Giant Sperm From Australia's Ancient Rainforests

The Riversleigh World Heritage Site, Queensland, Australia

Sexual reproduction changed the planet. Its gene shuffling power, the diversity of biological structures and the pace of animal evolution increased a thousandfold around 550 to 600 million years ago, resulting in two explosions of life; one during the Ediacaran period, the other a few tens of millions of years later during the Cambrian period.

From sponges to molluscs, vertebrates to echinoderms: all animals owe their evolutionary origins, at least in part, to sexual reproduction. Even plants took advantage of this remarkable new means of propagation, allowing complex organisms to survive multiple mass extinctions; to produce the abundance of life seen on the planet and in the fossil record.

The process is in some ways remarkably simple: two cells with half a genome, one from each parent, combine their genetic information to create a cell with a complete number of chromosomes, capable of developing into a fully grown member of that species. In most cases the incomplete precursors, properly known as gametes, are very small relative to the producer. Microscopic pollen grains come from even the largest trees. In humans sperm cells are the smallest in the body.

Yet in some cases gametes are giants. A well documented example are the gamete of ostracods: a group of marine crustaceans sometimes referred to as seed shrimps. Housed inside a hinged shell-like casing, ostracods have a rich fossil record. Usually just the shells are preserved, but palaeontologists working at the Riversleigh World Heritage Fossil Site, Queensland, Australia, found something rather unusual: the gigantic, 17 million year old sperm of seed shrimps, complete with cell nuclei.

The 17 million year old sperm in the ostracod gonads.
The nuclei are the little black dots

'These are the oldest fossilised sperm ever found in the geological record,' said Professor Mike Archer, of the UNSW School of Biological, Earth and Environmental Sciences. 'The Riversleigh fossil deposits in remote north-western Queensland have been the site of the discovery of many extraordinary prehistoric Australian animals, such as giant, toothed platypuses and flesh-eating kangaroos....It now makes us wonder what other types of extraordinary preservation await discovery.'

Examination of the fossils at the European Synchrotron Radiation Facility in Grenoble brought out the detail. At 1.3 millimetres in length they were longer than the ostracods themselves, which also displayed an incredible degree of preservation, complete with gonads, associated sperm and two muscular pumps known as the Zenker organs, responsible for transferring the sperm to the female organs. The next question was determining how such delicate structures survived for 17 million years.

The Riversleigh site was a cave, part of 'a vast biologically diverse rainforest.' The ostracods survived in a pool of water, 'enriched by the droppings of thousands of bats,' said Professor Archer. The researchers believe that the phosphate in the bat droppings helped to mineralise the soft tissues of the ostracods. Indeed it is thought that this process is responsible for the incredible degree of preservation seen in fossils found at Riversleigh, which include the soft tissues of eyeballs and the muscles of insects.

The cell is the standard unit of biological enquiry for the natural world. The fossil record is much more vague: a bone or maybe a set of footprints. Being able to make the shift to the cellular level has been one of the most important revolutions in palaeontology. From red blood cells in tyrannosaur femurs to plant tissues from Devonian Scotland, the Riversleigh sperm is a new line of enquiry, fast changing the way we view the fossil record. In time ancient cells may help shed light on the origins of sexual reproduction itself.