Description
Over 1000 specimens, representing organisms of all stages of maturity, have now been found in the White Sea area at the bottom of fine-grained sandstone layers. The large number of specimens, the small grain-size of the sediments and the variety of circumstances in which specimens were preserved provide detailed information about Kimberella′s external form, internal anatomy, locomotion and feeding style.
All of the fossils are oval in outline. Elongated specimens illustrate that the organism was capable of stretching in an anterior-posterior direction, perhaps by as much as a factor of two. The only type of symmetry visible in the White Sea specimens is bilateral; there is no sign of any of the kinds of radial symmetry that are normal in the Cnidaria, the group that includes jellyfish, sea anemones and hydras. The Australian fossils were originally described as a type of jellyfish, but this is inconsistent with the bilateral symmetry in the fossils. The White Sea fossils and the surrounding sediments also show that Kimberella lived on the surface of the sea-floor.
Kimberella had a dorsal integument that has been described as a non-mineralized "shell"; in the larger specimens this reached up to 15 cm in length, 5 to 7 cm in width, and was 3 to 4 cm high; the smallest specimens are only about 2–3 mm long. The shell was stiff but flexible, and appears to have been non-mineralized, becoming tougher as it grew larger (and presumably thicker) in more mature specimens. The deformation observed in elongated and folded specimens illustrates that the shell was highly malleable; perhaps, rather than a single integument, it consisted of an aggregation of (mineralized?) sclerites. At its highest point was a hood-like structure, forming what is thought to be the front. In some specimens, the inner surface of the shell bears stripes spanning the width of the creature; these may represent the attachment sites of muscles. Similar stripes around the edge of the shell may have been connected to muscles involved in retracting the muscular foot into the shell.
The long axis of the organism is marked by a raised ridge; the middle axis is slightly humped. Kimberella′s body had no visible segmentation but had a series of repeated "modules". Each module included a well-developed band of dorso-ventral muscles running from the top to the single, broad, muscular "foot", and smaller transverse ventral muscles from side to side on the underside of the body. The combination of the bands of dorso-ventral and transverse ventral muscles enabled Kimberella to move by making the foot ripple.
The body also had a frilled fringe that may have been part of the animal's respiratory system, performing a function similar to that of gills. The fact that the fringe extended well beyond the shell may indicate that Kimberella′s "gills" were inefficient and needed a large area, or that there were no effective predators on Kimberella and the shell's main function was to provide a platform for the muscles.
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