What do the dinosaurs, the dodo bird and the Tasmanian tiger all have in common? They're all extinct. Countless species have come and gone in the history of our planet, some leaving more of a mark than others. Sometimes the cause of a species' extinction is unknown. To understand and explain these deaths, scientists often work with numerous hypotheses and constantly hunt for more data to unravel the mysteries.
From the fearsome megalodon to the awkward elephant bird, here are some of history's most puzzling extinctions.