Bio 211 (DIVERSITY OF CHORDATE)

    Bio 211 (DIVERSITY OF CHORDATE) LECTURE NOTE FIRST SEMESTER   

2023/2024 SESSION

                                                    CONCEPT OF CHORDATES

The phylum chordate consist of animals with a flexible rod supporting their dorsal or back sides, the phylum name derives from Greek word CHORD means String. Most species within these phylum are vertebrates. However all vertebrate are chordate but not all chordate are vertebrates, because some chordate lack back bone but they have NOTOCHORD e.g. lancelets and tunicates.

The phylum derive its name from one of the common characteristics  of this group namely notochord (from Greek Noton means back  and from Latin Chorda meaning cord) .Animals belonging to all other phyla of animal kingdom are termed the non-chordate or invertebrates , since they have neither notochord nor back bone in their body. Chordates are of primary interest because human beings are members of this group. 

FOUR UNIQUECHARACTERISTICS OF CHORDATES

i.  Notochord: It is from this structure the phylum takes its name. The notochord lies between the dorsal nerve chord and the digestive tract. In some chordates it is a major supporting structure, while in vertebrate the notochord is present only in the embryo.

ii. Dorsal Hollow Nerve Cord: This runs along the top or dorsal side of the animal. In advanced vertebrate the nerve cord become differentiated into the brain and thee spinal cord

iii. Pharyngeal pouches or slits: These structures are located in the pharynx. The pharynx is a tube that joins the mouth to the digestive and respiratory tract. The pharyngeal slits are used to filter food particles from the water

iv.  Post anal tail: All chordate has post-anal- tail that are found opposite the head and extends down the anus.

Note that in some chordates all the four traits persists throughout life and serve important functions. However, in many chordates including human all the four traits are present only during embryonic stage

GENERAL CHARACTERISTICS OF CHORDATES

1. Chordates are the best known group of animals with thousands of species

2. They consist of both vertebrate and invertebrate and range in length from about a centimeter to over thirty (30) meter

3. They are bilaterally symmetrical with segmented body

4. Chordates have a complete digestive system and a closed circulatory system

5. Their nervous system is centralized with brain and spinal chord

CLASSIFICATION OF CHORDATES

1. Sub phyla Protochordata 

2. Sub phyla vertebrata

SUB PHYLA PROTOCORDATA:-They are the primitive chordates. They consist of three 3 major classes as follows;

i. Cephalochordata

ii. Urochordata

iii. Hemichordata

Cephalochordata (Head-notochord animals).

-They can be called lancelets or amphioxus.

-They are small, segmented, elongated and fish like chordates.

-They are filter feeders with numerous gill slits.

-The notochord extends from the head through the long axis of the body to the tail.

-Examples include Branchiostoma lanceolatum, B. bulcheri, B. californiense, Asymmetran lucayanum.  

Urochordata (Tail Notochord animals).

They are referred to as tunicates.

They possess notochord in the tail region.

They are filter feeders

Examples are sea squirts, tunicate of Salp species. 

Hemichordate (Half-Notochord animals).

They live in mud in the marine shallow water.

They are long ranging from 2cm – 1.5m.

Their notochord extend from the pharynx to the head region i.e half way of the length of the body.

They have well developed gill slits and are filter feeders.

Examples include Rhabdopleura, balanoglossus species.

2. SUB PHYLUM VERTEBRATA (Craniates);- They are refered to as the advanced chordates, they are divided into two(2) . super classes;

i. Super class Agnatha

ii. Super class Gnathostomata

Super class Agnatha (Jawless Vertebra):- Agnathans are the fish-like jawless vertebra with no scales. They have toothed round and sectorial mouth, mostly are parasitic, carnivorous and a lot are extinct.

Characteristics of Agnatha

They are jawless class of vertebrata

They do not possess teeth, paired appendages and exoskeleton.

Agnatha possess significant number of gill slits ranging from 7-14 pairs

The notochord persists into adult hood

Possession of 1 or 2 semi-circular ducts in the ear aiding for balance and orientation.

They lack specific genital ducts for reproductive purposes

They lack recognizable stomach in the digestive system

Examples include hagfish, lampreys 

Classification of the class Agnatha.

