Notes on Turtles (Testudines)

Evolutionary History

  • Turtles (Testudines) are unique members of the diapsid group.

  • Initially, the relationships of turtles to other reptiles were unclear, but they are now considered a sister group to basal archosaurs.

  • Turtles are highly derived tetrapods, showcasing distinct features such as:

    • A shell (composed of vertebral and rib elements)

    • An anapsid skull structure (no temporal fenestrae).

  • Stem turtles displayed traits resembling modern turtles but also retained some ancestral reptilian traits.

Structure & Function

Emergence of the Shell

  • The shell in turtles evolved from modified ribs:

    • Ribs broadened and elongated while intercostal muscles reduced in number.

    • The trunk vertebrae decreased but elongated over time.

Shell Structure

  • Composed of two primary parts:

    • Carapace (dorsal)

    • Plastron (ventral)

  • The carapace includes:

    • Integrated ribs and thoracic/lumbar vertebrae.

    • A structure that can fully or partially cover the ventral surface.

  • The shell is overlaid with:

    • Epidermal scutes which vary in shape, size, color, and patterns across species.

Protection

  • The shell serves mainly for protection:

    • Vulnerable parts of the turtle are hidden or can be retracted inside the shell.

    • Some species possess hinge joints that enable them to close their shells almost entirely.

Lung Ventilation

  • Turtles lack free-standing ribs that typically assist in lung ventilation.

  • Lung volume changes occur through:

    • Abdominal muscle contractions and relaxations.

  • Some turtles have additional respiratory systems engaging accessory organs.

Diversity

  • Turtles first appeared during the Mesozoic era; there are currently about 300 species.

  • Two main lineages of turtles:

    • Pleurodira: Neck retracts sideways; typically found in freshwater.

      • Families:

      • Chelidae (South American side-necked turtles)

      • Pelomedusidae (African mud turtles)

      • Podocnemididae (American side-necked turtles)

    • Cryptodira: Necks retract within the shell in a collapsable manner; evolved from pleurodires in the Jurassic period.

      • Families include:

      • Chelonioidea (sea turtles)

      • Testudinoidea (tortoises and pond turtles)

      • Kinosternoidea (Central American river turtles)

      • Trionychoidea (softshell turtles and their relatives)

Local Species in Manitoba

  • Two species of turtles are found:

    • Common snapping turtle (Family Chelydridae)

    • Western painted turtle (Family Emydidae)

Reproduction

  • Turtles are oviparous and typically lay eggs in nests.

  • Courtship behaviors include:

    • Various displays and synchronized movements.

  • Reproduction typically occurs annually; females of sea turtles usually emerge to lay eggs on land.

Hatching

  • Hatching often occurs synchronously, which may reduce predation risks.

    • Factors like vibrations from neighboring eggs can trigger hatching.

  • Hatchlings are not deeply buried, making it easier for them to emerge.

Sex Determination

  • In turtles, sex determination primarily relies on temperature rather than chromosomes:

    • Warm temperatures promote ovary development leading to females.

    • Cooler temperatures lead to testes development producing males.

Conservation Concerns

  • Global warming poses a significant threat to turtle populations, particularly affecting:

    • Temperature-dependent sex determination, causing male-to-female ratios to skew.

    • Other threats include pollution, fishing industry hazards, and poaching, greatly impacting their survival and reproductive success.