Erratus sperare, a new species of ancient marine arthropod from eastern Yunnan, China, had unique trunk appendages that represent an intermediate stage of biramous limb evolution.
The ecological reconstruction of Erratus sperare, showing the nektonic life style; the initial situation of trunk limbs, as flaps associated with weakly sclerotized endopods indicates a swimming rather than fully benthic habit. Image credit: Fu et al., doi: 10.1098/rstb.2021.0034.
Erratus sperare lived in what is now China during the Early Cambrian epoch, some 520 million years ago.
Its fossilized remains were discovered at the Chengjiang Fossil Site, a UNESCO World Heritage Site located in the Chinese province of Yunnan.
“The Chengjiang Fossil Site preserves an ancient underwater ecosystem which included the relatives of some well-known arthropod fossils like trilobites and anomalocarids,” said Dr. David Legg, a paleontologist in the Department of Earth, Atmospheric, and Environmental Sciences at the University of Manchester, and his colleagues.
Modern water dwelling arthropods have biramous limbs, legs that have two parts — one for breathing and one for walking — but how such specialized 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, paleontologists didn’t know how arthropods made the jump from these specialized flaps to the biramous limbs of modern arthropods.
Erratus sperare from the Early Cambrian Chengjiang Lagerstätte, China. Scale bars – 2 mm for (b) and 1 cm for others. Image credit: Fu et al., doi: 10.1098/rstb.2021.0034.
Erratus sperare provides the missing link between stem-Euarthropoda and Deuteropoda — arthropods that used such specialised flaps and those with biramous limbs.
According to Dr. Legg and his co-authors, the ancient arthropod had both limbs and flaps.
“Fish aren’t the only organisms that have gills! Arthropods have gills too… they just have them on their legs,” Dr. Legg said.
“When it came to arthropods, however, we just weren’t sure where these gills came from.”
“Thanks to this new fossil, Erratus sperare, we now have a much clearer idea,” he added.
“These gills also probably went on to evolve into the wings of insects and the lungs of terrestrial arthropods like spiders so were a very important innovation.”
The discovery of Erratus sperare is reported in a paper in the Philosophical Transactions of the Royal Society B: Biological Sciences.
Dongjing Fu et al. 2022. The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod. Phil. Trans. R. Soc. B 377 (1847): 20210034; doi: 10.1098/rstb.2021.0034