The creature appears to have been the first of its kind to stride on bendable, hard-covered legs.

THE GIST

Fossils of a creature resembling a "walking cactus" suggest how armor evolved in arthropods.

The fossil shows how legs of these animals may have appeared before their armored bodies.

Fossils of an ancient "walking cactus" suggest how ancestors of today's lobsters, insects, spiders and related groups went from squishy to spiky.

Dating back about 520 million years, the fossilized prickly creature is not a plant but a thumb-sized, wormlike animal with 10 pairs of long, sturdy legs, says Jianni Liu of Northwest University in Xi'an, China. Discovered in southwestern China, it probably scuttled along the bottom of shallow seas, she says. In the Feb. 24 Nature, she and her colleagues christen the species Diania cactiformis, in honor of its spiky look.

Its armored leggy look surprised Liu when she first saw it. "I fell in love with this strange guy," she says. "Later when I observed it carefully under the microscope, I realized it was not only a funny one but an important one."

The creature's 10 legs appear to have carried a hard, outer covering of armor and joints that let them bend. Those features would make the species the earliest known worm-with-legs to have a hardened outer covering and also the first to have jointed legs, Liu says. An armored outer skeleton and jointed legs today distinguish the arthropods, the major lineage including crustaceans, insects, spiders and mites. Thus the cactus sea creature might be a sister to arthropod ancestors.

"The significance of the find is that arthropods are, in terms of species, the most successful group on the planet," Liu says. "The secret of their success seems to be their legs." Ancient appendages evolved with diverse lifestyles, forming claws for example, or gilled structures for underwater life. Even legs for moving around diversified into paddles for swimming or launchers for jumping.

Liu points out that paleontologists pursuing the history of the remarkable arthropod legs have debated such puzzles as whether the armored bodies came before or after armored legs.

The presumably armored legs of this animal show how a legs-first scenario might look, but Liu notes that there's no evidence that this is a direct ancestor of modern arthropods.

Jan Bergström of the Swedish Museum of Natural History in Stockholm laments the fact that it's not a direct ancestor, but says the new fossil "fills a hole in the evolutionary mosaic."