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The Oldest Fossilized Animal Sperm Comes From a Worm That Lived 50 Million Years Ago

The discovery points to a new way that microscopic critters might be preserved in the fossil record

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Fossils are a good way to preserve hard structures — think of bones or teeth and less frequently cartilage or scales. Preserving soft tissues is far more rare, which is part of the reason that scientists got excited when they found 50 million-year-old spermatoza in Antarctica, reports Philip Oldfield for The Guardian.

The sperm cells came from a class of ancient earthworms and leeches called Clitellata. Since worms are all soft tissue, paleontologists have a hard time finding them in the fossil record, but Clitellata build distinctive cocoon-like egg cases that are robust enough to survive the fossilization process, the research team explains in their paper published in Biology Letters.

The team found the ancient sperm when they were looking for fossils in a deposit on Seymour Island, part of the Antarctic Peninsula. “It was an accidental find,”  says Benjamin Bomfleur, one of the authors, and a paleobotanist with the Swedish Museum of National History (SMNH). “We were analyzing 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.”

For The Guardian, Oldfield describes the find as "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."

Since the cocoon preserved the sperm structure (though not actual biological material like DNA) the researchers speculate that small, soft microorganisms, cells or sperm could be trapped in other specimens. The cocoons take several days to harden, reports Ker Than at National Geographic, so similar fossils around the world could yield future discoveries.

The discovery beats out the previous record-holders for oldest animal sperm yet discovered by a hefty 10 million years. Those sex cells belonged to an insect-like organism called a collembolan (also called springtails) found entombed in amber dating to the Late Eocene period. Another former contender that this worm sperm beats out was found inside fossilized shrimp from an Australian cave. Plant sperm fossils, however, date back to 400-million year-old fossils from Scotland, reports Sam Wong for Nature.

The worm sperm structure is very similar to those found in modern-day crayfish worms. “Surprisingly, modern crayfish worms are only known from the Northern Hemisphere,” study-coauthor Steve McLoughlin, also at SMNH, told National Geographic. “If our identification is correct then it implies that this group of animals had a much greater geographic range [50 million years ago] than they do today.”

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