Spiny trunks, stubby legs and “little wings”: paleontologists present further representatives of the bizarre creatures that roamed the seas around half a billion years ago. As they explain, the two new specimens are interesting pieces of the puzzle when researching the evolutionary history of arthropods. How exactly the freaks, around 462 million years old, fit into the picture remains questionable. The scientists are now hoping for more finds at the promising new site in Wales that may shed more light on the early evolutionary history of the arthropod.
Quantitatively speaking, it is the most successful of all animal groups: Around 80 percent of the animal species living today belong to the arthropods, such as insects, crabs and spiders. The evolutionary origin of this group is believed to lie in the so-called Cambrian explosion of life over 520 million years ago. Numerous fossils already give indications of early stages of development. But the relationship between the different fossil creatures still puzzles researchers. Because despite certain arthropod characteristics, these were sometimes very bizarre creatures, which are sometimes referred to as "weird wonders". Particularly famous is the bizarre predator Anomalocaris with its radial mouthparts and spiny appendages, as well as the enigmatic five-eyed trunk-creature Opabinia.
Now the group of primeval freaks is growing: The finds come from a newly discovered fossil deposit in which traces of living beings from the Ordovician have survived - the time around 40 million years after the Cambrian explosion. The site is a quarry in mid Wales surrounded by sheep pastures. At first only fossil sponges were discovered there. But then co-author Joseph Botting from the National Museum Wales in Cardiff came across something special: "I discovered something that at first looked like it was a creature with a tentacle sticking out of a tube," says Botting. He showed it to his colleague Lucy Muir and was enthusiastic: "It was obviously a real soft-body preservation - something that paleontologists dream of. We didn't sleep well that night because of the excitement," says the scientist.
Strange tiny creatures in sight
In fact, the find developed into a comprehensive investigation in cooperation with international colleagues. Another specimen was discovered at the site. As the researchers report, these are very remarkable small representatives of the "Weird Wonders". More detailed investigations using modern analysis methods revealed that the larger specimen measures 13 millimeters, while the smaller one is only three millimeters long. Nevertheless, the microscopic investigations provided insights into the partly well-preserved structures of the approximately 462-million-year-old beings.
Some features are similar to those of Opabinia, such as the triangular stump legs, which were probably used to interact with the sediment. A tail fan was also visible in the smaller specimen, as was observed in Opabinia's recently described sister group, Utaurora. The newly discovered beings also possessed the literally outstanding feature that Opabinia also possesses: a trunk. But in this case it was covered with spikes and the animals also had unusual, hard structures on their heads. These features are not previously known from any opabiniids and therefore indicate a possible relationship with radiodonts including anomalocaris, the researchers explain.
As they continue to report, it is not yet clear whether the two specimens are two species or just one. Because it could be that the smaller specimen is a young animal. "The size of the smaller one is comparable to that of some modern arthropod larvae - we had to take this possibility into account in our analyses," says senior author Joanna Wolfe from Harvard University in Cambridge (USA). The researchers therefore only used the larger specimen as the basis for the description of the new species. They named him Mieridduryn bonniae. The genus name Mieridduryn comes from the Welsh and means translated blackberry snout - with reference to the prickly trunk of the animal.
Notes on arthropod evolution
The paleontologists then performed phylogenetic analyses, comparing the new fossils with 57 other living and fossil arthropods to examine their place in evolutionary history. "The best supported position for our Welsh specimens, whether considered as one or two species, were more closely related to modern arthropods than to opabiniids. These analyzes suggest that Mieridduryn and the smaller specimen are not true opabiniids," says lead author Stephen Pates from the University of Cambridge (UK).
As the researchers explain, this interpretation in turn leads to an interesting connection to the evolutionary history of arthropods. It could mean that a proboscis—thought to represent a fused pair of head appendages—was not unique to the opabiniids. It may also have been present in the common ancestor of radiodonts and the ancestors of early modern arthropods. The scientists say that the proboscis may then have evolved back into the so-called labrum that covers the mouth of modern arthropods.
But as they emphasize, the classification remains unclear. Because it also seems possible that Mieridduryn was a representative of the Opabiniids. It is possible that its features, such as the spines and armor, were only due to a parallel development that made them appear similar to the radiodonts. But even then, the finds would be significant, the scientists point out: they would then be the youngest known opabiniids and the only ones discovered outside of North America. Whatever the conclusion, the fossils are an important new piece of the puzzle in the evolutionary history of arthropods, the researchers say. That is why they are now continuing their work in the quarry on the sheep pasture and are hoping for more finds. "Even the sheep seem to have noticed that we're on to something special here," Muir concludes.
Source: Harvard University, professional article: Nature Communications, doi: 10.1038/s41467-022-34204-w