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Early eocrinoids and brittle stars

Early eocrinoids and brittle starsEarly eocrinoids and brittle stars

NPL 14795
Found in the Erfoud region of Morocco

This intriguing limestone slab was extracted from Middle-Upper Ordovician (about 450 million years old) rocks in the Anti-Atlas part of the Atlas Mountains of Morocco. This slab was once part of an ancient sea floor that was uplifted and incorporated into mountains about 80 million years ago, long after the sea floor turned into thick layers of rock.

The fossils preserved in this slab include early echinoderms (spiny-skinned marine invertebrate animals) known as eocrinoids and ophiuroids (brittle stars). The brittle stars are clearly recognizable, with their five prehensile arms and compact central body. They look very similar to living species of brittle stars, most of which live in reef, shallow near shore and deeper water environments, and feed on small organisms suspended in the water.

Eocrinoids are more unusual, with long thin tapering stems and clusters of arm-like appendages at the opposite end. These animals are probably the eocrinoid Ascocystites, a suspension feeder that would have extracted food directly from the water column, filtering out micro-organisms with the help of those appendages.

Despite the name, eocrinoids were unrelated to crinoids (sea lilies) and differ in several morphological features from ancient and modern crinoids. However, like many fossil crinoids, their elongated stem allowed for attachment to hard substrates and the possibility of raising themselves higher off the sea floor.

There appear to be several larger individuals along with a number of much smaller ones. Those smaller animals may be juveniles of the larger form or a different species. Do you notice how the smaller individuals often seem to be in close contact with the brittle stars? We are left to wonder whether the brittle stars were feeding on the smaller eocrinoids.