50m-year-old worm sex cells are the oldest found by 10m years, but although incredibly well-preserved contain no organic material

This article is more than 5 years old

This article is more than 5 years old





The world’s oldest fossilised sperm has been discovered in the wall of a worm cocoon found in Antarctica, raising scientists’ hopes of recovering more extremely rare fossils of microscopic soft-bodied lifeforms.

The prehistoric sex cells belong to a class of earthworms and leeches called Clitellata, and date from 50m years ago, in the early Eocene period when the first horses, rhinos and sheep emerged.

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The fossil was able to form and survive so long because the sperm became trapped in the jelly-like wall of the Clitellata cocoon before it hardened. In a manner similar to bugs becoming trapped in amber, the creature was then fossilised and preserved over millions of years.



“Spermatozoa, being very transient and delicate, are hardly ever preserved in the fossil record,” said Benjamin Bomfleur, a palaeobotanist at the Swedish Museum of Natural History in Stockholm and an author of the study. “We have uncovered a new type of medium which we think will hold great potential for similar findings in future studies.”

Though spectacularly well-preserved, the fossilised remains do not contain any organic material, ruling out the prospect of a Jurassic Park-style effort to extract DNA from the material.



Facebook Twitter Pinterest A palaeontologists digging at the site where the fossil was found on Seymour Island, Antarctica. Photograph: Federico Degrange

“Its make-up is completely altered and there’s no original organic material left there,” said Bomfleur.

The fossilised sperm look similar to those of modern crayfish worms, the peculiar leech-like organisms that live on freshwater lobsters. The scientists are unsure if they are directly related though. “Crayfish worms are only found in the northern hemisphere,” said Bomfleur. “It would be very surprising to find a relation coming from Antarctica.”

The fossilised sperm was collected as part of a deposit found on Seymour Island, one of 16 islands in the Antarctic Peninsula. It was aged at approximately 50m years old, based on strontium isotope dating.

A single worm cocoon fragment, measuring 1.5 by 0.8mm, was extracted from the deposit and examined by scientists through a scanning electron microscope. The researchers went on to create a 3D computer model from sections of the fragment taken with an x-ray microscope.

The cocoon was found to consist of a solid inner wall, one fortieth of a millimetre thick, and a spongy outer layer of loosely interwoven cables of between 0.005 and 0.010 mm thick.

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To the surprise of the scientists, the images also revealed microscopic cells embedded in the cocoon wall, including some which were tightly coiled up with a drill-bit-like structure, and others that were rod-shaped with a finely grained texture and a whip-like tail.

“It was an accidental find,” said Bomfleur. “We were analysing the fragments to get a better idea of the structure of the cocoon. When we zoomed into the images, we started noticing these tiny biological structures that look like sperm.” Through working with biologists, the team were able to determine these were indeed fragments of 50 million-year-old Clitellata sperm.

The research, published in Royal Society Biology Letters, suggests these are the oldest fossilised sperm ever discovered, by a magnitude of 10m years. The previous oldest examples are collembolan spermatozoa found entombed in amber from the Late Eocene period, and crustacean sperm from cave deposits found in Australia.