CHORDATA: Main CharacteristicsChordates have the following features at some point in their life (although in some cases, these features may be present only in the embryo): Bilateral symmetry.Dorsal nerve cord (=spinal cord) - runs down the dorsal side, connecting the brain with the muscles and other organs.Notochord - stiff cartilaginous rod which supports the nerve cord.7Larval fish showing notochord and muscle blocks, or myotomes
Pharyngeal gill slits - a series of openings that connect the inside of the throat to the outside of the "neck". 8A primitive type of modern, extant chordates are the Cephalochordata (or lancelets), which live in sandPhoto: uic.eduDiagram of a lanceletPost-anal tail - an extension of the body past the anusBlood that circulates forward in a main ventral vessel and backward in a dorsal vessel.
Primitive Chordates9Pikaia(530 mya from the Middle Cambrian) was believed by some scientists to be the ancestor of all chordates, but had tentacles. It measured 5 cm in length.Photo: Y11evolutionBbc.co.uk.sciencePikaiaHaikouichthyesA more recent fossil find from China, Haikouichthyes, is now considered to be among the oldest possible true craniate, as it had a defined skull.The ancestor of all vertebrates??Not a vertebrate!
10VERTEBRATA: General Characteristicsruns along the dorsal surface from head to tail and forms the main skeletal axis of the body, surrounds and more or less replaces the notochord as the chief "stiffener" of the body in locomotion. General body plan: head, trunk, 2 pairs of appendages, and a post-anal tail (but highly modified in many vertebrates and sometimes the tail is absent altogether). Later animals developed a cranium, or brain case (“Craniata”).The first vertebrates were fishesSubphylum Vertebrata (e.g. fishes, reptiles, amphibians, birds, mammals) all have a vertebral column, or a chain of bony elements (vertebrae) that:
11The Earliest FishesSmall, slow-moving jawless fisheswere around filter-feeding 500 mya – the ostracoderms (‘shell skin’).Then evolution of jawed fishes~400 mya, including the armoured placoderms (‘plate-skin’). All extinct by the late Devonian.
12Devonian: “The Age of Fishes”Approx. 400 mya in the Devonian Period:All 5 major lineagesof fishes co-existed for these 48