This handout image released by Sophie Vrard and Julien Benoit on April 9, 2026, shows images of a Lystrosaurus - a herbivore that lived 250 million years ago - Did the distant ancestors of mammals, which lived 250 million years ago, lay eggs or were they viviparous? Synchrotron analysis of a Lystrosaurus embryo finally answers the question. It is a mystery that has preoccupied scientists for decades: synapsids, the stem group from which mammals evolved, are thought to have been egg-laying, even though among their current descendants only monotremes (platypuses and echidnas) have retained this trait. (Photo by SOPHIE VRARD AND JULIEN BENOIT / AFP via Getty Images) / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO/SOPHIE VRARD/JULIEN BENOIT/ HANDOUT / " - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS