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The Vertebrates The Vertebrates Chapter 34 Chordates • Chordates (phylum Chordata) are animals that include the vertebrates and several closely related invertebrates • They are united by having – – – – a notochord a hollow dorsal nerve cord pharyngeal slits or pouches A post-anal tail Hollow dorsal nerve cord • Develops from a plate from ectoderm – Rolls into a tube, dorsal to notochord • Unique to chordates, other animal phyla have solid nerve chords, mostly ventral • Develops into CNS – Brain and spinal cord Notochord • Skeletal structure present in all chordate embryos Ascidian (Tunicate) notochord – composed of cells derived from the mesoderm, defines the primitive axis of the embryo – in lower organisms, it persists throughout life as the main axial support of the body, while in higher vertebrates it is replaced by the vertebral column Pharyngeal slits or pouches • In all chordate embryos, series of pouches separated by grooves forms along sides of pharynx • Function as suspension-feeding devices in many invertebrate chordates • In aquatic vertebrates, these structures turn into gills • In tetrapods, pharyngeal pouches develop into parts of the ear and other structures in head and throat Muscular, Post-Anal Tail • Tail extends posterior to anus • In many species, lost during embryonic development • Contains skeletal elements and muscles – Provides propelling force in aquatic species The Vertebrates • • • • Chordates with a backbone or spinal columns Originated ~530 mya (Cambrian explosion) 52,000 species Vertebrata comprises the largest subphylum of chordates, and contains many familiar groups of large land animals – comprise fish (including lampreys), amphibians, reptiles, birds, and mammals (including humans) 4 wk human embryo 10-12 wk embryo Lampreys – oldest living lineage of vertebrates Origin of Craniates • • • • • Least derived craniate lineage is the Myxini – the Hagfish Skull made of cartilage Lack jaws and vertebrae Have small brain, eyes, ears Retain notochord as adults • Marine and freshwater environments • Most are parasites – Clamp round, jawless mouth onto plank of fish – Swim in snake-like fashion by pushing against it with their muscles • Use slime to protect themselves – Slime studied for biotech uses • Skeleton made of cartilage – also have cartilaginous pipe around notochord • Their relationships to other major living vertebrate groups — the similarly jawless lampreys, and the jawed vertebrates — remain contentious Next step: jaws and teeth • Jawed vertebrates are called Gnathostomes (“jaw mouth”) • Include sharks, rays, chimaeras, rayfinned fishes, lobe-finned fishes and land vertebrates • Jaw probably a result of transitions in feeding mechanisms – Data suggests mineralization of lower vertebrates began in the mouth – In more advanced vertebrates, the exoskeleton began to mineralize too Gnathostomes are characterized by: • • • • An additional duplication in Hox genes Forebrain enlarged Enhanced sense of smell and vision Aquatic gnathostomes have lateral line systems Class Chondrichthyes – Sharks, rays – “cartilage fish” – Skeleton composed of cartilage – Have some traces of bone Class Chondrichthyes – 2 subclasses – Sharks, rays – Ratfish • In scales, base of teeth, surface of vertebrae – Among largest and most successful predators in ocean – Sharks have acute senses Class Chondrichthyes – Sharks have streamline bodies – Fast swimmers • Poor manueverability – Gains buoyancy by storing oil in its liver • Still denser than water • Must consistently swim to stay afloat Sharks – Largest members are suspension feeders • Consume plankton – Many sharks are carnivores • Swallow prey whole • Tear it apart – Several rows of sharp teeth • Constantly being replaced Rays Osteichthyes – Flattened bottom dwellers • Use jaws to crush mollusks and crustaceans – Several defense mechanisms • Whip-like tail with venomous barbs • Generation of electric current – Bony fish clade – Ray finned • Class Actinopterygii – ray finned fishes – Lobe finned • Class Actinistia – coelocanths • Class Dipnoi - lungfish Osteichthyes – Bony endoskeleton • Calcium phosphate matrix – Breathe with gills • Have 5 pairs covered by operculum Ray finned fishes – Class Actinopterygii • Common fish • Sea horses • eels – Protective bony flap – Many control buoyancy with swim bladder • Internal air sac • Air pressure controlled by blood gas transfer Ray finned fishes – Fins supported by bony, flexible rays • Modified for different adaptations – Maneuvering – Defense Lobe finned fishes – Rod shaped bones surrounded by thick layer of muscle in pelvic and pectoral fins – Class Actinistia • coelacanths – Class Dipnoi • lungfish Class Actinistia – – – – – Coelacanths Primitive bony fish Once thought to be extinct “rediscovered” in 1938 Found in Indian Ocean Class Dipnoi – Lungfish – 3 genera – All found in southern hemisphere – Lungs supplement O2 from gills – Evolved to be able to breath in waters with low O2 Derived Characters of Tetrapods Tetrapods – Limbs supporting weight on land – Feet with digits – Bones of pelvic girdle fused to backbone – Clade with 4 limbs • Arms and legs – Evolved from lobe finned fins – – – – • Permits walking Amphibians Reptiles Birds Mammals Meet Tiktaalik Derived Characters of Tetrapods Reproduction – Pharyngeal gill slits disappear during development • Three major evolutionary innovations gave tetrapods the ability to reproduce successfully in terrestrial environments: • Slits give rise to parts of ears, glands • Ears adapted to detect airborne sounds – eardrum – Amniotic egg – Placenta – Elaboration of parental care Amniote Egg Membranes • Amnion – fluid filled layer acts as shock absorber • Surrounds embryo • Allantois – disposal sac for metabolic wastes • Chorion – gas exchange • O2 and CO2 diffuse freely through shell • Yolk sac – nourishes embryo • Albumen also nourishes embryo Amniotes – – – – Fertilization must occur internally Before shell is secreted Many snakes and lizards viviparous Eggs of most