Lecture 2: Vertebrate Origins

  1. Vertebrata are a subphylum of the Chordata
  2. Chordate characteristics
  3. Notochord at some stage of development.
  4. Dorsal hollow nerve cord.
  5. Pharyngeal gill slits present at some stage of development
  6. Endostyle (becomes thyroid gland in vertebrates). It is a ciliated glandular groove on the floor of the pharynx, that aids in filter feeding by secreting mucus, and just as in the thyroid, it is able to concentrate iodine.
  7. Muscular postanal tail
  8. Ventral heart with a closed circulatory system.
  9. Living bony or cartilaginous endoskeleton.
  10. Chordata include (note: hemichordates are in a separate phylum – these are the acorn worms)
  11. Urochordata – Tunicates
  12. Cephalochordata
  13. Vertebrata
  14. What defines a vertebrate?
  15. Presence of vertebrae
  16. They are cartilaginous in some fishes
  17. They are absent in hagfish
  18. Lampreys possess only rudimentary cartilaginous elements around the nerve cord. Note: at one time these organisms were considered to be degenerate.
  19. Presence of a cranium (hence the original name for the group: Craniata)
  20. Duplicated Hox gene (homeobox gene)
  21. Presence of embryonic tissue called the neural crest, which give rise to the epidermal placodes. These are the origin of the complicated sensory tissue characteristic of vertebrates.
  22. An interesting observation about vertebrates:
  23. Most animals (invertebrates) are small. Vertebrates however tend to be large. This means that diffusion is no longer sufficient for many bodily functions. This necessitates specialized structures and systems in vertebrates.
  24. Basal metabolic rates of vertebrates are much higher than those of invertebrates. What does this mean?
  25. Vertebrates are capable of anaerobic metabolism.
  26. What is the evolutionary history of the vertebrates?
  27. There are 3 general hypotheses
  28. Arthropod hypothesis
  29. Arthropods are a major animal group – common and therefore likely to have daughter groups.
  30. They share some characteristics with the vertebrates.
  31. If you turn an arthropod upside down, you have the basic vertebrate body plan.
  32. The body is segmented.
  33. There is a ventral nerve cord and a dorsal heart.
  34. Problem – the exoskeleton.
  35. This idea dates to 1818 by St. Hilaire.
  36. Annelid hypothesis
  37. Semper and Dohrn noted in 1875 that annelidshave the same basic body plan as vertebrates, only upside down, and they have an excretory system that is remarkable similar to that of some chordates.
  38. Problem – the nerve cord is ventral and bifurcates to go around the pharyngeal tube to a dorsal brain. If you turn the organism upside down, the brain is ventral and the mouth dorsal … a situation which does not show up in any vertebrate.
  39. Echinoderm – Hemichordate – Chordate Hypothesis.
  40. Both of the above hypotheses suffer from the fact that annelids and arthropods have spiral determinate cleavage while chordates have radial indeterminate cleavage.
  41. Both annelids and arthropods are protostomes while chordates are deuterostomes.
  42. Arthropods and annelids have shizocoelous coelom formation while chordates have enterocoelous coelom formation.
  43. Echinoderms have precisely the same characters as the chordates: radial indeterminate cleavage, deuterostomes, and enterocoelous coelom formation.
  44. Also, some echinoderm bipinnaria larvae resemble closely the tornaria-like larvae of some chordates in that both have sensory cilia at the anterior end, both have a complete digestive system with ventral mouth and posterior anus, and both have ciliated bands in loops.
  45. It is important to remember that the echinoderms we see today are probably very dissimilar from the echinoderms that were the actual ancestors to the chordates. Early echinoderms for example were not pentaradial. The diversity of echinoderms today is but a fraction of what was once there.
  46. Not all basal deuterostomes were asymmetrical or pentaradial. The calcichordata were bilaterally symmetrical, and may in fact be specialized echinoderms.
  47. Non-vertebrate Chordates
  48. Urochordates
  49. Tunicates (sea squirts)
  50. Sea squirts have sessile filter feeding adults and free swimming planktonic larvae. Larvae look similar to amphioxus – basic vertebrate body plan. Have pharyngeal gill slits, notochord, dorsal hollow nerve cord, muscular post anal tail.
