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Vertebrata

Vertebrata

The vertebrates are commonly called "animals with backbones," but this is a simplified description of a group of animals who are the most anatomically and functionally diverse on Earth. As with most major groups of animals, their beginnings are not known. However, scientists have constructed a theory of the origins of vertebrates that is generally well accepted by the academic community.

Scientists base their model of a hypothetical vertebrate ancestor on several primitive living vertebrates. At the forefront is Amphioxus, whose body shape is close to what scientists believe resembles that of the ancestral vertebrate. Many researchers have successfully examined the body of this small marine animal and it has become a popular organism for study in biology classes.

Based on Amphioxus, scientists believe that the first vertebrates had a fishlike body with individual segmented muscles along its entire length. They were small ocean dwellers who lived close to the bottom and used their muscles to contract and move their bodies and tail in a side-to-side motion. This action propelled them through the water. They had no distinct head. The brain of early vertebrates is a somewhat controversial topic among scientists, but most researchers agree that the early forms had a brain that was more complex than the simple brain of Amphioxus. However, the brain was not the highly complicated structure we see in most vertebrates today. The early brain probably carried out only the most basic of body and sensory functions. As with many animals with cerebral head ganglia, nerve tracts emerged from the brain and ran along the length of the body. Scientists assume the nerve tracts responded to sensory stimuli. The lateral line system, present in most fish, was most likely present in early vertebrates.

Just as in Amphioxus, a dorsal (top of body) hollow nerve cord ran the entire length of the body. This nerve cord was supported by an important evolutionary structure made of cartilage called the notocord. The notocord, and its role in the evolution of vertebrates, is one of the most important characteristics distinguishing vertebrates from nonvertebrates. Although there are many other characteristics that help to classify the group, the notocord is the structure from which the backbone is believed to have evolved and is the structure from which vertebrates get their name.

As vertebrates became more specialized and increased their ability to move and sense their environment, the brain and spine became more complex. In looking at the growth of fetal vertebrates it has been shown that the developing muscles place a strain on the cartilaginous notocord. As the strain on the notocord increases with growth of the muscles, deposits of bone replace the cartilage, giving the rod greater strength. This process of replacement eventually produces the bones known as the vertebrae. Each vertebra in a primitive vertebrate corresponds to an individual set of muscles. This pattern is harder to recognize in more derived vertebrates, like mammals and birds, but it is there nonetheless.

In primitive vertebrates, the mouth is a simple oral opening that leads to the gill slits and digestive system. As the vertebrates continued to evolve, the oral cavity was replaced by a more specialized mouth and gill apparatus. Although sharks and their relatives have a primitive type mouth and gills, bony fishes such as salmon and perch have developed complicated gills with a bony covering called an operculum. Sharks are primitive vertebrates in that they do not develop bony skeletons, but even so, the cartilage structure of vertebrates is easy to see.

As vertebrates become complicated in body structure, the mouth becomes a very characteristic structure. Many have teeth that are actually modified body scales. The increasing specialization of the teeth, such as the pointed, socket-bound teeth of the reptile or the many cusped teeth of the mammals, is a major trait on which groups of vertebrates are identified. Birds have no teeth whatsoever.

Early vertebrates had a simple mouth opening through which they gulped food like a frog or fish. This structure was not only poorly adapted for capturing and holding on to active prey but also prevented the animal from breathing while trying to feed. Hunting and swallowing quick prey, like some flying or hopping insects, was problematic to the ill-equipped vertebrates. As a response to increasingly swift food sources that were adapting to life on land, the vertebrates became swifter and more dangerous. The increased specialization of the mouth proved to be an advantage for the group in capturing food.

As they struggled with larger or stronger prey, it was necessary to bite and hold on to wriggling and unwieldy insects. It was hard for them to breathe and many primitive vertebrates were unable to capture these more agile animals.

A major evolutionary trend in the vertebrates was the development of the secondary palate in the mouth, a platform of bone that separates the nasal cavity from the mouth. Mammals, including humans, have a secondary palate that allows for breathing while feeding. This means that the hunter can bite and hold onto its prey and still breathe. It can chew or tear at its food instead of gulping like a crocodile. Lions are an excellent example of how the secondary palate helps the lion to bite and hold onto its intended victim until it is dead and then tear off portions for eating.

It is difficult to provide a generalized summary of the characters of all vertebrates. The group is extremely diverse and includes fish, sharks and rays, amphibians, reptiles, birds, and mammals. However, there are several characteristics that are common to all vertebrates and four that are completely exclusive to the group.

All vertebrates are bilaterally symmetricalthey have two sides which are identical to each other in one plane only. A vertebrate can be divided down the middle, or sagital plane , to produce two equal halves. However, if it is divided down the side, or transverse plane , the sides will not be identical.

