Nature Paper Reveals Evolution of Reproduction in Ancient Fish

20 October 2014

Fossil of male Microbrachius dicki.

Male fossil showing reproductive structures called ‘claspers’, made of bone and fused to the trunk plates of the fish. SAM P50601. Donated to the South Australian Museum by Roger Jones, UK. 

Ancient, super-breeding fish on display from today at the South Australian Museum are the stars of evolutionary research carried out by an international team including South Australian Museum Palaeontologist Dr Mike Lee, which has been published in the prestigious journal Nature.

The fossils on display are primitive, jawed fish called antiarch placoderms, and they were the earliest known vertebrate species to reproduce by internal fertilisation. They also probably bore live young. These tiny fish lived in ancient Scottish lakes, and are about 385 million years old, nearly twice as old as the first dinosaurs.

This research was led by Flinders University Professor John Long, who is also a South Australian Museum Honorary Research Associate.

The research team discovered that these antiarchs are the first vertebrate animals known to exhibit “sexual dimorphism”, meaning males and females are very distinct: males bore large L-shaped genital structures (called ‘claspers’) and the females had an ornamented pair of plates in her pelvic region, to receive the tip of the male clasper and lock it in place. This indicated they reproduced by copulation, with males transferring sperm along grooves seen in the claspers.

Many fossilised baby antiarchs are known, and all are very large – often up to one quarter the length of adults. This suggests the  young were nourished for a long time inside the mother, and born live.

“These discoveries reveal that some of the earliest and most primitive fish nevertheless possessed elaborate adaptations for mating and reproduction, similar those seen in living mammals such as ourselves,” said Dr Lee.

From today, South Australian Museum will show two of these globally-important fossils in the North Foyer. The exhibition also includes models made by the Museum’s Supervisor of 3D Design Jo Bain, an animated video which used these models as a starting point, along with a spectacular painting by Dr Brian Choo (Flinders University, and a co-author of the Nature paper). There will also be an accompanying online exhibition, where people can download a 3D file and print their own fish.

The Nature publication and associated Unlocked exhibition showcases the world-class research and collections at the South Australian Museum, as well as the enthusiasm of talented exhibition and outreach staff who share these discoveries with the public.