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"All mammals have platelets, which are necessary for survival because they arrest bleeding (haemostasis). No other order has platelets, even though the problem of bleeding exists. A solution to the enigma of why only mammals have platelets was recently proposed, linking the origin of the platelet in an egg-laying animal, which initially did not have platelets, to the evolution of the eutherian (haemochorial) placenta found in all mammals apart from monotremes and marsupials"
