group, the Agnathans, and a group that developed jaws, the Gnathostomatans. The modern representatives of the Agnathans are the hagfish and the lampreys.
The first jawed fishes, the Placoderms, are extinct but from this primitive ancestor, arose the present day cartilaginous and bony fish. The cartilaginous fishes, Chondrichthyes, include the sharks, skates and rays. The bony fishes, or Osteichthyes, include two groups, the ray finned and the fleshy finned fishes. The ray-finned fishes represent the majority of the present fish. The flesh finned fishes, the lung fish and the lobe finned fish, are a small almost extinct group that is believed to have provided the ancestral stock for the remainder of the vertebrates.
The lobe finned or lungfish gave rise to a once dominant, but now marginally successful group of terrestrial animals, the amphibians. The amphibians were the ancestors of the reptiles, and highly successful birds and mammals.
Agnatha: Lampfrey and Hagfish
Jawless vertebrates
carilaginous skeleton
lack paired appendages
notochord persist throughout life
The most primitive of the vertebrates evident in the fossil record are the jawless Ostracoderms. From this ancestral form two lines of vertebrates evolved. A jawless group, the Agnathans, and a group that developed jaws, the Gnathostomatans. The modern representatives of the Agnathans are the hagfish and the lampreys.
The first jawed fishes, the Placoderms, are extinct but from this primitive ancestor, arose the present day cartilaginous and bony fish. The cartilaginous fishes, Chondrichthyes, include the sharks, skates and rays. The bony fishes, or Osteichthyes, include two groups, the ray finned and the fleshy finned fishes. The ray-finned fishes represent the majority of the present fish. The flesh finned fishes, the lung fish and the lobe finned fish, are a small almost extinct group that is believed to have provided the ancestral stock for the remainder of the vertebrates.
The lobe finned or lungfish gave rise to a once dominant, but now marginally successful group of terrestrial animals, the amphibians. The amphibians were the ancestors of the reptiles, and highly successful birds and mammals.
Chrondrichthyes: Sharks, Skates, and Rays
Carilaginous fishes
Bony Teeth
leathery protective skin
gills (some must swim to circulate water)
internal fertilization
eggs or live birth
Sharks, skates, rays, and even stranger fish make up the Chondrichthyes, or "cartilaginous fish." First appearing on Earth almost 450 million years ago, cartilaginous fish today include both fearsome predators and harmless mollusc-eaters (harmless, that is, unless you are a mollusc). A number of shark and ray species are fished, commercially or for sport.
Members of the Chondrichthyes have a skeleton made of cartilage. Only their teeth, and sometimes their vertebrae, are calcified; this calcified cartilage has a different structure from that of true bone. Thus, preservation of the whole body of a cartilaginous fish only takes place under special conditions.

Not only does this class have internal fertilization and a reproduction strategy that reminds about what is seen in amniotes, they have also a relative brain development of its major divisions which reminds about what is found in birds and mammals. Their relative brain weight comes close to that of mammals, and is about ten times of bony fishes at the same size. There are not surprisingly some exceptions; the bony fishes mormyrids have a relative brain size to be compared with the brain size of humans, while the primitive Megamouth have a brain of only 0.002 percent of its body weight.
Digestive system - Their digestive systems are unique due to spiral valves, and with the exception of Holocephali they also have a cloaca

Because they don't have any bone marrow, the RBC must be produced somewhere else. The spleen and special tissue around the gonads is where we can find the production of red blood cells, as well as a special organ called Leydig's Organ and is only found in cartilaginous fishes, even if some have lost it. Another unique organ is named epigonal organ, and has probably a role in the immune system.
A spiracle is found behind each eye on most species. Their tough skin is covered with dermal teeth, also called placoid scales or dermal denticles, making it feel like sandpaper.
Characteristics of Sharks
1 . Sharks typically have a fusiform body (rounded and tapering at both ends). This body shape reduces drag and requires a minimum of energy to swim.
2. Sharks are generally drably countershaded. Countershading is a type of camouflage in which the dorsal side is darker than the ventral side. The result is that predators or prey do not see a contrast between the countershaded animal and the environment.
3. Fins - Fins are rigid, supported by cartilaginous rods. Sharks have five different types of fins.
a. Paired pectoral fins lift the shark as it swims.
b. Paired pelvic fins stabilize the shark.
c. One or two dorsal fins stabilize the shark. In some species, dorsal fins have spines.
d. A single anal fin provides stability in species where it is present; not all sharks have an anal fin.
e. The caudal fin propels the shark.
