Nectocaris
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Extinct (fossil)
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Nectocaris pteryx is the fossil of an animal of unknown affinity from the Middle Cambrian Burgess Shale near Field, British Columbia, Canada. It was described by Simon Conway Morris in 1976. The head has two eyes, one or two pairs of appendages, and an oval carapace or shield at the rear. No shell hinge is visible in the single known specimen. The rest of the animal consists of about 40 segments -- each with three short spines on the top and bottom. Long fins run the length of the top and bottom of the animal. The species is represented by a single specimen and is thought to be a hemichordate, chordate, likely an arthropod, less likely a crustacean, or very likely something else. The specimen individual is thought to be a free swimming animal that was caught by accident in a turbidity flow, trapping it beneath the layers of silt that preserved it. It is unclear whether growth required moulting (as with arthropods) or not. There appear to be no hard parts. The pair of short, straight appendages on the front of the head appear to be unjointed.
The curious combination of arthropod and vertebrate characteristics in Nectocaris has aroused a lot of interest. Other than this single fossil, there is no reason to think that these two phyla are closely related. Extensive molecular evidence, on the other hand, indicates that the arthropods are closely related to the nematode worms, onychophorans, and tardigrades. Vertebrates, in turn, are thought to be more closely related to echinoderms, and acorn worms.
