Transcript Chordates
Chordates (phylum Chordata) are deuterostome coelomates -Nearest relatives are echinoderms (the only other deuterostomes) Four features characterize chordates 1. Nerve cord 2. Notochord 3. Pharyngeal slits 4. Postanal tail All chordates have all four of these characteristics at some time in their lives Other characteristics also distinguish chordates 1. Chordate muscles are arranged in segmented blocks called somites 2. Most chordates have an internal skeleton against which the muscles work The Chordates Somites Phylum chordata can be divided into three subphyla 1. Urochordata Nonvertebrates 2. Cephalochordata 3. Vertebrata Tunicates are marine animals -Larvae are tadpole-like and have notochord and nerve cord -Are free-swimming but do not feed -Adults typically lose the tail and notochord -Are immobile filter-feeders -Many secrete a tunic (cellulose sac) that surrounds the animal Lancelets are scaleless chordates -Notochord persists throughout animal’s life -Have no distinguishable head -Feed on plankton using cilia-generated currents -Closest relatives to vertebrates Vertebrates: chordates with a spinal column -Distinguished from nonvertebrates by 1. Vertebral column – Encloses and protects the dorsal nerve cord 2. Head – Distinct and well-differentiated possessing sensory organs Vertebrates also have -Neural crest – A unique group of embryonic cells that forms many vertebrate structures -Internal organs – Liver, kidneys, endocrine glands, heart and closed circulatory system -Endoskeleton – Made of cartilage or bone -Makes possible great size and extraordinary movement The first vertebrates appeared in the oceans about half a billion years ago -Jawed fishes soon became dominant -Their descendants, the amphibians, invaded the land -Reptiles replaced them as the dominant land vertebrates. 300 MYA -Gave rise to birds and mammals 220 MYA Fishes are the most diverse vertebrate group -Over half of all vertebrates • • • • Vary greatly in size, shape, color and appearance 500 MYA Paired fins Jaws Fishes have the following characteristics 1. Vertebral column 2. Jaws and paired appendages 3. Internal gills 4. Single-loop blood circulation 5. Nutritional deficiencies The first fishes had mouths with no jaws Skull -Agnatha (Superclass) extant as hagfish (class Myxini) and lampreys (class Cephalaspidomorphi) Gill slit Anterior gill arch -Ostracoderms are now extinct The development of jaws occurred in the late Silurian period (438-408 MYA) -Jaws evolved from the anterior gill arches that were made of cartilage The Devonian period (408-360 MYA) saw two types of jawed fishes -Spiny fishes (Acanthodii) -Smaller fishes that dominated early on -Armored fishes (Placodermi) -Larger fishes that dominated late At the end of the period, these early fishes were replaced by sharks and bony fishes -These had more improved jaws In the Carboniferous period (360-280 MYA), sharks (Class Chondrichthyes) became the dominant sea predators -A light, flexible, calcified, cartilaginous skeleton, and paired fins made them superior swimmers Sharks were among the first vertebrates to develop teeth -Evolved from rough scales on mouth’s skin -Easily lost but continuously replaced Sharks (and bony fishes) have a fully developed lateral line system -Series of sensory organs under the skin that detects changes in pressure waves Most sharks have internal fertilization. Oviparous, ovoviviparous, and viviparous. Bony fishes (Osteichthyes) evolved at the same time as sharks about 400 MYA -However, they adopted a heavy internal skeleton made completely of bone Bony fishes are the most species rich group of all vertebrates (> 30,000 living species) They developed significant adaptations that enabled them to dominate life in the water -Swim bladder and gill cover Swim bladder -A gas-filled sac that allows bony fishes to regulate their buoyant density -Gas addition = Gas gland -Gas release = Oval body Gill cover -A hard plate, the operculum, covers gills -Its flexing permits water pumping over gills Ray-finned fishes (class Actinopterygii) -Parallel bony rays support & stiffen each fin -There are no muscles within the fins Lobe-finned fishes (class Sarcopterygii) -Have paired fins that consist of a long felshy muscular lobe -Supported by central co-jointed bones -Almost certainly the amphibian ancestors Amphibians (class Amphibia) are damp-skinned vertebrates -First vertebrates to walk on land Successful invasion of lands by vertebrates required several adaptations 1. Legs to support body’s weight 2. Lungs to extract oxygen from air 3. Redesigned heart to drive larger muscles 4. Reproduction in H2O to prevent egg drying 5. System to prevent whole body desiccation. The skin also aids in respiration. 6. Plumonary veins. Amphibians evolved from lobe-finned fish Ichthyostega was one of the first amphibians -Efficient limbs for land crawling -Improved olfactory and auditory structures -However, still fishlike -Spent much of life in water In 2006, a transitional fossil was found between fish and Ichthyostega -Tiktaalik lived about 375 MYA -Shoulder and limb bones like those of an amphibian -Fins like those of a lobefinned fish -Spent most of its life in water -Capable of moving onto land to capture food or escape predators Modern amphibians are descended from three families of the Tertiary period (65-2 MYA) -These accomplished a highly successful invasion of wet habitats all over the world Today there are about 5,670 species in 37 different families, comprising three orders: -Anura (“without tail”), Caudata (“visible tail”),Apoda (“without legs”) Over 7000 species of reptiles (class Reptilia) now live on Earth All living reptiles exhibit three key features 1. Amniotic eggs, which are watertight 2. Dry skin, which covers body and prevents water loss 3. Thoracic breathing, which increases lung capacity Reptiles, birds and mammals are amniotes The amniotic egg has four membranes Chorion – Allows O2 entry Amnion – Fluidfilled cavity Yolk sac – Provides food Allantois – Excretes wastes Reptiles dominated earth for 250 million years Reptiles are distinguished by the number of holes on side of the skull behind eye orbit: -0 (anapsids), 1 (synapsids), 2 (diapsids) The synapsids rose to dominance first -Pelycosaurs were first land vertebrates to kill beasts their own size -Therapsids replaced them about 250 MYA -May have been endotherms -Replaced by diapsids but gave rise to mammals. Archosaurs (Diapsids) were the first land vertebrates to be bipedal -Gave rise to dinosaurs about 220 MYA Dinosaurs dominated for over 150 million years -Became extinct 65 MYA -Asteroid’s impact Modern reptiles developed two important characteristics: 1. Internal fertilization: Sperm fertilizes egg before protective membranes are formed 2. Improved circulation: Oxygen is provided to the body more efficiently Comparing the circulatory systems of fish, amphibians, and reptiles. All living reptiles are ectothermic -Obtain heat from external sources There are four surviving orders of reptiles -Chelonia (turtles and tortoises) -Rhynchocephalia (tuataras) -Squamata (lizards and snakes) -Crocodylia (crocodiles and alligators) Crocodile Gharial In many ways, crocodiles resemble birds far more than they do other living reptiles -Build nests and care for their young -Have a four-chambered heart