ORDOVICIAN BIVALVE BIOGEOGRAPHY 39
nuculoids that have a posteriorly elongate shell
and the family Malletiidae includes all the
Ordovician nuculanoids. Excluding forms
related to Ctenodonta, the earliest malletiids
appear to be from the Gondwanan Middle
Ordovician and include the genus Cadomia de
Tromelin, 1877 (de Tromelin & Lebesconte
1877), first reported from Armorica, but more
recently reported from the Middle Ordovician
of Argentina (Sanchez 1986). The genus
Myoplusia Neumayr, 1884, shows multiple
muscle scars, and other malletiids include the
endemic Australian genera Johnmartinia,
Lophoconcha, Sthenodonta and Zeehania
(Pojeta & Gilbert-Tomlinson 1977). The genera
Nuculites and Palaeoneilo are very widely
reported from the Upper Ordovician (and
younger rocks) and appear cosmopolitan, but,
as commented upon by Pojeta & Gilbert-
Tomlinson (1977, p. 16), both genera are in need
of revision and probably include several distinct
generic level taxa.
Solemyoida
The other protobranch group to appear is the
Solemyoida, known first from the early Arenig
of South Wales (Cope 1996b), although as
pointed out by Cope (1997b) the Montagne
Noire specimen figured as a Cymatonotal by
Babin (1982a, p1. 11, fig. 17) could possibly
belong here too. The genus Ovatoconcha Cope,
1996b possesses the strongly anisomyarian ante-
riorly elongate shell and radial ornamentation
typical of later solemyoids; it also apparently
had a thick periostracal fringe that projected
beyond the calcified margin of the valves, as in
Recent solemyoids (Cope 2000). Pojeta (1988)
discussed the origin of the solemyoids and
derived them from ctenodontid nuculoids; the
earliest form he identified with the solemyoids
was the Baltic species Dystactella aedilis
(Eichwald, 1856) from the early Late Ordo-
vician of Estonia. Cope (2000) showed how the
presence of ligamental nymphs in Ctenodonta
and the Solemyoida, claimed as a synapomorphy
to the two groups by Waller (1990, 1998), was
independently derived, as Mid-Ordovician
ctenodontids apparently lack the nymphs
possessed by Late Ordovician ctenodontids. The
nymphs of Ctenodonta were probably necessi-
tated in the Late Ordovician as the umbones
migrated rearwards, thus shortening the liga-
ment and making stronger insertion essential
(Cope 2000). This therefore confirmed Cope's
earlier (19976) view that Pojeta's claimed
intermediate forms between solemyoids and
ctenodontids must be persistent intermediate
stocks, as fully evolved solemyoids were present
at least as early as the Early Arenig. No pre-
Arenig bivalves are known that could indicate
the origins of Ovatoconcha.
Autolamellibranchia
Cardiolarioidea
Cope (1995) reported that the palaeotaxodont
genus Cardiolaria had a dentition that was so
similar in some respects to that of the hetero-
conch Glyptarca that they were likely to be
phylogenetically related. He suggested that the
asymmetrical hinge in Cardiolaria could have
been evolved concomitantly with the evolution
of the filibranch gill and that the hinge was
designed to permit far wider valve opening than
that permitted by the nuculoid hinge. Cope later
(19976) proposed the family Cardiolariidae for
Cardiolaria and related genera, but still retained
them within the Palaeotaxodonta. Subsequently
as further cardiolariid taxa have been recog-
nized, the Cardiolarioidea was proposed as a
superfamily (Cope 2000) and it was established
that it was then possible to classify Ordovician
bivalve groups on the basis of their possessing
either a protobranch or a more advanced gill
type, thus reflecting the principal divisions of
many zoological classifications of the bivalves. It
was clear at this point that the subclass Palaeo-
taxodonta Korobkov, 1954 was a paraphyletic
combination of protobranch nuculoids and
autolamellibranch cardiolarioids, and should be
abandoned.
The earliest cardiolarioids known hitherto are
from the Arenig, but if the ideas of Cope (1995,
19976) on the evolution of the filibranch gill are
correct, they should predate other autolamelli-
branchs. Cardiolaria itself is best known from
the Middle Ordovician, where it is widespread,
particularly across European Gondwana, from
Portugal to Bohemia and into Morocco. It is
known earlier, from the Upper Arenig of
Armorica (Babin 1966) and also occurs in the
Arenig of Guadarranque, Extramadura, Spain
(fide Babin 1993). As earlier Ordovician faunas
are still known from so few localities, its lack of
earlier occurrence is not seen as a major
problem. Cardiolaria is also known from the
early Caradoc of the Central Andean Basin of
NW Argentina and Bolivia (Sanchez 19996).
One cardiolarioid has an extraordinarily wide
latitudinal range. Cope (2000) proposed the
family Eritropidae to include the distinctive
cardiolarioid Eritropis. Eritropis has a pro-
nounced posterior carina and was first described
from the Middle Ordovician (Llanvirn Series) of