The super class agnatha can further be classified two (2) classes:

Class Cyclostomes (myxinoidea).

They can also be called hagfishes (myxins) with 65 species.

They are eel-shaped marine animals.

They possess skull but lack vertebral column.

Example hagfish (myxins)

Class Ostracoderm

They are the ancient and extinct group of heavily armoured fishes. 

Their body is covered with heavy bony scales.

Example Lampreys

(ii) SUPER CLASS GNATHOSTOMATA:- The Gnathostomata includes all vertebrates with upper and lower jaws.it comprises a wide range of animals, from fishes to the various tetrapod classes, which have in turn been derived from a fish or fish like ancestor. The old classification of the Gnathostomata comprises of five (5) major classes

Class of Pisces

Class of Amphibians

Class of Aves

Class of Reptilians

Class of Mammalians

1 CLASS PISCES (FISHES):-These are true, jawed vertebrates with specific organs for respiration, excretion and blood circulation. The organisms belonging to this class are poikilothermic, meaning that they cannot regulate their own body temperature. Essentially, all the fish come in this category.

                                                         Characteristics of Pisces

They are all aquatics

They possess fins for locomotion and balance in water.

Internal skeleton is bony or cartilaginous.

They possess gills for respiration, but in few cases lungs as in lung fishes

Heart is 2-chambered, S-shaped with one auricle and one ventricle (single circulation)

Their body is distributed into a head, trunk and tail.

Their body is covered with scales while others are scaleless.

The lateral line system functions as a sensory organ to sense the disturbances in the nearby environment.

They may be herbivores or carnivores, oviparous or ovoviviparous.

They reproduce sexually but fertilization external.

Classification of Pisces:-Pisces are classified into three subclasses:

Chondrichthyes

Osteichthyes

Placodermi (extinct)

I. Subclass Chondrichthyes (Cartilaginious fishes)

They are found only in the marine environment.

Endoskeleton is cartilaginous.

Gills are the respiratory organs.

Swim/ air-bladders absent. E.g., Sharks, Skates, Rays etc.

II. Subclass Osteichthyes (Bony fishes)

They are found in marine water, freshwater or brackish water.

They have a bony endoskeleton.

Gills are covered with an operculum.

Air bladders are present in most of them.

E.g. Catfish, Lungfish, Sunfish, Clownfish, Salmon etc.

2. CLASS AMPHIBIANS: - These are multicellular vertebrates that live both on land and water. This class includes about 3000 species. They are the first cold-blooded animals to have appeared on land.

                                            Characteristics of Amphibians

They can live both on land and in water.(dual life)

They have two pairs of limbs for locomotion.

They has three chambered of heart

The skin is smooth and rough without any scales, but with glands that make it moist.

Respiration by lungs and skin. Gills might be present externally in some adults.

They are sexually reproduction and fertilization is usually external. However, in salamanders, the fertilization is internal.

        E.g. Frogs, Toads and Salamanders.

Classification of class Amphibians:-The Amphibians are divided into three orders

I-Order Anura: - e.g. frogs and toads

II-Order Urodela (Caudata):- e.g. salamanders and newts

III:-Order Apoda (Gymnophiona):-They are the extinct amphibians without tail and limbs look like worms .e.g. Caecilians

3. CLASS REPTILIANS: - Reptiles are believed to have evolved from amphibians, they often considered to be the most primitive types of vertebrates to have completely adapted to terrestrial life

                                              Characteristics of Reptiles

They are cold-blooded animals and hibernate during cold season.

Some are aquatics while others are terrestrials.

Their skin is dry, and rough to prevent water loss.

They has homodont dentition

Heart is 3 chambered. However, crocodiles have a 4-chambered heart.

They reproduce sexually and fertilization is internal, 

They are oviparous animals ie the female lay eggs.

Some they possess two pairs of limbs, each bearing claws. Except Snakes.

They possess a typical cloaca.

Examples, Snakes, Turtles, Lizards, Crocodiles.

