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Recent analyses using large-scale phylogenies suggest a radically different history for the evolution of live birth and egg laying in squamate reptiles (lizards and snakes) than traditionally understood. What is the ancestral condition for lizards and snakes? How frequently does live bearing evolve in egg-laying lineages? Can the eggshell ever re-evolve in live-bearing lineages? Answering these fundamental questions about the evolution of key physiological processes will require additional data from genomic, developmental, and fossil data.
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Pyron et al. (2015) studied this question.
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