RESULTS

As noted previously, the data used here vary from temporal resolutions at stage to epoch level. This variance causes some temporal smearing, i.e., artificially high diversities for some stages. The effect is presumed random and the majority of the records have a stage-level temporal precision. However, for the Early Ordovician temporal smearing is more serious. Especially for Baltica and Laurentia, diversity counts are problematic because the records detailing these faunas often have a temporal resolution at the epoch level. Furthermore records detailing the faunal developments of the Early Ordovician are often old, meaning that the original correlations are uncertain. This has the effect that some genera belonging to the Arenig are also reported for the Tremadoc and vice versa, artificially increasing the diversity. A more detailed study of these data is planned, however, this problem is the primary reason why the Early Ordovician data are only briefly described here.

There are common trends in all the diversity curves for the plates of the greater Iapetus region (Figure 1) despite the latitudinal difference in position from a lower latitude (Laurentia, Siberia), mid-latitude (Baltica, Avalonia depending on the time), higher latitude (European massifs) and polar (Gondwanan terranes, however, note that South America reached fairly far north [Cocks and Torsvik 2002]). The curves display a rapid diversity increase from approximately the Tremadoc-Abereiddian, followed by a diversity drop that lasted until the Costonian, where renewed diversification is rapid, especially on Avalonia Major and the European massifs (Figure 1).

The rest of the Caradoc and the Ashgill is generally marked by high diversities, with indications of a decline in diversity beginning in Ashgill on Laurentia Major and the Core Gondwanan terranes. This pattern is expressed to a lesser degree on Siberia and Avalonia Major. The European massifs and Baltica appear unaffected during this interval. At the Hirnantian-Rhuddanian transition, all plate groups except Baltica and Avalonia experienced dramatic reductions in diversity.

The European massifs suffered the worst, dropping from about 50 to only 10 genera. Avalonia experienced only a minor diversity decrease within this interval, however, displayed a similar reduction during the Rhuddanian-Aeronian (Figure 1). Baltica deviates further from this pattern – similar to Avalonia there is a small diversity decrease in the Hirnantian-Rhuddanian, but the dramatic reduction does not occur until the Telychian-Sheinwoodian. This is coeval with a facies change in the Scandinavian area from marine to terrestrial facies (Worsley et al. 1983). The orthides of Baltica did experience a reduction in diversity during the Ordovician-Silurian transition, however, already by the Rhuddanian new diversifications were characterizing the emptied niches, so that the net result was a faunal turnover, not a permanent reduction in diversity.

The development after the extinction event shows the most contrasts. Laurentia experienced renewed diversity increases from the Sheinwoodian-Gleedonian, but this is not seen on the other plate systems. Siberia, the European massifs and Baltica do not record the rising diversity seen on Laurentia, however, at the end of the Silurian diversity in the European massifs was increasing. The core Gondwanan terranes and Avalonia Major generally follow a declining pattern, the major diminution in diversity occurring together with the diversity drop in Laurentia during the Gleedonian-Gorstian.

Endemism 

The geographical range definition is used for endemism. Genera indigenous to certain plates or plate groups such as the peri-Gondwanan terranes or Avalonia Major are here termed endemic. How endemism is defined naturally affects levels of endemism. With the limits on the range of an endemic genera being defined as stated, the orthides of the GIOR display a high degree of endemism (Figure 2). During most of the Ordovician the total endemic rate at the generic level was around 50%. In the Late Cambrian there was about 75% endemic genera (not included in Figure 2), however, this dropped to about 46% in the Tremadoc, signifying a geographical expansion of taxa. This was coupled with a 344% increase (25 to 86 genera) in total diversity from the Late Cambrian to the Early Ordovician.

The Late Abereiddian-Llandeilian increase in endemicity is correlated with a significant diversity decrease for all terranes except the tropically placed (Cocks and Torsvik 2002) Laurentia and Siberia. In the Late Abereiddian, the global orthide diversity experienced a minor reduction, followed by an interval of high endemism and low diversity rates. Globally orthide diversity was about 25% lower than that in the time after the early Caradoc, and about 20% below Abereiddian diversity values. The situation persisted until sometime around the late Velfreyan-Costonian, where there was a drop in the degree of endemism corresponding with a surge in diversity for all plates. This increase, however, was relatively limited for Laurentia. Baltica appear to have experienced a more gradual diversity buildup, lacking the period of low diversity following the Llandeilian reduction. Globally the diversity increased about 25% from the late Velfreyan to the Costonian.

During the Late Ordovician Extinction Event, the proportion of endemic genera was markedly reduced. The reduction affected most genera with restricted geographical distributions (one to three plate groups in Figure 2). The relative proportion of genera with representative species on all the GIOR plates increased, while the actual generic diversity dropped dramatically in the Hirnantian-Rhuddanian.

During the Silurian, the number of endemic genera gradually increased to and even above the Ordovician average in the Gorstian-Ludfordian, where about 52.5% of the genera were endemic to one plate system. This level of endemism was the highest since the Velfreyan. There were, however, fewer orthide genera around (Figure 1). These two intervals of increasing endemicity correspond to the Early-Middle and Middle-Late Silurian boundaries (Figure 2). Only a very few genera were spread across the entire GIOR (total average approximately 3.3 genera for the Silurian versus 5.9 for the Ordovician (Appendix), however, these comprise a larger proportion of the total generic diversity following the diversity reduction at the Hirnantian-Rhuddanian boundary.

