A new method for analysing the interior of fossils has shown that millimetre-long mussel-like crustaceans called ostracods used giant sperm to mate.
Sperm of modern ostracods can reach 10 times their body length.
The technique, known as holotomography, imaged the large sperm pumps and seminal receptacles of well-preserved ostracod fossils found in Brazil.
The results, published in Science, show ostracods had already evolved the giant sperm 100 million years ago.
The samples came from the Santana Formation, which has already yielded a wealth of perfectly preserved fossils, particularly of fish.
Because they are scavengers, ostracods were also found in abundance near some of the fish, with their soft internal organs equally well preserved.
Renate Matzke-Karasz of the Ludwig Maximilians University in Germany took Harbinia micropapillosa ostracod fossils, part of the collection of the Natural History Museum in London, to the European Synchrotron Radiation Facility in France.
There, light from accelerated particle beams was used to image the interiors of the fossils, yielding a detailed picture of their inner structure.
What the team found was that the sexual organs of the fossils were remarkably similar to those of modern ostracods.
Both males and females have fully separate pairs of organs. The males have two sperm pumps, or Zenker organs, and the females have two vaginas and large seminal receptacles, separated by a lengthy duct that seems optimised to accommodate the sperm.
"It was very surprising that these structures were so similar to organs we know from modern ones," Dr Matzke-Karasz told BBC News.
"Now we know that it happened at least 100 million years ago - it developed quite early in the freshwater ostracods."
Ostracods are not the only animals that use giant sperm as a mating strategy - it has also been seen in the fruit fly Drosophila. However, because it costs a lot of energy to produce long sperm, it is still unclear to biologists why the strategy arises.
For Dr Matzke-Karasz, the results open a new avenue of research for palaeontologists.
"This holotomography is so helpful; it could bring out much, much more of fossils which are all over the world in museums, buried in the collections," she said.