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36 JOHN C. W. COPE

Altogether some nine or ten species are known

so far from Tremadoc rocks. From the lower part

of the Arenig Series, bivalves are known from

Salta Province, Argentina (Harrington 1938);

the Montagne Noire (Babin 1982a); Afghanis-

tan (Desparmet et al. 1971); originally reported

as Tremadoc but more probably of Arenig age;

Babin & Gutierrez-Marco 1991); the Moroccan

Anti-Atlas (Babin & Destombes 1992); Ramsey

Island, South Wales (Hicks 1873; Carter 1971);

and most recently from Llangynog, South Wales

(Cope 1996b), the latter locality yielding no

fewer than 20 species belonging to 18 genera,

representing well over half the species known

hitherto from rocks of this age. After the Arenig,

bivalves become commoner and Babin &

Gutierrez-Marco (1991) were able to report on

Middle Ordovician bivalves collected from no

fewer than 87 localities from Spain alone.

All the Early Ordovician bivalves known are

from Gondwana and it now seems clear

that they were thus geographically restricted

(Avalonia being so close to the Gondwanan

margins in Arenig times that, for the purposes of

palaeobiogeography, it can still be considered as

part of Gondwana). It is assumed that, although

they were able to migrate freely around the

Gondwanan margins, the width of the oceanic

areas around Gondwana precluded bivalve

larvae from crossing to other continental shelves

before the Mid-Ordovician.

The major radiations of the Early Ordovician

resulted in the appearance of most bivalve

groups by the end of that time. Cope (2000)

proposed a revised classification of the Bivalvia

recognizing that the division into the subclasses

based on gill grade, Protobranchia Pelseneer,

1889 and Autolamellibranchiata Grobben, 1894,

long used by zoologists, could now be applied to

the fossil record. The classification of the

Bivalvia used in this paper is essentially that of

Cope (2000) shown in Figure 1. The only differ-

ence between that figure and Cope (2000, fig. 2)

is that the Nucinelloidea are not shown, as they

do not appear until well after the Ordovician.

Continental reconstructions for the Ordovician

are shown in Figure 2.

Protobranchia

Nuculoida

Amongst the protobranchs the Nuculoida are

well represented in the Early Ordovician and

range from low-latitude areas like Australia, to

high-latitude areas such as the Montagne Noire

and the Moroccan Anti-Atlas. The simplest

forms, with undifferentiated taxodont dentition,

Fig. 1. Classification of the Bivalvia adopted herein,
showing the phylogenetic links between the major
bivalve groups. Modified after Cope (2000).

belong to the family Praenuculidae McAlester,

1969 (in Cox et al. 1969-1971). The earliest

praenuculids include forms like Paulinea and

Pemarnia from the Early Arenig of South Wales

(Cope 1996b); the latter also occurs in the Late

Arenig of the Welsh Borderland (Cope 1999).

Some other praenuculids also seem to be of

restricted geographical range, such as Fidera

Pojeta & Gilbert-Tomlinson, 1977, from the

Middle to Upper Ordovician of Tasmania.

However, Praenucula Pfab, 1934 itself occurs

widely in the Middle Ordovician of Europe from

Bohemia (e.g. P. expansa Pfab, 1934) westwards

to Portugal (e.g. P. riheiro (Sharpe, 1853)); and

has also been recorded from the Lower, Middle

and Upper Ordovician of the Moroccan Anti-

Atlas (Babin & Destombes 1992). Praenucula

and the praenuculid Palaeoconcha were

recorded from the upper Middle or lower Upper

Ordovician of Argentina by Sanchez (1990).

Dzik (1994) figured a minute juvenile nuculoid

from the Lower Llanvirn of Baltica that appears

to be a Praenucula, as indicated by Babin (2000);

this is notable in another context, as it is one of

the earliest bivalves to have reached Baltica.

Praenucula also occurs in the Upper Ordovician

of Laurentia and P.filistriata (Ulrich, 1894) from

Ohio was figured under the generic name
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