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Natural History of Sex

By John Upton

If you could exchange the orgasmic gift of mammal-style intercourse for the messy ritual of dribbling fluids together, would you?

If you were the lineage of the world’s fish, then the latest evidence suggests that the answer to that unlikely conundrum would be, ‘Yes,’ to the great surprise of scientists documenting backbone-baring animals’ earliest known instances of internal fertilization.

We’re not talking about pudgy, panting penetration following happy hour pitchers. We’re talking rock-hard prehistoric fish, dermal claspers and paired dermal plates — locked, hundreds of millions of years ago, in ocean-floor embraces.

Bow guppy bow wow.

antiarchs
Illustrated by Perry Shirley.

A new scientific study peers into the most intimate moments in the lives of fish of several closely related ancient fish genii — MicrobrachiusPterichthyodes and Bothriolepis. Each was an antiarch, which was a heavily-armored order of fish known as placoderms. Placoderms were early gnathostomes, an infraphylum also called boned vertebrates, which includes all the fish alive today.  Gnathostomes started the branch on our evolutionary family tree that blossomed into lizards, birds and apes.

These long-dead fish were caught in the act of possessing the organs needed for sexual penetration. In a question over which came first, be it fish intercourse or external spawning, the answer would seem to be the former.

(The word antiarch means “opposite anus,” by the by, because a scientist once mistook a fossil’s mouth for its ass. And, while we’re at it, one of the species studied was M. dicki. Because somebody couldn’t help themselves.)

We’ve included a sketch of this primal love making below. Just make sure there are no underaged fish fossils in the room.

Mating Microbrachius
Adapted from a figure in Nature showing Microbrachius mating.

Is there anything cuter than fish that lock arms like that at a time like that? In real life, the width of that embracing scene would have been an adorable inch and a half.

We’re not sure if those are expressions of ecstasy — or the shocked looks that are to be expected when scientific method walks through the doors of ancient history during the most intimate of hypothesized moments. But we did confirm with Flinders University paleontology professor John Long that those big holes are where the eyes and nostrils would be. Because we didn’t want to get anything ass-backwards.

By now you might be wondering such things as, What does this have to do with me? If I took the right medicine, could I lead a life like that? And, How can I not think about this next time I’m bonking my boyfriend, or rolling in the hay at that haystack that my wife and I both like; won’t somebody please come and scoop the vision of adorable ancient randiness from my braincase? Aarrrggghhhhh.

We don’t know the answers to all your implied imaginary questions. What we do know comes from a study published Sunday in Nature by a large team of scientists led by Long.

The scientists analyzed fossils of the three genii that we mentioned earlier, and found in the males what they believe to be evidence of dermal claspers. They also found what they take to be the paired dermal plates of their female counterparts. Place those together and you’ve got yourself some internal fertilization, Velcro style.

The researchers write that the claspers found in these fossils resemble those of ptyctodonts, an order of placoderms that came after the unfortunately-named antiarchs. Those similarities, they wrote, suggest that “all placoderm claspers are homologous,” and that “internal fertilization characterized all placoderms.”

And they say that implies that spawning, which is so popular with the gnathostomes of today, must be an evolutionary adaptation that began with internal fertilization, even though such a transformation had been regarded as implausible.

“The complex physiology of fish that internally fertilize precludes a reversion back to spawning in water,” Long told us. “Yet our analysis shows it was the most likely explanation.”