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Chapter   10 - TETRAPOD ORIGINS

Sarcopterygians --lobe-finned fishes (in a paraphyletic sense, monophyletically includes the tetrapods)

Division TETRAPODA

tetrapods are weird lobe-finned fish that are adapted to life on land

• Taxa [fig 8-13]
  1. AMPHIBIA (Gr. amphi = both or double; bios = life)
     • living amphibians are "tied to water" 
       1. for reproduction, anamniote eggs
       2. for respiration--skin must remain moist
  2. AMNIOTA (mammals, reptiles, and birds)

THE TRANSITION TO LAND

• Advantages to invasion of land
1. new food resources
2. avoidance of aquatic predators and competitors
3. Oxygen abundant
• Disadvantages (Obstacles)
  1. water becomes limiting factor in distribution (desiccation, respiration, reproduction)
  2. gravity - necessitates new morphological designs. Water provides buoyancy and allows for good support even though it is more difficult to move through than air.
  3. water has high heat capacity. Most aquatic animals, especially marine species do not have problems with the drastic temperature changes that occur on land.

Adaptations for Life on Land

Support and Locomotion [fig 8-1]

1. modified axial skeleton with zygapotheses
2. strengthened shoulder and pelvic girdles
3. loss of  skull bones, freeing the shoulder from the skull allowing a flexible neck; shoulder girdle supported by serratus musculature
4. attachment of ilium to sacral rib
5. evolution of paired limbs
   1. proximal limb elements are homologous with bones of rhipidistian fin
   2. elongate, laterally directed humerus and femur
   3. podials, metapodials, and phalanges

Respiration

1. Air versus water
   1. air has higher O₂ content - 20 X more per volume; faster diffusion (500,000 X)
   2. less energy for ventilation; up to 25% of total metabolism for fish, 1-2% for air breathers
   3. air is not hyper- or hypotonic (no salt gain or loss)
   4. air does not remove as much heat
   5. air causes problems with desiccation; structures located deep within body; long passageways moisturize air
2. structural adaptations
   1. absence of internal gills
   2. reduction and loss of operculum
   3. better developed lungs than were present in sarcopterygians
   4. three chambered heart with discrete systemic and pulmonary circulation
   5. three chambered heart with separation of blood in the ventricle [fig 9-23]
   6. loss of scales to allow for cutaneous respiration

Water Balance

1. Excretion
   1. urea is the principal nitrogenous waste; Latimeria and the living lungfish can synthesize urea in their livers; lungfish use urea; retention for water conservation during estivation
2. Dehydration
   1. still a problem for most amphibians

Sense Organs

1. Middle ear
   1. derived from spiracle
   2. hyomandibular (freed from jaw support) modified to columella [stapes]; initially the stapes is large and heavy and when it became solely involved in aerial sound transmission is questioned
   3. evolution of otic notch--supports a tympanum
2. Vision
   1. change in shape of lens, associated with different refractive indices of air and water

Reproduction

• usually external fertilization, eggs laid in water (spermatophore in salamanders)

EVOLUTION of TETRAPOD CHARACTERS in an AQUATIC HABITAT

Where?

All lobe-finned fish are initially marine in distribution and later invaded fresh water habitats.

Why?

Romer proposed climatic conditions. Devonian red beds indicate severe drought. Fish left the ponds that dried up in search of water.

Thompson proposed ecological conditions. Appearance of wetlands at margins of lakes or estuaries. Abundant food for fish that could live in very shallow water. Limbs are adaptation for locomotion in shallow water. E.g., pectoral fins of frogfishes (teleosts) are modified into structures that function like tetrapod limbs [fig 8-18]. Gradually amphibians took advantage of terrestrial resources.

Pough indicates early amphibians were adapted for aquatic environments. He suggests terrestriality is associated with dispersal of juveniles who would feed on terrestrial invertebrates and have fewer problems with gravity because of small size.

CLASS AMPHIBIA

a paraphlyletic group of non-amniotic tetrapods (recent amphibians are monophyletic--see below)

Characteristics

1. well-developed paired limbs [with digits]--may be secondarily lost
2. vertebrae with zygapotheses
3. skull freed from shoulder girdle,
4. pelvic girdle attached to a single sacral vertebra
5. hyomandibular modified to stapes
6. operculum absent
7. internal gill apparatus absent as adults

Upper Devonian Tetrapods

1. Acanthostega, Elginerpeton, Ichthyostega, Tulerpeton, Metaxygnathus
2. These five taxa are the earliest known tetrapods and retain many fish-like characters indicative of an aquatic lifestyle
   • median caudal fin rays present on tail
   • skull with canals for lateral line system
   • preopecular bone present (Acanthostega may have retained internal gills)
3. Other primitive characters

polydactyly (more than 5 digits in manus or pes; Acanthostega has 8) [fig 8-16]

4. labyrinthodont teeth - synapomorphy shared with osteolepiform
5. Ichthyostega from the Devonian of Greenland is one of the best known early tetrapods.

