Fossil fish reveal prehistoric lovemaking
Fish have been doing it for more than 380 million years and palaeontologists now know how, according to an Australian-led study.
In a paper published today in Nature, Dr John Long and colleagues describe the origins of internal fertilisation in an extinct fish known as placoderms.
In May 2008, the researchers announced in Nature they had discovered a 380 million-year-old fossil of a pregnant female placoderm (Materpiscis attenboroughi), complete with an umbilical cord and embryo.
The fossil, found in 2005 at Gogo Station in the Kimberley region of Western Australia, provided the oldest evidence of vertebrate sex.
But without the father, no one knew how the fish 'did the deed'.
Long, who is head of sciences at Museum Victoria, says the 2005 discovery confirmed the earliest evidence of internal fertilisation.
He says the discovery of the M. attenboroughi embryo, named after British naturalist Sir David Attenborough, lead to the latest finds.
A previous study of Gogo fish fossils collected by the Natural History Museum, London, in the 1960s, concluded the small skeletons inside the placoderms were the animal's last meal.
But closer inspection by Long and colleagues revealed they were the tiny bones of unborn fish from two adult species of arthrodires (Incisoscutum ritchiei) - the biggest and most diverse placoderm group.
The researchers then re-examined the fossils of male Incisoscutum fossils stored in Australia and the UK, which revealed how the ancient fish mated.
"Finding the first embryos last year was the Rosetta Stone to reinterpret existing material in Australian and overseas collections," says Long.
Male point of view
What his team found were structures on the fossilised pelvic girdle that allowed male fish to fertilise females, "the old fashioned way".
Dr Kate Trinajstic, a palaeontologist at the University of Western Australia, says the discovery opens up new windows to the past in collections around the world.
"The last find was more from the mother's point of view," she says. "This discovery has given us a better understanding of the male reproductive system.
"We have been able to show for the first time the anatomical connections in the place where we believe the male's pelvic elements developed to allow internal fertilisation."
Trinajstic says there is mounting evidence to suggest reproduction, internal fertilisation and live births occurred often and earlier than previously thought.
She says internal fertilisation was a driving force for the development of the pelvic girdle in these early fish.
Long says the pelvic girdle could have been a placoderm innovation, which resembles that of modern sharks.
In most vertebrates, including humans, the pelvic girdle supports the hind legs.
But in fish it supports the pelvic fin, which helps stabilise them in the water.
Fish hook-like bones called "claspers" were used to hold on to the female during copulation, says Long.
He says this complex form of mating may have also lead to the development of the jaw.
"Sharks have elaborate mating rituals where the males will bite females on the pectoral fin while they mate."