4. Head.
a. Eyes -are lateral on sharksSome species have an eyelidlike structure called a nictitating membrane. The nictitating membrane protects the eye from being injured by thrashing prey while the shark is feeding.
b. Nostrils - Sharks have ventral external nostrils. Some species have nasal barbers, sensory projections near the nostril
c. Mouth. - the mouth is usually ventral. The mouth may have labial folds or furrows. Teeth are modified, enlarged placoid scales. Sharks have numerous rows of teeth attached at their bases by connective tissue. Several rows of replacement teeth continually develop behind the outer row(s) of functional teeth. As the functional teeth fall out, replacement teeth take their place. Some species of sharks may shed as many as 30,000 teeth in a lifetime.
5. Gill slits - Sharks have five to seven pairs of lateral gill slits.
6. Spiracles - Some species of elasmobranchs have small openings called spiracles behind the eyes at the top of the head. These openings bring oxygen-carrying water into the gill chamber. Spiracles originate from rudimentary first gill slits and are reduced or absent in active, fast-swimming sharks.
7. Scales -Sharks have placoid scales, also called dermal denticles Placoid scales have the same structure as a tooth, consisting of three layers: an outer layer of vitro-dentine (an enamel), dentine, and a pulp cavity. Placoid scales are arranged in a regular pattern in sharks and an irregular pattern in batoids.
Unlike other types of scales, placoid scales do not get larger as the fish grows. Instead, the fish grows more scales.
- Like teeth, the shape of the scales is variable among species and can be used to identify the species.
- Placoid scales gave rise to teeth, stingrays'spines, and the dorsal spines of horn sharks (Heterodontus spp.) and dogfishes (family Squalidae).
- As a shark or batoid swims, placoid scales may create a series of vortices or whirlpools behind each scale. This enables a shark to swim efficiently.
- European cabinetmakers used the rough skin of a shark as sandpaper, called shagreen. With the denticles removed, shark skin is also used for leather.
Osteichthyes: Ray-finned and Fleshy-finned Fish
bony skeleton and jaws
swim bladder
lateral line system
leathery protective skin or scales
external fertilization
Bony fish (Osteichthyes) appeared at the same time as cartilaginous fish. They are the largest group, with over 29,000 species. These fish have an organ called a swim bladder, which gives the animal buoyancy and in some cases may assist in respiration. An additional autapomorphy of Osteichthyes is the operculum, a pair of flaps covering the gills which can be moved to provide oxygen to a fish even when it is remaining still. Other characteristics of bony fish include paired fins, dermal scales, numerous vertebrae, and many teeth.
Characteristics of Ray-finned Fish
This brilliant, spiny, and poisonous lionfish is just one of the world's 20,000+ living species of actinopterygians. The name means "ray-finned," for unlike the Chondrichthyes, the fins of the Actinopterygii are webs of skin supported by bony or horny spines. Most actinopterygians have complex skeletons of true bone (sturgeons and paddlefishes are exceptions).
Ray-finned fishes are the dominant aquatic vertebrates today, making up about half of all vertebrate species known. They are found in every aquatic habitat from the abyssal depths of the ocean to freshwater streams and ponds; a few can even crawl on land for short periods of time. Ray-finned fishes constitute a major source of food for millions of people.
Characteristics of Lobe-finned Fish
The Sarcopterygii are the lobe-finned fish. Tetrapoda evolved from sarcopterygian fish.
These early lobe-fins were fast-swimmers with a heterocercal tail, meaning that the tail fin was asymmetrical and larger on the dorsal side. In the living lobe-finned fish (lungfish and coelacanths) the tail is now symmetric, though the fossil record shows that this change happened independently in the two groups.
One of the most important characteristics of lobe-finned fish is the lobe in their fins. Unlike other fish, sarcopterygian fish has a central appendage in their fins containing many bones and muscles. The fins are very flexible and potentially useful for supporting the body on land, as in lungfish and tetrapods.
Additionally, the Sarcopterygii is known for having enamel on the teeth.
Amphibia: frogs, toads, salamanders
Must spend part of their early development in water
limbs extend laterally
lungs and skin for gas exchange (moist skin)
external fertilization
metamorphosis found in some species
Some of the physical features of amphibians, like the scales of gymnophions, suggest their fish ancestry. Other characteristics are more clearly related to those of their descendants--the reptiles, birds, and mammals. Amphibians are unlike fishes in that most types have limbs instead of fins and generally breathe through lungs and skin instead of through gills. Unlike reptiles, amphibians lack a scaly or armored covering and take in water and oxygen through their skin. Amphibians have developed in many different ways in order to survive in areas with widely varying climates, dangers, and food sources.
