236-Million-Year-Old Fossil Suggests Live Birth Evolved Earlier Than Thought

A fossil of Chiniquodon theotonicus, a mammal-like reptile about 236 million years old from northwestern Argentina, indicates that the evolution of live birth in mammalian ancestors may have occurred earlier than previously believed. Discovered in 2011, the fossil displays a neonatal line that signifies a growth spurt after birth. Researchers calculated its birth weight at 3.7 pounds and its weight at death at 26.4 pounds, resulting in a newborn-to-adult body mass ratio of 14 percent, comparable to modern mammals but greater than that of reptiles and birds. The study, published on August 12 in Frontiers in Mammal Science, suggests a shift in the timeline for viviparity by 95 million years, although experts debate the strength of this evidence.