Amphibians and reptiles lay eggs, but recently researchers have discovered that some species of lizards, frogs and snakes abandoned laying eggs to giving birth to live young. A European lizard and a skink species have also been caught using both types of reproduction.
Lizards and snakes have been discovered to give birth to live young about 175 million years ago. Further studies uncovered that many reptiles developed a pattern of switching from egg laying to live births. While most reptiles transition to reproducing by laying eggs, about 20 percent of modern scaled reptiles still give birth to live hatchlings. Scientists explain that switching from laying eggs to giving birth may be a form of adaptation to varying ecological conditions. Female frogs living in harsher climates may hold on to their eggs a little longer, until the embryos thin dramatically, giving way to birthing live young mostly covered only by a thin membrane.
Amphibians, such as frogs and toads, lay eggs in the water or in damp places. The Asian fanged frog, however, gives birth to live tadpoles instead of soft gel-like eggs. Research suggests that the fanged frogs are able to birth live young because fertilization occurs internally instead of the common external fertilization among amphibians. This sole species of fanged frog has evolved a penis-like tail that allows the male frog to transfer sperm inside the female fanged frog.