Success of vertebrates

Introduction

Vertebrate mean backbone and every species in the vertebrate classes has a backbone. Phylum chordatata contains the most familiar species, which includes humans. All chordates have several things in common that occurs in some stage of their development. They have pharynged slits, which are openings that connect the inside of the throat to the outside of the neck. These are often used as gills, and are only present in humans when they are at the early stage of development (foetus). A tail is also present, which extends past the anal opening. The main feature is the notochord, which is a rod that supports the nerve cord and this is present in all species. The nerve cord is a bundle of nerve fibres which connects the brain to every muscle and organ in the body. These nerve fibres are used to send messages to organs and muscles from the brain. In most species these features disappear with age. There are about 44.000 species in three subphylum’s groups (Matthew Morris15/05/2003).

Vertebrata is the largest subphylum with the more well known animals such as: mammals, reptiles, fish, aves, amphibians. Every animal with a back bone is present in vertebrate subphylum. All vertebrates have a skeleton of either bone or cartilage and there brain is protected by a bony cranium which consists of three parts. They have well developed hearts with three or four chambers and have a closed circulatory system. There are 41700 species in eight different groups and they are as follows: Amphibia (frogs, salanders), Aves (birds), Cephalaspidomorph (lamprey), chondrichthyes (hag fish), osteichthyes (bony fish) and reptilian (crocodiles, snakes, turtles).

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Agnatha which are also known as jawless fish is a lower class vertebrate and the best representative for this vertebrate is the marine lamprey (petromyzon). This fish is eel like in its appearance, but much more primitive in its structure than true eels, which are more developed bony fish. The lamprey body is very soft and scale less and its skeleton consist of just cartilage (it lacks bone completely). There are no traces of paired fins and most of all it is completely jawless. The lampreys rounded mouth cup forms an adhesive disc, which it uses to attach to other fish that it preys on as a blood sucker. It has a rough tongue like structure in the mouth that is good substitute for having no jaw bone. There is just one nostril opening which is situated on top of its head, and having a hypophsial pouch combined with it. The gills passages in typical fish are slits, but in the lamprey they are rounded pouches, which are connected by narrow tubes with the pharynx and body surface.

Cartilaginous Fishes (Chondrichthyes) date back to the Devonian period and fossels that were found resembled sharks. The animals of today are made up of about eight hundred species which include; sharks, skates and rays. The cartilaginous fishes got their name from the fact that their skeleton is made of cartilage, not bone. With their gills exposed to sea water, all marine fishes are faced with the problem of conserving body water because Sea water is about 3.5% salt, which is over 3 times that of vertebrate’s blood. The cartilaginous fishes solve that problem by maintaining a high concentration (2.5%) of urea in their blood (which is far higher than the 0.02% of other vertebrates) that is in osmotic balance with sea water. This ability develops late in embryo, so the eggs of these species cannot simply be released in the sea, but there are two solutions to this and they are: Enclose the egg in an impervious case filled with isotonic fluid before depositing it in the sea and, Retain the eggs and embryos within the mother’s body until they are capable of coping with the marine environment. Both these solutions require internal fertilization and the cartilaginous fishes were the first vertebrates to develop this. The pelvic fins of the male are modified for depositing sperm in the reproductive tract of the female.

Bony Fishes (Osteichthyes) as the name indicates their skeletons are made of bone and they are divided into two groups which are: ray-finned fishes (Actinopterygii) and lobe-finned fishes (Sarcopterygii). Ray-finned fishes have thin fins that are supported by the spine and there are around thirty thousand species. the only Lobe-finned fishes that are still around today are one possibly two species called coelacanth, which were thought to be extinct and several species of lungfish that are found in Africa, South America, and Australia.In additions to gills, these fishes had a pair of pouched outgrowths from the pharynx which served as lungs. They were inflated with air taken in through the mouth and may have provided a backup gas exchange organ when the water became too warm and stagnant to carry enough dissolved oxygen. Their kidneys were adapted for the hypotonic environment in which they lived. These animals diversified through the remainder of the Devonian period (which is often called the “Age of Fishes”). Some migrated to the oceans. In this more stable environment, their lungs became transformed into a swim bladder with which they could alter buoyancy. Their kidneys became transformed as well adapting them to their new hypertonic surroundings. The nostrils of bony fishes open only to the outside and are used for smelling. Some of the lobe-finned fishes developed internal openings to their nostrils. This made it possible to breath air with the mouth closed as modern lungfishes do. These rare modern lobe-finned fishes are the sole survivors of once-flourishing groups that also gave rise to the tetrapods — the four-legged vertebrates. In the Devonian (perhaps as early as 395 million years ago), the paired fins of some sarcopterygians moved under the body and developed limbs (complete with digits). This enabled them to venture out on land. So once again, evolution was opportunistic giving rise to the first land vertebrates, the amphibians.