Classification of Reptiles:-Reptiles are classified into four (4) orders

I-Order Squamata:- e.g Lizards and Snakes

II-Order Crocodilia:- e.g Crocodiles and Alligators

III- Order Testudines:- e.g Turtles, Tortoise and Terrapins

Iv-Order Sphenodontida:- e.g Tuataras which is the only living species found in Newzealand that have parietal eye on top of head.

4. CLASS AVES (Birds) Aves are also called social animals as they can build nests, show territorial behavior, and engage in courtship.

                                               

                                                  Characteristics of Aves

Birds are warm-blooded animals.

Their forelimbs are modified into wings.

Their hind limbs are adapted for walking, hopping, perching, grasping, wading and swimming.

Heart is 4-chambered

Their body are covered with feathers except legs.

There is no skin gland except the oil gland.

The lower and upper and jaws are modified into a beak and have no teeth.

They have sharp eyesight.

The alimentary canal has a crop and a gizzard. The crops help in softening food, and the gizzard helps in crushing the food.

They have a single ovary and oviduct on the left side. E.g; pigeon, parrot, owls, sparrow, penguins, humming birds etc.

Classification of Aves:-The class Aves is divided into two subclasses:

I-Subclass Archaeornithes:- This class of birds are extinct. They had a toothed beak with a long lizard-like tail.Example Archaeopteryx 

II-Subclass Neornithes:- These include extant as well as extinct birds. They have no teeth and a short tail.Example, Penguin, Grey Heron, Kingfisher, Duck, etc

5. CLASS MAMMALIANS: - Mammals from Latin “mamma” meaning “breast”, are group of vertebrate animals constituting the class Mammalia. It’s estimated that there are more than 5,500 living species, they are incredibly diverse and can be found in every major habitat.

Three (3) unifying characteristics of mammals

possession of fur(hair)

production of milk by mammary glands

presence of three(3) middle ear bones (malleus, incus, and stapes)

Characteristics of mammals

They are warm-blooded animals 

They are viviparous ie they give birth to younger ones.

They has two pairs of limbs ie forelimbs and hind limbs.

They possess mammary glands.

They possess external ear

They possess fur/hair which almost covered the bodies.

They has heterodont, dentition i.e., possess different types of teeth adapted for various feeding habits, such as incisors, canines, premolars, and molars.

They respire through lungs and exhibit one of the most advanced forms of Diaphragms.

Heart is four-chambered. 

The brain is well developed divided into cerebrum, cerebellum and medulla and possess 12 pairs of cranial nerves.

Examples Human beings, lion, elephants, rat, cat, monkey, kangaroo etc.

Classification of Mammals: - Mammalia has the largest class in the animal kingdom. Based on their reproduction, they are classified into three subclasses:

Subclass Eutheria

Subclass Metatheria

Subclass Prototheria

i- Subclass Eutheria ((Placental):-Mammals under this subclass give birth to young ones. The young ones are developed inside the mother and derive nutrition through the placenta from the mother. It consists of 19 orders, few of which are:

Order

Examples of Species

Primates

Humans, monkeys, apes

Carnivora

Lions, tigers, dogs, cats

Rodentia

Mice, rats, squirrels

Cetacea

Whales, dolphins

Artiodactyla

Cows, deer, pigs

Perissodactyla

Horses, rhinos, zebras

Chiroptera

Bats

Lagomorpha

Rabbits, hares

Proboscidea

Elephants

ii- Subclass Metatheria (Marsupials):- Mammals belonging to this sub-class give birth to immature young ones, hence they stay in their mother’s pouch until they mature. For eg., Marsupials and Kangaroos. They are divided into seven different orders:

Order

Examples

Notoryctemorphia

Marsupial modes

Diprotodontia

Kangaroo

Microbiotheria

Colocolo

iii-Subclass Prototheria (Monotremes):-consists of egg-laying mammals. It has one order with species.

Order: Monotremata, example: Duckbilled platypus, Echidna

EVOLUTIONARY ADVANCEMENT, ADAPTIVE RADIATION, SUCCESS AND SPECIAL FEATURES OF INTEREST IN VARIOUS GROUPS

Adaptive radiation is the rapid diversification of species as they adapt to new condition. Over the course of evolution, the appearance of new adaptations such as jaws and paired appendages has launched adaptive radiations in chordates groups. Adaptive radiation can produce species that are similar in appearance and behavior but not closely related, a condition known as Convergent Evolution, which has produced flying vertebrates as different as birds and bats.