reptiles, birds, some mammals have shells • Bird egg shell – hard, CaCO3, inflexible • Reptile eggs – usually leathery and flexible • Mammal eggs – leathery – Most mammalian eggs non shelled – Develop within mother Placenta • Most mammal females produce eggs that lack yolk • After fertilization, egg is retained, and mother produces a placenta within the uterus • Placenta is a specialized organ rich in blood vessels Parental care • Increases chances of offspring survival • Most extensive versions observed in mammals and birds – Mother and father feed and care for young – In mammals, mothers also lactate – Allows for gas exchange – Allows mother to provide nourishment – After a period called gestation, embryo emerges Amphibians – Salamanders, frogs, toads, Caecilians – Feeding: carnivorous • Sit and wait predators • Many have extendible tongues Survey of Tetrapods Amphibians • Movement: have well-developed limbs (4) – Ceacilians lack legs • Ectothermic – Do not use internally generated heat to regulate body temperature – Reproduction • Mostly oviparous • Frogs have external fertilization • Salamanders and caecilians have internal fertilization • Many caecilians are viviparous Reptilia • Feeding: – Most are carnivorous – Some fierce predators (snakes, crocodiles) – Turtles, some lizards are herbivores • Skin is waterproofed – Scales • Keratin built up in outermost layer – prevent breathing through skin • Rely on lungs for gas exchange • Ectothermic • Reproduction: – Mostly oviparous – Most reproduce sexually – Some have extensive parental care (crocodiles) The Origin and Evolutionary Radiation of Reptiles • The oldest reptilian fossils date to about 310 million years ago • The first major group to emerge were parareptiles, which were mostly large, stocky herbivores • As parareptiles were dwindling, the diapsids were diversifying • The diapsids consisted of two main lineages: the lepidosaurs and the archosaurs – The lepidosaurs include tuataras, lizards, and snakes – The archosaur lineage produced the crocodilians, pterosaurs, and dinosaurs The archosaur lineage produced the crocodilians, pterosaurs, and dinosaurs The lepidosaurs include tuataras, lizards, and snakes Reptiles provide stem for vertebrate evolution Reptilia - Birds • • Fossil record provides conclusive evidence that birds descended from a lineage of dinosaurs with a unique trait: feathers Molecular data beginning to support this idea as well – An adolescent female T. rex died 68 million years ago, but its bones still contain intact soft tissue, including the oldest preserved proteins ever found – A comparison of the protein's chemical structure to a slew of other species showed an evolutionary link between T. rex and chickens • Feathers – unique adaptation in birds – Used for display and insulation – Also furnish lift power, and steering required for flight Archaeopteryx (Cretaceous) • Reptile-like traits: – Teeth – Tail w/ many vertebrae – Wing claws Birds • Warm-blooded • Feeding: – Most are omnivores – Size and shape of beak correlates sharply with diet • Bird-like traits: – Wings – Feathers • Reproduction: – Oviparous – Provide extensive parental care • Contour • Down Mammals • Have fur or hair – Insulates body • Like birds, mammals are warm blooded – Result of oxidation of large amounts of food, generating heat – Endothermy is an adaptation that allows for high levels of activity Mammals • Evolved at a time when dinosaurs and other reptiles dominated the planet • Earliest mammalian fossil is 195my old • Mammals diversified after the dinosaurs became extinct – Diversified into small and large herbivores, predators, or marine hunters • Earliest mammals were small, and probably mostly nocturnal – Many modern species of mammal have good nocturnal vision and strong sense of smell • Have mammary glands – – – – Unique structure that makes lactation possible Gives ability to provide extensive parental care Mammals are also only animals with facial muscles and lips Only vertebrates with lower jaw made from a single bone Mammals form a monophyletic group Monotremata • • • • • Most ancient lineage of mammals Found only in Australia and New Zealand Egg-laying mammals Three species exist: 1 Platypus and 2 echidna Have leathery beak or bill Marsupiala Eutheria – Placental Mammals • Distributed worldwide • Most species-rich and diverse group of mammals • Divided into 18 lineages • Live in Australian region and in the Americas • Females have placenta, but young are born after a short embryonic period, poorly develop • Complete their development within attached to mother’s nipple – Six most species rich are: rodents, bats, insectivores (shrews, etc), artiodactyls (pigs, hippos, whales, etc), carnivores (dogs, cats, bears, etc), and primates • Reproduction: internal fertilization, viviparous – Extensive placenta – At birth, young are well developed • Size and structure of teeth correlate closely with diet – Typically within a pouch, called a marsupium A Key Eutherian Lineage: the Primates Homo sapiens • Two main groups – Prosimians (lemurs, tarsiers, pottos and lorises) – Anthropoids (New World Monkeys, Old World Monkeys, gibbons, and Great Apes – or Hominids) • Primates distinguished by having eyes on front of face • Hands and feet efficient at grasping – Anthropoids have opposable thumbs • Large brains relative to body size • Complex social behavior • Extensive parental care • Humans diverged from their closest relative (sister species), chimps, 6-7my ago • Since then, at least 14 human-like species (hominins) have existed • Several of these lived in Africa at the same time • Most lineages went extinct without leaving descendant populations • Humans are the only surviving representative of the initial “out of Africa” adaptive radiation You! (modern day) An evolving world • Earth is ancient, has undergone extensive changes • All forms of life undergo changes – species evolve at different rates – >90% of all life forms to live on Earth are extinct • Humans too, have evolved and continue to do so! Dinkenesh (Lucy) – 3.2 mya