Most amphibians are relatively small animals. Except for the salamander of Japan, the giants among them became extinct long ago. They vary in length from less than 2/5 inch (1 centimeter) to over 60 inches (150 centimeters). The West African Goliath frog grows to more than 1 foot (30 centimeters) in length and may weigh as much as a full-grown house cat. Most of the species have four limbs. The hands generally end in four fingers, and the feet in five toes. Although the limbless gymnophions crawl, most amphibians with legs move by jumping, climbing, or running.
The skulls are usually flat and wide, and the teeth, which grow in the jawbones and roof of the mouth, lack roots and are replaced intermittently. Amphibians do not chew with these teeth. They use their long, flexible tongues to capture their prey, which they then swallow whole.
The moist, supple skin of most amphibians provides protection and absorbs water and oxygen. The upper skin layer, called the epidermis, is regularly shed in a process called molting. The skin usually comes off in one piece and is then eaten by the animal.
The lower skin layer, called the dermis, of the typical amphibian often includes mucous and poison glands. The mucous glands help provide essential moisture to the body. The protective poison glands are quite often located in different places on different species--by the ears in certain toads, and behind the eyes of salamanders. These glands produce poisons that are toxic to natural enemies, such as birds and small mammals, but that rarely harm humans.
These glandular secretions give some amphibians distinct odors. The spotted salamander and the common toad smell of vanilla. Some frogs smell of onion, and the fire-bellied toad smells of garlic.
The skin's protective properties include the ability to change color so that the animal can hide when an enemy is nearby. Certain cells under the skin alter the color so that the amphibian can blend into its surroundings. Sometimes parts of the skin become brightly colored. The amphibian displays these colors to enemies to warn them to keep away.
The sense organs vary greatly, depending on the order and the species. The eyes are virtually useless in underground amphibians but are well-developed in other species. The sense of smell is generally good. Hearing ability varies according to the species. Some amphibians also have pores on their bodies, called lateral line organs, that are sensitive to vibrations in the water.
Amphibian Life cycle
For the purpose of reproduction most amphibians are bound to fresh water. A few tolerate brackish water, but there are no true sea water amphibians.
Several species have also adapted to arid and semi-arid environments, but most of them still need water to lay their eggs.
The larvae (tadpoles or polliwogs) breathe with exterior gills. After hatching, they start to transform gradually into the adult's appearance. Typically, the animals then leave the water and become terrestrial adults, but there are many interesting exceptions to this general way of reproduction.
The most obvious part of the amphibian metamorphosis is the formation of four legs in order to support the body on land. But there are several other changes:
The gills are replaced by respiratory organs
The skin changes and develops glands to avoid dehydration
The eyes get eyelids and adapt to vision outside the water
An eardrum is developed to lock the middle ear
In frogs and toads, the tail disappears
Reptillia: lizards, snakes, turtles, alligators, crocodiles
internal fertilization
deeper skull
long and flexible neck
scales to prevent water loss
the shelled amniote egg
efficient lungs and circulatory system
further development of limbs and skeleton
1. Reptiles have tough, dry, scaly skin offering protection against desiccation and physical injury. The skin consists of a thin epidermis, shed 3'l periodically, and a much thicker, well-developed dermis (Figure 3a). The dermis is provided with chromatophores, the colour-bearing cells that give many lizards and snakes their colourful patterns.
The characteristic scales of reptiles are formed largely of keratin. Scales are derived mostly from the epidermis; they are not homologous to fish scales, which are bony, dermal structures. In some reptiles, such as alligators, the scales remain throughout life, growing gradually to replace wear. In others, such as snakes and lizards, new scales grow beneath the old, which are shed at intervals. Turtles add new layers of keratin under the old layers of the platelike scutes, which are modified scales. In snakes the old skin (epidermis and scales) is turned inside out when discarded; lizards split out of the old skin leaving it mostly intact and right side out or it may slough off in pieces.
2. The shelled (amniotic) egg of reptiles contains food and protective membranes for supporting embryonic development on land. Reptiles lay their eggs in sheltered locations on land. The young hatch as lung- breathing juveniles rather than as aquatic larvae. The appearance of the 34- shelled egg (Figure 3b) widened the division between the evolving amphibians and reptiles and, probably more than any other adaptation, contributed to the evolutionary establishment of reptiles.
3. Reptilian jaws are efficient for crushing or gripping prey. The jaws of fish and amphibians are designed for quick jaw closure, but once the prey is seized, little static force can be applied. In reptiles jaw muscles became larger, longer, and arranged for much better mechanical advantage.
