By Zoë Corbyn

A clue to how arthropods--the group of more than a million invertebrate species that includes insects, spiders and crustaceans--evolved their distinctive jointed legs has been discovered in southwestern China.

Nicknamed the "walking cactus" because of its spiny appearance, the Diania cactiformis fossil find is reported in a paper published February 23 in Nature. The animal belongs to the Lobopodia, a now-extinct group of animals resembling worms with legs, which may have been a relative of today's velvet worms. But it is the first species of that group found to have the jointed legs typical of Arthropoda.

"A lot of scientists had long suspected that arthropods evolved from lobopodians," says Liu Jianni, a paleontologist from the Early Life Institute at Northwest University in Xi'an, China, and currently at the Free University of Berlin, who led the work. "But we did not have a single fossil we could point at and say, 'This is the first lobopodian with jointed legs'."

She adds, "Now, with the 'walking cactus', we do. It is important because it could be seen as a missing link from lobopodians to arthropods."

Legs before bodies

The creature, which dates from around 500 million years ago, is about 6 centimeters long. It resembles a thin, soft-bodied worm, similar to the lobopodians. But it is also arthropod-like in that it has jointed legs--ten pairs in total. The researchers believe the legs had hardened surfaces, not unlike the tough surfaces of the articulated limbs of crustaceans or insects.

The finding adds to our understanding of Arthropoda, a group that contains more than 80% of all known living animal species, and makes up the bulk of the tree of life. It could imply that arthropods developed hardened limbs before hardened bodies, say the researchers.

Paleontologist Simon Conway Morris of the University of Cambridge, UK, says the discovery of D. cactiformis provides important insights into the flourishing of new animal body plans during the period dubbed the Cambrian explosion, the sudden appearance--in geological time--of most major phyla around 530 million years ago.

Conway Morris says that the fossil "offers crucial evidence about the key step of moving from an effectively soft-bodied construction to an articulated exoskeleton--a breakthrough that would ultimately lead to such biological designs as the crab claw and the praying mantis".

Liu adds that the development of jointed legs was only one of three "critical steps" in the evolution of arthropods from lobopodians. She now hopes to find further Lobopodia fossils that show the development of both the arthropods' head and jointed body.