The notocord, or skeletal rod, and the dorsal hollow nerve cord are present in all vertebrates. These two characters are unique to vertebrates.

Other characteristics unique to the vertebrates are pharyngeal (at the sides of the pharynx, or throat) gill slits and a tail behind the anal opening. The presence of the tail may seem an obvious trait, but no other group of animals has a structure that can be identified as an actual tail behind the anal opening. Some insects have bodies that extend beyond the anus, but they do not have tails. Vertebrate tails can move and provide locomotion, or balance and support, as in birds and dinosaurs. In many vertebrate groups, such as monkeys, which use their prehensile tails for swinging through trees, the tail can act as an extra appendage.

The vertebrate circulatory system is always closed, but this is not unique to the group. However, the vertebrate heart is always located in a ventral position and the digestive system is complete. Surprising to many, certain vertebrates, especially extinct forms, have an exoskeleton . Early fishes, called ostracoderms and placoderms, had bony exoskeletons that protected their head and sensory areas.

The vertebrates have become a highly successful group of animals with an interesting and exciting evolutionary story. Because humans are vertebrates, they have a natural and continuing curiosity about their predecessors who, most likely, had their beginnings about 500 million years ago in the seas of Earth.

Brook Ellen Hall

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Vertebrates

Vertebrates

Vertebrates are any animals that have a backbone or spinal column. These animals are so named because nearly all adults have vertebrae, bone or segments of cartilage forming the spinal column. The five main classes of vertebrates are fish, amphibians, birds, reptiles, and mammals.

Vertebrates are the most complex of Earth's animal life-forms. The earliest vertebrates were marine, jawless, fishlike creatures with poorly developed fins. First appearing on Earth more than 500 million years ago, they probably fed on algae (single-celled or multicellular plants and plant-like animals), small animals, and decaying organic matter. The evolution of jaws, limbs, internal reproduction organs, and other anatomical changes over millions of years allowed vertebrates to move from ocean habitats to those on land.

All vertebrates have an internal skeleton of bone and cartilage or just cartilage alone. In addition to a bony spinal column, all have a bony cranium surrounding the brain. Vertebrates have a heart with two to four chambers, a liver, pancreas, kidneys, and a number of other internal organs. Most have two pairs of appendages that have formed as either fins, limbs, or wings.

[See also Amphibians; Birds; Fish; Invertebrates; Mammals; Reptiles ]

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vertebrate

vertebrate, any animal having a backbone or spinal column. Verbrates can be traced back to the Silurian period. In the adults of nearly all forms the backbone consists of a series of vertebrae. All vertebrates belong to the subphylum Vertebrata of the phylum Chordata. There are five classes of vertebrates: fish, amphibians, reptiles, birds, and mammals. General characteristics of vertebrate animals include their comparatively large size, the high degree of specialization of parts they exhibit, their bilaterally symmetrical structure, and their wide distribution over the earth. In addition to an internal skeleton of bone and cartilage or of cartilage alone, vertebrates have a spinal cord, a brain enclosed in a cranium, a closed circulatory system, and a heart divided into two, three, or four chambers. Most have two pairs of appendages that are variously modified as fins, limbs, or wings in the different classes. All animals without backbones are called invertebrates; these do not form a homogeneous group as do vertebrates.

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vertebrate

vertebrate Animal with individual discs of bone or cartilage called vertebra, which surround or replace the embryonic notochord to form a jointed backbone enclosing the spinal column. The principal division within vertebrates is between fish and partly land-adapted forms (amphibians), and the wholly land-adapted forms (reptiles, birds, and mammals, although some mammals, such as whales, adapted to a totally aquatic existence). Phylum Chordata; subphylum Vertebrata.

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Vertebrata

Vertebrata (kingdom Metazoa, phylum Chordata) The subphylum which includes the fish, Amphibia, Reptilia, Aves, and Mammalia. These appear successively in the fossil record, starting in the Ordovician (although traces of fish are now known from the late Cambrian). The lower chordates (e.g. the living Amphioxus) are soft-bodied and first appear in the Cambrian.

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vertebrate

ver·te·brate / ˈvərtəbrət; -ˌbrāt/ • n. an animal of a large group distinguished by the possession of a backbone or spinal column, including mammals, birds, reptiles, amphibians, and fishes. Compare with invertebrate. • adj. of or relating to the vertebrates.

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vertebrate

vertebrate Any one of a large group of animals comprising all those members of the phylum Chordata that have backbones (see vertebral column). Vertebrates include the fishes, amphibians, reptiles, birds, and mammals.

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vertebrates

vertebrates See CRANIATA.

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Vertebrata

Vertebrata See CRANIATA.

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vertebrate

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