Dept Palaeobiol./Swedish Museum of Natural History 
 
 The remains of long, thin cells preserved inside the 50-million-year-old fossilized cocoon of an unknown worm species represent the oldest animal sperm ever found, say researchers at the Swedish Museum of Natural History in Stockholm. 
 
 Benjamin Bomfleur and his colleagues spotted the sperm fragments when they used an electron microscope to examine the inner surface of the cocoon fossil, which had been collected by an Argentinian expedition on Seymour Island, which lies off the Antarctic Peninsula. Their findings are published today in Biology Letters1. 
 
 Because of their delicate nature, sperm cells are very rarely found in fossils. The previous oldest animal sperm was from springtails preserved in Baltic amber, about 40 million years old2. (Plant sperm fossils go back further, to 400-million-year-old specimens from early land plants in Scotland3.) 
 
 Surprising discovery 
 
 Bomfleur says that the discovery was a surprise — “we laughed”, he says, on seeing the microscope images — “but in retrospect, it makes sense that you would find them as common inclusions in fossil cocoons”. The cocoons are secreted by some worms, including earthworms and leeches, which deposit sperm and eggs inside. Each cocoon then hardens to form a protective case for the developing embryos. 
 
 The researchers do not know what kind of worm left the sperm. Scanning electron microscope images show helical structures resembling drill-bits and beaded tails, which are characteristic of sperm produced by crayfish worms, leech-like creatures that live on freshwater lobsters. But these animals are found only in the Northern Hemisphere, so it would be surprising if they had existed in Antarctica 50 million years ago, Bomfleur says. “It could be an extinct relative with similar types of sperm.” 
 
 There will be no extractable DNA left in the sperm fragments, Bomfleur adds, because the chemical make-up of the organic material would have changed from its original composition over such a long time. But his team expect that the cells they saw under the microscope are not merely mineralized outer casts of the original cell’s shape, but will retain their inner structure. 
 
 Taxonomic tool 
 
 The structure of sperm cells has been studied for decades as a way of determining evolutionary relationships between worm species. The team did not have access to ultra-high-resolution instruments that might reveal more detail about the sperm’s microstructure than can be seen under the electron microscope. But if the fine structure of sperm is commonly preserved inside cocoons and can be imaged, Bomfleur thinks that future studies could provide important insights into the evolution of the kinds of worms that secrete cocoons. “If it should turn out that we can get this information, all of a sudden we would basically unlock an entire fossil record for a group that hardly had any identifiable fossils before,” he says. Soft-bodied microorganisms that do not usually fossilize, including nematodes4, have also been found preserved inside cocoons, but few researchers have studied them. 
 
 Jakob Vinther, who studies invertebrate evolution at the University of Bristol, UK, agrees that cocoon fossils could represent an underexplored avenue for understanding the origin of earthworms and leeches. “I think we might have a really interesting system here that can be sort of a hidden window to the past,” he says. “There could be a lot of potential hidden gems inside those cocoons.” 

 It's time to call Guinness World Records: Researchers on an Antarctic expedition have uncovered sperm cells dating to a whopping 50 million years ago, making these the oldest known animal sperm cells, a new study finds. 
 
 The researchers found the sperm fragments embedded within the walls of a fossilized cocoon. The scientists said they suspect that an ancient relative of worms or leeches likely created the cocoon while mating, and released its sperm inside. 
 
 The sperm became trapped in the cocoon before the enclosure's walls hardened, the researchers said. Just as amber can entrap and preserve insects, the cocoon preserved the sperm cells while fossilizing over millions of years, the researchers said. [See Images of the Oldest Petrified Sperm from an Ostracod] 
 
 "Because sperm cells are so short-lived and fragile, they are vanishingly rare in the fossil record," said lead author Benjamin Bomfleur, a paleontologist at the Swedish Museum of Natural History. "Our discovery of sperm in a leech cocoon from Antarctica is the oldest record of fossil animal sperm and one of only a tiny number of such fossils in the geological record." 
 
