V. Vertebrates. A. Archetypal Vertebrate.

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1 History of Life 18 V. Vertebrates. A. Archetypal Vertebrate. 1. Active swimmers; bilaterally symmetric. a. Segmented trunk muscles run the length of the animal contract one side at a time. b. Notochord / vertebral column support. c. Muscles supported by ribs Early Cambrian jawless fish, Myllokunmingia recently discovered in China. that attach to vertebral centra that surround the notochord. 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.

2 History of Life 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 oxygenated 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. 2 nd 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.

3 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 The Vertebrate Body. W.B. Saunders. Philadelphia.

4 History of Life 21 B. Invertebrate chordates. 1. Hemichordates acorn (proboscis) worms and pterobranchs. a. Three part body structure b. Proboscis captures food and directs 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 mucous secreted by endostyle 3. Cephalochordates lancelets (Amphioxus). a. Closest living form to a primitive chordate. Segmented trunk musculature. Notochord. Gills / gill slits. Segmented gonads / nephridia (excretory structures). Invertebrate Chordates. Top. Acorn worm (Hemichordata).. Middle. Tunicate (Urochordata). A. Motile larva with notochord. B. Sessile adult with gill basket and endostyle. Bottom. Amphioxus. (Cephalochordata).

5 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 tunicates 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.

6 History of Life 23 Tunicate larva scenario of vertebrate evolution.

7 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 crossopterygians. C. Agnatha Jawless fish. 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 gill basket. d. Hemicyclaspis had flipper-like structures in lieu of pectoral fins. Vertebrate phylogeny according to Purves et al. Fossil Ostracoderms. From Romer (1964).

8 History of Life Contemporary cyclostomes (lampreys and hagfish) degenerate. a. Bony skeleton lost. b. Hagfish marine bottom scavengers. Lampreys anadromous. Sessile, filter-feeding urochordate-like larva. Adults parasitize real fish. D. Placoderms first jawed fishes st appear in Silurian; now extinct. 2. Jaws developed from gill arches. 3. Ossified skeleton / paired fins in some. 4. Several groups. a. Giant, giant parrot-beaked arthrodires with jointed neck and head armor. b. So-called spiny sharks acanthodians close to ancestry of modern fish. Devonian placoderms. A. Spiny shark; B. Arthrodire. C. Antiarch with pectoral flippers. From Romer (1964). Palaeoniscid chondrosteans. A. Extinct Palaeonsicus. B. Living Polypterus. From Romer (1964).

9 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 bladder. 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 marine 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.

10 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 depth. Crossopterygians. A. Rhipidisian from the Devonian. B. Living Latimeria. From Romer (1964).

11 History of Life 28 From fins to legs. Pectoral fin structure of recently discovered Tiktaalik is almost perfectly intermediate between 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 discernable wrist elements. From R. Dalton The fish that crawled out of the water. Nature (published online 5 April, 2006 doi 10: 1038/news ).

12 History of Life 29 H. Labyrinthodont Amphibia. 1. Rhipidistian ancestry indicated by tooth structure unique to Rhipidistians, Labyrinthodonts and Cotylosaurs (stem reptiles). 2. Shared characters: hinged braincase; internal nares; pineal eye. 3. A good example of a case where grades are useful (IMHO). I. Reptiles. 1. Dispensed with aquatic larval stage amniotic egg. 2. Four principle groups distinguished by temporal fossae. a. Anapsida no opening stem reptiles, turtles. b. Synapsida lower opening bounded above by postorbital and squamosal bones mammal-like reptiles. c. Parapsida upper opening bounded below by postorbital and squamosal extinct. Labyrthinthodont tooth in cross section. Note the elaborate infolding of the dentine and enamel. Labyrinthodontia. A useful paraphyletic group. d. Diapsida two openings rhyncocephalians (tuatara), Archosauria (dinsoaurs, birds), lizards and snakes.

13 History of Life 30. Reptile Skull Types (schematic). A. Anapsid type stem reptiles, turtles. B. Synapsid type mammal-like reptiles. C. Parapsid type extinct pleisiosaurs, etc. D. Diapsid type rhyncocephalians, dinosaurs, birds, snakes and lizards. From Romer (1964).

14 History of Life 31 Simplified schematic of the amniotic egg. Gas exchange with the external environment via porous shell, which is impermeable to water, but not air.

15 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 Mammallike 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 direct ancestors of mammals. mammals. Right. Overview of mammalian evolution. At present, it is generally believed that mammals constitute a monophyletic group, with therian (placental) mammals separating from nontherians (monotremes and marsupials) in the Jurassic. From Crompton, A. W. and F. A. Jenkins Mammals from reptiles: a review of mammal origins. Ann. Rev. Earth Planet. Sci. 1:

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17 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 process of dentary tooth bearing bone of the lower jaw. 1. Correlated expansion of a. Temporal fossa. b. Jaw adductor muscles. 2. Reduction of post-dentary bones on the lower jaw 3. Replacement of articular-quadrate joint with dentarysquamosal.

