Discovered in 1988, the fossil cave in the Riversleigh World Heritage Fossil Site in Queensland, Australia was already famous among paleontologists for its prodigious cache of bat fossils. Filled to the brim with skulls, bones, teeth and a vast quantity of petrified bat feces, or guano, the cave has now yielded another fascinating discovery: the world’s oldest sperm, found in fossilized ostracods (a tiny mussel shrimp) and perfectly preserved after some 17 million years.
Scientists aren’t sure exactly what went down in the cave in Queensland, Australia, some 17 million years ago, but they know they’ve discovered something big–something giant, in fact. Not only is the fossilized ostracod sperm they found the oldest ever, it’s also enormous. According to the researchers’ findings, published this week in the journal Proceedings of the Royal Society B: Biological Sciences, the sperm found coiled in the reproductive tract of a female ostracod recovered from the cave measured about 1.2 millimeters long–around 20 times the length of a human sperm cell.
Ostracods, which are still around today, have hinged shells like mussels and live in watery places, both fresh and seawater; they subsist on detritus found in the water. While the ostracod itself is only around 1 mm long, its sperm (which resembles angel hair pasta) can reach up to 1 centimeter in length. Only a few other animals, including some flies and moths, can boast such giant sperm, the exact evolutionary purpose of which is unclear. Though the ostracod may hold the title for oldest sperm discovered, it’s not the largest, as some species of fruitfly are known to produce sperm up to several centimeters long.
Amazingly, the reproductive processes and biology of these mussel shrimps haven’t changed much over millions of years. “We almost couldn’t believe our eyes,” the study’s lead author Renate Matzke-Karasz of German’s Ludwig-Maximilian-University told USA Todayof his team’s examination of a male ostracod fossil. Inside, they found “sperm (that) looked like little ropes, exactly how modern ostracod giant sperm look!”
The sperm remains coiled up inside a pump-like organ called a Zenker, until it’s released during mating and transferred to the female. The female ostracod found inside the fossil cave had apparently copulated just before she died and was fossilized. According to paleontologist Michael Archer of the University of New South Wales (UNSW), who led the team that collected the ostracods: “We don’t know how the instantaneous fossilization happened….But that we don’t know what happened is part of the fun.”
They also don’t know exactly how the tiny shrimp and their giant sperm were so perfectly preserved for at least 17 million years, but Archer and his UNSW colleague Suzanne Hand believe that bats played a key role in the process. In a press releaseannouncing their discoveries, Archer stated that 17 million years ago, the fossil cave site was located “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.”
The steady downpour of guano would have produced high levels of phosphorus in the water, which could have facilitated the mineralization of the animals’ soft tissue. According to Hand, “This amazing discovery at Riversleigh is echoed by a few examples of soft-tissue preservation in fossil bat-rich deposits in France.” One of these, a frog fossil discovered in the 19th century in southwestern France, appeared amazingly detailed and un-fossil-like, despite dating back as many as 34 million years. As Hand puts it, “[T]he key to eternal preservation of soft tissues may indeed be some magic ingredient in bat droppings.”