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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
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