In this article we will discuss about the examples of vertebrate chordates.
The first vertebrates were fishlike. Fishes are aquatic, gill-breathing vertebrates that usually have fins and skin covered with scales. The larval form of a modern-day lamprey, which looks like a lancelet, may resemble the first vertebrates- it has the three chordate characteristics (like the tunicate larva), as well as a two-chambered heart, a three-part brain, and other internal organs that are like those of vertebrates.
Small, jawless, and Finless ostracoderms were the earliest vertebrates. They were filter feeders, but probably were also able to move water through their gills by muscular action. Ostracoderms have been found as fossils from the Cambrian through Devonian periods, when the group finally went extinct. Although extant jawless fishes lack protection, many early jawless fishes had large defensive head shields.
1. Class Petromyzontiformes (Lampreys):
These long, eel-like, jawless fish are free-swimming predators on other fish. Lampreys hatch in freshwater and many live their lives entirely in freshwater. Some lampreys migrate to the sea, but must return to freshwater to reproduce. Lampreys have a sucker-like mouth that lacks a jaw.
2. Class Myxini (Hagfish):
Members of the class Myxini have a partial cranium (skull), but no vertebrae. Their skeleton is made of cartilage, as is that of sharks. Hagfish lack jaws, and for this reason used to be classified with the lampreys in a group called the Agnatha (“no jaws”) or the Cyclostomata (“round mouth”).
Fish-Vertebrates with Jaws:
The fish first appeared during the Cambrian Period. Whether fish first evolved in fresh or salt water is unclear from the fossil record. The jawless fish are the most primitive group, although they were a very important group during the Silurian and Devonian periods. Hagfish and lampreys are the only living members of this class today. They have long, cylindrical bodies with cartilage skeletons and no paired fins.
The first jawed fish were the Placoderms, an extinct group of Devonian-aged jawed fishes. Placoderms were armored with heavy plates and had strong jaws and paired pectoral and pelvic fins. Paired fins allow fish to balance and to maneuver well in water, which facilitate both predation and escape.
The evolution of jaws is an example of evolutionary modification of existing structures to perform new functions. Jaws are modified gill arches, and allowed the exploitation of new roles in the habitats: predators with powerful jaws. There are two classes of jawed fish- the cartilaginous fish and the bony fish.
3. Class Chondrichthyes (Cartilaginous Fish):
The class Chondrichthyes contains approximately 850 species of skates, rays, and sharks. They have jaws, lots of teeth, paired fins, and a cartilage endoskeleton. Cartilaginous fish first appeared during the Devonian Period and expanded in diversity during the Carboniferous and Permian before nearly disappearing during the great extinction that occurred near the end of the Permian. A large group of cartilaginous fish still survives today and is an important part of the marine fauna.
These fish have five to seven gill slits on both sides of the pharynx, and lack the gill covers found in bony fish. The chondrichthyian body is covered epidermal placoid (or toothlike) scales. Developmental studies show the teeth of sharks are enlarged scales.
The largest sharks are filter feeders. Basking and whale sharks eat tons of crustaceans (small krills, etc.) filtered from the water. Most sharks are fast- swimming, open-sea predators. The great white shark feeds on dolphins, sea lions and seals (and people sometimes). In other words, anything is wants to.
Rays and skates live on the ocean floor; their pectoral fins are enlarged into wing like fins; they swim slowly. Stingrays have a venomous spine. The electric ray family can feed on fish that have been stunned with electric shock of over 300 volts. Sawfish rays have a large anterior “saw” that they use to slash through schools of fish.
4. Class Osteichthyes (The Bony Fish):
There are about 20,000 species of bony fish, found both in marine and freshwater, comprising the class Osteichthyes. This class is divided into two groups- the lobe-finned (Sarcopterygii) and ray-finned fish (Actinopterygii). The bony fish have a bony skeleton. Most species in this class are ray-finned with thin, bony rays supporting the fins. A few fishes are lobe-finned and are thought to be related to the ancestors of amphibians.
a. Ray-Finned Fish (Actinopterygii):
The ray-finned fish include familiar species such as tuna, bass, perch and trout. Ray-finned fish are the most successful and diverse of the vertebrates (more than half of all vertebrate species belong to this group). Thin, bony supports with radiating bones (hence the term ray- finned) hold the fins away from the body.
Ray-finned fish obtain their food by filter feeding and by preying on insects and other animals. Their skin is covered by scales formed of bone. These scales are homologous to our own hair (and the feathers of birds), being derived from the same embryonic tissues. The gills in this group of fish do not open separately and are covered by an operculum.
