Science and Technology Palaeontology Splay-footed, not flat-footed
科技 古生物学 不是扁平足而是八字足
A new fossil shows that evolution does not always mean change
WHEN a coelacanth, a type of lobe-finned fish once considered the missing link between fish and amphibians, was found off the coast of South Africa in 1938, it came as a shock to palaeontologists. Until then, the most recent traces of such a creature had been in rocks dating from the last days of the dinosaurs, 65m years ago. It was, in its way, as surprising as if a live Tyrannosaurus had been found hiding in an obscure part of Montana. Now the same experience is hitting palaeontologists again—but this time in reverse. Instead of finding a "living fossil" identical to an ancient beast, they have found a real fossil identical to a modern one.
The fossil in question, a 100m-year-old specimen from north-east Brazil, belongs to the genus Schizodactylus. These are large, carnivorous, cricket-like insects whose feet splay out wildly in different directions. Modern Schizodactylus use their feet like snowshoes, to help them remain stable as they travel over sandy terrain in search of prey.
If the new fossil—whose discovery has just been published in ZooKeys by Sam Heads of the Illinois Natural History Survey and Léa Leuzinger of the University of Fribourg, in Switzerland—were merely similar to modern splay-footed insects, the find would not be particularly surprising: it simply demonstrates a phenomenon called evolutionary stasis, in which a specific type of body form hangs around for a long time. What is surprising is just how static Schizodactylus has been.
Evolutionary stasis is fairly common at the higher levels of the Linnaean system of biological classification (class, order and family). Natural selection hits on a good design. That design is then adopted in slightly different forms by species after species. The shelled bodies of turtles, for example, evolved between 250m and 200m years ago, while the body plans of scorpions have been around for more than 400m years. That does not mean, however, that a zoologist would mistake a 200m-year-old turtle or a 400m-year-old scorpion for any species now alive.
What is remarkable about the new find is that it is so similar to modern animals that it can be assigned to an existing genus—the lowest level of Linnaean classification above a species—rather than just to some higher taxonomic group. That is rare indeed. Even the modern coelacanth, on closer examination, had to be put in a different genus from any known fossil.
Clearly the body plan of Schizodactylus is not merely good, but optimal, at least for the environment the animal lives in. Alas for Schizodactylus, the sandy deserts it prefers have retreated from north-eastern Brazil and its optimality there has vanished. But its discovery shows better what this part of the world was like 100m years ago—and also illustrates an important point about evolution that is often forgotten in biologists' understandable focus on the development of novelty. The first rule of natural selection is: "If it ain't broke, don't fix it."