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History of Life-10-V-VII.Docx History of Life 18 V. Vertebrates. A. Archetypal Vertebrate. 1. Active swimmers; bilaterally symmetric. a. Segmented trunk muscles run the length of the ani- mal – contract one side at a time. Early Cambrian jawless fish, b. Notochord / vertebral col- Myllokunmingia recently dis- umn – support. covered in China. c. Muscles supported by ribs that attach to vertebral centra that surround the noto- chord. d. Hollow dorsal nerve (spinal) cord enclosed by vertebral arches extends backward to the tail from anterior brain – also hollow. e. Sense organs concentrated up front (cephalized): • Eyes: Two lateral (image forming) / One dorsal median (principally light-sensing). • Nose • Internal ears. f. No paired fins. History of Life 19 2. Mouth leads to pharynx with gill slits. a. Internal gills used for filter feeding as well as respiration. b. Water flows in through the mouth, over the gills and then out through the gill slits. c. No jaws. 3. Primitive bladder / lung connected to pharynx – used for buoyancy / respiration. 4. Ventral heart pumps blood anterior to the gills where it is ox- ygenated and then delivered to tissues via arteries, arterioles, capillaries and then back to the heart. a. Second loop may have conducted deoxygenated blood to bladder / lung as needed, in which case, atrium may have been partially divided. b. In most modern fish, bladder / lung has become a swim bladder disconnected from pharynx. c. 2nd arterial loop – if it existed – lost. 5. Internal organs including segmented nephrons, in coelom – supported by mesenteries attached to ventral ribs – “tube within a tube” construction. 6. Post-anal tail extends beyond the coelom. History of Life 20 Archetypal vertebrate in sagittal section. Note the dorsal nerve cord, notochord and pharyngeal gill slits. Vertebral column, ribs not shown. From Romer, A. S. 1964. The Vertebrate Body . W.B. Saunders. Phila- delphia. History of Life 21 B. Invertebrate chordates. 1. Hemichordates – acorn (probos- cis) “worms” and pterobranchs. a. Three part body structure b. Proboscis captures food and di- rects to mouth, which is in the collar. c. Pharyngeal gill slits in trunk used for gas exchange. 2. Urochordates – tunicates (sea squirts) a. Motile larva; sessile adult. b. Extract food from water passing through gill basket using mu- cous secreted by endostyle 3. Cephalochordates – lancelets (Amphioxus ). a. Closest living form to a primi- tive chordate. • Segmented trunk musculature. Invertebrate Chordates. • Notochord. Top. Acorn worm (Hemi- • Gills / gill slits. chordata).. Middle. Tuni- • cate (Urochordata). A. Mo- Segmented gonads / nephridia tile larva with notochord. (excretory structures). B. Sessile adult with gill basket and endostyle. Bot- tom. Amphioxus . (Cepha- lochordata). History of Life 22 b. More primitive than fish. Lacks • Brain / sense organs. • Respiration through skin. • Heart, capillaries, hemoglobin, RBCs • Sedentary. c. Filter feeder – endostyle secretes mucous as in tunicates. d. Fossils from Burgess Shale (mid-Cambrian). B. One theory of chordate origins is that they arose from tunicate larva via neoteny. 1. Retention of larval notochord & trunk muscles. 2. Integration of gill basket / visceral structures with somatic structures (notochord / trunk muscles). C. An alternative scenario is that tuni- cates are descended from a motile, Pterobranch. Note dorsal nerve cord and gill slit. bilaterian ancestor. In this case, 1. Sedentary habit of adults is a derived character 2. Larval motility reflects ancestral state. History of Life 23 Tunicate larva scenario of vertebrate evolution. History of Life 24 Early Vertebrate Evolution. A. Cephalochordate Origins. B. Phylogeny according to Purves et al . Rejects 1. Freshwater origins. 2. Tetrapod descent from crossopte- rygians. Vertebrate phylogeny ac- C. Agnatha – Jawless fish. cording to Purves et al . 1. Date to Cambrian; common by Ordovician. 2. Had bony skeletons. 3. Lacked jaws / paired fins. 4. Ostracoderms. a. External head armor; possible defense against sea scorpions. b. In some, small spines at points where paired fins develop in more advanced forms. c. Cephalapsis type flattened dorso-ventrally with expanded Fossil Ostracoderms. From gill basket. Romer (1964). d. Hemicyclaspis had flipper-like structures in lieu of pectoral fins. History of Life 25 5. Contemporary cyclostomes (lam- preys and hagfish) degenerate. a. Bony skeleton lost. b. Hagfish marine – bottom scaven- gers. • Lampreys anadromous. • Sessile, filter-feeding urochor- date-like larva. • Adults parasitize “real” fish. Devonian placoderms. A. D. Placoderms – first jawed fishes. “Spiny shark; B. Arthrodire. C. st 1. 1 appear in Silurian; now extinct. Antiarch with pectoral flippers. From Romer (1964). 