Ray-finned fish have a swim bladder, a gas-filled sac that regulates buoyancy and depth. Sharks lack this feature, which enables fish to “sleep” without sinking. The swim bladder acts much the way a ballast tank does on a submarine to control buoyancy.
Salmon, trout, and eels can migrate from fresh water to salt water, but must adjust kidney and gill function to the tonicity of their environments. In freshwater, the fish is hypotonic relative to its aqueous (watery) environment. Water is constantly flooding into the fish, and must be removed by the fish’s excretory system. In seawater, the fish is now hypertonic or isotonic relative to the seawater, requiring conservation of body water.
Bony fishes depend on color vision to detect both rivals and mates. Sperms and eggs are released into the water, with not much parental care for the newborn. Most fish have fertilization and embryonic development taking place outside the female’s body.
b. Lobe-Finned Fish (Sarcopterygii):
This group includes six species of lungfishes and one species of coelacanth that has muscular fins with large, jointed bones attaching the fins to the body. Lobe-finned fish have fleshy fins supported by central bones, homologous to the bones in your arms and legs.
These fins underwent modification, becoming the limbs of amphibians and their evolutionary descendants such as lizards, canaries, dinosaurs, and humans. The lungfish are a small group found mostly in freshwater stagnant water or ponds that dry up in Africa, South America, and Australia.
Coelacanths live in deep oceans. They were once considered extinct, although more than 200 have been captured since 1938. Mitochondrial DNA analysis supports the hypothesis that lungfish are probably the closest living relatives of amphibians.
The crossopterygian fish (represented by the marine extant deep-living coelacanth and extinct freshwater forms) are regarded as ancestors of early amphibians. Extinct crossopterygians had strong fins, lungs, and a streamlined body capable of swimming as well as traveling short distances out of water.
The term “tetrapod” (meaning four-limbed or four-footed) has historically been applied to the land vertebrates (amphibians, reptiles, dinosaurs, birds, and mammals). Most zoologists would accept that the Devonian lobe-finned fishes were ancestral to the amphibians.
Animals (both vertebrate as well as many invertebrates such as insects) that live on land use limbs to support the body, especially since air is less buoyant than water. Lobe-finned fishes and early amphibians also had lungs and internal nares to respire air.
5. Class Amphibia (Animals Move Ashore):
This class includes 4000 species of animals that spend their larval/juvenile stages in water, and their adult life on land. Amphibians must return to water to mate and lay eggs. Most adults have moist skin that functions in helping their small, inefficient lungs with gas exchange. Frogs, toads, newts, salamanders, and mud puppies are in this transitional group between water and land.
Following Amphibian features are not seen in bony fish:
I. Limbs with girdles of bone, those are adapted for walking on land.
II. A tongue that can be used for catching prey as well as sensory input.
III. Eyelids that help keep the eyes moist.
IV. Ears adapted for detecting sound waves moving through the thin (as compared to water) medium of the air.
V. A larynx adapted for vocalisation.
VI. A larger brain than that of fish, and a more developed cerebral cortex.
VII. Skin that is thin, smooth, non-scaly, and contains numerous mucous glands; the skin plays an active role in osmotic balance and respiration.
VIII. Development of a lung that is permanently used for gas exchange in the adult form, although some amphibians supplement lung function by exchange of gases across a porous (moist) skin.
IX. A closed double-loop circulatory system that replaces the single- loop circulatory path of fish.
X. Development of a three-chambered heart that pumps mixed blood before and after it has gone to the lungs.
Reproduction involves a return to the water. The term “amphibian” refers to two life styles, one in water, and other on land. Amphibians shed eggs into the water where external fertilization occurs, as it does in fish. Generally, amphibian eggs are protected by a coat of jelly but not by a shell. The young hatch into aquatic larvae with gills (tadpoles). Aquatic larvae usually undergo metamorphosis to develop into a terrestrial adult.
Amphibians, like fish, are ectothermic; they depend upon external heat to regulate body temperatures. If the environmental temperature becomes too low, ectotherms become inactive.
Salamanders more likely resemble earliest amphibians due to their S-shaped movements. Salamanders practice internal fertilization; males produce a spermatophore that females pick up. Frogs and toads are tailless as adults, with their hind limbs specialised for jumping.
