Anomalocarids

ANOMALOCARIDS

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  (Note: in this essay I use anomalocarid to refer to the animal group, and Anomalocaris to refer to the species A. canadensis (just as shark would refer to a group of marine predators and Hammerhead would refer to a particular species).

Anomalocarids are a group of (usually) predaceous arthropods found primarily in fossil beds of the Cambrian period.  As more and more fossils are unearthed, the diversity of anomalocarids becomes increasingly clear.  Some were the apex predator of their time, others gentle plankton-skimmers, some impaled actively swimming prey in the open water, others probed for worms in the mud.  There are, however, a few  features that unite them all:

Anomalocaris great appendages1.) GREAT APPENDAGES: Anomalocarids possessed a pair of spiny, multi-jointed feeding appendages on the front of the head.  There is considerable variation in size and structure of these limbs among the various species.  For instance, the "classic" anomalocarid, A. canadensis (the one people usually think of when they picture anomalocarids) had long, robust limbs with multi-pronged spines for capturing prey.  Laggania cambria, on the other hand, had stubbier appendages with long, delicate, saw-like spines used for capturing plankton.

On a historical side-note, the great appendages of A. canadensis, were originally discovered separate from the body due to decomposition before fossilization. As a result, the great appendages were originally described as completely separate organisms-- specifically, a species of unusual shrimp (hence the name, Anomalocaris, which means “strange shrimp”.)

 

Anomalocaris peytoia mouth2.) PINEAPPLE-RING MOUTH. The mouth of an anomalocarid is a ring formed from several wedge-shaped pieces.
Like the great appendages, pineapple-ring mouth fossils were discovered before a complete specimen of the whole animal was known. Thus the mouth too was originally misidentified as an independent organism—a jellyfish called Peytoia.

 

3.) SEGMENTED BODY WITH LATERAL FLAPS OR FINS FOR SWIMMING. Although researchers don’t know exactly how these fins moved in life, the most likely hypothesis is that they worked in a sequential sinusoidal motion like the wings of a stingray or the mantle of a squid or cuttlefish.

 Anomalocaris lateral flaps

 

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