4. Reptiles have some form of copulatory organ, permitting internal fertilization. Internal fertilization is obviously a requirement for a shelled egg, because the sperm must reach the egg before the egg is enclosed. The glandular walls of the oviducts secrete albumin (source of amino acids, minerals, and water for the embryo) and shells for the large eggs.
5. Reptiles have a more efficient circulatory system and higher blood pressure than amphibians. In all reptiles the right atrium, which receives unoxygenated blood from the body, is completely partitioned from the left atrium, which receives oxygenated blood from the lungs. Crocodilians have two completely separated ventricles (Figure ); in other reptiles the ventricle is incompletely separated. Even in reptiles with incomplete separation of the ventricles, flow patterns within the heart prevent admixture of pulmonary (oxygenated} and systemic (unoxygenated} blood; all reptiles therefore have two functionally separate circulations.
6. Reptilian lungs are better developed than those of amphibians. Reptiles depend almost exclusively on lungs for gas exchange, supplemented by respiration through the pharyngeal membranes in some aquatic turtles. Reptiles suck air into the lungs by enlarging the pleural cavity, either by expanding the rib cage (snakes and lizards} or by movement of internal organs (turtles and crocodilians}. Reptiles have no muscular diaphragm, a structure found only in mammals. Cutaneous respiration (gas exchange across the skin), so important to amphibians, has been completely abandoned by reptiles.
7. Reptiles have efficient water conservation. All amniotes have a metanephric kidney which is drained by its own passageway, the ureter. However, the nephrons of the reptilian metanephros lack the specialized intermediate section of the tubule, the loop of Henle that enables the kidney to concentrate solutes in the urine. To remove salts from the blood, many reptiles have salt glands located near the nose or eyes (in the tongue of saltwater crocodiles) which secrete a salty fluid that is strongly hyperosmotic to the body fluids. Nitrogenous wastes are excreted by the kidney as uric acid, rather than urea or ammonia. Uric acid has a low solubility and precipitates out of solution readily, allowing water to be conserved; the urine of many reptiles is a semisolid suspension.
8. All reptiles, except the limbless members, have better body support than the amphibians and more efficiently designed limbs for travel on land. Nevertheless, most mode~ reptiles walk with their legs splayed outward and their belly close to the ground. Most dinosaurs, however, (and some modern lizards) walked on upright legs held beneath the body, the best arrangement for rapid movement and for the support of body weight. Many dinosaurs walked on powerful hindlimbs alone.
9. The reptilian nervous system is considerably more complex than the amphibian system. Although the reptile's brain is small, the cerebrum is larger relative to the rest of the brain. Connections to the central nervous ( system are more advanced, permitting complex kinds of behaviour unknown in amphibians. With the exception of hearing, sense organs in general are well developed. Jacobson's organ, a specialized olfactory chamber present in many tetrapods, is highly developed in lizards and snakes. Odours are carried to Jacobson's organ by the tongue
Amniote Egg
Animals have been laying eggs for millions of years; snails, fish, and many other critters produce eggs from which their young hatch. The egg of the chicken is a special kind of egg. It has a shell to help prevent drying, and a series of membranes that surround the developing chick.

This kind of egg is unique to the amniotes, a group that includes turtles, lizards, birds, dinosaurs, and mammals The last name in that list, the mammals, may have surprised you since most mammals do not lay eggs, but the earliest mammals laid eggs, and a few, such as the monotremes, still do
Inside the egg are a series of fluid-filled membranes which permit the embryo to survive: the amnion, allantois, yolk sac, and chorion. Surrounding and protecting the embryo is the amnion, filled with amniotic fluid, and providing the embryo with a stable fluid environment.
The allantois performs two very important functions for the embryo, providing for gas diffusion, and removal of wastes. Food for the developing embryo comes from the yolk sac, which reduces in size as the embryo matures. Surrounding all the other membranes is the chorion, providing an overall enclosure for the young.
Around the chorion is the albumin, or "white" of the egg, and an outer shell protects the whole egg, preventing drying while still permitting air to reach the embryo. An air space, visible at the left of the above diagram, provides an extra internal buffer for environmental conditions
Age of Reptiles
The mesozic age is also known as the age of the reptiles because dinosaurs were the predominant animal
most of those species became extinct about 65 million years ago after a mass extinction
However, one lineage of dinosaurs became the birds, and an earlier lineage led to the mammals, who also became more abundant after the age of reptiles.
Other reptiles became the current orders, which include Chelonia, the turtles and tortoises, Crocodilia, the alligators and crocodiles, Squamata and Sauria, the snakes, and Rhynchocephalia, assorted other reptiles.
