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World's Oldest Fossil Sperm Found at Riversleigh

By John Long

Synchrotron imaging of a 16 million-year-old ostracod found in NSW has revealed the world’s oldest fossilised sperm.

Exquisite preservation of soft tissues in fossils includes many mind-boggling recent examples, from muscles and placental cords in 380 million-year-old fishes from Gogo, impressions of cranial veins and arteries in 520 million-year-old arthropods to an amazing a 320 million-year-old fossilised brain in a bizarre shark-like fish from the USA.

But if gold medals were given out for excellence in fossil preservation I’d be giving one to the tiny 16 million-year-old ostracods from the World Heritage Riversleigh Fossil deposits in north Queensland. Not only are the soft parts beautifully preserved in this fossil, but we know it was a male as it has sperm perfectly preserved. This is the oldest record known on the planet of fossilised sperm in any creature.

The sperm, described by John Neil, Mike Archer and a team of international collaborators in Proceedings of the Royal Society B, were preserved in a freshwater calcite-rich lake. The tiny sperm are tightly coiled bundles that, if unfurled, would be the length of the ostracod (1.3 mm), which is roughly equivalent to a bloke having sperm the length of a cricket pitch!

The sperm show remarkable preservation of the nucleus of each cell, as revealed by nanoscale scanning carried out at the European Synchrotron facility in Grenoble. Even the Zenker organs, which pump the giant sperm into the female, were preserved in these fossils.

Russel Garwood and colleagues from the UK have recently reported in Current Biology a Carboniferous-age harvestman fossil (Hastocularis) that has huge implications for understanding the diversification of life today. The new suborder Tetrophthalmi means that the living harvestman groups most like diversified in the Carboniferous Period.

Following on from the arthropod-fest in last month’s column, a paper in Palaeontology has reported the discovery of the oldest known terrestrial decapod – the groups that crabs and prawns belong in – from a Late Devonian site in Belgium. This creature, named Tealliocaris, looks a lot like a small yabby and was probably just as tasty!

Pterosaurs were flying reptiles that lived alongside dinosaurs, and are as close as reality gets to the mythical dragons of lore. Some interesting new papers are now changing our views about these long-gone animals.

Roger Benson and colleagues have reported in Nature Communications that pterosaurs grew at a constrained rate for their first 70 million years, and then began a sustained period of multilineage diversification characterised by major increases in size. This change was coincident with the origin of and diversification of the first birds about 160 million years ago, so it supports the controversial idea that competition drove evolutionary radiations.

Another paper by Brian Andres, James Clark and Xu Xing in Current Biology documents the oldest known pterosaur lineage from China. Kryptodrakon, whose wingspan measured 1.4 metres, pushes back the oldest known record of this group.

The ongoing debate about the origin of the modern fish fauna was shaken up by the recent publication in Nature of a 325 million-year-old fossil shark by Alan Pradel, John Maisey and colleagues. The skull of Ozarcus was CT scanned using synchrotron tomography, revealing it had a set of gill arches unexpectedly similar to that of a bony fish (osteichthyan). This finding lends support to other recent papers that propose sharks are much closer to modern bony fishes than previously surmised, mainly because placoderms are becoming firmly rooted as the most basal of all jawed vertebrates.

John Long is Strategic Professor in Palaeontology at Flinders University.