lepidosauria - squamates Flashcards
describe lizards
• 6,459 living species
• Range in size:
○ Geckos - 3 cm
○ Komodo dragon - 3 m
• Dry Epidermal scales - a few have dermal scales
• Leglessness evolve several times independently within lizards
lizard dentition? jaw?
• Dentition
○ Most homodont, some heterodont dentition
○ Most have acrodont (top of ridge) and few have pleurodont (medial side of jaw) unsocketed teeth
○ Teeth replaceable
Moveable quadrate - increased jaw mobility - kinesis
tail autotomy how?
• many lizards are capable of self-amputation of the tail to escape predators
a. Form fracture planes which are weak areas - have to go further up to an area that has not softened from the fracture
b. Use neural and circulatory modifications (sphincters and valves) to help stem blood flow - deliberately starve tail of nutrients - cells around area of weakness will die off
○ It is an active process: contact breaking at fracture plane
c. It grows as a cartilaginous tube - any subsequent autotomy must occur at anterior to previous amputations
costs of tail loss
a. Reduced growth rate in juveniles
○ Effects survival and reproductive success
b. Loss of fat storage
○ Tail normally stores fat, females absorb and deposit in yolk of clutch
○ Results in smaller clutch and reduced juvenile survival
c. Balance and coordination reduced
d. Social behavior
○ Decreased social status
describe amphibaenians
• Amphibaena - amphisbaenians
• Only true fossorial forms of squamates - highly specialized
○ Almost exclusive subterranean exisence
• Lack hindlegs; forelegs in most (they burrow, so limbs get in the way- some do not have limbs at all )
amphibaena skull? body wall/motion? teeth?
• Solid skull
○ Akinetic (jaw) : rigidly constructed and inflexible
○ Tunnel with head and body movements
• Small eyes
• Lack external ears
• Blunt, shovel, or keel snouted
• Highly segmented body wall
○ Have annuli: external circular rings
○ Use an “amphibaenian” motion: double walk
• A retillinear (straight line) type of motion
○
1 upper and 2 lower teeth
describe snakes
- Evolved in early cretaceous
- Legless squamates
- Approx 3,619 extant species
how are snakes specialized?
• Highly specialized with extreme elongation
• Lack of vertebral differentiation and specialization - duplication
• Elongation and reduction or loss of organs
○ Ie., one lung; kidneys and other organs displaced - makes room for them to eat prey since they are much longer than they are wide
• Elongation limited prey size, so they evolved flexible jaws
snake skull and jaws?
• Skulls and jaws are highly kinetic
○ Loss of lower temporal arch (quadratojugal)
○ In some also loss of upper arch (postorbital-squamosal)
○ A highly modified diapsid skull
○ Palate and jaw bones are ligament suspended
○ 8 independently moveable joints in skull
○ Right and left skull and jaws move independently
○ Expandable ligament in the lower junction
snake teeth? describe each dentition
○ Most are recurved and pleurodont - buried in the jaw
§ Ie. Shallow sockets
• Four main types
1. Aglyphous
○ Lack any conspicuously enlarged or modified teeth (ie. Fangs)
○ Depend on their muscles - ex. Python
2. Opisthoglyphous
○ 1 or more fangs near rear of maxilla
○ Teeth solid; may have grooves to conduct saliva and venom
○ e.g. family colubridae - typical snakes
○
3. Solenoglyphous
○ Long, hollow fangs at the front of maxilla
○ Folds back along roof of mouth
○ Have venom glands
○ Ie. Family viperidae
○
4. Proteroglyphous
○ Hollow fangs at the front of maxillae followed by several smaller teeth - have auxiliaries at the back
○ Fangs are short and permanently erect
○ Highly venomous- venom passes through hollow fangs
○ e.g. family elapidae- coral snakes, cobras, mambas, sea snakes
snake scales and sensory systems?
• Little differentiation of vertebral column
• Epidermal scales
○ Large ventral scutes are scales for locomotion
• Immovable eyelids
○ Generally poor vision except arboreal snakes
• Lack external ears and ear drums
○ Do have inner ears for vibration/ low frequency
• Extendable forked tongue - like an antenna- greater SA to pick up signals with
○ Function in chemoreception
○ Insert into Jacobson’s organ pits in roof of mouth - organ will detect stimuli and send the signal to the brain (pathway separate from nose!!!!)
snake swallowing adaptations
• Swallowing adaptations:
○ Kill prey before swallowing - construction or venom; minimizes injury
○ Swallows prey head first to reduce injury
○ Forward facing glottis; protruded while swallowing
○ Neck muscles - strong; force prey through esophagus to stomach
○ Frontal and parietal bones create a “cranial box” that protects the brain
○ Hypophyses (midventral projections from centra) on vertebrae protects spinal cord
describe constrictor hunting
• Constrictors (e.g. boas and pythons)
• Rely on olfaction to find prey
• large bodied slow moving snakes
• Grasp prey by neck, then coil rapidly
○ Short vertebrae and truck muscles for bending/coiling when squeezing prey
○ Method - wait for prey to exhale, then tighten (suffocate prey) or pressure stops heart of prey
describe venomous snakes hunting
• Venomous snakes and venom
• Venom evolved to immobilize prey
• This allowed for other evolutionary changes:
a. Loose jaw articulation for large prey
○ Immobilize prey therefore, weaker jaws
b. Thin, flexible ribs - large prey swallowed so ribs expand
c. More rapid locomotion
d. Venom can help digest prey - digestive enzymes, ie fer-de-lance
