Fossils come in to land

Abstract

Fossils found in rocks of the Ediacaran period in Australia have been previously characterized as early marine organisms. But a report suggests that these rocks are fossilized soils. So did some of these Ediacaran organisms in fact live on land, like lichens? A palaeontologist and a geologist weigh up the evidence. See Letter p.89 The smaller the bat, the higher the frequency of its echolocation calls. Until now this scaling has been attributed either to the bat's size or to the supposition that smaller bats seeking finer prey will need to use higher frequencies to pick out smaller objects. But that doesn't explain why small bats emit such high-pitched squeaks—up to 55 kHz, when 20 kHz would do the job, carry farther and take less effort to produce. Annemarie Surlykke and colleagues compare echolocation signals from six species of vespertilionid bat over a fivefold mass range. They show that the size–frequency relationship is governed by the need to maintain a focused, highly directional beam. As sound beams narrow with increasing emitter size and signal frequency, smaller mouths impose a constraint that forces smaller bats to use higher frequencies to achieve a sufficiently narrow beam. Exceptions prove the rule: phyllostomid bats show a much weaker correlation between size and frequency, but they emit sounds through specialized nasal structures, not through the mouth, and noseleaf dimensions do not scale with body size.