Two specimens of euarthropods from the Middle Ordovician Castle Bank Biota in Wales, the United Kingdom, have striking similarities to Opabinia, an iconic Cambrian marine animal with its distinctive frontal proboscis and five eyes.
An artistic reconstruction of the two new opabiniid-like animals, with Mieridduryn bonniae at the front and the smaller unnamed animal at the back. Image credit: Franz Anthony.
The two Opabinia-like marine animals lived during the Ordovician period, some 464 million years ago — approximately 40 million years after the Cambrian explosion.
They belong to Euarthropoda (chelicerates, myriapods and pancrustaceans), a group estimated to comprise more than three-quarters of all living animal species.
The larger specimen measured 13 mm, while the smaller measured a miniscule 3 mm; for comparison, Opabinia specimens can be 20 times as long.
Some of their features are also found in Opabinia, such as triangular, squishy lobopod ‘legs’ for interacting with the sediment, and — in the smaller specimen — a tail fan with blades similar in shape to Opabinia’s recently described sister, Utaurora.
However, other features recognized in the material, such as sclerites covering the head as well as the presence of spines on the proboscis, were not known from any opabiniid and instead hinted at possible radiodont — including Anomalocaris — affinities.
The differences between the two specimens led Dr. Stephen Pates from the University of Cambridge and colleagues to wonder were these due to changes during the growth of one species, or did they instead suggest that two distinct species were present in this new deposit?
They describe the new euarthropod species, Mieridduryn bonniae, with the larger specimen designated as the holotype.
The status of the smaller specimen was left open, reflecting these different possibilities.
“The size of the smaller specimen is comparable to some modern arthropod larvae — we had to take into account this possibility in our analyses,” said Dr. Joanna Wolfe, a researcher at Harvard University.
The authors also compared the new fossils with 57 other living and fossil arthropods, radiodonts, and panarthropods, to determine their place in the history of arthropod evolution.
“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,” Dr. Pates said.
“These analyses suggested that Mieridduryn bonniae and the smaller specimen were not ‘true’ opabiniids.”
Their results also suggested that a proboscis — thought to represent a fused pair of head appendages — was not unique to opabiniids, but instead was present in the common ancestor of radiodonts and deuteropods (more derived, modern arthropods), and through evolutionary time may have reduced to become the labrum that covers the mouth in modern arthropods.
However, the second-best-supported position for these specimens was as true opabiniids, so the team enquired a bit further to test the robustness of this first result.
“These Welsh animals are 40 million years younger than Opabinia and Utaurora, so it was important to assess the implications of some features, such as spines on the appendages or a carapace, evolving convergently with radiodonts in our analyses,” Dr. Wolfe said.
“If some, or all, the features shared between the Welsh animals and radiodonts were instead considered to have evolved convergently, the analyses strongly favored these specimens being considered true opabiniids, the first from outside North America and the youngest by 40 million years.”
“Whatever the eventual conclusion, the fossils are an important new piece in the arthropod evolutionary jigsaw.”
The findings were published in the journal Nature Communications.
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S. Pates et al. 2022. Ordovician opabiniid-like animals and the role of the proboscis in euarthropod head evolution. Nat Commun 13, 6969; doi: 10.1038/s41467-022-34204-w
