Since life changes over time, it’s tricky to name an oldest species. But some kinds of animals have remained relatively unaltered over many millions of years.

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Fossils suggest that, horseshoe crabs have barely changed in appearance since the Triassic period 230 million years ago (mya), and scientists know of older related species.

The oldest fossil discovered so far dates back 445 million years. In the last half a billion years there have been five major mass extinctions, and the Earth has changed its polarity many times. Yet the shallow coastal marine habitats preferred by horseshoe crabs have remained throughout, which could be why the crabs have changed so little.

Coelacanths, first swam in the ocean about 400 million years ago and were believed to be extinct until one was caught off South Africa in 1938.

Cockroaches have existed for 300 million years. A 9cm fossil cockroach – double the size of a modern American roach – was found in 2001 in Ohio. When it lived there the area was a vast tropical swamp.

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Main image: a horseshoe crab © Getty Images

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