Lecture 13 - Chordata

• One of these types of animals gave rise to vertebrates, one of the most successful groups of animals.
• Chordates are bilaterian animals that belong to the clade of animals known as Deuterostomia. 
• Two groups of invertebrate deuterostomes, the urochordates and cephalochordates are more closely related to vertebrates than to invertebrates.
• What are the five district characteristics define the chordates?

  1. Notochord
  2. Dorsal tubular nerve cord
  3. Pharyngeal pouches (gill slits)
  4. Endostyle
  5. Postanal tail
• The notochord is a flexible, rod-like structure derived from mesoderm.

  • The first part of the endoskeleton to appear in an embryo.
  • Place for muscle attachment.
• In chordates, the nerve cord is dorsal to the alimentary canal and is a tube.  

  • The anterior end becomes enlarged to form the brain.
  • The hollow cord is produced by the infolding of ectodermal cells that are in contact with the mesoderm in the embryo.
  • Protected by the vertebral column in vertebrates. 
• Pharyngeal slits are openings that lead from the pharyngeal cavity to the outside. They are formed when pharyngeal grooves and pharyngeal pouches meet to form an opening.
• In tetrapods, the pharyngeal pouches give rise to the Eustachian tube, middle ear cavity, tonsils, and parathyroid glands.
• • The perforated pharynx evolved as a filter feeding apparatus.
  • Later, they were modified into internal gills used for respiration. 
• The endostyle in the pharyngeal floor, secretes mucus that traps food particles.

  • Found in protochordates and lamprey larvae.
  • Secretes iodinated proteins.
• Homologous to the iodinated-hormone-secreting thyroid gland in adult lampreys and other vertebrates.
• The postanal tail, along with somatic musculature and the stiffening notochord, provides motility in larval tunicates and amphioxus.

  • Evolved for propulsion in water.
  • Reduced to the coccyx (tail bone) in humans.
• Phylum Chordata

  Two protochordate subphyla

  1. Subphylum Urochordata
  2. Subphylum Cephalochordata
• Tunicates (subphylum Urochordata) are

  found in all seas.
• Subphylum Urochordata

  In most species, only the larvae show all of

  the chordate hallmarks.

  • Tadpole larva.
• Subphylum Urochordata

  • Tunicates filter feed using the pharyngeal slits and a mucous net secreted by the endostyle.
• Subphylum Urochordata

  • Larvaceans are paedomorphic. Adults retain larval characteristics.
• Subphylum Cephalochordata

  • Cephalochordates are the lancelets, also called amphioxus.
• Subphylum Cephalochordata

  • Filter feeding is accomplished using pharyngeal slits and a mucous net secreted by the endostyle.
• Subphylum Cephalochordata

  • The dorsal, hollow nerve cord lies just above the notochord.
• Subphylum Cephalochordata

  • The circulatory system is closed, but there is no heart.
  • Blood functions in nutrient transport, not oxygen transport.
• Subphylum Cephalochordata

  • Segmented trunk musculature is another feature shared with vertebrates.
• Subphylum Cephalochordata

  • Many zoologists consider amphioxus a living descendant of ancestors that gave rise to both cephalochordates and vertebrates.
• Subphylum Vertebrata is a monophyletic group that shares the basic chordate characteristics with the urochordates and cephalochordates.
• Subphylum Vertebrata

  • The animals called vertebrates get their name from vertebrae, the series of bones that make up the backbone.
• Subphylum Vertebrata

  • There are approximately 52,000 species of vertebrates which include the largest organisms ever to live on the Earth.
  • Fishes, Amphibians, Reptiles, Birds, and Mammals
• Subphylum Vertebrata = Craniata

  • Craniates are chordates that have a head.
• Subphylum Vertebrata = Craniata

  • The origin of a head opened up a completely new way of feeding for chordates: active predation.
• Vertebrates have an endoskeleton made of cartilage or bone.
• Endoskeleton

  • All have a cranium to protect the brain.
• Endoskeleton

  • Almost all have vertebrae to protect the spinal cord.
• Neural Crest Cells

  • One feature unique to vertebrates is the neural crest, a collection of cells that appears near the dorsal margins of the closing neural tube in an embryo.
• Neural crest cells give rise to a variety of structures, including some of the bones and cartilage of the skull.
• The Origin of Vertebrates

  • Pikaia was an early chordate discovered in the Burgess Shale.
• The Origin of Vertebrates

  • The most primitive of the early vertebrate fossils are those of the 3-cm- long Haikouella.
  • Eyes and brain present, but no skull.
  • It is transitional in morphology between cephalochordates and vertebrates
  • Some hypothesize that this is the sister taxon of vertebrates.
• The Earliest Vertebrates

  Haikouichthys - The earliest known vertebrate fossils belong to two fishlike 530 million year old vertebrates. Recently discovered (1999) they push back vertebrate origins to the early Cambrian.
• The Earliest Vertebrates

  • Other early vertebrate fossils include the armored jawless fishes called ostracoderms from the late Cambrian.
• The Earliest Vertebrates

  • Heterostracans had dermal armor, but lacked paired fins.
  • Osteostracans had paired pectoral fins as well as dermal armor.
  • Anaspids were more agile and streamlined.
• The Earliest Vertebrates

  • Conodonts resemble amphioxus, but havegreater cephalization.
• The Earliest Vertebrates

  • Vertebrates lacking jaws are known as agnathans.