Vertebrates

Vertebrate is an Organism which has a Backbone or a Spinal Column. Vertebrates are members of the larger phylum Chordata and show all of the major Chordate features at some point in their life cycles: Notochord, Dorsal Hollow Nerve Cord, Pharyngeal Slits, and a Post-Anal Tail. Vertebrates have a long history on the Earth - more than 500 million years, from the late Cambrian up to today. The first vertebrates lacked Jaws, like the Living Hagfish and Lampreys. Jawed Vertebrates appeared 100 million years later, in the Silurian. The Vertebrates show affinities with other Chordates but share some characters that make them unique. Some have argued that many of the characters that describe Vertebrates have been derived from the same set of cells, the Neural Crest Cells. These cells appear early in development, and only Vertebrates have them. From Neural Crest Cells are derived the Skull and Jaw Bones. Vertebrates are fascinating group of animals mainly because we are intimately familiar with them and also because Homo Sapiens, i.e. Humans are also included within the Vertebrata. There are about 1.5 million named Species of Extant Animals (probably 3-30 million total), of which about 50,000 are Vertebrates. Vertebrate Animals include 8,000 Reptiles, 10,000 Birds, 23,000 Fish, and 5,000 Mammal Species.

History of Vertebrates

The first known Vertebrate Fossils, found at the Chengjiang locality in China, date back to the early Cambrian Period. These early Vertebrates, such as Haikouichthys, are small, tapered, streamlined animals showing eyes, a brain, pharyngeal arches, a notochord, and rudimentary Vertebrae. Vertebrates appear to have radiated in the late Ordovician, about 450 million years ago. However, most Ordovician fossil Vertebrates are rare and fragmentary, although available material suggests that ancestors of the Sharks and Jawed Fish were present along with various lineages of Armoured Jawless Fish. Vertebrates first evolved in the Ocean. The Invertebrate ancestors of Vertebrates had Gill Slits, but these were used primarily for Filter Feeding; these organisms took up most of the oxygen they needed through the skin.

As early Vertebrates grew larger and developed Bony Scales or Plates between their tissues and the water, they developed Gills for taking up oxygen from the water. By the middle Silurian Period, about 400 million years ago, the picture is clearer: the Armoured Jawless Fish were quite diverse, and the first Definite Jawed Fish had appeared. By the late Devonian Period, 360 million years ago, early Cartilaginous Fish and Bony Fish were diversifying. The late Devonian Period also marked the first Tetrapods - Vertebrates with true legs that could walk on land. By about 330 million years ago, in the Mississippian, several groups of land-dwelling Amphibians had appeared. The Oldest known Amniotes - close to the ancestry of all Reptiles, Birds and Mammals - appeared in the early Pennsylvanian Period, about 310 million years ago. Land Amniotes continued to diversify, and by the middle Pennsylvanian had split into several Taxa, two of which would go on to dominate the Mesozoic and Cenozoic: the Diapsids and the Synapsids.

Scientific Classification

Kingdom - Animalia.
Superphylum - Deuterostoma.
Phylum - Chordata.
(unranked) - Craniata.
Subphylum - Vertebrata.

Characteristics of Vertebrates

• Bilateral Symmetry - It refers to Symmetrical arrangement of an Organism or a body part, along a Central Axis, so that the body is divided into equivalent right and left halves by only one plane.

• Locomotor Appendages - Two pairs of jointed locomotor appendages, which can include fins (pectoral and anal/dorsal fins, as well as the forelimbs and hind limbs).

• Protective Cellular Skin - Outer covering of protective cellular skin, which can be modified into special structures such as scales, hair and feathers.

• Metamerism - Metamerism found in skeletal, muscular and nervous system. This includes ribs, vertebrae, muscles, and ganglia/peripheral nerves.

• Coelom - Coelom is a Cavity in the Mesoderm of an embryo that gives rise in Humans to the Pleural Cavity and Pericardial Cavity and Peritoneal Cavity. Well-developed Coelom or body Cavity completely lined with Epithelium (cellular tissue of mesodermal origin) that may be divided into 2 to 4 compartments.

• Internal Skeleton - Well-developed internal skeleton of Cartilage and/or Bone, separated into Axial Skeleton (skull, vertebrae, ribs, sternum) and Appendicular Skeleton (girdles and appendages).

• Bones - Vertebrates possess two types of bone.

1.  Dermal bone - Dermal Bone consists of bony structures (plates and scales) that develop in the skin. The bony armour of the earliest Jawless Fish was Dermal Bone; so are Shark Scales, Shoulder Blades, and the roof of Human skull. Dermal Bone does not form from Cartilage first and then calcify, but Endoskeletal Bone does; in fact, in Cartilaginous Fishes it may never form True Bone. Vertebrae, Ribs, Appendages, and the Jaw are Endoskeletal Bone. The Vertebrate Skull is actually a complex structure of both Endoskeletal and Dermal Bone.

2.  Cartilage - All Vertebrates have Cartilage in addition to Bone, or instead of Bone. Cartilage may be flexible, like the Cartilage in Human nose and ears, or hard and firm, like the Cartilage in Humans Larynx (voice box). Cartilage also covers the adjoining surfaces of Bones in movable joints. The Calcified Cartilage that makes up Shark Teeth and Vertebrae is not True Bone; it is dead when functional, whereas Bone is a living tissue.

• Brain - Highly developed Brain enclosed by Skull and Nerve Cord enclosed by Vertebrae - these provide advanced neural structures that are highly protected from damage.

• Neural Crest Cells - Probably the most important Vertebrate feature is the presence of Neural Crest Cells. In early development, as the Nerve Cord is forming, Neural Crest Cells leave the Nerve Cord and move through the body. These Cells form, or cause to form, many important Nerves, Neural Ganglia, and many head and facial features.

• Sensory Organs - Well-developed Sense Organs (eyes, ears, nostrils) located on the head (cephalization).

• Respiratory System - Respiratory System, including either Gills or Lungs, located closely to the Pharynx or Throat.

• Closed Circulatory System - Closed Circulatory System with Ventral Heart and Median Dorsal Artery.

• Genital and Excretory Systems - Genital and Excretory Systems closely related, utilizing common ducts and pathways. Most Vertebrate produce Eggs inside the body which upon being fertilised produce an Organism. While some like the Birds and Reptiles lay eggs and hatch them. However a number of Fishes and Snakes retain their eggs in their bodies, and the eggs hatch internally. Such Animals are called Ovoviviparous. Many vertebrates, such as Dogfish Sharks and almost all Mammals, have further modified the ancestral structures of the egg so that the embryo is not only retained inside the body of the female parent, but actively nourished through a special connection with the mother's body. This is known as being Viviparous.

• Digestive Tracts - Digestive tracts with two major Digestive Glands (liver and pancreas) that secrete into it.

• Presence of Two Sexes - With rare exceptions, Vertebrates have two separate sexes.

• Pair of Eyes - All Vertebrates have paired complex eyes.

• Multicellular and Eukaryotic Organisms - Almost all Vertebrates have Multiple Cells within their bodies and have cells with good or membrane-bound nuclei.

• Segmented Body - Vertebrates body have a plan of repeated structures known as Segments or Parts, such as Arms, Forearms, Hands, Thighs, Legs, Feet etc.

Geographical Range and Habitat

Vertebrates are found from the tropics to the polar regions, from the deep sea to high mountains, and even the air

There are Five Classes / Types of  Vertebrates.

These are:

• Mammals.

• Birds.

• Fish.

• Reptiles.

• Amphibians.

Invertebrates