Variety and Placentation in Squamate Reptiles
Why is it so successful?
What are selective pressures leading to evolution?
Do they all do it in the same way, or do they have different mechanisms? What functional
mechanisms might we want to get at?
How do pregnant females sustain embryos? Gas exchange, water, nutrients
How do placentas develop and function?
Selective pressures and constraints?
Patterns of evolutionary change
Reptile viviparity was long seen as simple
“not really viviparous” “lack true placenta” unlikely to be relevant to mammal evolution
viviparity has evolved over 90 times independently. Now, up to 115 separate origins of this pattern
(115 squamata, 6 extinct reptile, 8 lissamphibia, 23 fishes, 1 mammals). Distributed worldwide.
Actual number undoubtedly higher.
Sometimes, within the same species, if different populations, one population may be oviparous
instead of viviparous, etc.
>1800 extant squamate species viviparous (~20%), arisen >115 times in squamates, most origins
at low taxonomic level
What factors affect evolution?
Protect eggs from environmental hazard and predation
Permits use of