Nectocaris from http://www.coe.uga.edu/~pkeck/ediacara/pics
Another Dilemma...
How do you classify an organism that appears to be an arthropod at its anterior portion and a chordate at the posterior end? This is the exact dilemma that paleontologists face with the classification of Nectocaris pteryx, another curious organism of the Burgess Shale.
A swimming predator...
The body of Nectocaris is suggestive of an active, swimming predator due to its flat shape and fins. The rarity of Nectocaris fossils could be explained by this mode of locomotion. Because swimmers are not trapped in sediment often, their fossilization is infrequent. Furthermore, swimmers are usually predators, because rapid movements and the large expenditures of energy necessary to achieve such movements are not necessary for a filter feeder.
Arthropod?
Its form is suggestive of an arthropod because of the shield enclosing the head and the serial appendages along the body length. However, these appendages are not jointed, as one finds in other arthropods. Each of the 40 segments is connected to a single, continuous fin, but arthropod appendages are normally separate and distinct. Each of the 40 segments then contains three spines that enter the fins on the dorsal and ventral sides.
Pikaia, from http://www.coe.uga.edu/~pkeck/ediacara/pics
Chordate?
Fins which run the length of the body and are found dorsally and ventrally are, instead, characteristics of the first chordates, represented by Pikaia graciliens. Metamerism is also found in Chordata. The supporting fin rays are also reminiscent of Pikaia.
Perhaps with the discovery of additional fossils, the
classification of Nectocaris within existing groups may be possible. Since only
one fossil has been found, 3-dimensional sectioning has not been possible, and
thus only the surface anatomy has been described.
