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48 JOHN C. W. COPE

anomalodesmatans, which are primarily (but

not exclusively) infaunal, were also more diverse

at low latitudes, as too were the protobranch

groups. The rapid diversification of bivalves in

the Late Ordovician produced a large number of

species as well as genera, but there has been little

revisionary work on the faunas. For this reason,

Figure 3c shows the number of genera in the

Late Ordovician in three low-latitude areas.

Insufficient is known of higher-latitude faunas to

make meaningful comparisons.

The low-latitude shelves contained a high

proportion of Late Ordovician endemic genera.

Cope & Babin (1999) noted that out of 53

genera from the Laurentian Late Ordovician,

over 40% were endemic. However, some low-

latitude continental shelves were evidently in

sufficient proximity to each other to allow inter-

change of genera. For example the strongly

ribbed Paraphtonia Khalfin, 1958 from Kaza-

khstan also occurs in Siberia (Krasilova 1979)

and genera such as Cyrtodonta are close to

cosmopolitan. In stark contrast to these rich

low-latitude faunas, the Late Ordovician faunas

of high latitudes are quite impoverished and

entirely dominated primarily by nuculoids;

there are a few heteroconchs, but very few

pteriomorphs or anomalodesmatans. Although

there has been a lack of recent work on high-

latitude Late Ordovician faunas, it appears that

rates of endemism are lower, with Gondwana at

about 30% and Avalonia at about 14% (Cope &

Babin 1999).

These factors strongly suggest that latitudinal

temperature differences became more pro-

nounced in the Late Ordovician, heralding the

Late Ashgill glaciation. This glaciation pro-

duced a major eustatic sea-level fall that

exposed the low-latitude carbonate platforms.

The resulting extinction event had a profound

effect on bivalve stocks. The greatest losses were

amongst the epifaunal and semi-infaunal

groups, with major extinctions amongst the

cyrtodontids, ambonychiids and pterineids.

Other groups to be affected were the modi-

olopsids, amongst which the extinction of

the coral-boring genera Corallidomus and

Semicorallidomus ended a mode of life that was

not to re-evolve until the Late Jurassic. On the

other hand, at higher latitudes, the nuculoid-

dominated communities appeared to have

suffered least by the sea level fall, the infaunal

forms adapting to the new sea-levels with little

apparent difficulty; several genera survived

apparently little changed into the Silurian.

There has been very little recent work pub-

lished on Silurian bivalve faunas; this is a pity

as the bivalves have the potential to provide

much information about the recovery from the

latest-Ordovician mass extinction. One fauna

that has been studied is that of the Silurian of

Wales and the Welsh Borderland (Ratter 1999).

which is largely unpublished. Ratter's studies

show that in the Llandovery Series these mid-

latitude Avalonian faunas, predominantly from

siliciclastic facies, are dominated by nuculoids,

which make up half of the total of 18 recorded

species. The remainder of the bivalve fauna

consists, in order of diminishing importance, of

anomalodesmatans, heteroconchs and trigo-

nioids; there are no pteriomorphians. Diversity

levels appear much lower in the Early Silurian

and it seems that the Ashgill extinctions essen-

tially reduced the bivalves to something like

their Mid-Ordovician levels of diversity.

Thanks are due to L. C. Norton and L. E. Popov. both

of the Geology Department, National Museum of

Wales, for assistance in drafting of figures 1. 3 and 4.

and figure 2, respectively.

References

BABIN. C. 1966. Mollusques bivalves et cephalopodes
du Paleozoique armoricain. Imprimerie Commer-
ciale et Administrative. Brest.
BABIN. C. 1977. Etude comparee des genres Babinka
Barrande et Coxiconcha Babin (Mollusques
bivalves de l'Ordovicien) Interet phylogenetique.
Geobios. 10. 51-79.
BABIN. C. 1982a. Mollusqes bivalves et rostroconches.
In: BABIN. C.. COURTESSOLE. R.. MELOU. M..
PILLET, J.. VIZCAINO, D. & YOCHELSON. E. L. (eds)
Brachiopodes (articules) et mollusques (bivalves,
rostroconches, monoplacophores, gastropodes) de
I'Ordovicien inferieur (Tremadocien-Arenigien)
de la Montagne Noire (France meridionale).
Memoire de la Societe des Etudes Scientifiques
de l'Aude. Sival. Carcassonne. 37-49. pls 8-12.
BABIN. C. 1 982b. Tromelinodonta nov. gen.. Bivalve
lyrodesmatide (Mollusca) de l'Arenigien
(Ordovicien inferieur). Geobios. 15. 423—427.
BABIN. C. 1987. Pseudarca. Mollusque Bivalve sin-
gulier de l'Ordovicien superieur du Massif
armoricain. Annales de la Societe Geologique du
Nord. 105, 253-258.
BABIN. C. 1993. Role des plates-formes gondwani-
ennes dans la diversification des mollusques
bivalves durant l'Ordovicien. Bulletin de la
Societe geologique de France. 164. 141-153.
BABIN. C. 2000. Ordovician to Devonian diversifica-
tion of the Bivalvia. American Malacological
Bulletin. 15. 167-178.
BABIN. C. & BRANISA. L. 1987. Ribeiria. Peelerophon y
otros moluscos del Ordovicico de Bolivia. IV
Congreso Latinamerico de Paleontologia, Santa
Cruz. 1. 119-129.
BABIN. C. & DESTOMBES. J. 1992 (dated 1990).
Les Mollusques Bivalves et Rostroconches
ordoviciens de l'Anti-Atlas marocain: Interet
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