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Animal Evolution – The Vertebrates. Chapter 23. Chordates. Most are coelomate, bilateral animals All share four features: Notochord supports body Nervous system develops from dorsal nerve cord Embryos have pharynx with slits Embryos have tail that extends past anus. Lancelet Body Plan.

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Animal Evolution – The Vertebrates

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  • Most are coelomate, bilateral animals
  • All share four features:
    • Notochord supports body
    • Nervous system develops from dorsal nerve cord
    • Embryos have pharynx with slits
    • Embryos have tail that extends past anus
lancelet body plan
Lancelet Body Plan





invertebrate chordates
Invertebrate Chordates
  • Many of the animals that preceded vertebrates were like the simplest chordates – the urochordates
    • Sea squirts
    • Other tunicates
larval form of a sea squirt
Larval Form of a Sea Squirt


nerve cord


adult tunicate

atrial opening (water out)

oral opening (water in)

Adult Tunicate

pharynx with gill slits

  • Lancelets
  • Fish-shaped filter feeders that lie buried in sediments
  • Chordate characteristics of adult:
    • Notochord lies under dorsal nerve cord
    • Pharynx has gill slits
    • Tail extends past anus
hagfish body plan
Hagfish Body Plan


gill slits (twelve pairs)

mucus glands

trends in the evolution of vertebrates
Trends in the Evolution of Vertebrates
  • Shift from notochord to vertebral column
  • Nerve cord expanded into brain
  • Evolution of jaws
  • Paired fins evolved, gave rise to limbs
  • Gills evolved, gave rise to lungs
  • Cranium is a chamber of cartilage or bone that encloses all or part of a brain
  • First craniates evolved by 530 million years ago
evolution of jaws
Evolution of Jaws
  • First fishes lacked jaws
  • Jaws are modifications of the anterior gill supports
evolution of fishes
Evolution of Fishes

cartilaginous fishes

ray-finned fishes

lobe-finned fishes












jawed vertebrates

lungs or swim bladder



ancestral chordates

jawed fishes
Jawed Fishes
  • Most diverse and numerous group of vertebrates
  • Two classes:
    • Cartilaginous fishes
    • Bony fishes
cartilaginous fishes
Cartilaginous Fishes
  • Most are marine predators
  • Cartilaginous skeleton
  • Main groups:
    • Skates and rays
    • Sharks
    • Chimaeras (ratfishes)
bony fishes
Bony Fishes
  • Includes 96 percent of living fish species
  • Three subclasses:
    • Ray-finned fishes
    • Lobe-finned fishes
    • Lung fishes
evolution of amphibians
Evolution of Amphibians
  • Lobe-finned fishes arose during the early Devonian
  • Used their fins to travel over land from pool to pool
early amphibians
Early Amphibians
  • Lungs became more effective
  • Chambers of the heart became partially separated, making circulation more efficient
modern amphibians
Modern Amphibians
  • All require water at some stage in the life cycle; most lay eggs in water
  • Lungs are less efficient than those of other vertebrates
  • Skin serves as respiratory organ
living amphibian groups
Living Amphibian Groups
  • Frogs and toads
  • Salamanders
  • Caecilians
evolution of reptiles
Evolution of Reptiles
  • Reptiles arose from amphibians in the Carboniferous
  • Adaptations to life on land
    • Tough, scaly skin
    • Internal fertilization
    • Amniote eggs
    • Water-conserving kidneys
reptilian radiation
Reptilian Radiation
  • Adaptive radiation produced numerous lineages
  • Extinct groups include:
    • Therapsids (ancestors of mammals)
    • Marine plesiosaurs & ichthyosaurs
    • Dinosaurs and pterosaurs
living reptiles
Living Reptiles

Four orders made it to the present day:




Snakes and lizards

crocodile body plan
Crocodile Body Plan









turtles and tortoises
Turtles and Tortoises
  • Armorlike shell
  • Horny plates instead of teeth
  • Lay eggs on land
  • Only two living species
  • Live on islands off the coast of New Zealand
  • Look like lizards, but resemble amphibians in some aspects of their brain and in their way of walking
lizards and snakes
Lizards and Snakes
  • Largest order (95 percent of living reptiles)
  • Most lizards are insectivores with small peglike teeth
  • All snakes are carnivores with highly movable jaws

