Prehistoric Undersea Sex Was No Fluke

Matt Kaplan
for National Geographic News
February 25, 2009

Sex in prehistoric oceans was likely quite common, a new study says. It was previously unknown whether long-ago sea creatures laid eggs that were fertilized outside the body or engaged in internal fertilization, aka sex.

It may sound like fun, but seeking evidence of sex is perhaps one of the most difficult endeavors in paleontology.

Detecting actions in the fossil record is extremely tough. Unlike the animals themselves, which often have hard bits that fossilize, ancient actions can be discovered only in subtle ways.

Sex itself rarely leaves much behind for the ages. Sperm and shell-less eggs, all soft and squishy, do not fossilize, for example.

Yet John Long at Museum Victoria, Australia, and a team of researchers have found evidence for sex in the form of embryos inside fossils of placoderms—380-million-year-old armored fish that looked like swimming DustBusters.

Evidence for ancient underwater sex is not new—it had been found before in a small and obscure placoderm fish group called ptyctodontids.

But nobody knew if that meant that sex was unique to that group and that most other placoderms were fertilizing eggs outside of the body—or if it was a sign that sex was much more typical.

"We have now found that the major and most diverse group of placoderms, the arthrodires, were engaging in sexual intercourse. This kind of mating behavior was far more widespread than anyone realized," Long said.

(See also: "First Sex" Found in Australian Fossils?")

Findings to be presented in tomorrow's issue of the journal Nature.

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