Tiny fossil crustaceans found in Australia contain remarkably preserved giant sperm that are between 16 million and 23 million years old — the oldest reproductive cells ever discovered.

Renate Matzke-Karasz at Ludwig Maximilian University in Munich, Germany, and her colleagues examined ancient freshwater sediments in Queensland and used synchrotron X-rays to image the internal structure of fossil ostracods, crustaceans around 1 millimetre in length. The authors found well-preserved detail of soft tissue, including internal organs and sperm clusters around 1.2 mm long. The team even identified nuclei in some sperm.

Many modern ostracods have huge sperm relative to their body size, and these fossils show that the trait evolved long ago, although it is not clear why.

Proc. R. Soc. B http://doi.org/ssh (2014)