‘Strange wonder’ fossil discovery adds a piece to the puzzle of arthropod evolution

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Artist’s reconstruction of the Ordovician Mieridduryn bonniae fossils. Credit: Original artwork by Franz Anthony

Fossils of the Welsh ‘strange wonder’ provide new clues to the history of arthropod evolution.

The most famous fossils from the Cambrian explosion of animal life, which occurred over half a billion years ago, stand in stark contrast to their modern counterparts. These “strange wonders”, like the five eyes Opabinia with its distinctive frontal proboscis and fearsome apex predator Anomalocaris with their radial mouthparts and spiny feeding appendages, they have become icons of popular culture. However, they have only recently been recognized as extinct evolutionary stages that are crucial to understanding the origins of one of the largest and most important animal phyla, the arthropods (a group that includes modern crabs, spiders, and millipedes).

Two new specimens with striking similarities to Opabinia are described in a recently published article in the journal

The Cambrian explosion, or biological big bang, refers to a time interval about 530 million years ago in the Cambrian Period, when nearly all major animal phyla began appearing in the fossil record.

The quarry is well known as one of several local sites producing new species of fossil sponges. “When lockdown started, I thought I’d take one more trip to collect a few last sponges before finally putting them in writing,” Botting said, “of course, that was the day I found something sticking out of a tube instead with tentacles.”

“This is the kind of thing paleontologists dream of, real soft-body preservation,” Muir said. “We didn’t sleep well that night.” That was the beginning of a large and ongoing investigation that turned into an international collaboration, with lead author Dr. Stephen Pates (University of Cambridge) and senior author Dr. Joanna Wolfe (Department of Organismic Biology and evolutionary science at Harvard University).

Among the fossils unearthed so far are two very unexpected remnants of the “strange wonders” of the Cambrian. Pates met with Botting and Muir to study the specimens using crowd-funded microscopes to examine the tiny specimens. The largest specimen measured 0.5 inch (13 mm), while the smallest measured a minuscule 0.12 inch (3 mm). for comparison, Opabinia samples can be 20 times longer).

Extensive studies during this visit revealed further details in the new specimens. Some of these features are also found in Opabiniasuch as the triangular and soft lobopod “legs” to interact with the sediment and, in the smallest specimen, a tail fan with blades similar to Opabiniathe recently described sister, Utaurora. However, other features recognized in the material, such as the sclerites covering the head and the presence of spines on the proboscis, were not known in any opabiniid and instead suggested a possible radiodont (including Anomalocaris) affinity. Did the differences between the two specimens lead the researchers to question whether they were due to changes during the growth of one species or did they instead suggest that two distinct species were present in this new deposit?

The Ordovician Period spans 41.6 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the beginning of the Silurian Period 443.8 Mya.

The authors describe the new taxon, Mieridduryn bonniae, with the largest specimen designated the holotype. The state of the smallest specimen has been left open, reflecting these different possibilities. “The size of the smallest specimen is comparable to that of some modern arthropod larvae—we had to account for this possibility in our analyses,” Wolfe said.

The genus name Mieridduryn comes from the Welsh language and translates as “thorn snout”, reflecting the spiny proboscis in the new material. It is pronounced like “i-airy-thein. “Many scientific names are made using Latin or Greek words,” Muir said, “but we really wanted to honor Wales, where the specimens were discovered, and so we chose to use the Welsh language.” The name of the species bonniae pays tribute to the landowners’ granddaughter, Bonnie. “Landowners have been very supportive of our research, and Bonnie has been following our progress avidly, even participating in some of our Zoom updates,” Botting said.

The researchers used phylogenetic analyses, comparing the new fossils to 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, considered as one or two species, was more closely related to modern arthropods than to opabiniids. These analyzes suggested it Mieridduryn and the smaller specimens weren’t “true” opabiniids,” Pates said.

Crucially, these results suggested that a proboscis, thought to represent a fused pair of head appendages, was not unique to opabiniids, but was instead present in the common ancestor of radiodonts and deuteropods (more derived modern arthropods), and through time evolutionary may have shrunk to become the lip covering the mouth in modern arthropods. However, the second best-supported position for these specimens was as true opabiniids, so the authors investigated a bit more to test the robustness of this first result.

“These Welsh animals are 40 million years younger Opabinia And UtauroraWolfe said, “so it was important to evaluate the implications of certain features, such as spines on appendages or a carapace, evolving convergently with radiodonts in our analyses.” If some, or all, of the characteristics shared between Welsh animals and radiodonts were thought to have evolved convergently, the analyzes strongly favored these specimens being considered true opabiniids, the first outside North America and the youngest of 40 million years. Whatever the final conclusion, fossils are an important new piece in the arthropod evolutionary puzzle.

These small but scientifically powerful fossils are some of the earliest finds of this important new Ordovician fauna. Botting and Muir continue their work in the small quarry in the sheep field and more will follow. Muir added, “Even the sheep know we’re doing something special here, they usually come to see.”

Reference: “Ordovician opabiniid-like animals and the role of the proboscis in euarthropod head evolution” by Stephen Pates, Joseph P. Botting, Lucy A. Muir, and Joanna M. Wolfe, November 15, 2022, Nature communications.
DOI: 10.1038/s41467-022-34204-w

Contributors to the crowdfunding appeal for the microscopes (including a Holloway Grant from the Warwickshire Geological Conservation Society) are thanked. Additional funding was provided by a University of Cambridge Herchel Smith Postdoctoral Fellowship, Chinese Academy of Sciences PIFI Fellowships (2020VCB0014 and 2018VCB0014). This work was also supported by the National Science Foundation DEB #1856679.

The specimen is kept at the Amgueddfa Cymru—National Museum Wales.

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