Vertebrates evolutionary history

• Chordates

  Bilaterian animals and within bilateria, they belong to the clades of animals known as deuterostomia
• Characters of Chordates

  Notochord

  • A long lotidunal flexible rod located between the digestive tube and the nerve cord
  • It is composed of large fluid-filled cells encased in fairly stiff, fibrous tissue
• Characters of Chordates

  Dorsal hollow nerve cord

  • The nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube located dorsal to the notochord
• Characters of Chordates

  Pharyngeal slits or cleft

  • In all chrodate embryos, a series of pouches seperated by grooves forms along the sides of the pharynx.
  • In most chordates, these grooves (known as pharyngeal clefts) develop into slits that open to the outside of the body
• Characters of Chordates

  Muscular post anal tail

  • Chrordates have a tail that extends posterior to the anus.
• Lancelets ( cephalochordata)

  • The most basal ( earliest diverging) group of living chordates
  • Obtained their name from their bladelike shape.
• Tunicates (urochordata)

  • The chrodate characters of tunicated are nost apparent during their larval stage
  • In many species the larva uses its tail muscles and notochord to swim through the water in search of a suitable substrate on which it can settle, guided by cues it receives from light-and gravity-sensitive cells.
• Hox genes control the development of major regions of the vertebrates brain
• Craniates

  • Chrodates with a head are known as craniates
  • Craniates possess two or more sets of hox genes which led to additional genetic complexity
• Derived characters of craniates

  • Neural crest- a collection of cells that appears near the dorsal margins of the closing neural tube in an embryo.
• Origin of Craniates

  • The most pimitive of the fossils are the 3 cm long Haikouella which in a way resemble lancelets
  • For example, it had a well-formed brain, small eyes, and muscle segments along the body, as do the vertebrae fishes.
• Haikouella was discovered in southern china in 1999.
• Hagfishes

  • Most basal group of craniates is Myxini.
  • Have a skull made of cartilage, but they lack jaws and vertebrates
  • Have a small brain ,eyes, ears, and a nasal opening that connects with the pharynx.
  • Their mouths contain toothlike formations made of the protein keratin.
• Craniates with backbones

  • Vertebrates
  • They underwent another gene duplication, this one involving a group of transcription factors gene called the DIx family.
  • The vertebrae of some vertebrates are little more that the small prongs of cartilage arrayed dorsally along the notochord while majority of vertebrates, posses vertebrae enclosing the spinal cord and have taken over the mechanical roles of the notochord.
• Lampreys ( petromyzontida )

  • The most basal lineage of living vertebrates.
  • There are about 35 species of lampreys inhabiting various marine and freshwwater environments
  • As larvael, lampreys live in freshwater streams
  • The skeleton of lampreys is made of cartilage but it has no collagen, instead, it is stiff matrix of other proteins.
• Fossils of early vertebrates

  Conodonts

  • Cone tooth
  • Slender, soft-bodied vertebrates with prominent eyes controlled by numerous muscle
  • Most conodonts were 3-10 cm in length, although some may have been as long as 30 cm.
• Fossil of early vertebrates

  • Vertebrates with additional Innovations emerged during the Ordovician, Silurian, and Devonian periods
  • Had paired fins, as in lampreys, an inner ear
  • Lacked jaws but had a muscular pharynx, which they may have used to suck in bottom-dwelling organisms or deritus
  • They were also armored with mineralized bone covering their body.
• Jawless armored vertebrates

  • Pteraspis and Pharyngolepsis were of many genera of jawless vertebrates that emerged during Ordovician, Silurian, and Devonian Periods.