 The researchers discovered the cocoon while sieving sediments for small vertebrate remains during an expedition in Antarctica, Bomfleur said. The research team then used a scanning electron microscope to examine the fossil's surface, and the particles on it, at very high magnifications, he said. 
 
 Researchers found the fossilized cocoon at this snowy location in Antarctica. Credit: Federico Degrange 
 
 The scientists also used high-power X-rays from a particle accelerator in Switzerland to image the cocoon's internal structure. These analyses revealed that the cocoon held bacteria and sperm cells, Bomfleur said. 
 
 However, the early Eocene specimen didn't hold entire sperm cells, just pieces of them. 
 
 "All we found are fragments of the drill-bit–shaped 'head regions,' ornamented midpiece regions that presumably contain the nuclei, and very long, whiplike tails, some attached to the midpiece regions," Bomfleur told Live Science in an email. 
 
 It's challenging to compare sperm fragments to the sperm of modern species, but the drill-bit-shaped head regions "do appear strikingly similar to those of this one peculiar group of leechlike worms that is today only found living symbiotically on crayfish in the Northern Hemisphere," Bomfleur said. 
 
 He added that the next-oldest known fossil of animal sperm is also from the Eocene, dating to about 40 million years ago. Researchers found that specimen, which belonged to a relative of insects called a springtail, in a piece of Baltic amber, Bomfleur said. 
 
 However, the oldest known fossil sperm cell doesn't belong to an animal, but to a plant called a chert found in Scotland. The plant dates to the early Devonian, about 410 million years ago, Bomfleur said. 
 
 The new study is a "well-done investigation," said Renate Matzke-Karasz, a geobiologist at Ludwig-Maximilian-University in Germany. 
 
 The finding of the "entombed micro-treasures" may help illuminate the family tree of worms and leeches, of which little is known because fossils of these soft-bodied creatures are rare, Matzke-Karasz said. 
 
 "I am sure a broader search for more such fossils will help create a clearer view on the evolution of these animals," she said. 
 
 The study will be published online Wednesday (July 15) in the journal Biology Letters. 
 
 Follow Laura Geggel on Twitter @LauraGeggel. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science. 

 Spermatozoon A 50 million year old spermatozoon (sperm cell) found in Antarctica. Department of Palaeobiology, Swedish Museum of Natural History 
 
 We all know about how mosquitos trapped in amber: 
 
 Can lead to dinosaur biker gangs: 
 
 (Or, at the very least, awesome memes) But what about worm sperm trapped in a cocoon? It probably won't lead to insane theme parks with a death wish, but it could change our understanding of ancient microbes. In a paper published today in Biology Letters, Benjamin Bomfleur and his colleagues announced the discovery of 50 million year old sperm cells found in a cocoon in Antarctica. This is the oldest animal sperm ever discovered, but what's really exciting is where it was found: in a tiny cocoon in Antarctica. These cocoons aren't like cocoons used by butterflies to make their lovely transformation from lowly caterpillar to gorgeous winged creature. Instead these cocoons are integral to the mating process of many different species of worm and leech. After mating, the worms secrete a sticky gel like cocoon that contains the egg and sperm. Over time, it hardens, trapping the cells inside. 
 
 Leech Diagram Benjamin Bomfleur 

 
 
 A fossilized sperm cell (Department of Palaeobiology, Swedish Museum of Natural History) 
 
 Benjamin Bomfleur can’t help but laugh a little when talking about this latest, record-setting discovery. 
 
 “A 50-million-year-old worm sperm from Antarctica?” he said between chuckles. “Who would have thought that’s possible?” 
 
 And yet, what Bomfleur, of the Swedish Museum of Natural History, and other researchers stumbled upon was a serious discovery: the oldest animal sperm ever found. 
 
 As it goes with many great discoveries, this wasn’t what they were looking for. While on an expedition on Seymour Island in the Antarctic, Thomas Mörs, a paleobiologist at the same museum, was looking for signs of small mammal bones when he discovered a fossilized cocoon. Remembering that cocoons like this often carried notable remains of plants, he passed it along to Bomfleur, a fellow palaeobiologist. Bomfleur took it and started looking for plant remains. That’s when he noticed the long, fragile remains of what appeared to be a sperm. 
 