venom gland

hollow fang

  • Only birds have feathers
  • Arose from reptilian ancestors
    • Feathers are highly modified reptilian scales
amniote eggs
Amniote Eggs
  • Like reptiles, birds produce amniote eggs
  • Inside the egg, the embryo is enclosed in a membrane called the amnion
  • Amnion protects the embryo from drying out
adapted for flight
Adapted for Flight
  • Four-chambered heart
  • Highly efficient respiratory system
  • Lightweight bones with air spaces
  • Powerful muscles attach to the keel
  • Hair
  • Mammary glands
  • Distinctive teeth
  • Highly developed brain
  • Extended care for the young
mammalian origins
Mammalian Origins
  • 200 million years ago, during the Triassic, synapsids gave rise to therapsids
  • Therapsids were the reptilian ancestors of mammals
  • The first mammals had evolved by the Jurassic
three mammalian lineages
Three Mammalian Lineages
  • Monotremes
    • Egg-laying mammals
  • Marsupials
    • Pouched mammals
  • Eutherians
    • Placental mammals
living monotremes
Living Monotremes
  • Three species
    • Duck-billed platypus
    • Two kinds of spiny anteater
  • All lay eggs
living marsupials
Living Marsupials
  • Most of the 260 species are native to Australia and nearby islands
  • Only the opossums are found in North America
  • Young are born in an undeveloped state and complete development in a permanent pouch on mother
living placental mammals
Living Placental Mammals
  • Most diverse mammalian group
  • Young develop in mother’s uterus
  • Placenta composed of maternal and fetal tissues; nourishes fetus, delivers oxygen, and removes wastes
  • Placental mammals develop more quickly than marsupials
earliest primates
Earliest Primates
  • Primates evolved more than 60 million years ago during the Paleocene
  • First primates resemble tree shrews
    • Long snouts
    • Poor daytime vision
  • Apes, humans, and extinct species of their lineages
  • In biochemistry and body form, humans are closer to apes than to monkeys
  • Hominids
    • Subgroup that includes humans and extinct humanlike species
from primates to humans
From Primates to Humans

“Uniquely” human traits evolved through modification of traits that evolved earlier, in ancestral forms

trends in lineage leading to humans
Trends in Lineage Leading to Humans
  • Less reliance on smell, more on vision
  • Skeletal changes to allow bipedalism
  • Modifications of hand to allow refined hand movements
  • Bow-shaped jaw and smaller teeth
  • Longer lifespan and longer period of dependency
adaptations to an arboreal lifestyle
Adaptations to anArboreal Lifestyle
  • During the Eocene, certain primates became adapted to life in trees
    • Better daytime vision
    • Shorter snout
    • Larger brain
    • Forward-directed eyes
    • Capacity for grasping motions
  • Earliest known is A. anamensis
  • A. afarensis and A. africanus arose next
  • All three were slightly built (gracile)
  • Species that arose later, A. boisei and A. robustus, had heavier builds
  • Exact family tree is not known
humans arise
Humans Arise
  • First member of the genus Homo is H. habilis
  • Lived in woodlands during late Miocene
homo erectus
Homo erectus
  • Evolved in Africa
  • Migrated into Europe and Asia about 1.5 million - 2 million years ago
  • Had a larger brain than H. habilis
  • Was a creative toolmaker
  • Built fires and used furs for clothing
homo sapiens
Homo sapiens
  • Modern man evolved by 160,000 years ago
  • Had smaller teeth and jaws than H. erectus
  • Facial bones were smaller, skull was larger
homo neanderthalensis
Homo Neanderthalensis
  • Early humans that lived in Europe and Near East
  • Massively built, with large brains
  • Disappeared when H. sapiens appeared
  • DNA evidence suggests that they did not contribute to modern European populations
where did h sapiens arise
Where Did H. sapiens Arise?
  • Two hypotheses:
    • Multiregional model
    • African emergence model
  • Both attempt to address both biochemical and fossil evidence
multiregional model
Multiregional Model
  • Argues that H. erectus migrated to many locations by about 1 million years ago
  • Geographically separated populations gave rise to phenotypically different races of H. sapiens in different locations
  • Gene flow prevented races from becoming species
african emergence model
African Emergence Model
  • Argues that H. sapiens arose in sub-Saharan Africa
  • H. sapiens migrated out of Africa and into regions where H. erectus had preceded them
  • Only after leaving Africa did phenotypic differences between races arise
earliest fossils are african
Earliest Fossils Are African
  • Africa appears to be the cradle of human evolution
  • No human fossils older than 1.8 million years exist anywhere but Africa
  • Homo erectus left Africa in waves from 2 million to 500,000 years ago