2 The Ordovician seas
Before going any further, click on 'View document' below and read pages 68-71 from Douglas Palmer's Atlas of the Prehistoric World.
Collecting seashells on an Ordovician beach would have been a rather curious experience. Whilst most shells were made of similar materials to those found on a modern beach, the detailed form of many would have been quite unfamiliar, and all the species have long been extinct.
Have a look at the panoramic illustration on pp. 70-71 of the Atlas. From this and the Atlas, pp. 68-71, think about which organisms appear most unfamiliar to you.
There are many organisms that probably seem unfamiliar, and when reading about them you may want to refer to Section 1.3. Brachiopods, e.g. Strophomena, are superficially clam-like animals, and although not extinct, are much rarer now than in the Palaeozoic; most people have never seen a living one. Graptolites, e.g. Orthograptus, are extinct colonial animals that mostly drifted in the ocean currents (Figure 2). Trilobites, e.g. Triarthrus, are extinct marine arthropods. Conodonts, e.g. Promissum, are extinct, and jawless (agnathan) fish, e.g. Sacabambaspis, are extinct except for hagfish and lampreys (see Section 1.4). The straight-shelled nautiloid cephalopods, e.g. Endoceras, are extinct and only distantly related to today's Nautilus, which has a spiral shell. The groups still very much with us include horseshoe crabs, snails (gastropods), e.g. Cyclonema, and corals (although Palaeozoic corals were significantly different from modern corals). Bivalves (not shown) were much rarer in the Ordovician seas than they are today.
All this fauna was marine. Very little, if any, animal life had made it out of the sea onto dry land by the end of the Ordovician. Trace fossils of an unknown, possibly millipede-like animal (Atlas, p. 71) are rare evidence that invertebrates were exploring the edge of the land by a freshwater route. There is fossil evidence that bryophyte-like plants and fungi had begun to colonise land environments back in Ordovician times. Apart from a small advance guard, however, the main invasion of freshwater and land environments by plants and animals did not really get going until the Silurian Period.