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Scientists estimate that there may be between 3 and 30 million species of animals on our planet. If you're just starting to learn about animals and wildlife, it would be an overwhelming task to try and memorize species, one by one. Instead, an understanding of the main groups of animals provides a good foundation for further learning. In this article, we'll take a look at six groups of animals, their characteristics and the types of organisms that belong to each group. Once you have a understanding of these six groups, you'll be on your way to grasping the basic characteristics of virtually every animal on the planet.
The first animals to evolve were invertebrates. Fossil evidence of invertebrates dates back to the late Precambrian, 600 million years ago. Invertebrates evolved from single-celled microorganisms. Since then, invertebrates have diversified into countless forms. An estimated 97% of all species are alive today are invertebrates.
Invertebrates are not a true taxonomic class of animals, they are united more by what they lack (a backbone) than by shared characteristics. Invertebrates include animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.
2. Fish were among the first vertebrates to evolve. The earliest known fishes were the ostracoderms, a now-extinct group of jawless fish that appeared in the Cambrian Period, about 510 million years ago. Other early fish include the conodonts and the agnanthans (the hagfish and the lamprey). Fish later evolved jaws and diversified into a number of lineages including cartilaginous fish, ray-finned fish and lobe-finned fish.
The ray-finned fish are the most diverse of all vertebrate groups, with some 24,000 species. There are about 810 species of cartilaginous fishes and 8 species of lobe-finned fish. Like invertebrates, fish are not a true taxonomic class.
3 .Amphibians were the first vertebrates to make the move from life in water to life on land. Despite their early colonization of terrestrial habitats, most lineages of amphibians have never fully severed their ties with aquatic habitats. The first amphibians evolved from lobe-finned fishes approximately 370 million years ago during the Devonian Period.
Amphibians include newts and salamanders, frogs and toads, and caecilians. There are between 5000 and 6000 species of known amphibians alive today. Amphibian species are in decline around the world due to a variety of threats including of invasive species, habitat destruction, disease, climate change, and toxins.
4.Reptiles are cold-blooded vertebrates that diverged from ancestral amphibians about 340 million years ago during the Carboniferous Period. Early reptiles included organisms such as Hylonomus, Petrolacosaurus, Archaeothyris, and Paleothyris. The oldest evidence of reptiles is a set of footprints found in Nova Scotia. Two characteristics distinguish early reptiles from amphibians: scales and the ability to lay hard-shelled amniotic eggs.Reptiles include turtles, squamates, crocodiles, and tuataras. There are about 8,000 species of reptiles alive today. Of the four reptile groups, the squamates (amphisbaenians, lizards and snakes) are the most diverse with nearly 7,600 species.
5. Mammals display a remarkable array of adaptations that enable them to inhabit a wide range of habitats. Mammals range in size from the minute Bumblebee Bat which measures a mere three centimeters in length, to the magnificent blue whale, which can measure 33 metres head to tail. Some of the better-known mammal groups include carnivores, rodents, elephants, marsupials, rabbits, bats, primates, seals, anteaters, cetaceans, odd-toed ungulates, and even-toed ungulates.
6. Birds evolved from reptiles during the Mesazoic Era about 150 million years ago. Today, more than 9,000 species inhabit virtually every terrestrial habitat on the planet. Birds have a number of characteristics that sets them apart from other vertebrates such as feathers, bills, and a furcula.
Birds, best known for their ability to fly, are unmatched in their command of the skies. Albatrosses glide over the vast open sea, hummingbirds hover motionless in mid-air, and birds of prey capture prey with pinpoint accuracy. But not all birds are aerobatic experts. Some species such as ostriches, kiwis and penguins, lost their ability to fly long ago in favor of lifestyles suited more for land or water.
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