6. Class Reptilia:
This class of 6000 species includes the snakes, lizards, turtles, alligators, and crocodiles. Reptiles that lay eggs lay an egg surrounded by a thick protective shell and a series of internal membranes. Reptiles have internal fertilization- their gametes do not need to be released into water for fertilization to occur.
The amniotic egg is a superb adaptation to life on land. While amphibians need to lay their eggs in water, their descendants (reptiles) were not as strongly tied to moist environments and could truly expand into more arid areas. Reptiles were the first land vertebrates to practice internal fertilization through copulation and to lay eggs that are protected by a leathery shell with food and other support for the growing embryo.
The amniote egg contains extraembryonic membranes that are not part of the embryo and are disposed of after the embryo has developed and hatched. These membranes protect the embryo, remove nitrogenous wastes, and provide the embryo with oxygen, food, and water.
The amnion, one of these extraembryonic membranes, creates a sac that fills with fluid and provides a watery environment in which the embryo develops. The embryo develops in a “pond within the shell”.
Archosauria (Birds and Dinosaurs):
Cladistic analyses place the birds, alligators, and dinosaurs in the same clade, the Archosauria (or “ruling reptiles”). This group is a major group of diapsids (vertebrates that have two openings in their skulls) that have single openings in each side of the skull, in front of the eyes (antorbital fenestrae), among other characteristics.
This helps to lighten the skull, provides more room for muscles and other tissues, and allows more skull flexibility when eating. Other typical archosaurian characteristics include another opening in the lower jaw (the mandibular fenestra), a high narrow skull with a pointed snout, teeth set in sockets, and a modified ankle joint.
The ancestral archosaurs probably originated some 250 million years or so ago, during the late Permian period. Their descendants (such as the dinosaurs) dominated the realm of the terrestrial vertebrates for a most of the Mesozoic Era. The birds and crocodilians are the last living groups of archosaurs.
7. Class Aves (Birds with Feather):
The class Aves (birds) contains about 9000 species. Birds evolved from either a dinosaurian or other reptilian group during the Jurassic (or possibly earlier). The earliest bird fossils, such as the Jurassic Archaeopteryx or Triassic Protavis, display a mosaic of reptilian and bird features (teeth in the bill, a jointed tail, and claws on the wing are reptilian; feathers and hollow bones are bird-like).
Archaeopteryx, once considered the first bird. The fossil is from the Solenhoefen Limestone (Jurassic) of Germany.
The distinguishing feature of birds is feathers- which provide insulation as well as aid in flight. Modern birds appeared during the early Tertiary, and have adapted to all modes of life- flying (condors, eagles, hummingbirds), flightless-running (ostriches, emus), and swimming (penguins). Birds exhibit complex mating rituals as well as social structure.
8. Class Mammalia:
Class Mammalia contains around 5000 species placed in 26 orders (usually).
The three unifying mammalian characteristics are:
2. The presence of three middle ear bones
3. The production of milk by mammary glands
Milk is a substance rich in fats and proteins. Mammary glands usually occur on the ventral surface of females in rows (when there are more than two glands). Humans and apes have two mammary glands (one right, one left), while other animals can have a dozen or more. All mammals have hair at some point during their life. Mammalian hair is composed of the protein keratin.
Hair has several functions:
2) Sensory function (whiskers of a cat)
3) Camouflage, a warning system to predators, communication of social information, gender, or threats.
4) Protection as an additional layer or by forming dangerous spines that deter predators.
Modifications of the malleus and incus (bones from the jaw in reptiles) work with the stapes to allow mammals to hear sounds after they are transmitted from the outside world to their inner ears by a chain of these three bones.
Mammals first evolved from the mammal-like reptiles during the Triassic period, about the same time as the first dinosaurs. However, mammals were minor players in the world of the Mesozoic, and only diversified and became prominent after the extinction of dinosaurs at the close of the Cretaceous period.
Mammals have since occupied all roles once held by dinosaurs and their relatives (flying- bats; swimming- whales, dolphins; large predators- tigers, lions; large herbivores- elephants, rhinos), as well as a new one (thinkers and tool makers- humans). There are 4500 species of living mammals.
I. Mammals developed several adaptations that help explain their success.
II. Teeth are specialised for cutting, shearing or grinding; thick enamel helps prevent teeth from wearing out.
III. Mammals are capable of rapid locomotion.
IV. Brain sizes are larger per pound of body weight than most other animals’.
V. Mammals have more efficient control over their body temperatures than birds.
VI. Hair provides insulation.
VII. Mammary glands provide milk to nourish the young.