2. Jaws developed from gill arches. 3. Ossified skeleton / paired fins in some. 4. Several groups. a. Giant, giant parrot-beaked ar- throdires with jointed neck and head armor. b. So-called “spiny sharks” – acanthodians – close to ancestry Palaeoniscid chondrosteans. A. Extinct Palaeonsicus . B. of modern fish. Living Polypterus . From Romer (1964). History of Life 26 E. Bony fish – two principle groups. 1. Actinopterygii – ray fins. 2. Sarcopterygii – lobe fins. F. Actinopterygii. 1. Chondrosteii. a. Palaeoniscids. • Living example Polypterus – has paired, ventral lungs connected to throat, as opposed to a single, dorsal blad- der. • Suggests lungs a primitive bony fish trait. b. Paddlefish and sturgeons – mostly cartilaginous skeletons, feeble jaws – sensitive rostrum anterior to the mouth. 2. Holostei. a. Mid-Mesozoic origins. b. Spread from freshwater to ma- rine environments. c. Surviving forms are garpike and bowfin. 3. Teleosts. Chondrosteans. A. Paddlefish. B. a. Vast majority of living fishes. Sturgeon. From Romer (1964). b. Replace Holosteans by end of Mesozoic. c. Primitive forms include herring and salmonids. d. Advanced forms tuck pelvic fins under pectoral. History of Life 27 G. Sarcopterygii. 1. Lungfish (Dipnoi) 2. Crossopterygii. a. Rhipidistians (Devonian). • Dominant FW predators. • Ancestral to tetrapods. • Fins evolved into limbs. b. Coaelacanths. • Secondarily marine. • Surviving Latimeria lives at Crossopterygians. A. Rhipidisian from the Devonian. B. Living Lat- depth. imeria . From Romer (1964). History of Life 28 From fins to legs. Pectoral fin structure of recently dis- covered Tiktaalik is almost perfectly intermediate be- tween that of rhipidistian lobe fin fishes and the legs of labyrthinthodont amphibians. To the lobe fin humerus (red), radius (blue) and ulna (green), Tiktaalik adds dis- cernable wrist elements. From R. Dalton. 2006. The fish that crawled out of the water. Nature (published online 5 April, 2006 | doi 10: 1038/news060403-7). History of Life 29 H. Labyrinthodont Amphibia. 1. Rhipidistian ancestry indicated by tooth structure unique to Rhipid- istians, Labyrinthodonts and Coty- losaurs (stem reptiles). 2. Shared characters: hinged brain- case; internal nares; pineal “eye.” Labyrthinthodont tooth 3. A good example of a case where in cross section. Note the grades are useful (IMHO). elaborate infolding of the dentine and enamel. I. Reptiles. 1. Dispensed with aquatic larval stage – amniotic egg . 2. Four principle groups distin- guished by temporal fossae. a. Anapsida – no opening – stem reptiles, turtles. b. Synapsida – lower opening – bounded above by postorbital and squamosal bones – mam- mal-like reptiles. c. Parapsida – upper opening – bounded below by postorbital Labyrinthodontia. A use- and s quamosal – extinct. ful paraphyletic group. d. Diapsida – two openings – rhyncocephalians (tuatara), Ar- chosauria (dinsoaurs, birds), lizards and snakes. History of Life 30 Reptile Skull Types (schematic). A. Anapsid type – stem rep- tiles, turtles. B. Synapsid type – mammal-like reptiles. C. Par- apsid type– extinct pleisiosaurs, etc . D. Diapsid type – rhyn- cocephalians, dinosaurs, birds, snakes and lizards. From Romer (1964). History of Life 31 Simplified schematic of the amniotic egg. Gas exchange with the ex- ternal environment via porous shell, which is impermeable to water, but not air. History of Life 32 VII. Evolution of Mammals. A. Synapsid reptiles antecedent to mammals. Include 1. Pelycosaurs (late Carboniferous, Permian). a. So-called “sail lizards” b. Dimetrodon , Edaphosaurus . 2. Therapsids (late Permian, early Triassic). a. Anomodonts – herbivorous forms. b. Theriodonts– “Mammal- like” reptiles i. Name - “beast-tooth” – reflects differentiation of teeth as observed in A therocephalian (cynodont sister mammals. clade) close to the ancestry of ii. Include cynodonts – di- mammals. rect ancestors of mammals. Right. Overview of mammalian evolution. At present, it is generally believed that mammals constitute a monophyletic group, with therian (placental) mammals separating from non- therians (monotremes and marsupials) in the Jurassic. From Crompton, A. W. and F. A. Jenkins. 1973. Mammals from reptiles: a review of mammal origins. Ann. Rev. Earth Planet. Sci. 1: 131-155. History of Life 33 History of Life 34 B. True mammals (and 1 st dinosaurs) appear in Triassic. C. Theriodont evolution reflects acquisition of more active life style . 1. Locomotion . a. Legs tucked under the body. b. Chest deepened – anterior ribs expanded; posterior ribs lost. 2. Alternating contraction of trunk muscles replaced by “back and forth” motion of legs. 3. Tooth differentiation . a. Incisors, canines, premolars, molars; cusps on molars. b. Precise occlusion. 4. Bony secondary palette. D. Hypertrophy of the coronoid
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