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Chapter   12 - TURTLES

Subclass ANAPSIDA or DIAPSIDA

The ancestry of Turtles

• Turtles are so distinct from living and fossil reptiles that are ancestry has been an enigma. Two anapsid families, pareiasaurs and procolophonids, have been considered the two most likely ancestral groups.

  Family Pareiasauridae (upper Permian)

  large herbivores with massive limbs, barrel like rib cages;  [fig. 9.11g]

  Family Procolophonidae (upper Permian - upper Triassic)

  small to medium herbivores; triangular skull; orbital margin of skull embayed for jaw muscles; Procolophon [fig. 9-11f]

• Molecular and new interpretation of fossils indicates that turtles are diapsids.
  • Mitochondrial evidence favors a close relationship of turtles to archosaurs (crocodiles and birds)
  • Morphological evidence links turtles to lepidosaurs (tuatara, lizards, and snakes) (deBraga and Rieppel 1997).

Superorder Testudinata

• includes the extinct Proganochelys and the Order CHELONIA  [extant turtles]

1. temporal openings absent
2. large posttemporal fossae
3. beaked jaws
4. a shell [fig 12-3]
5. plastron (ventral) and carapace (dorsal) of shell; shell produced by dermal plates fused to ribs and vertebrae
   1. expanded ribs
   2. shell overlaid with scales (scutes).
   3. shoulder girdle inside the rib cage
   4. clavicles and interclavicles incorporated into plastron

Everyone Recognizes a Turtle

The shell (although a key to success) limits the diversity of the group [fig 12-1]

shell morphology reflects ecology

Classification of turtles

Order PROGANOCHELYDIA

Proganochelys (upper Triassic)

Characteristics

1. palate with denticles (teeth) on the vomer and pterygoid
2. well-developed shell, but could not retract head beneath shell--cervical vertebrae unspecialized

Order CHELONIA

330 species

Characteristics

1. cervical vertebrae allow retraction of head
2. teeth absent, jaws covered by keratinized "beak"

Suborder PLEURODIRA (upper Triassic to Recent)

• "side-necked" turtles; South America, Africa, Madagascar, Australia, New Guinea
• two families, 65 species of aquatic turtles [fig 12-1i]

Characteristics

1. jaw muscles pass over trochlear process of the pterygoid [fig 12-2c]
2. head retracts by neck bending horizontally
3. fusion of pelvic girdle to carapace and plastron.

Habits

1. semiaquatic or aquatic
2. long necked forms feed on fishes, short necked forms feed on mollusks
3. Chelus - the matamata [fig 12-1j]: flaps of skin, shape of skull, growth of algae make it very cryptic. flaps of skin sense vibrations; opens mouth, expands throat, carrying in prey.

Suborder CRYPTODIRA (upper Jurassic - Recent)

• "s-necked" turtles [hidden-necked]
• Northern continents, South America, Africa

Characteristics

1. jaw muscles pass over trochlear process of the otic capsule [fig 12-2b]
2. suture attaches shell to pelvic girdle.

Classification [Table 12.1]

Superfamily Chelydroidea

1. snapping turtles
2. 2 families (including Chelydra, Macroclemys), 3 species
3. snapping turtles [fig 12-1f] are slow swimmers

RESPIRATION

1. turtles can not use ribs to ventillate lungs
2. lungs are connected to viscera [fig 12-5]
3. muscles contract to force the viscera up to expel air; other muscles increase the volume of the visceral cavity allowing the viscera to settle down, expanding the lungs
4. the pharynx of soft shelled turtles contains fringe-like structures used for underwater respiration
5. cloaca used for respiration in the Australian turtle Rheomys leukops; turtle pumps water in and out of cloacal bursae.

CIRCULATION

1. three chambered heart
2. ventricle can shift blood between pulmonary and systemic circuit
3. intercardiac shunt shifts deoxygenated blood to the systemic circuit
   1. lizards and crocodiles use shunt to increase blood flow to skin to warm more rapidly
   2. shunt is used in reptiles during periods of apnea (no breathing) to stabilize oxygen levels of blood
   3. reduces blood flow to lungs during periods of diving or when head and limbs are withdrawn into shell and can not respire

TEMPERATURE REGULATION

• basking by pond turtles raises T_body: may speed digestion, growth, egg production
• leatherbacks can maintain a body temperature 18° C or higher above water temperature using countercurrent exchange in flippers

REPRODUCTION

• marine turtles return to their nesting beaches [fig 12-9]; often 1000's of km's from their feeding grounds
• mechanisms?: magnetic field, polarized light, sun and stars, wave direction; perhaps olfaction
• sex of turtles is determined by incubation temperatures; the higher the temperature, the more females [fig 12-13]

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