 No expert in sperm himself, Bomfleur sent pictures of the specimen along to Marco Ferraguti, who happens to be an expert in annelid sperm. 
 
 Through radiometric dating, it was determined that the cocoon and its contents were at least 50 million years old, making the sperm the “oldest fossil animal spermatozoa yet identified.” 
 
 After comparing the pictures with his “enormous collection” of sperm pictures, Ferraguti determined the specimen was likely the product of a “crayfish worm.” 
 
 Now retired, Ferraguti made a point of mentioning that he finds it amusing that he gets to be part of such an eye-catching study because, while in the field, he found it “really, really difficult to tell people the work [he does] is interesting.” 
 
 What allowed this sperm to survive for so long was the biology of its creator. 
 
 The ancient worm secretes a cocoon, about 2 millimeters around, which then typically serves as a protective housing for an egg and sperm when the worm reproduces. The cocoon is formed by a sticky mucus that takes several days to harden, but once it does, biological material, such as sperm, can be trapped along its walls. 
 
 
 
 General location of Cocoon fossil. (Federico Degrange) 
 
 This can preserve all kinds of different materials for centuries. 
 
 Bomfleur likened it to amber preservation, though it’s unlikely this could be used for the creation of a Spielberg-esque, Jurassic worm (sorry). 
 
 “These things are — they are flexible,” Mörs said, comparing the cocoon to similar fossils typically formed by plants. “I didn’t expect at the beginning that these were from animals.” 
 
 The downside, Bomfleur said, is that the sperm was not completely intact. It was broken into fragments, making it difficult to learn much about the anatomy of this specific worm. 
 
 However, the research team also found remnants of clam shells and small animals within this cocoon. 
 
 The evolutionary history of Citellata (earthworms, leeches, etc.) is largely unknown, the team wrote in their paper, which was published in Biology Letters this week. Finding remnants of this type of species in the Northern Hemisphere is particularly rare, the study said, indicating that the evolutionary history of these worms is more complex than once thought. 
 
 More importantly, Bomfleur explained that though worms themselves don’t fossilize well, sperm can be a better clue as to the creator of a given cocoon. This discovery should encourage future researchers to look for similar structures. 
 
 “I think we might have a really interesting system here that can be sort of a hidden window to the past,” Jakob Vinther, who studies invertebrate evolution at the University of Bristol, told Nature. “There could be a lot of potential hidden gems inside those cocoons.” 
 
 While fossilized sperms are a rare find, Seymour Island is a known hot spot of noteworthy fossils, Mörs said — he’s been there three times already. The only part of Antarctica that is completely free of ice cover, the island is a perfect location for extraction. Mörs said other fossils originating from the mainland of the continent often wash up onto the island, bringing a variety of fossils, including small mammal bones and shark teeth. 
 
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– They were looking for mammal bones, but scientists exploring an island in the Antarctic instead stumbled across an unexpected milestone—the world's oldest animal sperm. They found it inside a fossilized cocoon made by some type of worm (possibly a crayfish worm) 50 million years ago, reports Nature. It beats by 10 million years the previous record-holder, a specimen from an insect relative called the springtail. “A 50-million-year-old worm sperm from Antarctica?” Benjamin Bomfleur of the Swedish Museum of Natural History marvels to the Washington Post of his team's discovery. “Who would have thought that’s possible?” These types of cocoons aren't of the butterfly variety but were instead used in the mating process, explains Popular Science. They're secreted by the worms to hold the sperm and egg, and they often trap other things inside as they harden. "That's what makes this research so exciting," writes Mary Beth Griggs. "By taking a closer look at the cocoons, researchers can get a better idea of what kinds of microorganisms were around in the past." Because sperm cells are so delicate, it's rare to find any this old. Still, the age doesn't come close to the oldest sperm on record, reports Live Science. That hails from a plant found in Scotland and goes back 410 million years. (A bioethicist thinks men should freeze sperm samples at age 18.)