A century later, researchers describe second opabiniid ever discovered

An international team of researchers led by Harvard University confirm that a specimen previously considered a radiodont is in fact an opabiniid. The new study inProceedings of the Royal Society Bused novel and robust phylogenetic methods to confirmUtaurora comosaas only the second opabiniid ever discovered and the first in over a century.

Utaurora comosa发现,在500年前威尔士n Wheeler Formation of Utah, was first described in 2008 as a radiodont. Co-lead author Stephen Pates, former postdoctoral fellow in the Department of Organismic and Evolutionary Biology (OEB) at Harvard, first encountered the specimen at the University of Kansas Biodiversity Institute & Natural History Museum while a graduate student. Pates was studying the diversity of radiodonts and felt this specimen did not exactly fit with a true radiodont. Upon joining senior author Professor Javier Ortega-Hernández's lab in OEB, Pates worked with co-lead author Jo Wolfe, postdoctoral fellow in OEB who studies the relationships of fossil and living arthropods,to determine whereUtaurorabest fit in the tree of life.

Opabiniids are the first group to have a posterior-facing mouth. Their dorsal intersegmental furrows are precursors to full body segmentation and their lateral swimming flaps precursors to appendages.Utaurorashares characters and morphology with both radiodonts andOpabinia. WhileUtaurora'santerior structure and eyes were poorly preserved --Opabiniais most recognizable from its frontal proboscis and five eyes, the intersegmental furrows along the back and the paired serrated spines on the tail were fully observed.

Limited morphological observations led Pates and Wolfe to use phylogenetic analysis comparingUtaurorawith 43 fossils and 11 living taxa of arthropods, radiodonts, and other panarthropods.

"The initial phylogenetic analysis showed it was most closely related toOpabinia," Wolfe said. "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."

UnlikeOpabinia,which was discovered in the Cambrian Burgess Shale of British Columbia in Canada,Utaurorawas found in Utah and, though still Cambrian, is a few million years younger thanOpabinia. "This meansOpabiniawas not the only opabiniid,Opabiniawas not as unique a species as we thought," Pates said.

When Utaurora was first described as a 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 over 10 new species of radiodonts, making it possible to see that opabiniids and radiodonts are slightly different.

"We also have more phylogenetic tools to interrogate our results," Pates said. "Based on the morphology alone you could make a case forUtaurorabeing a weird radiodont and also for bringing back the 'dinocarid' concept. But our phylogenetic dataset and analyses supportedUtaurora asan opabiniid in 68% of the trees retrieved by analyzing the data, but only in 0.04% for a radiodont."

"Wonderful Lifeand the description of these fossils happened before current evolutionary paradigms. The similarities betweenOpabiniaandAnomalocarisweren't really understood yet," Wolfe said. "Now we know that these animals represent extinct stages of evolution that are related to modern arthropods. And we have tools beyond qualitatively comparing morphological features for a more definitive placement within the animal tree of life."