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 鈥渕other fish鈥� 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 鈥渆xtra long bone鈥� with 鈥渁 long lobe projecting backwards鈥�, 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鈥檚 cloaca, a posterior opening also used for expelling waste, in a similar way to today鈥檚 sharks, says Long.
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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鈥檚 team now thinks that they were growing embryos. 鈥淭he fish bones and armoury were not broken and crushed, as you鈥檇 expect if they were stomach contents,鈥� says Long.
While the original 鈥渕other fish鈥� 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.
