The discovery of an unusual 6-centimetre-long creature in China — nicknamed the ‘walking cactus’ — may add to our understanding of arthropod evolution.
Credit: Jianni Liu
An artist's reconstruction of the "walking cactus" fossil species.
Credit: Jianni Liu
CAMBRIDGE: An international team of paleontologists may have found the evolutionary key to the most diverse species on earth - in the form of a walking, sea-dwelling, armoured ‘cactus’.
The study, published in Nature, documents the discovery of an unusual worm-like creature from Cambrian-era China. The fossil suggests the species to have developed robust, hardened legs before the acquisition of an armoured torso, raising questions about the evolution of arthropods in general, from scorpions to wasps to butterflies.
“Arthropods are the richest species on earth - there are over one million described species. Thus, the origins of arthropods is always a hotly contested issue,” said lead author Jianni Liu, an earth scientist at Northwest University in Xi'an, China. “But until now we didn’t have a single fossil with appendaged joints indicating an early arthropod. Our fossil shows a link - that’s the important point.”
Absence of suggestive fossils
The evolutionary stems of most modern species were born 500 million years ago in the Cambrian explosion - a period of rapid, complex evolution that diversified what was otherwise relatively small and simple life. Mineralised organisms became common for the first time, allowing for fossilisation.
Despite the richness of the era, however, there has been a notable absence of suggestive fossils that connect arthropods to a common ancestor.
Liu and her colleagues suggest that they may have found this link, situating lobopodians - a group of small, sea-faring animals originating in the Early Cambrian, resembling modern velvet worms or water bears - as potential ancestors of arthropods in general.
Arthropod or lobopodian?
Although the ‘walking cactus’ (Diania cactiformis) is, in many ways, a typical lobopodian, it is remarkable for also sharing features with arthropods. Most notably, its limbs are armoured, implying that it is close the point of arthropodisation.
“When I got the fossils, I was so surprised,” said Liu. “I couldn’t understand a creature with a soft body but hardened limbs. It’s really on the way to arthropods.”
The ‘walking cactus’ lived mostly in the sea, no more than 200 metres away from the shore at any given time, Liu said. Although data is not readily available, the researchers speculate that it was a kind of sediment feeder, extracting nutrients from the soil like certain worms today.
Did hard legs precede armour?
Another possibility is that it used its spindly legs to catch creatures and eat them, similar to numerous modern arthropods.
These two potential feeding habits highlight the dilemma of the finding: how far along is the arthropodisation process in this lobopodian? And can we confidently claim it to be a common ancestor for arthropods in general?
