The Origin and Evolution of Vertebrates

Ch. 34

Chordates

• Bilaterally symmetrical • Deuterostomes

Derived Characters of Chordates

• Notochord • Dorsal, hollow nerve cord • Pharyngeal slits/clefts • Muscular postanal tail

Notochord

• A longitudinal, flexible rod located between the gut and nerve cord • Present in all chordate embryos • Composed of large, fluid filled cells encased in a stiff, fibrous tissue • Extends throughout most of the length of the animal as a simple skeleton • Jointed skeleton develops around notochord – Remains seen in intervertebral disks

Dorsal Nerve Cord

• Hollow, dorsal to the notochord • Brain and spinal cord develops from this primitive nerve cord

Pharyngeal Slits or Clefts

• Opening of pharynx to the outside through several pairs of slits • Permits water to enter the mouth to exit without passing through the entire digestive system • May serve as suspension feeding devices in invertebrates • Modified for gas exchange in aquatic vertebrates • Become part of the ear (eustachian tube) in terrestrial vertebrates

Muscular Postanal Tail

• Extends beyond anus • Contains skeletal elements and muscles • Propulsive force in aquatic species

Subphylum Cephalochordata

• Most basal (earliest ) group • Lancelets • Adult – possess all 4 chordate characteristics • Marine filter feeders – use pharyngeal slits • Feeble swimmers with fishlike motions • Live in sand

Subphylum Urochordata

• Tunicates (sea squirts) • Cloaked in a celluloselike tunic • Sessile • Some are colonial • Filter feeders – incurrent siphon  excurrent siphon • Larva – free swimming slits in pharynx – possess all 4 chordate characteristics • Adults – lack a notochord, a nerve cord, and tail – only possess pharyngeal slits 

Derived Characters of Craniates Craniates • Chordates with a head – Brain at anterior end of dorsal hollow nerve cord, eyes, sensory organs, skull – Allowed for complex movement and feeding patterns Neural Crest • Collection of cells that appears near the dorsal margins of the closing neural tube in an embryo • Teeth, bones of skull, dermis of face, types of neurons, sensory capsules

Derived Characters of Craniates • Gill slits that allow water to be pumped through the slits for sucking in food and gas exchange • Higher metabolic rate • Extensive muscular system • Heart with at least 2 chambers • Red blood cells with hemoglobin • Kidneys to clean waste from blood

Agnathans

• Two Classes – Myxini – Petromysontida • Jawless vertebrates –oval, slitlike mouth • Lacked paired fins • Bottom-dwellers • Some were active and had paired fins

Myxini (Hagfish)

• Basal group • Skull of cartilage • Retain notochord • Small brain, eyes, ears, and a nasal opening • Scavengers • Slime glands

Derived Characters of Vertebrates

• More extensive skull • Vertebrae of bone • Dorsal, ventral, and anal fins in aquatic species • Pronounced cephalization – Brain and sense organs at anterior end – Attached to the nerve cord

Derived Characters of Vertebrates Vertebrate Skeleton Axial skeleton • cranium, vertebrae, ribs, sternum, hyoid • main support for larger body size and fast movements Appendicular skeleton • girdles and two pairs of appendages (fins, legs, or arms)

Derived Characters of Vertebrates

• Closed Circulatory System • Ventral chambered heart – 2-4 chambers • Arteries – carry blood away from the heart – blood is oxygenated in lungs/gills • Capillaries – site of gas exchange and diffusion of other substances • Veins – carry blood back to the heart

Derived Characters of Vertebrates

• Greater metabolic demands • Generation of ATP consumes oxygen – adaptations exist such as lots of mitochondria in muscle tissues • More active  larger amounts of organic molecules needed – adaptations for feeding, digestion, and nutrient absorption • Closed circulatory system

Vertebrate Diversity

• Class Agnatha – Now split into Myxini and Petromyzontida • Class Chondrichthyes • Class Osteichthyes – Now split into Actinopterygii, Sarcopterygii (Actinistia and Dipnoi) • Class Amphibia • Class Reptilia • Class Aves • Class Mammalia

