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(repeat) We all may retreat to our protective shells, but evolution has perfected the calcite variety to give some critters permanent defense against predators.\xa0So why did squids and octopuses lose their shells?\xa0Find out what these cephalopods gained by giving up the shell game.\nPlus why Chesapeake Bay oyster shells are shells of their former selves.\xa0What explains the absence of the dinner-plate sized oysters of 500,000 years ago, and how conservation paleobiology is probing deep time for strategies to bring back these monster mollusks.\nAlso, was the Earth once encased in a giant, continental shell?\xa0A new theory of plate tectonics.\xa0Land ho!\nGuests:\n\n\n Rowan Lockwood\xa0\u2013 Conservation paleobiologist at the College of William and Mary.\xa0\n\n\nAl Tanner\xa0\u2013 Ph.D. student in paleobiology at the University of Bristol, U.K.\n\n\nMike Brown\xa0\u2013 Professor of Geology, University of Maryland\n\n\nLearn more about your ad choices. Visit megaphone.fm/adchoices