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Sexual habits of the ‘dinosaurs of the sea’ revealed

News story

A Perth-led international research team has found that the males of some ancient fish species — known as dinosaurs of the sea — had erectile elements used to impregnate females, just like modern day sharks.

The research led by Curtin University of Technology’s Dr Kate Trinajstic, published in the prestigious scientific journal Nature, demonstrates that some fish species as far back as 380 million years ago engaged in penetrative sex and gave birth to live young.

These findings, involving the identification of claspers in a fossil of a male Incisoscutum, exceed the oldest known date for vertebrates by approximately 200 million years.

The Incisoscutum is an extinct species of placoderm, a primitive, shark-like armoured fish.

Placoderms were the dominant creatures of the Earth’s oceans for around 70 million years, until their extinction about 360 million years ago.

Dr Trinajstic, of Curtin’s new Centre for WA Organic and Isotope Geochemistry, said their discovery shed light on the evolution of vertebrates on Earth, including our own species.

“This discovery provides a link in the chain of evolution that scientists have been uncovering for over a century,” she said.

“It is the first time that we have been able to determine the sex of the fossil fish we study.

“We now know that nearly all the fish we have collected are female, with only a few males represented. This suggests that males and females lived separately for most of the time and only came together to reproduce.

“Another interesting discovery was that live births in vertebrates have been occurring on Earth much longer than previously thought, and that some species of placoderms had shark-like claspers.
“These are modifications of the pelvic fins that form the penis-like organ found in sharks. They are grooved organs that are used to deposit sperm into the genital duct of sexually receptive females.

“It is also possible to use the mating habits of modern sharks to understand how ancient placoderms reproduced.

“When most species of shark mate, the males hold the female in position by biting onto her fins, before arching his body so that his pelvic fins are brought close to hers.
“This is quite advanced mating behaviour. We may have found the first evidence of complex foreplay.

“For researchers, this sort of finding has broad implications in the study of the evolution of complex vertebrates, including humans.”

The research was the result of collaboration between Curtin, the University of Western Australia, the Museum of Victoria, the Natural History Museum, London, and Sweden’s Uppsala University.