Life in the Palaeozoic
Life in the Palaeozoic

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Life in the Palaeozoic

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.

View document [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

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.

Figure 2
Figure 2 Specimens of the graptolite Diplograptus in Ordovician shales (field of view 3.5 cm). Graptolites often look like tiny saw blades lying on the rock surface

Take your learning further371

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses372.

If you are new to university level study, we offer two introductory routes to our qualifications. Find out Where to take your learning next?373 You could either choose to start with an Access courses374or an open box module, which allows you to count your previous learning towards an Open University qualification.

Not ready for University study then browse over 1000 free courses on OpenLearn375 and sign up to our newsletter376 to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371