18 History of Life 35 Hylonumus. Primitive anapsid from early Pennsylvaian Note sprawling gait. Length about 25 cm. From Ostrom, J. H A history of vertebrate success. Pp In, Sachopf, J. W. Major Events in the History of Life. Bartlett Jones. London. Thrinaxodon. An advanced early Triassic cynodont. Length about 60 cm. From Ostrom, J. H A history of vertebrate success. Pp In, Schopf, J. W. Major Events in the History of Life. Bartlett Jones. London.

19 History of Life 36 Table 1. Some Reptile-Mammal Comparisons # Character Reptile Mammal 1 Skull Fenestrae None Massive. 2 Braincase Loosely attached Firmly attached 3 Secondary palette None Complete, bony 4 Dentition Undifferentiated Differentiated 5 Cheek teeth Uncrowned points Crowned and cusped 6 Tooth replacement Continuous At most, once 7 Tooth roots Single Molars doublerooted 8 Jaw articulation Articularquadrate Dentary-squamosal 9 Lower jaw Many bones Single bone 10 Middle ear bones Stapes 11 External nares Joined Separate 12 Occipital condyle Single Double 13 Cervical ribs Long Reduced 14 Lumbar ribs Present Absent 15 Diaphragm Absent Present 16 Limbs Sprawled out Under body Stapes, incus, malleus

20 History of Life Scapula Simple Large spine for muscles 18 Pelvic bones Unfused Fused 19 Sacral vertebrae Two Three of more 20 Toe bone #s Body temperature Variable Constant

21 History of Life 38 E. Conversion of articular and quadrate bones to malleus and incus (middle ear bones) and angular to tympanic annulus. 1. One of the great examples of evolutionary transition. 2. In reptiles, the lower jaw consists of the toothbearing dentary plus postdentary bones. 3. Reptilian jaw hinge involves articular (jaw) and quadrate (skull). Probainognathus. An advanced cynodont with a double jaw articulation. Modified from Carroll, R. L Vertebrate Paleontology and Evolution. W. H. Freeman. NY. 4. With expansion of dentary, a. Post-dentary bones reduced. b. Dentary-squamosal hinge develops. c. Articular (malleus) and quadrate (incus) become middle ear bones. 5. Amazingly, intermediate forms with two functional joints have been found.

22 History of Life 39 Double jaw articulation in Probainognathus. The jaw joint is composed of quadrate and squamosal (upper) and articular and dentary (lower). Because the reptilian elements are still functional, this animal is considered to be a reptile. From Carroll (1988).

23 History of Life 40 F. All of these changes believed to have been driven by selection for 1. Higher activity levels, BMR, possibly reflecting greater reliance by mammals on aerobic respiration. 2. Enhanced high frequency hearing. 3. Questions remain as to degree of parallelism vs. synapomorphy in various therapsid lineages. G. Therapsid extinction. 1. Therapsids were dominant tetrapods during the Permian. 2. Numbers decline during Triassic. 3. Two hypotheses: a. Outcompeted by dinosaurs. b. Opportunistic replacement mediated by extinction. 4. Fossil record does not permit us to distinguish between competive exclusion and environmentally-mediated extinction, but a. Considerable size overlap between therapsids and thecodont ancestors of dinosaurs. b. Virtually no overlap between mammals and dinosaurs. c. During the second half of the Trias, the lineages leading to mammals got small; while those leading to dinosaurs got large.

24 History of Life 41 Synapsid and Diapsid Reptiles in the Permian and Triassic. From M. J. Benton Dinosaur success in the Triassic: A noncompetitive ecological model. Quart. Rev. Biol. 58:

25 History of Life 42 H. Mid-Mesozoic gap. 1. Mammalian grade of organization achieved by late Triassic. 2. For the most part, mammals remained small and insignificant for 150 million years. 3. Was what would otherwise have been a smooth progression from pelycosaurs to therapsids and cynodonts to placental mammals disrupted by the dinosaur interregnum? I. Dino-enthusiasts point to the following postulated traits that could have resulted in archosaurian superiority: 1. Endothermy and high metabolic rate at least in small predators. 2. Sopohisticated social behavior: a. Flocks / herds. b. Pack hunting. c. Maternal care / family groups. d. Rapid locomotion. Deinonychus. A relative of Velociraptor, star of Jurassic Park. 3. Perhaps best explanation is that Mesozoic mammals remained small because larger ones would have been eaten.