Vertebrate Diversity

Classes Grouped by… • Column • Jaws • Lungs • Legs • Amniotic egg • Milk • Feathers

Vertebrate Diversity

Lampreys (Petromyzontida) • Most basal lineage of vertebrates • Marine and freshwater • Most are parasites • Cartilage skeleton • Notochord is the axial skeleton

Derived Characters of Gnathostomes • Vertebrates with jaws • Enlarged forebrain • Lateral line system in aquatic vertebrates

Class Chondrichthyes

• Sharks, Rays, and their relatives • Cartilage fishes • Skeleton strengthened by calcified granules • Tail for propulsion • Dorsal fins are stabilizers • Pectoral and pelvic fins – lift • Most are carnivorous some are suspension feeders • Sharp, colorless vision

Class Chondrichthyes

• Teeth evolved from modified scales (placoid) • Lateral line system • External fertilization – oviparous, ovoviviparous • Internal fertilization via claspers – few are viviparous • Cloaca is present

finned fishes • Bony skeleton • Scales buoyancy – oviparous

Osteichthyans

• Ray-finned and lobe • Mucus glands in the skin • Lateral line system • Operculum covering gills • Swim bladder for • External fertilization

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Osteichthyans

Class Actinopterygii – ray finned • Major source of protein for humans

Osteichthyans

• Sarcopterygii – lobe finned – Rod shaped bones surround by a thick layer of muscle in their pelvic and pectoral fins Actinistia • Coelancanths Dipnoi • lung fishes • ancestor of amphibians and all other tetrapods

Derived Characters of Tetrapods

• Four feet – Limbs with digits • Head separated from body by a neck • Pelvic girdle bones fused to backbone

Amphibians •“both ways of Life”

Class Amphibia

Order Urodela (“tailed ones”) • salamanders, newts Order Anura (“tail-less ones”) • frogs, toads Order Apoda (“legless ones”) • caecilians

Class Amphibia

• Metamorphosis • Tied to water for reproduction • Dioecious – sexually, external fertilization – Amplexus • 3 chambered heart • Cutaneous and pulmonary respiration

Derived Characters of Amniotes

Amniotic egg • Adaptation for a terrestrial habitat • Prevents desiccation Monophyletic group the amniotes • Reptiles, birds • mammals (although most have dispensed with the shell)

Amniotic Egg

Class Reptilia

Order Chelonia • turtles Order Squamata • lizards and snakes Order Crocodilia • alligators and crocodiles

Class Reptilia

• Keratin scales • Gas exchange via lungs • Most are dioecious – internal fertilization – oviparous (most); some are ovoviviparous • Ectothermic • 3 chambered heart with a partially divided ventricle

Class Aves

• Birds – Archeopteryx • Anatomy modified to enhance flight – Bones have a honeycombed structure – No urinary bladder – One ovary – no teeth (reduces weight) – gizzard

• Endothermic • 4 chambered heart

Class Aves

• Dioecious – internal fertilization – incubation • Feathers made of keratin • Sternal keel

Class Aves

Ratites • flightless b/c no keel • ostrich, rhea, emu, kiwi, cassowary Carinates • sternal keel present • fly • exception - penguins

Derived Characters of Mammals

• Hair made of keratin • Endothermic – Fat layer under skin • High metabolic rate • Mammary glands • Large brains, capable learners • Four chambered heart • Differentiated teeth • Efficient lungs – Alveoli, diaphragm • Dioecious – internal fertilization – most are viviparous

Monotremes

Oviparous Australia and New Guinea Platypuses, echidnas (spiny anteater) Milk comes from fur b/c they lack nipples

Marsupials

Marsupiam (maternal pouch) Opossum, koala, kangaroos

Eutherians

Placental Mammals Viviparous Placenta Longer gestation

Convergent Evolution

Derived Characters of Primates • Hands and feet adapted for grasping • Opposable thumb • Large brain • Short jaws • Forward facing, close together eyes • Well developed parental care

Derived Characters of Humans • Large Brain • Bipedal locomotion • Language, symbolic thought, artistic expression, manufacture and use of tools