The First Vertebrates

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The First Vertebrates

• Phylum Chordata

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• Kingdom
• Phylum
• Class
• Order
• Genus
• Species

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5 Kingdoms10-100 million species
Monera
Protista
Fungi
Plantae
Animalia
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Kingdom Animalia

• 3-30 million animal species
• A few of the 33 phyla

Annelida
Echinodermata
Arthropoda
Mollusca
Porifera
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What makes an organism an animal?

• Eukaryotic (cells have a nucleus)
• No cell walls
• Multicellular
• Requires oxygen
• Heterotrophic (cant make its own food like
  plants do)

Susan Middleton
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Phylum Chordata

• 3 Subphyla
• 3.7 of animal species

Urochordata
Cephalochordata
Vertebrata
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What makes an animal a chordate?(5
characteristics that all chordates have at some
point in their livesmany only as embryos)

• Notochord
• Used to provide rigidity to body
• Present in vertebrate embryos, replaced by
  vertebral column later

Vertebral (spinal) column
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What makes an animal a chordate?

• Dorsal tubular nerve cord
• Dorsal back
• Tubular tube-shaped
• Protected by notochord
• Think spinal cord

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What makes an animal a chordate?

• Pharyngeal gill slits
• Openings between the pharynx (throat) and the
  outside
• For filter-feeding in primitive chordates
• Gas exchange in fish and larval amphibians
• Only present in embryos of terrestrial chordates
  (and not used for breathing in fetuses)

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What makes an animal a chordate?

• Segmented, muscular, post-anal tail
• Post-anal means extending out beyond the
  digestive system
• Yes, humans, too!

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What makes an animal a chordate?

• Endostyle
• Ciliated, glandular groove on the floor of the
  pharynx (throat region) that secretes mucus for
  trapping food particles
• In vertebrates it is the thyroid gland

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Subphylum Urochordata2000 species

• Animals called tunicates, or sea squirts
• Marine filter feeders
• Most are sedentary as adults
• Swimming larvae that resemble tadpoles

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Subphylum Cephalochordata22 species

• Animals called lancelets
• Less than 5 cm long
• Marine filter feeders
• Most are sedentary adults
• Swimming larvae that resemble tadpoles

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Subphylum Vertebrata7 Classes
Agnatha
Amphibia
Aves
Chondrichthyes
Reptilia
Mammalia
Osteichthyes
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What makes a chordate a vertebrate?

• Distinct vertebral column (instead of just a
  notochord)
• Distinct head region (other 2 subphyla do not
  have heads)
• Distinct skull
• In general, vertebrates are larger and more
  active than other chordates

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According to the theory of evolution
How did this
evolve from this?
sea squirt larva
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Theories of vertebrate evolution

• There are NO transitional fossils to provide
  evidence of an intermediate between invertebrates
  and vertebrates
• 2 popular theories of vertebrate evolution.

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The Amphioxus TheorySubphylum Cephalochordata

• Vertebrates may have arisen from an ancestor like
  the lancelet species Amphioxus
• Small, eel-like animals that burrow in the sand
• Not a popular theory

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Cephalochordata vs. Vertebrates

• Similarities
• Notochord allows fish-like propulsion
• Closed circulatory system
• Nerve chord expanded at end
• Spinal nerves similar to vertebrates

• Differences
• Excretory system completely different
• Notochord extends too far anteriorly
• Not enough cephalization (head not big enough)

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The Tunicate Theorysubphylum Urochordata

• Adult sea squirts are very different from
  vertebrates
• BUT the young are tadpole-like

Adult tunicate
Larval tunicate
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Paedogenesis

• Reproduction by a juvenile
• Occurs when a larva does not metamorphosize for
  some reason, but becomes reproductively mature as
  it is
• Occasionally occurs in insects and amphibians

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The tunicate theory

• If paedogenesis occurred in sea squirt larvae,
  and the larvae reproduced successfully
• then natural selection may have reinforced
  this characteristic and the first vertebrate
  species would have evolved.
• The very first vertebrates were thought to have
  been small animals that could absorb oxygen
  through their thin skin.

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Earliest evidence of vertebrates

• 500 million year old fossils from China were
  found in late 1990s
• 2 species called Myllokumingia and Haikouichthys
• 3cm long, fish-shaped

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Vocabulary

• Cartilage
• Multicellular
• Heterotrophic
• Eukaryotic
• Notochord
• Vertebral Column
• Dorsal
• Ventral
• Tubular

• Pharyngeal gill slit
• Larva
• Endostyle
• Sedentary
• Paedogenesis