Ordovician

The Ordovician period probably saw a greater expansion within the animal kingdom than any other similar time interval. The tri-lobites, which were the dominant marine animal group of the Cambrian, continued to diversify during the Ordovician but, in proportion to the other animals present, the trilobites no longer had their former dominance. However, new developments, like large raised eyes in phacopids and numerous long spines in odonto-pleurids, suggest that some trilobites spread out into new ecological niches. The same is true for the inarticulate brachiopods, which continued in about the same relative abundance as in the Cambrian.

By contrast, the articulate brachiopods increased greatly in abundance, in numbers of genera, and in the colonization of the sea floor in areas of deeper water. The brachiopods were, and are, all suspension feeders, circulating water containing organic matter through their mantle cavities in order to extract their food. This food mainly originated from marine phytoplankton, which was most abundant in shallow waters. There is little doubt that in the Lower Palaeozoic (as at the present day) there was a gradual decrease in the quantity of food for suspension-feeding benthos with increase in the distance from shore. Perhaps the gradual colonization of the deeper environments by the brachiopods during the Ordovician may be related to some increase in efficiency of the brachiopod feeding mechanisms. Although the most primitive articulates (the orthides) probably had a fairly simple lophophore (the orthide lophophore is not preserved fossil) they diversified to a great extent. Other articulate groups to develop from the orthides in the Ordovician were the rhynchonellides, the spiriferides and the pentamerides; probably all of these had more advanced lopho-phores than the orthides. The strophomenides developed in a different way: many of these lost the use of their pedicles in the adult and developed curved shell shapes which allowed them to rest on soft substrates without sinking.

In the Cambrian, most of the graptolites belong to the order Dendroidea. The first dendroids were sessile, but in the Late Cambrian (Tremadoc) Dictyonema flabelliforme and other pelagic forms appeared, enabling fast world-wide dispersal, useful for zonal dating. The Early Ordovician saw a great expansion of pelagic graptolites with the rise to dominance of the order Graptoloidea.

The molluscs are more varied in their modes of life than most other phyla. The Cambrian bivalves were mostly deposit feeders, but in the Lower Ordovician considerable diversification too'k place and the teeth (which are of value in classification) show that most modern bivalve orders appeared at this time. Some of these new forms developed their gills so that they could feed on material suspended in the seawater. This adaptation is frequently accompanied by a byssal mode of attachment. Deposit-feeding forms never had a byssus and usually nestled in the sand (they could not burrow to any great depth, as bivalves did not develop siphons until the Upper Palaeozoic).

The gastropods were probably mainly bottom-dwelling scavengers; they only diversified to a limited extent during the Ordovician. The third major group of molluscs are the cephalopods; during the Lower Palaeozoic they are mainly represented by the nautiloids. Most Palaeozoic nautiloids had straight shells (orthocones), but in the Ordovician curved forms appeared and also forms with other shapes. The nautiloids also showed diversification in the shape of the aperture and in many internal features. Most of this nauti-loid radiation occurred quite quickly in the LcAwer Ordovician. Nautiloids (like Nautilus today) all have shells divided by calcareous plates into gas chambers; modern cephalopods are known to be able partially to fill these chambers with liquid in order to adjust their buoyancy. It is probable that most nautiloids swam near the sea floor scavenging organic material. Like most modern scavengers, the nautiloids were seldom as abundant as the suspension feeders (brachiopods and bivalves), but in some Ordovician carbonate environments (like parts of Scandinavia and North America), where perhaps algae were present in quantity, nautiloids were the most abundant animals with shells.

Many bryozoan orders appeared for the first time at the base of the Ordovician. Bryozoans are similar to the lophophore-bearing brachiopods in many ways, except that they are colonial; like the brachiopods, bryozoans are often more abundant in shallower waters where perhaps the food supply for these suspension feeders was greatest. In the Palaeozoic, bryozoans were sufficiently abundant to form the major proportion of certain reefs.

There are two large groups of Palaeozoic corals: the Tabulata and the Rugosa. The tabulates appeared in the Late Cambrian (Tre-madoc); they were all colonial corals with very simple internal structures, but with a wide variety of colony shapes. Solitary rugose corals appeared for the first time in the Arenig in shallow carbonate environments. Rare compound rugose corals are first known in the Caradoc, but they do not become common until the Silurian.

The echinoderms are mainly represented by Crinozoa in the Ordovician. Of these, the most widespread group is the Crinoidea, which first appeared in the Cambrian, but became abundant in the Ordovician. Crinoids were either cemented to a hard substrate or anchored in sediment by a stem made up of numerous cylindrical plates. The cup at the head of the stem contained the mouth, digestive tract and reproductive organs; it was supplied with food by currents set up along the arms (usually five, ten or twenty in number). Crinoids, like the other Ordovician suspension feeders, were usually more abundant in shallow water environments. Other groups of Crinozoa can also be locally very abundant; for example, the cystoids are a major component of some Middle and Upper Ordovician limestones, particularly in North America and Scandinavia. Echinoids are very rare indeed during the Ordovician.

The Agnatha are a primitive group of fish-like animals: some fragments which occur in the Arenig may belong to this order, but more complete fossils assigned to this group did not appear until the Caradoc. True fish (Pisces of Tarlo, 1967) did not develop until the Silurian.

The Ordovician can be divided into five series:

Ashgill (after a stream in the English Lake District) Upper

Caradoc (after a hill in Shropshire)

Middle

Llandeilo (after a town in Dyfed) Llanvirn (after a farm in Dyfed)

Arenig (after a mountain in Gwynedd) Lower

The Ordovician System was first defined by Lapworth (1879) to resolve the differences between Sedgwick (who wished to include it in his Cambrian System) and Murchison (who claimed it for his Silurian System). Lapworth defined the base of the Ordovician as the base of the Arenig Series in north Wales; this is still the normal practice in Britain (and the one followed in this book), though many geologists in Europe and North America include the underlying Tremadoc in the Ordovician.

