THE discovery of tiny fragments of land plants that date back 450 million years has prompted a re-evaluation of even older fossils. The conclusion is that plants first colonised the land around 50 million years earlier than we thought.
Until now, the earliest conclusive evidence of land plants consisted of tiny branching structures found in rocks in Wales, dated to around 425 million years ago. But many rocks laid down during the previous 50 million years contain spores that cannot be identified with certainty. Some palaeobiologists argued that the spores came from primitive land plants such as liverworts, while others said they were merely from aquatic algae.
Now fossilised fragments of plants have been found in 450-million-year-old rocks from Oman (Nature, vol 425, p 282). The fragments, all less than half-a-millimetre across, are of structures called sporangia, in which spores develop. They closely resemble those of modern liverworts 鈥� convincing evidence that the unidentified spores did come from land plants, says Charles Wellman at the University of Sheffield in the UK, whose team found the fossils.
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Liverworts are an ancient group of land plants related to mosses. The uniformity of the spores that geologists have found suggests that for the first 50 million years or so, the same liverworts thrived all over the world. Because they are so numerous and well dispersed, the spores provide a reliable picture of what is going on, says Paul Kenrick, an expert on plant fossils at the Natural History Museum in London. 鈥淭he fact that we don鈥檛 find them before 475 million years is very telling,鈥� he says.
But while the first plants were widespread, this early world was far from green. Most of the land would have been barren and rocky, without a hint of life, says Wellman. Only permanently moist patches, such as lake edges and stream banks, would have harboured the pioneers, visible from afar only as a green sheen.
