Diversity: All of these assorted chordates are united because they are descended from a common ancestor that had three features that were passed on to all of its descendants. These three characteristics can be used to distinguish chordates from other animals.First, chordates have a collection of nerve fibers, called a nerve cord, which runs down their back sides connecting the brain to the organs and muscles. The second characteristic is a notochord , which is a stiffened rod that runs underneath the nerve cord. The notochord is used by many chordates as an aid for swimming. Muscles pull the notochord one way and then it springs back, propelling the chordate forward through the water. Life History: The chordate life cycle begins with fertilization (the union of sperm and egg). In its primitive form, fertilization occurs externally, in the water. Asexual reproduction takes place in tunicates and in some vertebrates (females of some fish and lizards can reproduce without fertilization). Hermaphroditism (possessing both male and female reproductive organs) is found in tunicates and some fishes, but otherwise the sexes are separate. Larvae (very young forms that differ considerably from the juveniles and adults), when they do occur, differ in structure from the larvae of nonchordates. Internal fertilization, viviparity (giving birth to young that have undergone embryological development), and parental care are common in tunicates and vertebrates. Ecological Roles: Chordates are well represented in marine, freshwater and terrestrial habitats from the Equator to the high northern and southern latitudes. The oldest fossil chordates are of Cambrian age. The earliest is Yunnanozoon lividum from the Early Cambrian era. These fossils are highly significant because they imply the contemporary existence of the tunicates and craniates in the Early Cambrian during the so-called Cambrian Explosion of animal life. Human Impacts: Chordata is a large and diverse group of animals, with roughly 50,000 living species species included. The majority of chordates belong to a group called Vertebrata. Vertebrates have backbones that are composed of vertebrae. Some examples of vertebrates are sharks, fish, dinosaurs, and human beings.A second group of chordates, called Urochordata, consists of animals found mostly in oceans. Urchordates include sock-shaped pyrosomes that grow up to 10 meters (32.8 feet) long, sack-shaped sea squirts that live attached to the seafloor, and tadpole-shaped larvaceans that build their floating houses out of mucus. Work Cited: http://www.britannica.com/EBchecked/topic/114462/chordate/49559/Evolution-and-paleontology http://www.encyclopedia.com/topic/Chordata.aspx https://www.boundless.com/biology/textbooks/boundless-biology-textbook/vertebrates-29/chordates-171/characteristics-of-chordata-663-11884/ |