In “Superfreak of Evolution: The Lizard With a Humanlike Placenta” (Discover, April 9, 2012), Ed Yong reports
Whereas virtually all cold-blooded reptiles supply embryos with nutrients from a large egg yolk, five-inch-long Trachylepis ivensi females ovulate small, yolk-poor eggs that implant in the uterus. As the fetus develops, its tissues become intimately entangled with the blood vessels of its mother, providing ready access to nutrients and oxygen in the mother’s blood. Sound familiar? “The fetal tissues actually invade the uterine ones, much like in humans,” Blackburn says. “It’s totally unexpected.”
It doesn’t sound like a very advanced placenta, but it is definitely the same idea.
We tend to think that placentas are “better” because they are more complex systems. Which raises the question, better for what? The reptiles’ numbers game (lay many eggs, maybe guard them but maybe not) clearly works.
The placental mammals’ system reduces the number of offspring possible and creates a (literally) heavy parenting load. But it works too. It may be essential for high intelligence, but most mammals are not in fact highly intelligent. Many are not as smart as clever birds.
One wonders if this is what happened: A common ancestor of reptiles and mammals used a system like ivensi’s, but the reptiles lost theirs (except for a few holdouts) and the mammals refined theirs ….