Cocoons don't just protect developing eggs and larvae; they can also preserve fossils for hundreds of millions of years. Scientists have discovered a 25-micrometer-long, teardrop-shaped protozoan (left) trapped in the wall of an egg case produced by a leech between 200 million and 215 million years ago. The Triassic-era relic's coiled stalk and large, horseshoe-shaped nucleus make it an ancient doppelgänger of the modern-day Vorticella (right), a group previously unknown in the fossil record , the researchers report online today in the Proceedings of the National Academy of Sciences. As soft-bodied organisms, protozoans aren't typically preserved as fossils. But egg cases—the walls of which quickly harden from a protein-and-sugar-rich mucus secreted by the invertebrates—of leeches and their kin are surprisingly common, the researchers explain. Like amber, which starts out sticky and then hardens, the tiny egg cases (one produced by Hirudo medicinalis, the medicinal leech, inset) can trap and then preserve soft-bodied organisms that would otherwise be quickly lost to decomposition. Although a few teams have previously described small fossils, such as spores and microorganisms entombed in such egg cases, the cocoons have largely been ignored by paleontologists and could therefore serve as an unrecognized yet bountiful source of microbial fossils.
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*Correction, 4 December: This article has been updated to reflect that the fossil, as well as the modern-day microorganism Vorticella, is a protozoan and not a bacterium.