The world's oldest and best preserved sperm has been discovered in 17 million-year-old fossils unearthed in far north Queensland.

The sperm of an ancient species of freshwater shrimp, which were found inside a rock taken from the Riversleigh World Heritage area, were not only hardy, but huge compared to the animal's body size.

The not-so-little swimmers were as long as the crustacean itself, which was only a couple of millimetres in length.

Palaeontologist Michael Archer, one of the three researchers who dug up the fossils in 1988, said: ''99.9 per cent of organisms who have sperm have, relative to their body size, small sperm [because] you've got to tuck them away in a little jiblet somewhere in your body.''