How do palaeontologists classify strange animals like Opabinia regalis?

Like Opabinia, many fossils found in the Burgess Shale are difficult to classify. Part of the problem is that some species are poorly known - i.e., there is not enough well-preserved fossil material to describe the anatomy of the animals with certainty. Opabinia fossils are rare, with only 42 specimens currently (as of 2011) known from all collections, but overall its anatomy is relatively well preserved. Thus difficulties in interpreting the affinities of Opabinia are more related to its bizarre anatomy: Opabinia displays only some of the traits associated with familiar groups and possesses a combination of traits that remains at odds with what we know for any living or extinct organisms.

Because of the jointed claws on its proboscis, Opabinia is classified, along with another iconic animal from the Burgess Shale, Anomalocaris canadensis, as a primitive arthropod in a group called the anomalocaridids (see learning object Anomalocaris canadensis). Today, arthropods are the most diverse of all animal groups, a distinction they have probably held over the last 500 million years. Characterized by a segmented body, a rigid, articulated external covering (exoskeleton), and jointed limbs, arthropods are represented today by spiders, shrimps, insects, and millipedes. While Opabinia regalis does not closely resemble modern arthropods, it is believed to represent one of the most primitive species in the evolution of this group.
 

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