Thursday, January 8, 2015

Helicoplacus, an Early Cambrian echinoderm

Helicoplacus gilberti Durham & Caster, 1963

This post concerns a group of very rare fossils called helicoplacoids. They belong to the phylum Echinodermata (i.e., includes starfish, sea urchins, sand dollars, sea cucumbers, etc.). Helicoplacoids, however, do not remotely look like echinoderms. Instead of having fivefold symmetry (= pentaradial or pentameral), they all look like the above-sketched specimen of Helicoplacus gilberti. They are characterized by an oblong body (most are about 3 cm long). Near one end of the body is the spiral food groove that acted like a mouth. Their "skin" was covered in spirals of overlapping calcareous plates, which were not sutured together tightly like on most echinoderms. Many "specimens," therefore, consist of small concentrations of easily disarticulated (scattered) calcareous plates.

Helicoplacoids are the earliest well-studied fossil echinoderm. They are only known from Lower Cambrian strata, around 525 million years ago, and they apparently lasted for 15 m. y.

A nearly complete specimen of Helicoplacus gilberti, 2.8 cm long, from Lower Cambrian strata, White Mountains, California. The food groove area is poorly preserved.

It is currently believed that Helicoplacus was a suspension feeder that lived in burrows in muddy substrate of shallow-marine waters. They might have extended their flexible bodies outward to feed. All the specimens found in the White Mountains of California are in siltstone that has been slightly metamorphosed. Their calcareous plates have been either weathered away or dissolved because only molds of the plates are now present.

Although some fragmental specimens of Helicoplacus are found in a few areas other than the White Mountains of California, the only complete specimens of this genus are found in these mountains.

The above picture shows a small cluster of H. gilberti from Lower Cambrian strata, White Mountains, California. The length of this slab is 8 cm.

This picture shows a large cluster of H. gilberti from the White Mountains. The longest dimension that incorporates all the specimens is 15.5 cm. This is an astonishing slab of specimens, the likes of which have never been shown before. These fragile-looking specimens must have undergone a short distance of post-mortem transport before being concentrated and deposited together. 

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