TRENDS IN THE EVOLUTION OF VERTEBRATES

A Shift from notochord to vertebral column

Nerve cord expanded into the brain

Evolution of jaws

Paired fins evolved which gave rise to limbs

Gills evolved which gave rise to lungs

                                            EVOLUTION OF PISCES

The first vertebrates to evolve is the bony fish. Before that there were jawless fish. The amphibians evolved next after fish and it is the first vertebrates to live on land.

                                       EVOLUTION OF AMPHIBIANS

Amphibians evolved from lobe-finned fishes during the early Devonian about 400 million years ago

They used their fins to travel over land from pool to pool and their lungs become more effective

Chambers of the heart become partially separated making circulation more efficient

Their lungs are less efficient than those of other vertebrates hence their skin serves as respiratory organ.

                                             

                                                  EVOLUTION OF REPTILES

Reptiles arose from amphibians in the carboniferous period 280-350 million years ago. Adaptation to life on land include: tough scaly skin, internal fertilization, amniotic egg, water conserving kidneys. Adaptive radiation in reptiles produced numerous lineages and extinct groups which include:

Therapsids (ancestors of mammals)

Marine Plesiosaurs and Icthyosaurs

Dinosaurs and Pterosaurs.

The living reptiles to the present day include:- Crocodilians, Turtles and Tortoise, Tuataras, Snakes and Lizards.

                                          EVOLUTION OF AVES (BIRDS)

Aves evolved from reptilian ancestors. Only birds have feathers and the feathers are highly modified reptilian scales. Aves produce amniotic egg and have four chambered heart with highly efficient respiratory system. They have. Powerful muscles attached to the keel. 

Evidence that Birds are reptiles is that;

Birds and Reptiles have scales

Birds and Reptiles have lay egg

Birds and Reptiles have many similar bones including hips, feet and toes.

                                          EVOLUTION OF MAMMALS

Mammals arose about 200 million years ago during the Triassic Synapsids (extinct reptiles) gave rise to Therapsids. The Therapsids were the reptilian ancestors of mammals. The first mammals have evolved by the Jurassic (extinct terrestrial reptiles from 190-135 million years ago) the special features of mammals include:

Hair

Mammary glands

Distinctive teeth

Highly developed brain

                 

                THREE GROUPS OF MAMMALS

Monotremes mammal: they are egg laying mammals

Marsupial mammals: they are pouched mammals. Their young are born in an undeveloped state and complete development in a permanent pouch in mother

Placental mammals: They are the most diverse mammalian group. The young developed in mothers’ uterus

TRANSITIONN FROM WATER TO LAND AND THE PROBLEM OF LAND LIFE IN AMPHIBIANS

Transition from water to land in Amphibians means significant changes to both external and internal features. Amphibians are group of vertebrate that has adapted to live in both water and on land.

The first tetra pods faced major problems in moving from the water on to the land. Air breathing was in fact not the key hurdle to cross, but rather weight and structural support. New modes of locomotion had to evolve, as well as new ways of feeding, of sensing prey and predators, of water balance and of reproduction.

When all these problems of life on land are considered^ may seem surprising that vertebrates ever left the protection of the water. One classic theory (e.g. Romer 1966) was that fishes moved on to land in order to escape from drying pools. The Devonian was supposedly a time of seasonal droughts, and the freshwater fishes probably found themselves often in stagnant and dwindling pools. The conclusion of this viewpoint is that terrestrial locomotion evolved as a means of staying in the water! This suggestion has been criticized as there is limited evidence for droughts, and because the theory would explain only moderate terrestrial adaptations, not the much-modified tetrapod limb for example. A simpler hypothesis is that vertebrates moved on to land because there was a rich and untapped supply of food there. Waterside plants and terrestrial invertebrates diversified in the Late Silurian and Devonian, and it was inevitable that some group of organisms would exploit them sooner or later.