Amphibians in its class included all cold blooded species that are in between the evolutionary development of fishes and reptiles. They Include 4 living species which are the frogs, toads, salamanders and newts. Amphibians were the first vertebrates to move from a water habitat to a earthly one, and they are also the ancestors of all reptiles, birds, and mammals. Although there are a few species that live their lives in water, most spend a lot of time on land. Most Amphibians are species of the class Amphibia and are vertebrates easily recognised by their ability to live on land and water. amphibians have an aquatic larval, or tadpole stage that metamorphises into an adult. Amphibians are believed to have evolved from either the lobe-fin fishes (Crossopterygii) or the lungfishes (Dipnoi). These fishes had an advantage over other fishes by the fact that they had lungs. They could breathe on land which ment that when there was a shortage of water they could use there fins to pull themselves onto to land to search for another water source and in time they became less dependant on water.

Reptiles came into being about 315 million years ago, when amphibians developed two unique features, which were; skin and an egg covering that helped stop water loss and this is how they evolved into reptiles. These two adaptations allowed reptiles to become a dominant life form on land, as where amphibians are dependant on water to survive. The main success for reptiles was being able to produce offspring on land. Most reptiles lay eggs that are hard and brittle, or covered with paper like shell. Some reptiles like snakes and lizards are ovoviviparous, which means they give birth to living offspring that hhatch from an egg from inside the female and then she gives birth.

Birds have been termed glorified reptiles but are treated as a different class (aves). They are a far removed species from the general reptilian group because from that group there was a flying species called the pterosaurs. The aves are not descended from pterosaurs, they are descendants from another flying species called archosaur which had feathers instead of membrane. In birds we see a group of vertebrates that in a lot of ways is not considered a high level class of species like mammals. Birds can be trained but seem relatively much less capable of learning by experience than mammals. On the other hand they show innate behaviour patterns of a complexity unknown to mammals. A lot of these patterns are related to social behaviour for example, courtship, nest building and rearing there young.

Mammals are vertebrates that have hair, a four-chambered heart and mammary glands (sweat glands), which is where the name mammal came from because they are the only animals that have sweat glands. Mammals first came about 200 million years ago during the Jurassic Period and there are about five thousand four hundred living species of mammals today that differ greatly in size, form and adaptations. Mammals inhabit every country and have occupied a wide variety of places, which include grasslands,

wetlands, scrublands, seas and oceans, below ground, forests, mountaintops, Polar Regions and deserts. Mammals range in size from the minute bumblebee bat which measures a mere three centimeters in length, to the massive blue whale, which can measure up to 33 meters from head to tail, which makes it the largest animal alive today. Although mammal’s species vary in form, they do share some rare characteristics and they are; that their lower jaw bone which carries the teeth attaches directly to the skull. In other vertebrates, the jaw bone is one of multiple bones that does not attach directly to the skull. Mammals also have a unique arrangement of three bones, which are; the incus, malleus and stapes, that located in the middle of the ear. These bones turn sound vibrations into neural impulses. Two of these bones, the incus and malleus, originated as bones within the jaw. Another feature which is unique to mammals is that they have two lumps on the base of the skull (known as a double occipital condyle) which are used to hold the skull in the top neck vertebra, but in other vertebrates, the base of the skull has only a single lump. Hair is also unique trait to mammals because no other animals in the other class of vertebrates have true hair and all mammals have hair covering at least some part of their body at some time during their life. Hair grows from skin cells called follicles and it is made of a protein called keratin. Hair serves many functions which differ in different animals and they are to insulate, to conceal, to signal, to protect, and to sense the immediate surroundings. Insulation is to keep heat in the body, but it also helps to protect the body from too much heat as in the case of diurnal desert animals such as the camel. The coloring of hair on animals helps them to conceal themselves from predators or prey because some animal’s fur matches their habitat. Hair also provides by its color a means of signaling other members of one’s own species (e.g., the white tail of the white-tailed deer, flashed by a fleeing animal to signal danger) or members of other species like the skunk which has a big white stripe down its back which is warning to predators. The hair also serves to protect the skin from abrasion and from excessive UV radiation.

The success of vertebrates lies in the evolution and adaptation which helped vertebrates to survive on land, in water and in different climates all around the world.

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