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450 million year old fossils

Selenopeltis trilobite slab

450 million year old fossils
450 million years ago

This slab of sandstone, taken from the 450 million year old Ordovician rocks of the Mecissi-Alnif area in eastern Morocco, contains three types of trilobite – a long-extinct marine creature known only from the fossil record. The trilobites on this slab would have lived in cool, relatively shallow waters on the floor of an ancient ocean on the north-west edge of the southern supercontinent of Gondwana, which extended from the South Pole to north of the Equator.

Selenopeltis is the most common genus of trilobite on the slab, and is characterised by a series of long spines along its body. Calymenella is large, dome-shaped and with no spines. Dalmanitina is smaller, with a large spine extending backwards from the end of the tail. It is likely that all these trilobites were predators or scavengers, feeding on small invertebrates or their remains. The spiny armour of Selenopeltis was undoubtedly protective, possibly against the large, predatory squid-like creatures and fish that had evolved by this time.

The Ordovician Period ran from around 485 million to 444 million years ago and ended with a mass extinction brought by relatively rapid environmental changes. Cooler climates generated a great ice sheet centred on the South Pole, at the time occupied by the land that is now northern Africa, and this sheet spread almost completely around the Earth. Then the climate warmed and ice-cap melting caused rapid sea-level rises, reducing oxygen levels in the water. Many species found it difficult to quickly adapt to these changes and Selenopeltis, Calymenella and Dalmanitina all died out. A small number of trilobite families survived this mass extinction and continued to roam the seafloors until about 250 million years ago.