ORDOVICIAN BIVALVE BIOGEOGRAPHY 47
Fig. 4. Pie diagrams to illustrate abundance of bivalve individuals composing the faunas of which the species
diversity is shown in Figure 3a, b. Precise numbers are available in the sources quoted in Figure 3a, b for most
species, but for a few species numbers have been estimated from published information. (a) Compare with
Figure 3a. Note the dominance of the heteroconchs in both faunas and the small percentage of the diverse
pteriomorphians in the Welsh fauna. (b) Compare with Figure 3b. Note the dominance of the heteroconchs at
high and median latitudes and the dominance of the nuculoids at low latitude.
species (40% of the fauna), for Avalonia (Wales
and the Welsh Borderland) four species (31%)
and for low-latitude Gondwana (Australia) 11
species (36.7%). The most pronounced latitudi-
nal differences lie with the pteriomorphs and the
heteroconchs. For the former, Spain has two
species, one of which is a dubious record
(13.3%), Wales one (7.5%), whilst Australia has
12 (40%). For the latter, Spain has six (40%),
Wales four (31%) and Australia one (3.3%).
Thus the preference of the pteriomorphian
groups for low latitudes and heteroconchs for
high latitudes is clearly apparent by Mid-
Ordovician times. The apparently increased
diversity of the high-latitude faunas of Spain
when compared to the median latitudes of
Avalonia probably reflects nothing more than
the fact that the faunas from Spain were
collected from 87 localities, whereas those from
Wales and the Welsh Borderland were from nine
localities.
Proportions of each bivalve clade based on
abundance of individual bivalves is shown in
Figure 4b. Here again there are some differences
when compared with Figure 3b. Perhaps the
most surprising is that nuculoids are far more
abundant at low latitudes than at high latitudes:
in the equatorial latitudes of Australia they
account for no less than 59.8% of individuals.
Comparative figures for Wales and Spain are
1.2% and 18.6% respectively. Pteriomorphians
are shown to be far more abundant at low than
at high latitudes and heteroconchs are confirmed
as essentially preferring the median and high
latitudes, displaying a dramatically reduced
significance in the Australian faunas (3.3%)
when compared to either Spain or Wales, where
heteroconchs make up close to three-quarters of
individuals in the bivalve faunas.
In the Late Ordovician a second major
radiation of bivalves began; this was linked to the
development of low-latitude carbonate
platforms, particularly in Baltica and Laurentia,
but a smaller expansion of pteriomorph
groups also occurred in Kazakhstania and
Siberia. This major radiation was principally
amongst the semi-infaunal and epifaunal groups,
producing a plethora of new genera of cyrto-
dontids, ambonychiids and byssally attached
modiolopsoids, together with pterineids. The
