PLACODERMS, ARMOUR-PLATED FISH PIONEERED IN PENETRATIVE SEX

Could a long-extinct fish rewrite history of sex? Fish thought to be ancestral to all vertebrates had already evolved an organ for internal fertilization, even though some of their descendants reverted to doing it externally.

A study of Microbachius fossils revealed the first evidence of their primitive sexual organs. Showed that to transfer sperm, males had grooved L-shaped claspers which were held in place by small paired bones on the female.

These tiny fish (Microbrachius dicki) lived in ancient Scottish lakes and are 385 million years old – nearly twice the age of the first dinosaurs. (Microbrachius means little arms, and dicki… welll)

The new discovery implies that external fertilisation evolved from internal fertilisation involving sexual intercourse, and not the other way around. In any case, the team recognizes that more evidence is needed to show that this is not just a specialized sub-groups of placoderms.

• Both illustration by Brian Choo/ Flinders University
• 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
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