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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
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