For decades, scientists assumed that cynodonts, the ancient ancestors of modern mammals, laid eggs like most reptiles and the egg-laying monotremes of today. However, a new study published in *Frontiers in Mammal Science* has overturned this hypothesis. Researchers examining the bone microstructure of a *Chiniquodon theotonicus* specimen from the late Triassic period discovered a neonatal line—a growth ring formed immediately after birth. This finding, identified by graduate student Maria Miceli Baro, marks the first time embryonic tissue has been observed in a cynodont fossil.