Vertebrate

A vertebrate is an animal with an internal skeleton and a spinal column, or backbone. Vertebrae are the bones that make up a vertebrate's spinal column. The vertebrae surround and protect the spinal cord, which replaces the notochord found in the more primitive members of the phylum Chordata. Fish, amphibians, reptiles], birds, mammals, primates, rodents and marsupials are all vertebrates which belong to the sub-phylum Vertebrata. Although vertebrates are a minor portion of the animal kingdom, they tend to dominate their environment due to their size and mobility. http://virtualology.com/Phylumchordata/Vertebrata.org/ http://www.kidport.com/RefLib/Science/Animals/AnimalIndexV.htm

Fossil records show that about 550 million years ago, primitive invertebrate chordates appeared. http://www.mansfield.ohio-state.edu/~sabedon/campbl34.htm These organisms are all sea creatures that are suspension feeders that take in food though gill slits and trap the food in a mucous net. Examples of these organisms are lanclets, sea squirts, salps and larvaceans. http://www.anselm.edu/homepage/bpenney/teaching/bi102/Unit2/Echinodermsandbasalchord.html

It was not until 500 million years ago, during the Ordovician period, that vertebrates appeared, being distinguished from primitive chordates by having a segmented backbone. http://animals.about.com/od/evolution/a/vertebrateevolu.htm Early vertebrates lacked jaws and were detritivores or parasites. Lampreys are examples, whereas hagfishes are intermediate between the primitive chordates and vertebrates, because they have skulls but no backbone. http://www.anselm.edu/homepage/bpenney/teaching/bi102/Unit2/Echinodermsandbasalchord.html

Further evolution with vertebrates was facilitated by the development of jaw bones and the development of bone for the making of limbs. From 443 to 417 million years ago, during the Silurian period, cartilaginous fishes like sharks appeared, with bony fishes first appearing towards the end of that period. http://animals.about.com/od/evolution/a/vertebrateevolu.htm Fossilized footprints indicate that the first land vertebrates appeared around 380 million years ago. http://www.sciencedaily.com/releases/2010/01/100107114420.htm It is hypothesized that tetrapods evolved during periods of drought, when there was a need to move to new water sources. http://sci.waikato.ac.nz/evolution/AnimalEvolution.shtml