Oldest animal sperm unearthed in Antarctica
Department of Palaeobiology/Swedish Museum of Natural History

Oldest animal sperm unearthed in Antarctica

From the frigid soil of Antarctica, researchers have excavated a prehistoric gem: 50-million-year-old sperm. The fossilized cells, reported today as the oldest animal sperm ever collected, belong to a class of cocoon-producing worms called clitellate annelids, more commonly known as earthworms, leeches, and their relatives. Using scanning electron microscopy, researchers found a “network of interwoven cables,” nestled within the wall of a cocoon, which they later determined was the rare preservation of clitellate annelid spermatozoa (pictured above), they report today in Biology Letters. Even more surprisingly, the sperm cells appeared strangely similar to those of present-day, leechlike worms called Branchiobdellids. Why so strange? Branchiobdellids attach to and depend on freshwater crayfish exclusively in the Northern Hemisphere; a fossil from the Antarctic would hugely expand their past geography and pose quite a conundrum in how these leeches evolved. The next step: sharper imaging. The authors say high-resolution confirmation of the fossil would not only allow a peak into the evolutionary history of clitellate annelids, it would blow the door wide open. And with such a sparse fossil record of these soft-bodied microorganisms, researchers plan to keep scouring for cocoons. Basically, the more ancient worm sperm, the better.