Mammal Ancestors May Have Given Birth to Live Young 236 Million Years Ago

A team of researchers has made a groundbreaking discovery about the reproductive habits of ancient mammal ancestors. By analyzing a fossil of Chiniquodon theotonicus, a cynodont that lived 236 million years ago, they found evidence of a growth line in the animal's leg bone that suggests it went through a growth spurt after birth. This finding, combined with comparisons to modern animals, suggests that Chiniquodon's reproductive strategy was similar to that of placental mammals, which give birth to live young. The discovery has significant implications for our understanding of the evolution of mammals and the origins of viviparity. The researchers used a microscope to examine thin slices of the fossil's leg bone and noticed a curious growth line. They concluded that this line recorded the moment the ancient animal went through a growth spurt after its birth. By reconstructing the size of the leg bone and the animal itself at the time the growth line formed, the team calculated that the animal would have weighed 1.68 kilograms. When the animal died as a full-grown adult, it was just shy of 12 kilograms. The researchers compared this figure to similar ratios obtained from a wide variety of modern animals, including 1869 mammals, 2619 non-avian reptiles, and 782 birds. They found that Chiniquodon grouped with extant placental mammals, suggesting that its reproductive strategy was similar to that of these animals. This is significant because it suggests that mammalian ancestors may have been giving birth to live young 90 million years earlier than previously thought. The findings are convincing, according to Mike Benton at the University of Bristol, and the team's approach to solving the puzzle is impressive, according to Oliver Griffith at Macquarie University. However, Griffith notes that the evidence is indirect and that we may never get direct evidence of mammalian eggs or a fossilized mother during pregnancy. The discovery has significant implications for our understanding of the evolution of mammals and the origins of viviparity. It suggests that the reproductive strategy of mammalian ancestors was more complex and varied than previously thought, and that the evolution of viviparity may have occurred earlier than previously believed.