THE DEVELOPMENT OF THE CLEIDOIC EGG IN THE AMNIOTA

The amniotic egg is a terrestrially adapted egg, characterized by a shell and extra-embryonic membranes. The amniotic egg is 'air-breathing', meaning that it exchanges gases with the environment, unlike more primitive egg types. Amniotic eggs were an important adaptation for land dwelling species, such as reptiles, birds and even some mammals. Species that use an amniotic egg are called amniotes.

                                                Diagram of Amniotic Egg 

An amniotic egg includes a shell and four extra- embryonic membranes:

Amnion

Yolk sac

Allantois

Chorion

The amnion: - is a membrane surrounding the embryo. It encases the embryo in amniotic fluid, which provides a stable environment for the embryo to develop. 

The yolk sac: - encloses the yolk, which provides nutrients for the developing embryo. As the embryo matures and uses the water and nutrients contained in the yolk, the yolk sac shrinks.

The allantois: - functions to remove nitrogenous waste from the embryo and facilitates gas exchange.

The chorion: - is the outermost membrane and encloses the embryo and other membranes. The chorion also helps facilitate gas exchange with the outer environment.

 The hard shell of the egg encases all of the membranes, embryo, and a white fluid called albumin. Albumin provides water and protein to the developing embryo, such as the egg whites of chicken eggs we are familiar with. The rigid layer of the shell prevents the embryo from drying out and also helps protect the embryo from mechanical stress in the environment.

The amniotic egg evolved over 300 million years ago and was one of the main distinctions between amphibians and higher animals, such as reptiles, birds and mammals. Amphibians were restricted to wet environments that could keep the eggs moist. With the evolution of the amniotic egg, animals were able to expand on land. Birds evolved a hard calcium carbonate shell on their egg and mammals evolved to eliminate the shelled egg entirely. Instead, mammals have evolved an internal reproduction system that has similar embryonic layers inside the mother as part of the uterus. First of all, organisms that use an amniotic egg are able to colonize more areas on land. Prior to the evolution of the amniotic egg, amphibians and fish were limited to wet environments that could support their eggs. With the development of the shell, organisms were no longer restricted and could explore more areas on land. This also reduced competition for small pools of water to lay eggs in and allowed for greater expansion of the vertebrate line. Without being tied to pools, eggs and larvae were also better equip to hide from predators and survive better. This created selective pressure for organisms that use amniotic eggs. Amniotic Egg Examples Let's look at some amniotic egg examples. Amniotic eggs evolved in organisms that developed after amphibians. There are three main groups of animals that use amniotic eggs:

Reptiles, Birds and Mammals.

Reptiles were the first amniotes and the first organisms to develop internal fertilization. Prior to reptiles, amphibians typically released sperm and egg into the water and depended on a wet environment to reproduce. Reptiles typically have leathery amniotic eggs. This gives reptiles more protection than amphibians, whose eggs lack a proper shell, but still requires them to keep their eggs in relatively moist environments. Examples of reptiles include snakes and lizards.

THE DOMINANCE OF MAMMALS OVER OTHER CHORDATES.

Animals belonging to class Mammalia are referred to as mammals. Mammals are one of the most evolved species in the animal kingdom categorized under vertebrata. They exhibit advanced characteristics which set them apart from all other animals. They are characterized by the presence of mammary glands through which they feed their younger ones. They are distributed worldwide and have adapted well to their surroundings from oceans, deserts and Polar Regions to rainforests and rivers etc. Let us have a detailed look at the characteristics and classifications of the animals belonging to the class Mammalia. Characteristics of Mammals following are a list of distinct characteristics of mammals that separates them from other classes:

1. Mammals are warm-blooded animals who give birth to their younger ones.

2. They are the most dominant form of animals found in almost all types of habitats.

3. They have mammary glands that help them produce milk to feed their younger ones

4. Their skin possesses oil glands (sebaceous glands) and sweat glands (sudoriferous glands).

5. The fur of hair throughout the body which helps animals adapt to their environment.

6. They are heterodont, i.e., possess different types of teeth.

7. Mammals also possess cervical vertebrae.

8. The mammals respire through lungs.

9. Mammals have a four-chambered heart.

10. The brain is well developed divided into cerebrum, cerebellum and medulla. 

11. The mammals can lay eggs also. They are known as viviparous.