Sperm longer than the body of the crustacean that produced it have been discovered at the Riversletigh World Heritage Fossil Site in north-western Queensland. Astonishing as this may be to non-zoologists, the most unexpected aspect of the find is that the preserved seed is 17 million years old.
The Riversleigh Heritage Area is, for Australian paleontology, the gift that keeps on giving. The soft limestone of the area has preserved fossils intact, rather than compressing them, providing site across 100km2 area where extinct species 5-25 million years old have been revealed.
While the area is known primarily for its giant marsupials and monotremes other fascinating creatures lie thick in the ground.
Professor Mike Archer of the University of New South Wales helped bring Riversleigh to the world's attention with a series of discoveries in the 1980s that rewrote ideas about marsupials, but now he has found something very different. “These are the oldest fossilised sperm ever found in the geological record,” Archer says. “We have become used to delightfully unexpected surprises in what turns up there. But the discovery of fossil sperm, complete with sperm nuclei, was totally unexpected. It now makes us wonder what other types of extraordinary preservation await discovery in these deposits."
It took 26 years between the collection of some fossil ostracods and publication in the Proceedings of the Royal Society B, a consequence of the shortage of funding that frustrates efforts to study the extraordinary finds being rung from the area.
However, when La Trobe University's John Neil finally examined the find he realized that soft tissue had fossilized along with the hard shell. Drawing in some of the leading specialists in the field from around the world Neil and his colleagues found the sexual organs of the tiny crustaceans could still be seen.
Ostracods are a common but tiny creatures also known as seed shrimps. Most live in the oceans, but freshwater species such as these are not uncommon. The oldest evidence of parental care comes from ostracods. They often produce large sperm, in some cases six times the length of the parental body when uncoiled. The Riversleigh ostracods did not quite take things to such extremes, but the 1.3mm sperm are slightly longer than the preserved bodies within which they can be seen curled up. Also visible are the Zenker organs, relatively huge muscular pumps required to trasfer the large sperm to the females.
"About 17 million years ago, Bitesantennary Site was a cave in the middle of a vast biologically diverse rainforest. Tiny ostracods thrived in a pool of water in the cave that was continually enriched by the droppings of thousands of bats," says Archer. It is thought that the phosphorus from the bat droppings may have helped the soft tissues fossilize. Similarly preserved soft tissues have been found in caves in France also inhabited by bats. Riversleigh has seen other examples of soft tissue preservation, including the tissue of mammalian eyeballs and the cells of leaves.