Question corner | A missing link

How did arthropod breathing system evolve?

Arthropods, the group of animals that includes creepy crawlies like spiders and woodlice, are the largest phylum in the animal kingdom and are found everywhere from the deepest ocean trench to the top of Mount Everest.

New research shows the newest addition to the group is a 520-million-year-old (about 10 times as old as the dinosaurs) organism called Erratus sperare (Philosophical Transactions of the Royal Society B: Biological Sciences).Erratus sperare was discovered in the Chengjiang Fossil Site, a UNESCO World Heritage Site located in Yunnan, China. The Chengjiang Fossil Site preserves an ancient underwater ecosystem which included the relatives of some well-known arthropod fossils like trilobites and anomalocarids.

Modern water dwelling arthropods have biramous limbs, legs that have two parts – one for breathing and one for walking – but how such specialised limbs evolved was a mystery. Some of the earliest fossil arthropods, like Anomalocaris, had swimming flaps that may have doubled as gills, but until now researchers didn't know how arthropods made the jump from these specialised flaps to the biramous limbs of modern arthropods.

Erratus sperare provides the missing link between arthropods that used such specialised flaps and arthropods with biramous limbs. It has both legs and flaps.

Now with the new fossil, researchers have finally solved the riddle. The gills also probably went on to evolve into the wings of insects and the lungs of terrestrial arthropods like spiders.