  51. Adults however, look very different. How could this lead to vertebrates?
  52. Paedomorphosis – retention of juvenile morphology in the reproductive adult. This is an example of heterochrony.
  53. Alternatively, we may be derived from the sessile adult stage.
  54. Chordates are unique in having innervation of 2 types: segmented innervation and non-segmented innervation. It may be that we were originally nonsegmented (like the sessile adults) and later our morphology was over-run by the newly derived segmented components.
  55. Also, chordates have allorecognition. Invertebrates do not. However, echinoderms have allorecognition, as do some colonial organisms. Perhaps it is a means of preventing fusion of non-identical organisms. The ancestors of echinoderms may have been colonial and sedentary.
  56. Cephalochordates
  57. Fish-like in appearance and totally marine.
  58. Best know example is ampioxus (lancelet).
  59. Has segmented myomeres, and many homologies with vertebrates.
  60. Conodonts
  61. Perhaps the earliest ‘vertebrates’, although this issue has not been resolved.
  62. The importance of bone
  63. There is some question about when bone evolves as a vertebrate character.
  64. Hagfish and lampreys have no bone (they do have inner ear ossicle)
  65. Nature of early bone has some implications for physiology – ion & fluid regulation.
  66. What is the function of early bone?
  67. May serve a protective function. There were large invetebrate aquatic predators, and the armor of ostracoderms and placoderms may have prevented predation.
  68. Unfortunately, the bony armor is below the skin, and thus susceptible to injury.
  69. Perhaps it was used as a mineral sink? This is related to an early hypothesis about where vertebrates evolved.
  70. Did vertebrates have a freshwater or saltwater origin.
  71. Romer and Smith argued for a freshwater origin.
  72. This is based on the idea that bone is a mineral sink.
  73. Phosphates and calcium were probably a hot commodity in the Silurian.
  74. Bone armor may have prevented osmosis.
  75. Although all fossils were found in marine sediments, they argued that they had washed down into the sea.
  76. All fossils are marine.
  77. All old vertebrate groups are marine.
  78. Kidney function was probably co-opted from other mineral regulation.
  79. note: do fish drink?
  80. Prevailing view today is that vertebrates have a marine origin.
  81. Vertebrate ancestry
  82. Ostracoderms
  83. Oldest fossil vertebrates (except for conodonts)
  84. First discovered in Ordovician rock in Russia and the U.S.
  85. Belong to agnathan / cyclostomate group of vertebrates.
  86. Major radiation was in the Silutian and Devonian, but were extinct by the end of the Devonian. Why?
  87. Ostracoderm morphology
  88. No jaws.
  89. No paired fins.
  90. Heavy bony armor.
  91. Placoderms
  92. Less developed bony armor
  93. Paired fins and thus probably more active swimmers.
  94. Had jaws and were thus capable of predaceous lifestyle
  95. First appear in Silurian, major radiation in the Devonian, and extinct by the end of the Permian.
  96. One Placoderm group, the acanthodians, had bony scales like modern fishes.
  97. Placoderms may have given rise to, or had a common ancestor with 2 major groups: the Chondrichthyes and the Osteichthyes.
  98. Chondrichthyes
  99. No bone, probably underwent reduction from the Placoderm condition, or may represent true underived vertebrate condition. Could this be an example of neoteny or paedomorphosis? They have a living endoskeleton, but it is made of cartilage.
  100. Completely predaceous life style.
  101. Well formed independent jaws – consider the teeth.
  102. They have a spiracle.
  103. They have internal fertilization – eggs with cases, and yolks. Some are ovoviviparous.
  104. The holocephalans (chimaeras) have an upper jaw that is fused to the brain case, and a flap of skin that covers the gill region.
  105. Osteichthyes
  106. They have a bony skeleton, probably a retention of the ostracoderm or placoderm condition.
  107. They have bony scales and opercula
  108. Origin was in the Devonian, they split almost immediately into 2 groups: the Actinopterygians and the Sarcopterygians.
  109. Actinopterygians: Chondrosteans (sturgeons), Holosteans (bowfins and Garpikes) and Teleosts (modern bony fishes)
  110. Sarcopteryginas: Dipneusti (lungfishes), crossopterygians and ceolocanths.
  111. Sarcopteryginas have intenal nostrils like all land vertebrates, they have fleshy lobed paired fins, and were probably ancestral to the labyrinthodontia.