Aves: Birds
Feather and scales
air sacs
keeled sternum
toothless beak
large muscular gizzard
hollow bones
efficient lungs and hearts
Class Aves consists of all of the different birds. Birds are characterized by having feathers, forelimbs without claws that have evolved into wings, and a lack of teeth.

All parts of birds are modified to fly. For instance, the feathers of birds have hollow shafts and they have few vertebrae in the tail, both adaptations for less weight.
Connecting barbules in the feather provide and strong breast muscles provide rigidity and strength needed for flight. However, each bird has wings specified for a different task. Some birds flap repeatedly to remain in flight, while others are adapted to soaring on wind currents.
It is believed that birds evolved from two-legged reptiles 150-200 million years ago. Amniotic eggs, scales on the legs, keratin toenails, and even feathers (which evolved from scales) are parts of the bird that evolved from reptiles.
A main difference between birds and reptiles is that birds are endothermic, while reptiles are ectothermic. This means that birds heat themselves, while reptiles obtain heat from their external environment. Birds are easily able to heat themselves because of a high rate of metabolism. There are currently 27 orders of birds, with about 9,000 species.
Adaptations for Flight
centralization of weight
efficient metabolism
visual acuity
motor development of brain
Flight muscles (red fibers)
Skeletal - keeled sternum, fused bones, pygostyle, uncinate process, furcula
Flight muscles contain red fibers that have an extraordinary capacity for sustained work

The skeleton is strong and lightweight
-A large keeled sternum (or breastbone) houses the flight muscles and empowers the wings
-The bones of the hand and fused and thus reduced to better support and maneuver the powerful primary flight feathers
-the pygostyle, made of fused taile vertebrea, supports and controls tail feathers (used for breaking and steering)
-Horizontal rib projections, the uncinate processes, overlap other ribs and strengthen the thorax to withstand flight
The furcula, wishbone, compresses and rebounds like a spring in rhythm with the beat of the wings

Birds produce large external eggs, the most elaborate reproductive cells of any animal. Using eggs for reproduction, is another important adaptation for flight.
Avian Diversity
300 billion birds of 9648 species inhabit the earth
range in size from 2 to a 1000 grams
2,031 species in 23 orders are found in North America
Mammalia: Mammals
4 chambered heart
7 cervical vertebrae (neck bones)
Most are viviparous though some are oviparous
teeth embedded in the jaw bone
Well developed brain
Possess hair which is made of keratin. The evolution of mammalian keratin is believed to be independent of reptilian keratin. Hair provides insulation . Endothermic. The majority of the heat energy is used to maintain their high body temperature. 4 chambered heart. Mammary glands are used to produce milk to nourish their young. Female glands are the only functional glands. The diaphragm is a muscle that separates the thoracic cavity from the abdominal cavity. 7 cervical vertebrae (neck bones) are present in most mammals. Most are viviparous though some are oviparous. An extended gestation period (uterine development) is common in most placental mammals. Teeth are imbedded in the jaw bone and come in a variety of forms. Well developed brain.

Although mammals share several features in common (see Physical Description and Systematics and Taxonomic History), Mammalia contains a vast diversity of forms. The smallest mammals are found among the shrews and bats, and can weigh as little as 3 grams. The largest mammal, and indeed the largest animal to ever inhabit the planet, is the blue whale, which can weigh 160 metric tons (160,000 kg). Thus, there is a 53 million-fold difference in mass between the largest and smallest mammals! Mammals have evolved to exploit a large variety of ecological niches and life history strategies and, in concert, have evolved numerous adaptations to take advantage of different lifestyles. For example, mammals that fly, glide, swim, run, burrow, or jump have evolved morphologies that allow them to locomote efficiently; mammals have evolved a wide variety of forms to perform a wide variety of functions.
Mammalia: Monotremes
Characterized by the duckbilled platypus and the spiny anteaters, lay eggs and maintain some reptilian characteristics. They do not contain true mammary glands, but produce a fatty sweat (milk) from glands in the skin. The milk collects and drips down tufts of hair into the offspring's mouth. They are found in Australia and New Guinea.
Mammaila: Marsupials
These mammals contain a pouch (marsupium). Opossums,koalas,kangaroos, and other examples live in Australia as a result of the break up of the super continent Pangea. The young are born during the early stages of development. The new born crawls up to the mother's pouch, where it clings on to a nipple and hangs there until it fully develops.
Mammalia: Placental Mammals
These mammals are the most abundant and diverse of the class. The placenta, a reproductive structure, is housed in the uterus of the female. Here the offspring receives all that it needs to develop into a fully developed organism. This period of development (gestation) varies with the species of mammal. Whales gestate for 2 years, while a mouse develops in 21 days.