Cluster Analysis

As noted above, Early Ordovician data are somewhat uncertain in their spatio-temporal precision. However, a few general patterns are obvious from the association analyses performed on the separate and combined Tremadoc and Arenig datasets (Figure 3, top left). The three European massifs that include orthide data form a separate faunal group, while the remainder of the analyzed plates form an undifferentiated cluster, whose internal organization is not very well resolved and varied between the indices used (Dice, Raup-Crick and Simpson) for generating the dendrograms. The supercluster was consistent, and the Early Ordovician orthides appear fairly widespread in their distribution within the GIOR.

During the Arenig-Llanvirn, a gradual shift in the organization of the faunal associations occurred. At the same time data quality increases markedly. Up to and including the Llanvirn (Figure 3, top right), Avalonia still groups with the core Gondwanan terranes for all three indices, however, with the onset of the Late Ordovician (Figure 3, mid left), the microcontinent changes association to associate with Baltica and Laurentia. Baltica and Laurentia differentiate by the Llanvirn, suggesting that Baltica had then drifted enough northwards to attain a faunal composition different from the Gondwanan. The peri-Gondwanan terranes of Iberia, Armorica and Bohemia retained their individual faunal composition. Siberia appears oddly placed in the Llanvirnian dendrogram, however, analysis of the orthide data indicates that Siberia shared faunal elements with both Laurentia and the Gondwanan terranes, which gives the placement in the dendrogram.

By the Caradoc (Figure 3, mid left), a clear differentiation into three faunal associations had emerged, largely defined by latitudinal placement. The peri-Gondwanan terranes retain their independent status, and the core Iapetan terranes of Baltica, Avalonia and Laurentia have separated from South America and Africa. This pattern is essentially the same for the Ashgill (Figure 3, mid right).

Siberia is grouped somewhat differently depending on the index used, either with the Gondwanan core or with the Laurentia-Baltica-Avalonia group. The position of the Precordillera in the cluster and ordination analysis is tentative because of the small amount of genera available for analysis.

By the Llandovery (Figure 3, bottom left), the associations have been altered. The core Iapetan terranes now form part of a well-isolated cluster with the core Gondwanan terranes. The European massifs association is retained, however, marked diversity reductions have severely diminished orthide stock on Apulia and Iberia. This pattern is retained later into the Silurian (Figure 3, bottom right). Siberia continues its intermittent placement. Siberia branches off the larger associations near the base of the dendrograms, and the affinities of the Precordillera are somewhat uncertain during this interval. However, it is clearly not Laurentia-related.

Ordination Analysis 

While there are differences between the NMDS and PCO results, these are mostly in the detail. The differences are fairly well correlated with the signal strength in the faunal data, i.e., the differences were largest in the diagrams for the temporal intervals of the lowest diversity, e.g., the Tremadoc. The individual PCO-eigenvectors (PCO can be expressed as a distance-based method like NMDS or as an eigenanalysis) generally did not account for much of the variance in the datasets, typically around 20-40% of the variance was accounted for by the 1st axis, while the 2nd accounted for about half of the first axis. This suggests that there is no single gradient that fully explains the variation of the dataset. The Late Ordovician plots were the best resolved, corresponding with the largest amount of data available. The ordination diagrams generally display the same dramatic change in overall faunal patterns from the Hirnantian to the Rhuddanian as the classification diagrams. The faunas change from being divided into relatively well-defined groups to a single large association with some more or less detached faunal elements (Figure 4).

During the Costonian (Figure 4, top left), Laurentia, Avalonia and Baltica formed an individual association. Avalonia held an intermediate position between the core Gondwanan terranes and Laurentia/Baltica, suggesting shared faunal elements. Likewise, the position of Laurentia and South America possibly indicate some shared faunal elements. These relationships are well known (Fortey and Cocks 2003) and documented, and it is, therefore, not surprising. Baltica appears to be the most faunally isolated of the three core Iapetan plates. The core Gondwanan terranes are positioned together, as are the peri-Gondwanan terranes. By the Hirnantian (Figure 4, top right), Bohemia is indicated close to Avalonia and Baltica, perhaps indicating an intermittent position between these and the peri-Gondwanan terranes.

Siberia is in a rogue position between the three plate groups. Siberia and Avalonia probably retained some of their common Gondwanan elements, which explain the faunal similarity. The two core Gondwanan terranes still appear to form a separate group.

In the Rhuddanian (Figure 4, bottom left), half the plates form a closely knit group consisting of the core Gondwanan terranes as well as Laurentia, Avalonia and Baltica, however, apparently Siberia retained a different faunal composition. This might, however, be an artifact of low diversity from the craton during the Rhuddanian, not a true pattern. The terranes separated from this group are Apulia-Armorica, which have almost identical faunas. Bohemia retains a position somewhere between this plate pair and the large group. The Precordillera remains unattached to any of the groups. This pattern is retained in the Telychian. For the Gorstian (Figure 4, bottom right), the same pattern is basically retained; however, there is some separation into subgroups. Most of the peri-Gondwanan terranes remain marginal to the large group, and Siberia appears separate also. The two Gondwanan core elements remain tightly connected, and the old Baltica-Laurentia-Avalonia group is also well constrained. Bohemia shared faunal elements with all three groups, however, as indicated by the classification analysis, the terrane is now firmly placed with the Gondwanan and core Iapetan faunas.