Diversification and biogeography of bivalves during the Ordovician
Period
JOHN C. W. COPE
Department of Earth Sciences, Cardiff University, PO Box 914, Cardiff CF10 3YE, UK
(e-mail: [email protected])
Abstract: Bivalves have a wide distribution in the Lower and to a lesser extent Middle
Cambrian rocks, but they have not yet been certainly identified in the Upper Cambrian.
Recent discoveries have significantly increased our knowledge of Lower Ordovician
bivalve faunas and their explosive radiation from the Early Ordovician apparently
coincides with the evolution of the feeding gill. Early Ordovician faunas were confined to
the siliciclastic facies of Gondwanan shelf seas; most genera were clearly latitudinally
constrained, but others apparently migrated over wide latitudes. By the Mid-Ordovician,
bivalves had begun to escape the confines of Gondwana and marked latitudinal differences
in the composition of the faunas became apparent, with pteriomorphians showing clear
preference for low latitudes, whilst heteroconchs clearly preferred median to high latitudes;
surprisingly, nuculoids were both most diverse in terms of species and most abundant as a
percentage of individuals within the bivalve populations at low latitudes. It was in the Late
Ordovician that bivalves colonized the low-latitude carbonate platforms of Laurentia and
Baltica, leading to a second major diversification particularly within the pteriomorphian
bivalves, which developed semi-infaunal and epifaunal habits; they became the dominant
low-latitude bivalve group. The latest-Ordovician eustatic regression which exposed the
low-latitude carbonate platforms resulted in a major reduction in the epifaunal and semi-
infaunal bivalves involving extinction of many genera, including the only Ordovician boring
bivalves.Thirty years ago Pojeta (1971) produced his
classic work on Ordovician bivalves that illus-
trated something of the early diversity of this
long-neglected fossil group. At the time of this
publication, some revisionary work had been
done on Late Ordovician faunas from North
America (e.g. Pojeta 1962, 1966), but save the
works on French faunas of the Montagne Noire
(Thoral 1935) and Armorica (Babin 1966) little
work had been done in the first 70 years of the
twentieth century on Early Ordovician bivalve
faunas. Phylogenetic links were necessarily
sketchy (Pojeta 1971) and Cambrian bivalves
were of doubtful interpretation. Pojeta's (1971)
reservations about Lamellodonta, the earliest
bivalve accepted by the Treatise (Cox et al.
1969-1971), were confirmed when the genus was
shown to be based upon a distorted brachiopod
(Havlicek & Kriz 1978). Pojeta later (1973,
1975) went on to demonstrate the bivalve affini-
ties of the widely dispersed Lower Cambrian
Fordilla, whilst Jell (1980) described another
Early Cambrian genus Pojetaia from Australia,
subsequently found in Bornholm, China and
Morocco and found to extend into the Mid-
Cambrian. Other Mid-Cambrian forms have
been described from New Zealand (McKinnon
1982), Bornholm (Berg-Madsen 1987; Hinz-
Schallreuter 1995, 2000) and Morocco (Geyer &
Streng 1998). The lack of bivalves from the
Upper Cambrian remains an embarrassing
hiatus in the study of early bivalve evolution.
Berg-Madsen (1987) recorded a Tuarangia from
a Polish erratic that contained conodonts she
regarded as Upper Cambrian, but Hinz-Schall-
reuter (2000) pointed out that tricuspidate west -
ergaardodinids also occur in the Middle
Cambrian. Pojeta (1980) figured a rather fea-
tureless shell from the Upper Cambrian of
Maryland, but this single specimen has no
uniquely bivalve characteristics. Pojeta (2000)
has recently reviewed the Cambrian history of
the Bivalvia.
Lower Ordovician bivalves are much more
diverse, much larger in size and more common
as fossils. Cope (1995, 1997b) correlated these
factors with the evolution of the feeding gill in
the latest Cambrian or earliest Ordovician.
Bivalves remain, however, very rare fossils in
Lower Ordovician rocks. Thus from the
Tremadoc, bivalves are known from four areas:
Salta Province, Argentina (Harrington 1938);
the Montagne Noire (Thoral 1935; Babin
1982a); central Australia (Pojeta & Gilbert-
Tomlinson 1977; originally described as
probably of Arenig age but later shown to be
late Tremadoc by Shergold et al. 1991) and
the Moroccan Anti-Atlas (pers. obs. 2001).
From: CRAME, J. A. & OWEN, A. W. (eds) 2002. Palaeobiogeography and Biodiversity Change: the Ordovician
and Mesozoic-Cenozoic Radiations. Geological Society, London, Special Publications, 194, 25-52.
0305-8719/02/$15.00 © The Geological Society of London 2002.