TEMNOSPONDYLI (Pennsylvanian-Cretaceous)

• it is widely accepted that the three orders of living amphibians (Lissamphibia) are derived from the temnospondyls [Figs. 8-20d, e, f, g]
• Evidence summarized in Carrol [2002, Journal of Paleontology, 75:1202-1213] suggests that caecilians are most closely related to lepospondyl microsaurs and that salamanders and frogs are derived from different families of dissorophoid temnospondyls.  However, pedicellate teeth are present in the earliest caecilian, as well as frogs and salamanders, but are absent in microsaurs. 

10. REPTILOMORPHA
• skull and vertebral characters differentiate from the Batrachomorpha (temnospondyls and lyssamphibia)
• ANTHRACOSAUROIDEA
• 11.  BATRACHOSAURIA
  • SEYMOURIAMORPHA [fig 10-16b]
  • 12. COTYLOSAURIA
    • characters
      1. sacrum with more than one vertebra
      2. loss of lateral line system
      3. robust claws on forelimbs for digging
      4. modern atlas-axis complex
    • DIADECTOMORPHA [fig 10-16a]
    • 13. AMNIOTA
      • transverse pterygoid flange for pterygoideus jaw muscle, etc.

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TEMPORAL OPENINGS AND AMNIOTE CLASSIFICATION [fig. 10-21]

Anapsids

dermal skull roof completely covered with bone; turtles are typically included as anapsids; this is primitive for the amniotes.

Synapsids

single opening low on the cheek, bordered by the jugal, squamosal, and postorbital; condition found in mammals

Diapsids

two pairs of temporal openings; one pair ventral to the postorbital and squamosal (as in the synapsids); the second pair dorsal to the postorbital and squamosal and lateral to the parietal

Euryapsids

derived from the diapsid condition by loss of the lower temporal opening; evolved independently in plersiosaurs (lower temporal bar lost) and ichthyosaurs (opening closed by enlargement of bones of the cheek region

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AMNIOTE TAXONOMY

numbers refer to numbered nodes in figure 10-2, pp. 266-267

Paleozoic amniotes are illustrated in fig 10-18




14. Synapsida (includes Mammalia) [fig 10-18b]
15. Sauropsida
    • Mesosauridae
    • Reptilia
      17. stem parareptile (Anapsida)

          Milleretidae, etc.

          Testudines (may be diapsids)

      18. Eureptilia
          Captorhinidae, etc.
          19-20. Diapsida

          • stem taxa
          • 21. Sauria
            • Archosauromorpha [Crocodylia, dinosaurs, and Aves]
            • Lepidosauromorpha [lizards and snakes]

AMNIOTA

• Characteristics
  1. Amniote (cleidoic) egg [fig 10-19]
     1. Amniotes are fully terrestrial and lay eggs on land . Amniotes not "tied to water" for purposes of reproduction.
     2. exclusively internal fertilization, No larval stages. No metamorphosis
     3. four extraembryonic membranes
        1. amnion--inner part of a double membrane that surrounds the embryo; located directly outside of the embryo; encloses the developing embryo (fluid-filled sac)
        2. chorion--outer membrane which is part of the double membrane; located outside of the amnion; serves as a surface for gas exchange through the shell
        3. allantois--a chamber which stores nitrogenous wastes [uric acid] produced by the embryo during development
        4. yolk sac--contains the embryo's food and energy reserves in the form of lipids
        5. Albumin--protein source and holds water
  2. More effective jaw musculature [fig 10-22]
     1. greatest strength when nearly closed, for crushing prey.
     2. pterygoid with a transverse flange for attachment of large pterygoidius muscle
  3. Skeletal structure improved--greater strength and agility
     1. improved locomotion due to rotation of limbs to support body
     2. two sacral vertebrae
  4. otic notch absent

CLASS REPTILIA

Characteristics

1. a variety of skull characters [fig 10-2]
2. skin tough, leathery, and covered with keratinous scales.
   1. very few to, usually, no glands in integument; glandless skin is resistant to desication
   2. "dry" skin of reptiles serves no respiratory function except during embryo development.
3. well-developed lungs
4. Heart usually 3-chambered; 4-chambered in crocodiles
5. Ectothermic heterotherms
6. Paired metanephric kidneys; uric acid is the main nitrogenous waste
7. Very well-developed nervous system, especially within the cranium and with regard to optic nerves and sense organs; very well-developed visual sensory system
8. 7000 species total, world wide. 300 species in the U.S.

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