SEXUAL intercourse was far more common in early vertebrates than anyone imagined. So suggests a new study of ancient shark-like creatures called placoderms.
Last year, , Australia, and colleagues found an embryo complete with umbilical cord inside a placoderm fossil from the Gogo formation in Kimberley, Western Australia. This “mother fish” pushed back evidence of internal fertilisation and live birth by 200 million years to 380 million years ago. But how placoderms managed to mate, considering some orders could grow to be 6 metres long and all were heavily armoured, had been a mystery.
Now, Long and a different team think they have the answer. They examined the pelvic anatomy of three 380-million-year-old placoderm fossils belonging to the order Arthrodira and found a previously unnoticed “extra long bone” with “a long lobe projecting backwards”, says Long. The shape of the lobe indicates that it articulates with cartilage, similar to the erectile claspers of modern-day sharks, he says (Nature, ). These claspers would have been used to channel sperm into the female’s cloaca, a posterior opening also used for expelling waste, in a similar way to today’s sharks, says Long.
Advertisement
The team also re-examined two other arthrodire fossils from the same region. Small skeletons inside the specimens had been thought to be the debris of a cannibalistic dinner. But Long’s team now thinks that they were growing embryos. “The fish bones and armoury were not broken and crushed, as you’d expect if they were stomach contents,” says Long.
While the original “mother fish” was from an obscure placoderm order, the arthrodires are from the largest. This raises the question of whether sexual intercourse evolved once, prior to the orders branching off, or many times independently, says , an expert on fish evolution at the Australian National University in Canberra.
