Chordata

2. Chordata • Deuterstome coelomates • Four features characterize: • Dorsal, hollow nerve cord • Develops from ectoderm > develops into CNS: brain and spinal chord • Notochord • Fluid-filled cells encased in stiff, fibrous tissue • Pharyngeal slits/pouches • Filter feeding • Postanal tail • Urochordata and Cephalochordata are nonvertebrate subphyla

3. In tetrapods… Notochord develops into the nucleus pulposus in invertebral disks

5. Urochordata • Tunicates and salps • Larvae have notochord and nerve cord • Plainly exhibit all of the basic characteristics of chordates • Difficult to discern evolutionary relationship by examining an adult • Secretes tunic, a tough sec composed mainly of cellulose

6. Tail and notochord are resorbed; nervous system degenerates • Informal name “sea squirts” b/c some species shoot a jet of water through excurrent siphon

7. Cephalochordata • Lancelets – resemble a lancet – a small, two-edged surgical knife • Notochord runs entire length of the dorsal nerve cord and persists throughout the animal’s life • Retains chordate characters as adult • Filter feeders • Closest relatives to vertebrates • Hox genes for swollen tip of anterior dorsal nerve cord are expressed in the same anterior-to-posterior order in lancelets and vertebrates • Vertebrate brain an elaboration of simple nerve cord tip?

8. Chordates with a head = craniates • Brain at anterior end of dorsal nerve cord, eyes and other sensory organs, and a skull • Possess two clusters of Hox genes • Neural crest – a collection of cells that appears near the dorsal margins of the closing neural tube • Cells give rise to teeth, some bones and cartilage of the skull, inner layer of skin of facial region, several types of neurons

9. Transition to craniates Haikouella – 530 million years old (Cambrian explosion), 3-cm-long; suspension feeder; resembled craniates in that it had a small brain, eyes, and muscular segments, tooth-like denticles. Did not have a skull.

10. Haikouichthys – had a skull made of cartilage. Classified as a true craniate.

11. Hagfish

12. Hagfish • Skull of cartilage but lack jaws and vertebrae • Use segmented muscles to exert force against notochord • Small brain, eyes, ears, and a nasal opening • Tooth-like formations of keratin in mouth • Secretes mucus as defense • Not considered fishes

13. Nom nom

14. VertebratesCephalaspidomorphi • Gene duplication allowed development of innovations in the nervous system and skeleton • Vertebrae takes over mechanical role of notochord • Lampreys are oldest living lineage of vertebrates • Mostly parasitic • Skeleton of cartilage with no collagen • Pairs of cartilaginous projections partially enclose nerve chord

15. Conodonts“cone teeth” • 3-10-cm-long; soft-bodied with prominent eyes controlled by numerous muscles • Barbed hooks made of mineralized dental tissues • Dental elements used to date strata • Other innovations such as paired fins, an inner ear with two semicircular canals, and armor made of mineralized bone • Extinct by end of Devonian

16. Vertebrate skeleton evolved as a structure made of unmineralized cartilage • Mineralization began after lampreys diverged from other vertebrates, and mineralization began in the mouth (conodont dental elements)

17. Gnathostomes“jaw mouth” • Grip food, slice it up. • Evolved from skeletal rods that supported anterior pharyngeal slits • Remaining slits used for respiration (since suspension feeding was no longer necessary) • Additional duplication of Hox genes (four, as opposed to one cluster) • Lateral line system – organs sensitive to vibrations

18. Paired fins and tail allowed more efficient swimming • Jaws enabled gnathostomes to grab prey or bite off chunks • Earliest gnathostomes (placoderms and acanthodians) are extinct

19. Chondrichthyes“cartilage fish” • Sharks, rays, and relatives (ratfish) • Skeleton composed predominately of cartilage • Does not mean lineage is primitive in the evolution of vertebrates • 750 living species • Large amount of oil in liver for buoyancy, but sinks of stationary • Locomotion and respiration, although some have spiracles that force water across shark’s gills • “sleep swimming”

20. Sharks and Rays • Largest sharks and rays are suspension feeders • Sharks have sharp vision but cannot distinguish colors • Nose for olfaction, not breathing • Detect electric fields generated by the muscle contractions of animals • No eardrums; sound is transmitted by body to hearing organs of inner ear

21. Sharks • Eggs fertilized internally • Oviparous and ovoviviparous – eggs that hatch outside the mother and young that are born after hatching within the uterus • Viviparous – develop within uterus and obtain nourishment prior to birth via mother’s blood, or by eating other eggs • Sibling rivalry. • Cloaca – opening for digestive, urinary, and reproductive tract

22. Osteichthyes“bony fish” • Includes bony fishes and tetrapods • Aquatic osteichthyans are informally called fishes • Osteichthyans have ossified (bony) endoskeleton w/ hard matrix of calcium phosphate • Breathe by drawing water over 4 or 5 pairs of gills located in chambers covered by protective bony flap, operculum

24. Transfer of gas from blood to swim bladder allows control of buoyancy • Evidence suggests lungs came first then evolved into swim bladders… • Skin is covered by flattened, bony scales and mucus that reduces drag

25. Ray-FinnedActinopterygii“ray wing/fin” • Bass, trout, perch, tuna, herring, salmon • Maneuvering, defense • Originated in fresh water • Taste great.

26. Just dance • Birth

27. Ray-FinnedSarcopterygii • Evolved during Devonian • Rod-shaped bones surrounded by a thick layer of muscle in their pectoral and pelvic fins • Used fins to swim and walk underwater • Three lineages survive: coelacanths, lungfishes, and the lineage that gave rise to tetrapods, including humans!

28. Lungfish

29. The Shape of Life