ORDOVICIAN BIVALVE BIOGEOGRAPHY 45
and Siberia; Cuneamya is also recorded from
Kazakhstania (Khalfin 1958). Cuneamya
appears earliest, however, in the Middle
Ordovician of Argentina (Sanchez 1990).
Another Ordovician group may also be
anomalodesmatans: this is the elongate orthono-
tians which were elevated to subclass level by
Pojeta (1978). Of the Ordovician genera Pojeta
(1978) included in the Orthonotia, Psiloconcha
was later transferred to the Solemyoida (Pojeta
1988) and the elongate Cymatonota, Soleno-
morpha and Prothyris would now be regarded
by most workers as anomalodesmatans. Brev-
iorthonota Krasilova, 1979, from the Upper
Ordovician of Siberia, also belongs here. The
earliest possible Cymatonota was recorded from
the Early Arenig of the Montagne Noire by
Babin (1982a) but as discussed above, there is a
possibility that this specimen may be a solemyid.
The orthonotids were included in the Anom-
alodesmata by Cox et al. (1969).
Waller (1990) suggested that the ligamental
nymphs and outer prismatic shell layer of the
modiomorphoids suggested that they were
linked to the anomalodesmatans and this sup-
ported Carter & Aller's (1975) suggestion that
the calcified spicules of the modiomorphoid
periostracum indicated that their affinities lay
with the anomalodesmatans. Fang & Morris
(1997) showed that the modiomorphoideans as
defined by Cox et al. (1969) included both the
Devonian modiomorphoids and the Ordo-
vician-Silurian modiolopsoids, a group with
which they had frequently been confused, but
which lacked the ligamental nymphs. The latter
are now regarded as pteriomorphians and are
discussed above.
Summary and conclusions
Although fairly widely dispersed during the
Early and Mid-Cambrian, it is suspected that
during the Late Cambrian bivalves may have
become restricted to Gondwana, but they are so
far unknown with certainty from that Epoch.
In the Tremadoc Epoch of the Ordovician
Period bivalves are known from Gondwanan
high latitudes (Morocco, Montagne Noire),
median latitudes (Argentina) and low latitudes
(Australia). The most diverse fauna so far
discovered is that of the Amadeus Basin in
Australia (Pojeta & Gilbert-Tomlinson 1977)
which already contains pteriomorphians that are
not known with certainty from higher latitudes
in rocks of that age. The Early Ordovician initial
diversification of the bivalves was restricted to
Gondwana (including Avalonia at this time) and
by the Mid-Arenig all the principal clades of
bivalves had evolved. Some forms were
decidedly high-latitude in distribution, whilst
others were just as clearly confined to low
latitudes; most areas had some endemic genera.
Comparison between the high latitude of the
Montagne Noire and the lower latitude of
Avalonia for the Early Arenig (Fig. 3a) shows a
far greater diversity in Avalonian latitudes.
Avalonia has at least 18 genera belonging to all
the major bivalve groups, with great diversity of
pteriomorphians and to a lesser extent hetero-
conchs, whereas nuculoids and heteroconchs are
equally important in the less diverse faunas of
higher latitudes. Even though Avalonian was
not far north of the Gondwanan margin in Early
Arenig times, the difference in pteriomorphian
diversity is pronounced. It is a pity that there are
no known low-latitude faunas of this age with
which to make further comparisons. It thus
seems that climatic gradients played some part
in controlling bivalve diversity in the Early
Ordovician, but the fact that a few genera occur
at both high and median latitudes suggests that
such gradients may not have been particularly
strong at this time. In some cases the gradual
migration of a species across Gondwana can be
tracked through time, whilst in other cases the
same genus appears at widely separated areas,
or latitudes, simultaneously.
Figure 4a shows abundance of bivalve species
belonging to each major clade for the faunas of
South Wales and the Montagne Noire; compari-
son of Figure 3a with Figure 4a shows some
surprising differences. Most notable for Wales is
the fact that the diverse pteriomorphians make
up only 9% of individuals, with the fauna totally
dominated by heteroconchs (76.6%). For the
Montagne Noire the fauna is again dominated
by heteroconchs (56.6%) whilst the pterio-
morphians make up only 0.4% of the bivalve
population there.
Early Ordovician bivalves seem particularly
controlled by facies, occurring virtually exclus-
ively in siliciclastic sediments and being most
abundant in very shallow-water silty muds
(Cope & Babin 1999). Even the deeper-water
facies, such as the muds of the Montagne Noire,
were certainly deposited in water less than 50 m
deep (Babin 2000). These Early Ordovician
bivalves were predominantly infaunal, although
there may have been some semi-infaunal forms.
In the Mid-Ordovician bivalves managed to
migrate from Gondwana and reached both
Baltica and the Laurentian margin in early Mid-
Ordovician times. Once established here they
spread to reach all continental shelves by the
earliest part of the Late Ordovician. Mid-
Ordovician faunas are thus, again, virtually
