Opabinia Regalis - Classification

Classification

Considering how paleontologists' reconstructions of Opabinia differ, it is not surprising that the animal's classification is still debated. Walcott, the original describer, considered it to be an anostracan crustacean, while Leif Størmer, following earlier work by Percy Raymond, thought that it belonged to the so-called "trilobitoids". After his thorough analysis Whittington concluded that Opabinia was no arthropod, as he found no evidence for arthropodan jointed limbs, and nothing like the flexible, probably fluid-filled proboscis was known in arthropods. Although he left Opabinia′s classification above the family level open, the annulated but not articulated body and the unusual lateral lobes with gills persuaded him that it may have been a representative of the ancestral stock from which both the annelids and arthropods arose.

In 1985 Derek Briggs and Whittington published a description of Anomalocaris, also from the Burgess Shale. Swedish palaeontologist Jan Bergström suggested that the two animals were related, as they shared lateral flaps with gills, stalked eyes and other features; and he classified them as primitive arthropods, although he considered that arthropods are not a single phylum.

	onychophorans, including Aysheaia and Peripatus armored lobopods, including Hallucigenia and Microdictyon anomalocarid-like taxa, including modern tardigrades as well as extinct animals like Kerygmachela and Opabinia Anomalocaris arthropods, including living groups and extinct forms such as trilobites

Simplified summary of Budd's "broad-scale" cladogram (1996)

In 1996 Graham Budd found what he considered evidence of short, un-jointed legs in Opabinia. His examination of the anomalocarid Kerygmachela from the Sirius Passet lagerstätte, about 518 million years ago and over 10M years older than the Burgess Shale, convinced him that this had similar legs. He considered the legs of these two genera very similar to those of the Burgess Shale Aysheaia and the modern onychophorans, which are regarded as closely related to the ancestors of arthropods. After examining several sets of features shared by these and similar lobopods he drew up a "broad-scale reconstruction of the arthropod stem-group", in other words of arthropods and what he considered to be their evolutionary "aunts" and "cousins". One striking feature of this family tree is that modern tardigrades may be Opabinia’s closest living evolutionary relatives.

Although Zhang and Briggs (2007) disagreed with Budd's diagnosis that Opabinia’s "triangles" were legs, the resemblance they saw between Opabinia’s lobe+gill arrangement and arthropods' biramous limbs led them to conclude that Opabinia was very closely related to arthropods. In fact they presented a family tree very similar to Budd's except that theirs did not mention tardigrades.