Fishy sex a-Gogo

Last year, Museum Victoria’s Dr John Long and his colleague Dr Kate Trinajstic from the University of WA announced “the oldest mother ever discovered”, a ptychtodontid fish fossil called Materpiscis attenboroughi from the Gogo Formation in WA, carrying an embryo and what they suspect is an umbilical cord.

That discovery pushed back the known record of live birth, or viviparity, some 200 million years.

Now they have taken a look at some other fossil fish from the Upper Devonian period, and have found similar evidence of viviparity in another, much larger class of placoderms, the arthrodires.

Examining an Incisoscutum ritchiei specimen held in London’s Natural History Museum, they say that what was thought to be a small fish that had been eaten by Incisoscutum is in fact an embryo.

And by closely examining the pelvic girdle of the fossils, they have found evidence of a basipterygium, cartilage at the base of the pelvic fin that is used by cartilaginous fish, namely sharks and rays, to copulate.

This comes as a big surprise, as these fish do not show signs of sexual dimorphism, where males and females have differing body shapes, Long says.

The team also re-examined other placoderm fossils from the Mt Howitt site in Victoria. One fish, Austrophyllolepis, also showed these long structures attached to the pelvic fin.

Most modern sharks are viviparous and use a clasper to insert sperm into the cloaca of a female. Most fish, on the other hand, are oviparous, spawning eggs that are fertilised externally.

The team’s finding, published in Nature, is that these ancient fishes were viviparous, used a similar method for copulation as sharks and were at it long before anyone realised.

They also say the findings confirm that internal reproduction and live birth were much more widespread than previously thought.