Opabinia Regalis - Description

Description

= Eyes = Gut = = Gills Opabinia head – foremost lobes and gills cut away to show proboscis passing food to mouth.

Opabinia looked so strange that the audience at the first presentation of Whittington's analysis laughed. The length of Opabinia regalis from head to tail ranged between 4 centimetres (1.6 in) and 7 centimetres (2.8 in). The animal also had a hollow proboscis, whose total length was about one third of the body's and which projected down from under the head and then curved forwards and upwards. The proboscis was striated like a vacuum cleaner's hose and probably flexible, and it ended with a claw-like structure whose inner edges bore spines that projected inwards and forwards. The head bore five eyes: two on stalks near the front and fairly close to the middle of the head, pointing upwards and forwards; two larger eyes, also stalked, near the rear and outer edges of the head, pointing upwards and sideways; and a single eye with a shorter stalk between the larger pair of stalked eyes, pointing upwards. It has been assumed that the eyes were all compound, like arthropods' lateral eyes, but this reconstruction, which is not backed up by any evidence, is "somewhat fanciful". The mouth was under the head, behind the proboscis, and pointed backwards, so that the digestive tract formed a U-bend on its way towards the rear of the animal. The proboscis appeared sufficiently long and flexible to reach the mouth.

The main part of the body was typically about 5 millimetres (0.20 in) wide and had 15 segments, on each of which there was a pair of lobes (flaps) pointing downwards and outwards. The lobes overlapped so that the front of each was covered by the rear edge of the one ahead of it. The body ended with what looked like a single conical segment bearing three pairs of overlapping lobes that pointed up and out, forming a tail like a V-shaped double fan.

= Body and lobe = = Gill "blades" = "Leg" = "Claw" = Gut Opabinia cross-section based on Budd (1996)

Interpretations of other features of Opabinia fossils differ. Since the animals did not have mineralized armor nor even tough organic exoskeletons like those of insects, their bodies were flattened as they were buried and fossilized, and smaller or internal features appear as markings within the outlines of the fossils.

Whittington interpreted as gills some markings on the upper surfaces of all lobes except the first on each side, and thought that these gills were flat underneath, had overlapping layers on top, and attached to the bases of the lobes. Budd (1996, 2011) thought they consisted of separate "blades" attached along the front edges on the undersides of the lobes. He also found marks inside the lobes' front edges that he interpreted as internal channels connecting the gills to the interior of the body, much as Whittington interpreted the mark along the proboscis as an internal channel. On the other hand Bergström (1986) interpreted both the "gills" and the lobes as part of a covering over the upper surface of the body.

= Body = Coxa (base) = Gill branch // = Gill
filaments = Leg
branch Structure of a biramous limb

Whittington found evidence of near-triangular features along the body, and concluded that they were internal structures, most likely sideways extensions of the gut. Chen et al. (1994) interpreted them as contained within the lobes along the sides. Budd thought the "triangles" were too wide to fit within Opabinia’s slender body, and that cross-section views showed they were attached separately from and lower than the lobes, and extended below the body. He later found specimens that appeared to preserve the legs' exterior cuticle. He therefore interpreted the "triangles" as short, fleshy, conical legs. He also found small mineralized patches at the tips of some, and interpreted these as claws. Zhang and Briggs (2007) analyzed the chemical composition of the "triangles", and concluded that they had the same composition as the gut, and therefore agreed with Whittington that they were part of the digestive system. Instead they regarded Opabinia′s lobe+gill arrangement as an early form of the biramous limbs which are seen in trilobites and crustaceans and which may be the original form in all arthropods. However, this similar chemical composition is not only associated with the digestive tract; Budd and Daley suggest that it represents mineralization forming within fluid-filled cavities within the body (consistent with hollow lobopod legs).