Science Interviews

First Fish Sex

Nicky - Geologist John Long and his team have discovered how ancient vertebrates have sex and even how their penis functioned.  This research, combined with early finds suggests that vertebrates separated into males and females in primitive fish almost four hundred million years ago.  Scientists have known ancient fish had sex since their discovery of a primitive mother fish with a fossilised embryo inside it in the Gogo region of Western Australia last year.  The mother fish, a species called ptyctodon is from a group of extinct fish called placoderms.  Since then scientists have pieced together fossil records of another species of placoderms called arthrodires and revealed they too contained embryos.  These fossilised embryos are the oldest records of live birth in vertebrates.  Because live birth can’t occur without sex.  Scientists knew that primitive fish were doing the deed but how some of them were doing hit has remained a mystery until now.  John Long, from the Museum of Victoria:

John - Now looking at more of this amazing fossil fish material from the Gogo fossil sites in Australia we’ve found that the biggest group of these armoured fish placoderms – the biggest group, the arthrodires – also have  embryos inside them and they were also fertilising by males copulating with the females.  This is something that we would not have expected.  When you look at the group that the mother fish belongs to, the ptyctadons, they actually show sexual dimorphism.  The males have claspers and the females don’t.  Claspers are what we see in sharks and rays today, how they copulate.  The arthrodires, on the other hand, their pelvic fins up and till now have been always depicted as very simple structures just like simple fins.  It may just go right back and look again, and look hard at these fossils to see if anything had been overlooked.  We found it.  It was another fantastic eureka moment when you make a big discovery that for a hundred years has been completely overlooked.  We found that these pelvic fins in these arthrodires had an extra articulation and that articulation meant that had a long lobe attached to the fin that was directed behind the fish in a rearward direction.  That looks exactly like the claspers in modern sharks.  We found out not only that this major group were having internal fertilisation but we found out how they were doing it.  We believe that this shows the origin of the beginning of sexual dimorphism in vertebrates.

Ben - So structures found on fossilised fish can tell us a great deal about the evolution of sexual dimorphism. That’s different characteristics for males and females and the more we understand about these ancient animals the better we understand our own evolutionary history.  That was John Long from Museum Victoria.

March 2009