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A Very Ancient Group of Jawed Fish Were Having Surprisingly Intimate Intercourse

Many fish later ditched this clumsy method in favor of external fertilization

An artist's impression of an antiarch mating scene. (Photo: Brian Choo, Flinders University)
smithsonian.com

For many fish species today, copulation is a hands-off affair. Females release a torrent of eggs into the water, and males douse the eggs in a cloud of sperm. This seems like a pretty basic way of reproducing—shoot your gametes into the environment and hope for the best—and it has worked well enough for many species for millions of years. 

External fertilization, however, seems to be a more recent development in sexual strategies than the more intimate alternative—internal fertilization. According to research published yesterday in the journal Nature, a very ancient group of jawed fish called the placoderm antiarchs were getting intimate long before their future ancestors opted for external fertilization.

This implies that internal fertilization might have evolved right alongside vertebrates—perhaps it served as the reproductive strategy of choice for the earliest examples of our most ancient relatives, around 385 million years ago or earlier. 

An international team of researchers reached this conclusion after examining fossil antiarchs and spotting some previously overlooked telltale anatomy: male copulatory claspers and female dermal plates, similar to what is found in modern-day sharks. The researchers think that antiarch males used those bony protuberances to grasp the female from the side, and that her two dermal plates helped facilitate that exchange. "This implies that external fertilization and spawning . . . must be derived from internal fertilization, even though this transformation has been thought implausible," the team concludes. 

Whether antiarchs' choice reproductive strategy represented the norm for all early vertebrates or whether they alone had figured out the anatomical secret for internal fertilization remains unknown. Either way, though, the team points out that the finding has "startling significance" for vertebrate evolution—including our own. 

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