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The first reptile with a true placenta

Species: Trachylepis ivensii
Habitat: Angola, Zambia and the Democratic Republic of the Congo, being very hard to find

In evolution, as in life, some things are easier than others. It seems to be pretty straightforward to evolve complex eyes, which have turned up dozens of times.

Similarly, for some groups of animals it's easy to stop laying eggs and start giving birth to live young. Backboned animals have evolved live birth no fewer than 132 times, and nowadays a fifth of lizards and snakes give birth. Human mothers may disagree, but live birth is clearly not that difficult.

What is difficult, however, is nourishing unborn young the way mammals do. A female mammal allows each embryo to burrow deep into the wall of her womb, where it takes nutrients straight from her blood. This intimate arrangement was long thought to have only evolved once, in mammals.

Not so. It now appears that it evolved at least twice: once in mammals, and once in an obscure African lizard called Trachylepis ivensii.

These lizards don't look like rule-breakers. T. ivensii are fairly typical skinks, one of around 1200 species. Adult females grow to 9 to 14 centimetres long, plus tail. They are rarely seen: only a few specimens have been collected. "We don't know much about them," says Daniel Blackburn of Trinity College in Hartford, Connecticut.

Blackburn began studying T. ivensii in earnest after his colleague Alexander Flemming of Stellenbosch University, South Africa, found nine females preserved in a museum. Blackburn and Flemming worked together to dissect them and find out how their young developed.

They focused on the oviducts – tubes that run from the ovaries to the outside. Live-bearing reptiles release eggs into the oviducts, where they develop into babies before being born. The question is, how were the developing embryos fed while in the oviduct?

All live-bearing reptiles have a basic placenta, but unlike its mammalian counterpart the embryo doesn't get much food that way. It can't: although it nestles up against the oviduct wall, the embryo remains inside a remnant of eggshell that acts as a barrier. Instead, it is nourished by a large yolk.

A very few reptiles, including T. ivensii, break this rule. Their eggs are small, with little yolk, so they must get lots of food from their mothers via the placenta. But only T. ivensii allows the embryo to implant itself in the oviduct wall. "It's unprecedented," Blackburn says.

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TAGGED: BIOLOGY, EVOLUTION


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