Silurian

The marine faunas in the Silurian were dominated by benthic brachiopods and pelagic graptolites. Although trilobites occur in many environments, they were far less important than in the Or-dovician. Molluscs, bryozoans, ostracodes and crinoids are also present, but only common in some habitats. Towards the end of the Silurian, the land was just beginning to be colonized by plants, but animals with hard parts were absent from the land, although some arthropods and worms may have become terrestrial before the end of the period.

The Iapetus Ocean (Fig. e), which in the Ordovician separated the Canadian Shield (including Scotland) from the Baltic Shield (to which England was attached), became progressively narrower during the Silurian. Therefore, in their drifting (pelagic) larval stages, the brachiopods and trilobites were able to cross the ocean freely, and the faunas became the same on both sides of the ocean. In the Lower Silurian these brachiopods and trilobites were world-wide in their distribution; no ocean was wide enough to act as a permanent barrier to their pelagic larvae. Most ostracodes, however, do not have any pelagic larval stage — their eggs hatch out on the sea floor — and, as a result, even a narrow ocean, like the Iapetus in the Silurian, was a barrier to their migration.

The Lower Silurian brachiopods of Britain were one of the first groups of fossils from which communities have been described. Ziegler et al.(1968, 1968a) have concluded that these communities are more closely related to varying depths of water than to varying types of sediment. The evidence for this is, first, a consistent community sequence from the shore to deep water as shown by maps of community distributions at any one time (Fig. d), second, the fact that in the transgressive Lower Silurian of the Welsh Borderland different local successions show the same vertical sequence of communities (from shallow to deep), and, third, that like modern bottom marine communities, there is often a general progressive increase in the diversity of animal species with depth.

Because many communities occur in a variety of clastic sediments it appears that other factors also affected the distribution of brachiopods in the Silurian. Brachiopods are suspension feeders,

Silurian Marine Community Reconstruction

Fig. e. Geography of the North McKerrow and Ziegler, 1972; Ziegler, .are shown for late Llandovery or early

Atlantic region in Silurian times (after 1970). The shorelines and volcanic areas Wenlock time. The Welsh Basin (Fig. d)

Early Silurian Iapetus Ocean

lies to the north-west of the Mendip volcanics shown in southern England. The Iapetus Ocean was narrow enough to permit free migration of brachio-pods and other benthos with pelagic larvae.

attached to objects on the sea floor by pedicles, or lying loose on one of their valves. Their food supply originates from primitive microscopic plants, which may have been more abundant in shallow water. Many brachiopod species decrease in size away from the shore, suggesting that a corresponding decrease in food supply could have controlled their size. The food supply may also have been a significant factor in the control of the distribution of the communities, with only the more efficient feeders thriving in deep water.

Food supply alone cannot explain the diversification of Silurian brachiopods into communities. There must have been some advantages for those brachiopods which lived in the deeper water communities, and stability of environment and in particular constancy of sedimentation rates and of temperature would be among such benefits.

Brachiopods make up as much as 80 per cent of the preserved Silurian benthos in clastic marine sediments. Apart from competition for food, there was probably little interaction between one brachiopod and its neighbours on the sea floor. Each brachiopod community therefore did not form a discrete unit; there is little correlation between the ecological distribution of any two species, and in areas with an even sea floor there is a continuous gradation of changes in the brachiopod distribution from the shore to deep water, with few natural breaks.

The Silurian is divided into four Series, named Llandovery, Wenlock, Ludlow and Pridoli. The lower three were originally defined by Murchison, and correspond approximately to major litho-logical groups in the Welsh Borderland. The Llandovery takes its name from the town in Dyfed, where the Lower Silurian is rich in brachiopods; the Wenlock and Ludlow are named from Much Wenlock and Ludlow, which are towns in Shropshire. The name Pridoli comes from an area in Czechoslovakia, where marine conditions were continuous from the Silurian into the Devonian. The Pridoli Series corresponds to the Downtonian freshwater sediments of Wales and the Welsh Borderland. Recent estimates of dates from radioactive isotopes suggest that the Silurian lasted for about 27 million years, from 438 to 411 million years ago. The period is sub-divided into 34 graptolite zones, which are on average each a little less than a million years long. Since the classic work of Lapworth (1878) the graptolites have been the chief basis of Silurian time correlations, especially on a world-wide scale. Other animal groups which have been used to correlate the Silurian include conodonts, ostracodes, acritarchs and brachiopods. These have been especially useful in shallow water deposits where grap-tolites are uncommon.

In time correlation, brachiopods are most reliable when the evolution of individual species has been studied. Assemblages of

Fig. f. The evolution of the brachiopod Eocoelia during the late Llandovery (aftei Ziegler, 1966). The diagrams on the right show moulds o] the interior of the pedicle valve. The early species had strong ribs and no grooves on its teeth; the later species show gradual changes toward! smooth forms with strong grooves on their teeth. Most Silurian benthic genera do not show such well marked evolutionary changes, over a period of time lasting several million years, but the few examples that do (like Eocoeliaj are of great significance in dating the beds in which they occur. Eocoelia extends from the western United States to the U.S.S.R. and the changes are synchronous over this large brachiopods reflect environment rather than time. Very similar assemblages can occur in different places at different times in a similar environment, and are thus virtually useless for correlation. But the evolution of a genus reflects genetic changes through time within a similar habitat. Most Silurian brachiopod lineages show no detectable change with time, but those lineages which do show changes are very useful in correlation. The most useful are Eocoelia (Fig. f (after Ziegler 1966)), which lived in shallow

Silurian Graptolite Zones

water (Upper Llandovery — Lower Wenlock), Stricklandia, which was in deeper water (Lower Llandovery — Lower Wenlock) and Resserella sabrinae which was also in deeper water (Wenlock and Lower Ludlow). The age relationships of these lineages and the graptolite zones are known by means of the occasional graptolites which have been recorded amongst shelly faunas. Fig. f shows the changes in Eocoelia from strongly ribbed forms at the base of the Upper Llandovery to virtually smooth forms by the Lower Wen-lock, and through the same time interval there were also changes in the dentition between the two valves. Most of the localities in Fig. d have been dated by Eocoelia and Stricklandia, but those in the deeper water areas have been correlated by graptolites.

In the Wenlock and Ludlow Series, the successors of the Llandovery Communities form similar gradational sequences from the shore to deep water. These Upper Silurian Communities include many genera which survived from the Llandovery in the same general habitat, but there are also many differences. For example, the Eocoelia and Stricklandia lineages became first rare and then died out during early Wenlock time, and genera which are less abundant in the lower Silurian occupied the vacant habitats and became much more common.

In the case of the Clorinda Community, the two genera Clorinda and Dicoelosia occur together through much of the Silurian, but Dicoelosia is more prevalent during the Wenlock and Lud-

Time J Ec°*">uP

Lingula

Eocoelia-Salopina

Pentamerus-Sphaerirhynchia

Stricklandia-Isorthis

Clor in da-Dicoelosia

Visbyella

Graptolite

Pridoli and Ludlow

Lingula

Salopina

Sphaerirhynchia

Isorthis

Dicoelosia

Visbyella

Graptolite

Wenlock

Lingula

Salopina

Sphaerirhynchia

(sometimes Homoeospira in clastics)

Isorthis

Dicoelosia

Visbyella

Graptolite

Eocoelia

Late Llandovery

Lingula

Eocoelia

Pentameroides

Costistricklandia

Clorinda

Marginal Clorinda

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