Chapter  3 - Jawless Vertebrates

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What is a fish?

• A fish is an aquatic vertebrate that respires using gills, is ectothermic, has fins, and has a skin that is usually with scales.  The taxonomic term for fish is Pisces, which, as you should know, is a paraphyletic group.

•  PISCES: the fishes: they may be paraphyletic, but they are still fish.  This Florida Museum of Natural History web site gives a basic introduction to the major groups of fishes (with good photographs showing characteristic features).

  • Web Site with more information
    1. Here is a terrific web site on FISHES from the Biomedia section of the Biological Sciences Website; Glasgow University Zoology Museum.  

  "Why fishes instead of fish" by Clark Hubbs

  • "Fishes or Fish?
    • Although people generally use the word "fish" for both singular and plural, most scientists usually make a distinction between them.  From the scientist's point of view, then, what's the difference between "fish" and "fishes"?
    • Both words can refer to more than one fish. The word "fishes" refers to more than one KIND of fish:   If you have a tub with three bluegills in it, say: "I have several fish in this tub."
    • If you have a bucket with a bluegill, a largemouth bass, and a paddlefish in it, say: "I have several fishes in this tub."
    • And remember, a linguist will tell you that all languages constantly change over time and space, which means that what was "right" yesterday in Texas may not be "right"
      tomorrow in Maine! "
    • (Hubbs, Fishes of Texas, http://www.tmm.utexas.edu/exhibits/fish/)

An Introduction to Fishes -- Comparative Anatomy, Auburn University

  The Earliest Evidence of Vertebrates

SUBPHYLUM CRANIATA (Fig. 3-3, 3.4)

Learn  more about craniates from the UCMP or from the Tree of Life

Characteristics

1. distinct head and brain (cranium)
2. neural crest cells in association with
3. epidermal placodes that form specialized [paired] sense organs-pronounced cephalization 
4. one or more semicircular canals
5. gills (for respiration) in pharyngeal slits
6. duplication of the Hox gene complex
7. Endoskeleton of living tissue, primitively cartilage, later endochondral bone
8. Unique excretory (paired kidneys) and circulatory systems

Taxa

• the craniates include the hagfish and the vertebrates.

Class AGNATHA

• In traditional classifications, the hagfish, lampreys, and extinct armored jawless fish are grouped together in the paraphyletic Class Agnatha.
•   It is based on the following primitive craniate characteristics
  1. jaws absent (there is a mouth, but it lacks internal cartilaginous or bony supports)
  2. paired limbs absent (a single pair may be present in fossil forms)
  3. one or two semicircular canals
  4. vertebrae unossified, persistent notochord
  5. living forms lack bone, external bony skeleton may be present in fossil forms
  6. cartilaginous endoskeletons are present
  7. larval forms resemble lancelets
  8. Living forms are elongate, scaleless, slimy parasites and scavengers
• here's more on agnathans from Biomedia, Glasgow University Zoology Museum

INFRAPHYLUM MYXINOIDEA

• the most primitive group of craniates, includes the living and fossil hagfish. (Fig. 3-5)
• middle Pennsylvanian, Recent
• 6 genera, 60 species
• exclusively marine
• elongate (eel-like)
• scaleless
• many mucous glands present for antipredator defense [an enticing picture of hagfish slime from Safari Splash]

  feed on polychaete worms, shrimp, and dead or dying fish attach to fish, form a knot in the tail and pass it forward to rip off flesh.  Image © BIODIDAC. used with permission usually enter coelomic cavity and feed on soft parts

• most "eel skin" is bycatch of scavenging hagfishes
• Table 3.1 lists features of hagfish that are more primitive than the condition in lampreys and gnathostomes and includes the following:
  1. anterior nasal opening; pineal not exposed
  2. single semicircular canal
  3. poorly developed eyes (lensless)
  4. no vertebrae
  5. unsupported fin rays
  6. partially developed cartilaginous braincase and gill supports (derived from neural crest)
•  
• Almost all you need to know about hagfish from Oceanlink.  Check out the slime
• Learn  more about hagfish from the UCMP or from the Tree of Life
• the lowly hag from Safari Splash  
• the economic importance of hagfish skins from Sea Grant

INFRAPHYLUM VERTEBRATA

• Characteristics
  1. 2 or 3 pairs of semicircular canals
  2. mesonephric kidneys
  3. dermal bone
  4. neural arches
  5. electroreceptors and lateral-line sensory canals (with neuromast cells)
  6. brain with optic tectum, cerebellum, and oculomotor nerves
  7. fin rays associated with musculature
• Learn  more about vertebrates from the UCMP or from the Tree of Life

• Taxa
  1. Vertebrates first appear in the early Cambrian.   Two genera, Myllokunmingia and Haikouichthys are known from China.  They lack bone, but have skeletons of cartilage.  Haikouichthys looks like a lamprey, Myllokunmingia looks like a hagfish.
  2. lampreys
  3. OSTRACODERMS
     1. extinct Paleozoic (Cambrian to Devonian) jawless fish with an external skeleton of bone . (Figs. 3-1, 3-2, 3-7, 3-8)
     2. The ostracoderms include the Thelodonti, Anaspida, Galeaspida, Pituriaspida, and the Osteostraci.  The Osteostraci are the sister taxa of the jawed vertebrates (gnathotomes).
  4. gnathostomes (jawed vertebrates).

Order PETROMYZONTIFORMES (upper Mississippian, middle Pennsylvanian, Recent)

• lampreys (Fig. 3-6)
  1. have a skeleton of cartilage, previously interpreted as descended from the ostracoderms, jawless fish with an external bony armor, which lampreys have secondarily lost.
  2. currently researchers suggest the absence of a dermal skeleton in lampreys and absence of other features is primitive.
• Characters
  1. dorsal nasal opening
  2. pineal eye
  3. two semicircular canals
  4. cartilaginous braincase
  5. branchial skeleton, vertebrae (neural arches), and fin rays
  6. seven gill pouches open directly to exterior
  7. osmotic regulators
  8. ammocetes larva

Ventral view of oral sucker and mouth of the lamprey. Note the horny rasping teeth.   Image © BIODIDAC. used with permission  Many lampreys are parasites of bony fish. Image © BIODIDAC. used with permission Information on sea lamprey in the Great Lakes from the USGS.   Sea and River Lamprey from selected fishes of New York State A Lamprey-Based Undulatory Robot from John Ayers.  Learn how lampreys swim. Europeans have long considered lampreys a delicacy.   The lamprey has, so far, failed to whet North American appetites.  Click here for more information and recipes. Learn  more about lampreys from the UCMP or from the Tree of Life

The Origin of Bone and Other Mineralized Tissue

• see pages 49-50

The Origin of Fins

• see pages 68-69 (figs. 3-11, 6-12)
• pitch - tilt up and down (around z axis)
  • resisted by paired fins: pectoral and/or pelvic;
• side to side
  • roll (around x axis - long axis) and yaw (around y axis)
  • resisted by dorsal, anal, and caudal fins

A Problem Posed by Gills

• see pages 69-71
• Jon Mallatt's scenario of the evolution of vertebrate gills and jaws (Fig. 3-12).
  • Read Carl Zimmer's excellent summary of why jaws initially evolved for better bbreathing, not better biting.

The Transition from Jawless to Jawed Vertebrate Evolution of the vertebrate jaw from the anterior visceral arches (Fig. 3-9) image to right  © BIODIDAC. used with permission Generalized jawed vertebrate (Fig. 3-10) Derived features of gnathostomes (Table 3.2) Mallatt's hypothesis for the evolution of vertebrate gills and jaws (Fig. 3-12)

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