The second species of opabinid has been discovered which is a soft-bodied arthropod with a segmented exoskeleton that lived on the seafloor during the Miaolingian (509-497 million years ago).
The original opadiniid, Opabinia regalis, was first described over a century ago in 1912 and has several physical characteristics – not least the five eyes protruding on stalks from its head, a backward-facing mouth, and its hollow, tubular proboscis.
“The initial phylogenetic analysis showed it was most closely related to Opabinia,” says paleontologist Jo Wolfe from Harvard University.
“We followed up with more tests to interrogate that result using different models of evolution and data sets to visualize the different kinds of relationships this fossil may have had.”
U. comosa was first described as a radiodont in 2008. It’s a few million years younger than Opabinia, and was found in a different location – Utah rather than Canada.
The team compared the Utaurora fossil with 43 other fossils, plus 11 living taxa, covering arthropods, radiodonts, and other panarthropods.
Originally, it was thought that opabiniids and radiodonts had the same common ancestor and were grouped as so-called ‘dinocarids’.
“Based on the morphology alone you could make a case for Utaurora being a weird radiodont and also for bringing back the ‘dinocarid’ concept,” says paleontologist Stephen Pates, from the University of Cambridge in the UK.
“But our phylogenetic dataset and analyses supported Utaurora as an opabiniid in 68 percent of the trees retrieved by analyzing the data, but only in 0.04 percent for a radiodont.”
The researchers observed the intersegmental furrows along the back and the paired serrated spines on the tail. It was found that a new species came into existence, half a billion years after its demise.
“Dissection of the phylogenetic support demonstrates that while evidence for radiodont paraphyly is weak, Utaurora can be confidently reassigned to Opabiniidae,” the researchers said.
“The weirdest wonder of the Cambrian no longer stands alone.”
The Opabinia shares a scientific history with another group of arthropods known as Anomalocaris, and both were originally described as ‘weird wonders’ of the Middle Cambrian age.
“Now we know that these animals represent extinct stages of evolution that are related to modern arthropods,” says Wolfe.
“And we have tools beyond qualitatively comparing morphological features for a more definitive placement within the animal tree of life.”
The research has been published in the Proceedings of the Royal Society B Biological Sciences.