During the Ordovician (named after the Ordovices, a Celtic tribe that once inhabited north Wales) new organisms replaced those that died during the Cambrian. This was one of the largest adaptive radiations in Earth’s history: the number of families of known marine invertebrates more than doubled.
The trilobites, inarticulate brachiopods, and archaeocyathids were succeeded by articulate brachiopods, cephalopods and crinoid echinoderms (‘sea lilies’). Bryozoans (‘moss animals’), tiny colonial animals that build skeletons of calcium carbonate appeared as did the first coral reefs. Molluscs were abundant, especially bivalves, gastropods, and the nautiloid cephalopods which became the top predators – some of which had shells of over ten metres in length and were the largest animal that up until that time had ever lived.
The jawless fish agnathan appeared during the mid-Cambrian and two groups of jawless fish are extant – the hagfish and the lampreys. During the Middle Ordovician the first armoured agnathans appeared, the ostracoderms. Often less than 30 cm (1 ft) long they fed by sieving planktonic organisms from the water. Also during the Middle Ordovician the first certain eurypterids (aquatic scorpion-like arthropods) appeared, although a possible ancestor dates back to the Middle Cambrian.
Fossil spores from land plants have been identified in Late Ordovician sediments. Plant cover would protect the soil from erosion, enrich it with organic material, and release oxygen into the atmosphere to open new possibilities for life out of the water. The fossil record suggests that towards the end of the Ordovician a massive extinction took place.
Leave a Reply