[This essay was originally posted on August 13, 2011]

The opening sentence of F. Robin O’Keefe and Luis Chiappe’s new paper in Science this week is a simple statement of fact that threw me for a loop. “Viviparity, or birthing live young,” the paleontologists write, “is common among reptiles, having evolved over 80 times among extant clades.” Think about that for a moment. According to the traditional typology I was taught in my elementary school days, reptiles lay eggs and mammals give birth to live young. Yet monotremes – mammals such as the platypus and the echidna – lay eggs, and clearly reptiles have evolved the ability to produce to live young