Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: The first monotremes may have evolved about 150 million years ago. Early monotreme fossils have been found in Australia. An example is a genus called Steropodon , shown in Figure below . It may have been the ancestor of the platypus. Early monotremes retained some of the traits of their therapsid ancestors. For example, they laid eggs and had a cloaca. These traits are still found in modern monotremes. Answer: platypus
Output:
The steropodon may have been an ancestor of which animal?