GETTING to grips with the fossil record can be a headache. When it comes to understanding how the various species fit in, palaeontologists need patience, creativity 鈥� and a touch of humour. Take the seemingly clear-cut case of extinction which, it turns out, isn鈥檛 always as clear-cut as you might think. Animals that have apparently vanished from the fossil record can seem to reappear after a long hiatus. Often the vanished creature or a close relative has indeed returned. Sometimes, though, the new discovery is a different species entirely, but closely resembles one that went before. Palaeontologists describe such a case as an Elvis taxon.
A taxon is any type of biological group, but how did Elvis enter the palaeontological vocabulary? The story starts in 1983 with another piece of inventive categorisation, when David Jablonski and Karl Flessa gave the name 鈥淟azarus taxon鈥� to creatures that reappeared in the fossil record after a long absence. Unlike the biblical story, in which Lazarus was raised from the dead, no one thought miracles were involved, just gaps in the fossil record. Normally it is assumed that a taxon is gone for good if no trace is found for millions of years, but it may merely have become rare, or moved somewhere where it left no fossils. This is what happened with the coelacanth, a fish closely related to amphibians, which palaeontologists thought had died out with the dinosaurs until a South African trawler caught a living one in 1938.
So where does Elvis come in? Evolution sometimes converges, shaping different taxa so they look very much alike. For example, a number of distinct lines of predators have evolved the long, curved and deadly teeth best known from the sabre-toothed tiger of the Ice Age. Some invertebrates evolve shells that look like earlier forms. These can all be mistaken for Lazarus taxa until closely examined. To emphasise the distinction, Doug Erwin and Mary Droser coined an alternative term for imitators in 1993: 鈥淩ather than continue the biblical tradition favored by Jablonski, we prefer a more topical approach and suggest that such taxa should be known as Elvis taxa, in recognition of the many Elvis impersonators who have appeared since the death of The King.鈥� (Palaios, vol 8, p 623)
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鈥淓volution sometimes shapes different groups to look alike鈥�
Like Lazarus taxon, the term caught on. So have others, created in a similar vein, such as the 鈥淶ombie effect鈥�, which applies when hard fossils such as dinosaur teeth are washed out of sediments and deposited in rocks millions of years younger 鈥� so in a sense they become walking dead.
Palaeontologists are still looking for a 鈥淛immy Hoffa taxon鈥�, a label that doctoral student Roberto Takata suggested to the Dinosaur Mailing List to describe so-far-undiscovered bones that must be hidden somewhere, invoking the case of the erstwhile head of US labour union the International Brotherhood of Teamsters, who disappeared in 1975 but whose body is yet to be recovered.
