Computerized databases of palaeontological information provide the only practical means for investigating large-scale palaeobiological patterns and processes over long time intervals (Benton 1999; Markwick and Lupia 2002). Furthermore, large palaeobiogeographical databases comprise a complementary method for the reconstructions of palaeoplate positions and movements, especially for the early Palaeozoic where continental palaeomagnetic evidence is less secure (Lees et al. 2002). Such models also allow for the comparison of biofacies and the relationship between faunal associations and physical factors, such as water depth and substrate type (Cocks and McKerrow 1984; Bernasconi and Stanley 1994) across two or more margins. The use of large databases to analyze palaeodiversity, extinction and survival trends is, however, problematic because it assumes a number of conditions. For example it assumes that taxonomic and spatial errors are random (Benton 1999) and that sampling intensity is constant and independent of rock-volume variations (Smith 2001). If errors in taxonomic databases are random, this means that faunal signals cannot be created by the errors, however, they can be degraded due to the random noise the errors generate in the dataset (Raup 1991).

Brachiopods are ideal for palaeobiogeographical studies, because the animals were sessile, because most of them lived in relatively shallow water, because their shells accumulated in large numbers, and because they are commonly robust, with features that can survive transport, burial and diagenesis. Furthermore, their taxonomy is relatively stable (Rudwick 1970; Williams and Harper 2000).

Brachiopods were especially important elements of the benthic fauna during the early-middle Palaeozoic. The orthide (Order Orthida: Schuchert and Cooper, 1932) brachiopods were, together with the strophomenides, the first important rhynchonelliformean groups to diversify in great numbers. During the Ordovician, the diversity and fossil abundance of the orthide and strophomenide brachiopods dominated benthic assemblages. The orthides were particularly abundant during the Middle-Late Ordovician, however, the order ranged through the entire Palaeozoic (Williams and Harper 2000; Harper and Gallagher 2001). More than two hundred localities distributed globally have yielded orthide brachiopods, especially sites in North America, Europe, North Africa and China.

High orthide diversities were maintained through most of the Ordovician, following an initial rapid diversification during the Early Ordovician. After a dramatic, approximately 50%, reduction in global orthide diversity during the Late Ordovician extinction event, a steady decline in orthide diversity continued throughout the Silurian. The order briefly peaked during the Early Devonian, but gradually died out during the rest of the Palaeozoic. The last orthide is reported from around the Permian-Triassic boundary.

A general overview of the orthides from all the major terranes of the Ordovician-Silurian Palaeozoic Greater Iapetus Ocean Region (GIOR) (including both the Rheic and Tornquist oceans and adjacent plates) (Mac Niocaill et al. 1997) in time and space is presented here based on data from a new global database. Data are recorded at the generic level of taxonomic resolution.

The orthide data have been evaluated using a range of statistical and graphical techniques. The multivariate methods (Jongman et al. 1995; Etter 1999) include Principal Coordinates Analysis (PCO or PCoA, MDS) (Davis 1986), Non-Metric Multi Dimensional Scaling (NMDS) and UPGMA (paired linkage) cluster analysis (Krebs 1989), based on the Dice similarity coefficients. Diversity curves as well as the endemic properties of the orthide genera are also considered. The results have been validated using similarity indices, which are calculated differently than the Dice index, in order to assess the strengths of the faunal signals apparent from the analyses. The two validation indices were the Simpson and the Raup-Crick indices (Raup and Crick 1979). These results are compared with plate tectonic models for the region (e.g., McKerrow et al. 1991; Cocks and Torsvik 2002; Torsvik and Rehnström 2003). Analyses were performed using the statistical package Palaeontological Statistics (PAST^©, Ø. Hammer and D.A.T. Harper; Hammer et al. 2001) and Microsoft EXCEL (^©Microsoft). Taken together, the results provide a broad overview of the history of the Iapetan orthides and their associated plates and terranes.