180 PALAEOZOIC/Ordovician Articulated brachiopods became numerous enough to form shell beds and evolved into the orders that would dominate the Palaeozoic shallower seas Echinoderms saw radiations in the starfish, sea lilies, and the earliest sea urchins By the mid-Ordovician vertebrates with phosphatic external skeletons had become quite varied Conodonts (now considered to be related to vertebrates) (see Microfossils: Conodonts) became abundant, and showed a variety of feeding apparatuses It is likely that the first land plants, allied to liverworts, had appeared by Llanvirnian times, although their fossil record is dominated by spores alone Moreover, because of the diverse palaeogeography of the time, various groups of animals evolved differently on different palaeocontinents, adding to the total biodiversity Climatic zonation aided this differentiation If there was a watershed between Cambrian-style faunas and those characterizing later strata, it occurred at the base of the Whiterockian, when taxa typical of younger strata increased at the expense of those with an archaic history extending back into the Cambrian It has also been shown that there was an increase in the complexity of trace fossils and bioturbation through the period, with younger Ordovician animals burrowing deeper in the sediment and making ‘galleries’ The kind of tiering characteristic of later infaunal assemblages had its origins in the Ordovician It is possible that increased predation drove these innovations forwards Allowing for fluctuations in the fossil record, there was an increase in global biodiversity through the Ordovician until the Caradocian (Figure 3) The enrichment process was interrupted by the extinction event at the end of the Ordovician, but communities were able to re-establish comparable complexity in the Silurian The End-Ordovician Extinction and Glaciation The mass extinction at the end of the Ordovician is one of the major events in the fossil record, being exceeded only by those at the end of the Palaeozoic and at the Cretaceous–Tertiary boundary While the ultimate controls on the latter events are still a subject of dispute, the end-Ordovician extinction coincided closely with a short-lived but very extensive glaciation, and it is natural to link the biological event with the physical event Exactly how the secondary effects associated with the glaciation caused the extinction is still the subject of research The ice age climaxed in the final subdivision of the Ordovician, the Hirnantian ‘stage’ It was evidently short-lived, and current evidence indicates that the Hirnantian may have been as brief as 0.5 Ma There is good reason to suppose that the climate Figure Changes in biodiversity through the Ordovician, showing changes in the diversity of taxa belonging to the different ‘evolutionary faunas’ (EF) identified by JJ Sepkoski Maximum diversity is reached late in the Ordovician The numbers 1a d, 2a c etc., refer to finer scale temporal subdivisions used in cali brating this diagram Reproduced from Webby BDE, Paris F, Droser ML, and Percival IG (eds.) (2004) The Great Ordovician Biodiversification Event New York: Columbia University Press was deteriorating before the glacial climax Evidence for glacial deposits is widespread near the Ordovician south pole in North Africa, and they have also been discovered in South America and the Arabian Peninsula All these areas were part of the Gondwanan continent in the Ordovician, and the scenario of an extensive ice-sheet expanding outwards from a Gondwanan core is supported by evidence on the ground In Morocco, there are spectacular glacial pavements, which are associated with tillites and finer-grained diamictites Evidence of ice movement away from the polar centre has been obtained from striae, roches moutone´ es, and the fining directions of glacial deposits (see Sedimentary Processes: Glaciers) The glacial event does not seem to have involved multiple advances and retreats, as in the Pleistocene, although a double ice-advance pulse has been claimed Correlation within non-marine sediments is difficult at this early time, however The effects of the glaciation extended well beyond Gondwana Oceanic water became tied up in the icesheet, resulting in a general marine regression This event was accompanied by climatic cooling, such that the tropical carbonate belt became extremely restricted This obviously affected the many organisms adapted to warm carbonate environments, which either died out or were pushed into small refugia Clastic deposits are typical of the Hirnantian, but in many localities the interval was one of general scouring and erosion A striking faunal change occurs