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
