Researchers have found evidence that an ancient creature, a relative of mammals from 236 million years ago, gave birth to live young. This discovery was made by examining a fossil of a species called Chiniquodon theotonicus, found in Argentina. Inside the fossilized bone, scientists found a microscopic growth ring, known as a neonatal line. This line forms when a baby is born or hatches, indicating rapid growth.
By studying this ring, researchers calculated the weight of a newborn Chiniquodon theotonicus. It was about 14 percent of its mother's body weight. This ratio is very similar to that of modern placental mammals, like humans and dogs, which suggests live birth. Before this, scientists thought live birth only started about 160 million years ago in later mammals. This finding pushes that date back by at least 90 million years.
The harsh environment of the Triassic period, when this creature lived, likely favored live birth. With many predators and extreme weather, keeping a developing baby inside the mother's body offered better protection than laying eggs. This suggests that important mammalian traits, like fur and milk production, might also be much older than previously thought. The discovery raises questions about whether live birth evolved once or twice in history.