Lecture 11: Lepidosaurs: Tuatara, Lizards and Snakes

Lepidosauromorpha

• Largest group of non-avian sauropsids

Divided between two subgroups:

1. Sphenodontidae

2. Squamata

Ectothermy

Rely on external sources of heat to raise their body temp

Thermoregulation in lepidosaurs

• Ectotherms

• Gain heat from the sun (heliothermy) via:

  • Directly (radiation)

  • Indirectly (radiation via reflection

  • Conduction

  • Convection

• Lose heat through

  • Conduction

  • Convection

  • Evaporation

• Physiological processes (e.g., vasodilation) can speed up heating or slow down cooling

Lepidosaur Characteristics

• Bodies are covered in keratinous scales (derived from the epidermis) and are relatively impermeable to water

  • Outer layer of epidermis is shed periodically as growth occurs (cyclical shedding)

  • Growth is also determinate (grow ceases at a certain size) (vs indeterminate as in testudines and some other groups of tetrapods)

• Transverse cloacal slit (vs. vertical slit in other sauropsids

  • No copulatory organ in Tuatara (plesiomorphic condition)

  • Paired hemipenes in squamates

• Diapsid skulls

Sphenodontidae

• Commonly known as Tuatara

• Vulnerable status

• Sister group to Squamata

  • Two rows of teeth in the upper jaw, creating a shearing effect when the jaw is closed

Order Squamata

• Snakes and Lizards

• Highly modified diapsid skull

  • Lack temporal bar

  • In snakes, the postorbital and squamosal are further reduced

Serpents

Snakes

Two major lineages:

1. Scolecophidia

2. Alethinophidia

Diversity not evenly distributed between them

Superfamily Colubroidea

• Largest subgrouping of the Alethinophidia

• Contains three families

  • Colubridae

  • Viperidae

  • Elapidae

Characteristics of Snakes

• Highly modified diapsid skulls; no temporal fenestra remain

• Incredibly kinetic skulls that allow extensive jaw movement

  • Possible due to 8 mobile joints

• Lower jaw of the left and right sides are connected only by muscle and skin allowing them to spread apart and move forwards or backwards independent of each other

• Skin along the ventral surface of the head is also very stretchy

  • Allows snakes to eat large prey

• No limbs

Lateral Undulation (Serpentine) movement

Body is thrown into a series of curves

Snake presses backwards at each curve causing forward movement

Sidewinding Movement

Concertina movement

Rectilinear movement

• Used predominantly by heavy-bodied snakes (e.g., Boidae)

• Alternate sections of the ventral surface are lifted off the ground and pulled forward by muscles that originate on the ribs and insert on the ventral scales

• Waves of contraction move from anterior to posterior and the snake moves forward

• Slow but effective

Thermal Prey Detection

Some snakes detect prey by thermal cues

Opisthoglyphous snakes

Snakes have one or more small hollow fangs near the rear of the max (Colubridae) - so-called rear-fanged snakes

Proteroglyphous snakes

Hollow fangs located at the front of the max that are permanently erect (Elapidae)

Solenoglyphous snakes

The hollow fangs are the only teeth on the max; mobile, can be erected (when mouth open) or folded (when mouth closed) (Viperidae)