Early Sex

The planet’s first acts of copulation, discovered in an ancient species of fish, were performed sideways

Last year, an Australian palaeontologist stumbled across a strange single fossil bone of placoderms, a species of ancient fish believed to have existed some 385 million years ago, in Estonia. This strange grooved bone, belonging to the Microbrachius dicki, from the antiarch group of placoderms, was unlike anything he had ever seen before. With a group of researchers, the palaeontologist examined previously discovered fossils in museums, and found more instances of such bones attached to the body. Writing in Nature, the researcher has revealed his discovery: the bone was a sex organ, its most primordial example.

About 8 cm long, Microbrachius lived in ancient lake habitats in Scotland, as well as parts of Estonia and China. According to the study, the male members of this ancient species developed bony L-shaped genital limbs called claspers to transfer sperm to females. The females, it was found, developed small paired bones to lock the male organs in place for mating. The male’s organ was nearly as long as his body and rigidly fixed, and it used its small jointed arm-like appendages to achieve the appropriate mating position. The fish are believed to have mated sideways.

The discovery means that sex with internal fertilisation evolved much earlier in the history of vertebrates than previously thought. Since the oldest bony fishes, which follow placoderms in the evolutionary chart, show no evidence of internal fertilisation, the researcher concludes that at some point the internal fertilisation method got lost, before some of their descendants ‘re-invented’ such organs for a similar function. For instance, modern sharks and rays have claspers along the inner part of their pelvic fins that they use to deposit sperm in females.

In a press release put out by Flinders University, Long says, “Placoderms were once thought to be a dead-end group with no live relatives but recent studies show that our own evolution is deeply rooted in placoderms, and that many of the features we have, such as jaws, teeth and paired limbs, first originated with this group of fishes… Now, we reveal they gave us the intimate act of sexual intercourse as well.”

About The Author

Open