Global warming could lead to extinction within 50 years of lizard species whose embryos are retained with their mothers' bodies, British researchers say. Dozens of lizard species characterized by viviparous reproduction, as opposed to egg-laying species, are threatened, scientists at the Universities of Lincoln and Exeter said. "Lizards' reproduction is largely linked to climatic temperatures and viviparous species are usually found in cold environments," Lincoln researcher Daniel Pincheira-Donoso said in a Lincoln University release Wednesday. "When reptiles initially moved to colder areas they needed to evolve emergency measures to succeed in these harsh places, and we believe viviparity is one of these key measures, he said. "However, this transition is mostly one-directional and unlikely to be reversed. Rapid changes in the environment's temperature would demand rapid re-adaptations to secure the species' survival." Viviparity allowed lizards in the past to invade and adapt to live in cold environments, and was a key trait for their evolutionary success, but now it could be a factor in multiple events of extinction, the researchers said.