Many British geologists regard the Caradoc Series as part of the Upper Ordovician, but for the purposes of this book we group the series as above. During the Arenig and Llanvirn, brachiopods, bry-ozoans and molluscs were largely confined to the shallower parts of the shelf. In the Llandeilo and Caradoc these three groups expanded in numbers and in genera and the crinoids became important; there was also an ecological expansion of these bottom dwellers into the deeper parts of the shelf, a region hitherto mainly inhabited by trilobites. In the Late Ordovician, this process con tinued more slowly, with slightly deeper areas being colonized by bottom dwellers. By the end of the Ashgill, evolution of the common brachiopod and trilobite genera, sometimes accompanied by a change of ecological niche, had produced a new set of animal communities.

The Ordovician period lasted from about 519 to 438 million years ago: this interval represents over half of the total time of the Lower Palaeozoic (which lasted from about 570 to 411 million years ago).

The faunal provinces seen in the British Cambrian rocks continue into the Ordovician (Fig. c), with Scottish and most American bottom-dwelling faunas remaining very different from those in the equivalent communities in England and Wales. This difference is due to the great width of the Iapetus Ocean which extended from west of Norway, along the border between Scotland and England, through the centre of Ireland and of Newfoundland, and down the northern Appalachians to Connecticut. In the Arenig, the ocean was wide enough to prevent most animals from crossing. The only groups which were widespread on both sides are Dictyonema (which had crossed in the Tremadoc), some related pelagic dendroid graptolites and some conodont-bearing animals (conodonts are microscopic structures probably representing parts of the feeding apparatus of some unknown extinct animal). Many conodont-ophorids are geographically restricted and were probably nekton or benthos (Barnes and Fahraeus, 1975), but the widespread distribution of some forms suggests that they belong to pelagic animals; they were certainly among the first to be able to cross freely the gradually closing Iapetus Ocean. Correlation by means of the less restricted conodonts (Bergstrom and Cooper, 1973) shows that few graptolites crossed this ocean until the appearance in Wales of Didymograptus bifidus (whose ancestors are known only in North America) at the start of the Llanvirn. Though some graptolites could cross freely after the Llanvirn, many others remained provincial until later in the Ordovician.

Brachiopods and trilobites were both bottom-dwelling groups, but with free-swimming larval stages. There are occasional records of certain genera crossing the Iapetus Ocean prior to the Caradoc, but the vast majority of forms were different on both sides of the ocean. The agnostid trilobites, for example, seem to appear more often on both sides of the ocean than other groups: it may be that they were pelagic, or had longer-lived larval stages than most other trilobites. In the Caradoc, many brachiopod genera are common to both sides of the ocean, although the species are in most cases different. This suggests that while there was some connection between the species, the ocean still formed a barrier to free migration, and that after the original crossing further mixing was prohibited.

It has been suggested that oceanic islands may have played an

Ordovician Continents

Ordovician

Ordovician Trilobites

Fig. c. The world during the Ordovician (land areas and positions of continents after Barnes and Fahraeus, 1975, and McKerrow and Ziegler, 1972; Lower Ordovician trilobite provinces after Whittington and Hughes, 1973).

important role at this time, both as links between the continents where the pelagic larvae could settle and as evolutionary centres where the ecological pressures would be less than on the continents. Some unusual Ordovician brachiopod associations have been found in volcanic areas in the northern Appalachians (Neuman, 1972) where some forms occur earlier than on the adjacent continental areas.

During the Llandeilo, the volcanic rocks in the Lake District may have been part of an island arc off the continental areas of England; these volcanic rocks are followed by Caradoc sediments which contain brachiopod and trilobite faunas similar to those of the Welsh Borderland. Later in the Caradoc, and in the Ashgill, genera appear which previously were present at Girvan (in southern Scotland) on the other side of the Iapetus Ocean from the Lake District. By the end of the Ashgill nearly all brachiopod and trilobite species are common to both sides, showing that the ocean was now narrow enough for the pelagic spat to cross freely. This distance was probably around 2,000 to 3,000km (McKerrow and Cocks, 1976).

Many small bivalved ostracodes are different from most bottom-dwelling marine crustaceans in that they do not have a pelagic larval stage. Ostracodes are still alive today and it is known that they can lay their eggs on the sea floor from which their young hatch out directly. They have thus no means of crossing very deep water, no matter how narrow this water may be. The Iapetus Ocean did not close completely in Britain nor the northern Appalachians until the Devonian, but it closed slightly earlier to the west of Norway. It is not until late Silurian times that the ostracode faunas are similar on both sides; this is thus probably the time when the Iapetus Ocean first closed between Norway and Greenland. When the Iapetus Ocean finally closed altogether in the Lower Devonian, the freshwater fish faunas became the same on both sides. This record of progressive mixing of different animal groups thus provides the best documentation of the closing Iapetus Ocean. North America and the land around the Baltic Shield (of which England was a part) were only two of the four or five large continents in the world during the Ordovician (Hughes, 1973); other oceans opened and closed at different times, but so far the palaeon-tological record of these has not been fully studied.

The examples that follow are only a small selection of Ordo-vician communities. Apart from the Lower Ordovician Carbonate Community (8), they are drawn from Wales and the Welsh Borderland. Similar communities occur to the east of the Appalachians, but the majority of North America is in a different faunal province. Bretsky (1970) and Walker (1972) have described some Ordovician brachiopod communities from the North American faunal province east of the Appalachians.

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