ABOVE: An illustration of Erratus sperare shows pairs of flaps and leg-like appendages on the underside of its body, which represent a precursor to the branched limbs in modern arthropods. Fu et al., Phil Trans R Soc B, 2022


When researchers working in southwestern China in the mid-2010s unearthed two fossil arthropods representing a single species that lived roughly 520 million years ago, David Legg was part of the team tasked with determining the animal’s phylogeny. Legg, a paleontologist at the University of Manchester in the UK, thought the fossils resembled a museum specimen he’d seen with similar features that suggested the newfound animal, which the researchers named Erratus sperare, might represent a missing link in the evolution of arthropod limbs. 

Comparing these specimens with other arthropods, the team noted that Erratus’s body plan—including stubby legs fused with flaps on the underside of its trunk—differed from earlier arthropods, which had rows of flaps to propel them, and from more-recent fossils bearing two-part limbs used for both locomotion and breathing. Erratus appears to fall in the middle: the flaps and legs are fused together but not yet fully joined, a precursor to modern branched limbs. The group also noticed fine wrinkles on the flaps, a hint that these appendages may have doubled as gills. “This new animal shows us what the most primitive gills in arthropods look like,” says Legg, though he admits that other researchers might interpret these features differently. 

John Paterson, a paleontologist at the University of New England in Australia who was not involved in the work, says that while it’s possible the flaps functioned as gills, the evidence is limited because the fossils’ finer details aren’t preserved. Nevertheless, Erratus represents the earliest evidence for branched limbs in arthropods, “which is pretty important,” says Paterson. The discovery of more fossils could explain how the animal’s two-part limbs were attached, he adds. “Once we know that, we can say a lot more about . . . how these different types of appendages evolved.” 

D. Fu et al., “The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod,” Phil Trans R Soc B, 377:20210034, 2022.