Halkieria

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Halkieria is a genus of early molluscs that lived from the Lower to Middle Cambrian. Fossils of Halkieria have been found on many continents and are a major part of the small shelly fossil record. The best known species is Halkieria evangelista, from the Sirius Passet in North Greenland. Complete specimens of this animal were collected in 1989 and described in 1990, with a more detailed study published in 1995.

Halkieria looked like a slug with chain-mail armor. It ranged from about 1.5 to 8 centimeters long and was bilaterally symmetrical. The animal had no obvious swimming abilities and lived on the sea floor, probably moving by rippling its muscular sole. At each end there was a cap-shaped shell plate with growth lines. The upper surface was covered with about 2,000 hard plates called sclerites, arranged in three zones.

- Palmates: leaf-shaped sclerites in the center of the back.
- Cultrates: blade-shaped sclerites on either side of the palmates, pointing toward the middle.
- Siculates: slim, sickle-shaped plates along the outer edges.

The sclerites connected to form a protective shell over the back. They appeared to grow by adding material to the shell plates at the edges, while individual sclerites kept growing in a constant size pattern. Some evidence suggests there were fine ribs or canals in the sclerites, and the whole scleritome may have included muscles still present at burial.

The shells and sclerites were probably made of calcium carbonate, though some have debated whether they were wholly organic. The presence of three-dimensional preservation in Halkieria fossils argues against a purely non-calcified organism.

There are hints that the rear end of Halkieria housed gills, and there are traces of a gut in the back half of some fossils. A few specimens have been interpreted as having a radula (the toothed tongue typical of molluscs), but this is not certain, and in some cases may reflect dislocated sclerites.

Most Halkieria fossils are known only from the scattered sclerites. The only species for which the full scleritome is well understood is Halkieria evangelista from Greenland. Because of this, H. evangelista is used as a model to recognize and reconstruct other Halkieriids from their sclerites. The discovery and study of H. evangelista helped scientists understand the design and evolution of these early armored molluscs.