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46 JOHN C. W. COPE

Fig. 3. Pie diagrams to illustrate the faunal composition and diversity of bivalve faunas in the Ordovician.
(a) Early Ordovician. Comparison between the numbers of species comprising faunas of the Llangynog Inlier.
Wales (Cope 1996b) and the near-polar Montagne Noire (Babin 1982a). Note the far greater abundance of
pteriomorphians in Wales, (b) Mid-Ordovician. Comparison between the numbers of species comprising the
faunas of high-latitude Spain (Babin & Gutierrez-Marco 1991), mid-latitude Avalonia (Cope 1999) and low-
latitude Australia (Pojeta & Gilbert-Tomlinson 1977). Note the increased importance of pteriomorphians at
low latitudes whilst the heteroconchs are much more diverse at median to high latitudes. (c) Late Ordovician.
Comparison of three low-latitude bivalve faunas to show faunal composition and generic diversity: Laurentia
(data summarized from various sources), Baltica (primarily after Isberg 1934) and Siberia (Krasilova 1970.
1979).

exclusively Gondwanan and are dominated by

two groups, the nuculoids and the heteroconchs,

although there are a few pteriomorphs and even

fewer anomalodesmatans. The preference of

bivalves for shallow-water siliciclastic facies

persisted through Mid-Ordovician times and it is

noteworthy that one of the earliest Laurentian

faunas, from Minnesota, and probably of

Early Caradoc age, was in a sandstone facies

(Sardeson 1896).

Comparison of Mid-Ordovician faunas from

high, median and low latitudes (Fig. 3b) reveals

a real difference in both bivalve diversity and

faunal composition. The number of species at

equatorial latitude is twice that of the highest

latitude. Surprisingly, the percentage of nucu-

loid species in the faunas is remarkably similar

across the latitudes, but the diversity is clearly

greater at low latitudes. For high-latitude

Gondwana (Spain) there are six nuculoid