The late Stephen Jay Gould, a former professor in Harvard's Department of Organics and Evolutionary Biology, popularized the "strange wonders" of the stem group of arthropods Opabinia and Anomalocaris found in the Cambrian Burgess Shale in his book Wonderful Life and made them hallmarks of pop culture.
While the "Cambrian Horror" Anomalocaris—with its radial mouth and prickly grasping appendages—is a radial animal with many relatives, the Five-Eyed Opabinia— with its distinctive frontal proboscis device—remains the only opabiniid to have been found.
An international team of researchers led by Harvard University confirmed that a specimen previously thought to be radiodont was actually an opabiniid. The new study, published in Proceedings of the Royal Society B, uses a new, powerful phylogenous approach to confirm that Utaurora comosa is the second opabiniid ever discovered, and the first in more than a century.
Utaurora comosa was discovered in the Middle Cambrian Wheeler Formation 500 million years ago in Utah, and it was first described as a radioactive organism in 2008. Stephen Pates, co-first author of the study, a former postdoctoral researcher in Harvard's Department of Organisms and Evolutionary Biology (OEB), first encountered the specimen while in graduate school at the University of Kansas' Biodiversity Institute and Museum of Natural History. Pates was studying the diversity of radiodont at the time and felt that the specimen did not exactly match the real radiodont animals. After joining senior author Javier Ortega-Hernández in the OEB's lab, Pates collaborated with co-first author Jo Wolfe to study the relationship between fossils and living arthropods to determine the optimal location of Utaurora in the tree of life.
Opabiniids were the first group to have a posterior mouth. Their dorsal internodal sulcus is a precursor to systemic segmentation, and their lateral swimming lobes are precursors to appendages. Utaurora shares common features and morphologies with radiodonts and Opabinia. Although Utaurora's anterior structure and eyes are poorly preserved —Opabinia is most easily identifiable from its frontal probes and five eyes—pairs of jagged spines along the dorsal internodal groove and tail are fully observed.
Limited morphological observations led Pates and Wolfe to use phylogenetic analysis to compare Utaurora with arthropods, copepods, and other panpods from 43 fossil and 11 living taxa.
Wolfe said: "The initial phylogenetic analysis showed that it was most closely related to Opabinia. We then conducted more tests and used different evolutionary models and datasets to examine this result to visualize the different kinds of relationships that this fossil might have. ”
Unlike Opabinia, Utaurora was found in the Cambrian Burgess Shale in British Columbia, Canada, and while also Cambrian, it is millions of years younger than Opabinia. "This means that Opabinia is not the only opabiniid, Opabinia is not a unique species as we think it is," Pates said. ”
When Utaurora was first described as radiodont in 2008, scientists thought opabiniids and radiodonts formed a monophyletic group called dinocarids. But over the past 10 to 15 years, scientists have discovered more than 10 new radiolarian species, which makes it possible to see slightly different places between opabiniids and radiodonts.
"We also have more phylogenetic tools to look at our results," Pates says, "and based on morphology alone, you can show that Utaurora is a strange radiodnt, and you can also show that the concept of dinocarid is being re-proposed." But our phylogenetic dataset and analysis support Utaurora's 68% of trees retrieved by analyzing data as an opabiniid, but only 0.04% of trees are radiodont. ”
"Wonderful Life and the descriptions of these fossils occurred before the current evolutionary paradigm. The similarities between Opabinia and Anomalocaris were not really understood at the time. We now know that these animals represent a phase of extinction in evolution associated with modern arthropods. And we have tools that go beyond qualitative comparison of morphological characteristics, allowing for a more definitive localization in the animal's tree of life," Wolfe said.