The vast majority of mammals alive today—including us—give birth to live young, but this reproductive mode isn’t unique to mammals nor universal among them. In fact, it evolved independently many times in other groups of vertebrates, and some mammal species actually lay eggs.
For years, scientists believed cynodonts—a major group of mammal-like reptiles that gave rise to all modern mammals—probably laid eggs, too. After all, egg-laying is common among reptiles, as well as the most ancient lineage of living mammals: the monotremes, which include platypuses and echidnas. However, efforts to determine whether cynodonts laid eggs or gave birth…
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