Exquisite microfossils brought to light by dissolving 850 million-year-old rocks could represent the most ancient fungi ever discovered. If confirmed, they would add to the known diversity of early life and provide a new calibration point for the molecular clocks used to date major evolutionary events. The discoverer also argues that more enigmatic fossils--dating to around 1400 million years ago--are the same fungi, but other experts would like more evidence for that claim.
Fungi are simple eukaryotes (organisms made of cells with a nucleus), and recent studies have surprisingly suggested that they are more closely related to animals than to plants. Fungi have been conclusively identified in the Devonian Period, roughly 380 million years ago, but are suspected to have a much longer fossil record. The new examples were found by paleontologist Nicholas Butterfield at the University of Cambridge, United Kingdom. He dissolved samples of shale from Victoria Island, Canada, that are known to contain microfossils. After sieving the slurry, he painstakingly picked out tiny fossils under a microscope.
Just 30 to 400 microns long, the fossils come in a variety of shapes. Most have a rounded body covered with tiny, multicellular filaments. The key feature, as Butterfield describes in the current issue of Paleobiology, is that these filaments joined to form networks of loops. Such hyphal fusion is diagnostic of modern "higher" fungi. Yet the fossils, whose central bodies are unusually large, clearly don't belong to any living fungi group. Still, several morphological features lead Butterfield to speculate that these new fossils are the same as previously described fossils from China and Australia. Some of these enigmatic eukaryotes, called Tappania, are almost 1.5 billion years old, but they hadn't been identified as fungi before. "I can almost put my hand on my heart and say we've got a fungus at 1400 million years," Butterfield says.
Other experts say the evidence is strong, but not entirely conclusive, that the Canadian fossils are fungi. The filamentous loops could be the result of convergent evolution, a fact that Butterfield acknowledges. Emmanuelle Javaux of the University of Liège, Belgium, part of the team that first described the Australian fossils, thinks it possible that Butterfield's fossils could be related to the older Tappania. But she also notes that the older Australian fossils have features absent from the Canadian specimens and lack hyphal fusion. The question might be answered by higher resolution images and chemical analyses, she says.