48 JOHN C. W. COPE
anomalodesmatans, which are primarily (but
not exclusively) infaunal, were also more diverse
at low latitudes, as too were the protobranch
groups. The rapid diversification of bivalves in
the Late Ordovician produced a large number of
species as well as genera, but there has been little
revisionary work on the faunas. For this reason,
Figure 3c shows the number of genera in the
Late Ordovician in three low-latitude areas.
Insufficient is known of higher-latitude faunas to
make meaningful comparisons.
The low-latitude shelves contained a high
proportion of Late Ordovician endemic genera.
Cope & Babin (1999) noted that out of 53
genera from the Laurentian Late Ordovician,
over 40% were endemic. However, some low-
latitude continental shelves were evidently in
sufficient proximity to each other to allow inter-
change of genera. For example the strongly
ribbed Paraphtonia Khalfin, 1958 from Kaza-
khstan also occurs in Siberia (Krasilova 1979)
and genera such as Cyrtodonta are close to
cosmopolitan. In stark contrast to these rich
low-latitude faunas, the Late Ordovician faunas
of high latitudes are quite impoverished and
entirely dominated primarily by nuculoids;
there are a few heteroconchs, but very few
pteriomorphs or anomalodesmatans. Although
there has been a lack of recent work on high-
latitude Late Ordovician faunas, it appears that
rates of endemism are lower, with Gondwana at
about 30% and Avalonia at about 14% (Cope &
Babin 1999).
These factors strongly suggest that latitudinal
temperature differences became more pro-
nounced in the Late Ordovician, heralding the
Late Ashgill glaciation. This glaciation pro-
duced a major eustatic sea-level fall that
exposed the low-latitude carbonate platforms.
The resulting extinction event had a profound
effect on bivalve stocks. The greatest losses were
amongst the epifaunal and semi-infaunal
groups, with major extinctions amongst the
cyrtodontids, ambonychiids and pterineids.
Other groups to be affected were the modi-
olopsids, amongst which the extinction of
the coral-boring genera Corallidomus and
Semicorallidomus ended a mode of life that was
not to re-evolve until the Late Jurassic. On the
other hand, at higher latitudes, the nuculoid-
dominated communities appeared to have
suffered least by the sea level fall, the infaunal
forms adapting to the new sea-levels with little
apparent difficulty; several genera survived
apparently little changed into the Silurian.
There has been very little recent work pub-
lished on Silurian bivalve faunas; this is a pity
as the bivalves have the potential to provide
much information about the recovery from the
latest-Ordovician mass extinction. One fauna
that has been studied is that of the Silurian of
Wales and the Welsh Borderland (Ratter 1999).
which is largely unpublished. Ratter's studies
show that in the Llandovery Series these mid-
latitude Avalonian faunas, predominantly from
siliciclastic facies, are dominated by nuculoids,
which make up half of the total of 18 recorded
species. The remainder of the bivalve fauna
consists, in order of diminishing importance, of
anomalodesmatans, heteroconchs and trigo-
nioids; there are no pteriomorphians. Diversity
levels appear much lower in the Early Silurian
and it seems that the Ashgill extinctions essen-
tially reduced the bivalves to something like
their Mid-Ordovician levels of diversity.
Thanks are due to L. C. Norton and L. E. Popov. both
of the Geology Department, National Museum of
Wales, for assistance in drafting of figures 1. 3 and 4.
and figure 2, respectively.
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