Abstract
Once considered ‘weird wonders’ of the Cambrian, the emblematic Burgess Shale animals Anomalocaris and Opabinia are now recognized as lower stem-group euarthropods and have provided crucial data for constraining the polarity of key morphological characters in the group. Anomalocaris and its relatives (radiodonts) had worldwide distribution and survived until at least the Devonian. However, despite intense study, Opabinia remains the only formally described opabiniid to date. Here we reinterpret a fossil from the Wheeler Formation of Utah as a new opabiniid, Utaurora comosa nov. gen. et sp. By visualizing the sample of phylogenetic topologies in treespace, our results fortify support for the position of U. comosa beyond the nodal support traditionally applied. Our phylogenetic evidence expands opabiniids to multiple Cambrian stages. Our results underscore the power of treespace visualization for resolving imperfectly preserved fossils and expanding the known diversity and spatio-temporal ranges within the euarthropod lower stem group.
Highlights
Euarthropods have conquered Earth’s biosphere, comprising over 80% of living animal species [1]
As most of these early euarthropods did not possess mineralized hard parts, we rely on remarkable fossil deposits such as the Burgess Shale, which preserve soft-bodied components of ancient biotas, to reveal critical data on the extraordinary diversity, disparity and evolution of Cambrian euarthropods [3]
We evaluate its phylogenetic position using both maximum parsimony (MP) and Bayesian inference (BI) and further interrogate the support for alternative relationships for U. comosa by visualizing the frequency and variation of these alternatives in treespace [34,35]
Summary
Euarthropods (e.g. chelicerates, myriapods and pancrustaceans including insects) have conquered Earth’s biosphere, comprising over 80% of living animal species [1]. Two of the most peculiar Burgess Shale animals, Anomalocaris and Opabinia, illustrate the complicated history of research of many Cambrian soft-bodied taxa—a result of their unfamiliar morphologies compared to the occupants of modern oceans [4,5,6]. Both Anomalocaris and Opabinia possess compound eyes, lateral swimming flaps, filamentous setal structures and a tail fan [7,8,9,10]. The treespace approximates the RF distances between trees [34]
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