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(^128) MARTIN ABERHAN


considerable drop in species diversity from late

Pliensbachian to early Toarcian times. This

Pliensbachian-Toarcian extinction apparently

was of global extent, clearly exceeds background

levels of extinction and also affected several

other groups of organisms at various taxonomic

ranks (e.g. Raup & Sepkoski 1984; Hallam 1987;

Little & Benton 1995; Aberhan & Fursich 1997).

For these reasons the Pliensbachian-Toarcian

extinction can be termed a second-order mass

extinction.

The Hispanic Corridor is a postulated,

shallow marine connection between the eastern

Pacific and the western Tethys, which preceded

the birth of the Atlantic Ocean by many millions

of years (Smith 1983). Although there is little

direct geological evidence for its existence,

palaeontological data suggest that a narrow

seaway was established by Early Jurassic times

(e.g. Damborenea & Mancenido 1979; Hallam

1983; Smith & Tipper 1986). Biogeographic

analysis of Early Jurassic pectinoid bivalves,

combined with a review of the relevant

literature, supports the view that the Corridor

presumably was open from Pliensbachian times

onwards (Aberhan 2001). The apparently

bidirectional faunal exchange through the

Corridor is consistent with the establishment of

a megamonsoonal circulation for Pangaea,

which may have caused seasonal alternation of

flow directions within the Corridor (Aberhan

2001). Thus, the Hispanic Corridor was in

operation during the Early Jurassic extinction

and recovery intervals, and could have pro-

moted changes in regional diversity at opposite

ends of the Corridor.

Database and methods

This study is based on species-level data for

bivalves from two regions, western South

America (Colombia, Peru, Chile, Argentina)

and NW Europe (Great Britain, Sweden,

Denmark, Belgium, Luxemburg, Germany,

Switzerland, northern France). The South

American data (see Appendix 1) are based on

extensive field work and many years of detailed

taxonomic work (e.g. Aberhan 1994) and also

include a wealth of literature citations. The NW

European data (see Appendix 2) mainly stem

from the work of Hallam (1976, Appendix; 1987,

Appendix). This database was modified to

include recent work by Johnson (1984), Hodges

(1991, 2000), Muster (1995), Rohl (1998) and

Harries & Little (1999), and also takes into

account my examination of museum collec-

tions in The Natural History Museum,

London; GoldfuB-Museum, Bonn; Museum fur

Fig. 1. Bivalve diversity (expressed as number of
species) for various Jurassic stages and substages. (a)
Andean basins, (b) NW Europe. Het.. Hettangian;
Aal.. Aalenian.

Naturkunde, Berlin; Sedgwick Museum,

Cambridge; and Staatliches Museum fur

Naturkunde, Stuttgart. Whilst the NW Euro-

pean data are resolved to zonal level, the South

American data were pooled in substages due

to the lack of more precise biostratigraphic

information. To permit a comparison, the data

were processed at the substage level for both

regions.

Species diversity is expressed as the number

of species per substage. Total extinction rates for

the various substages are calculated as the

number of species making their last appearance

divided by the duration of the substage (sub-

stage durations after Gradstein et al. 1994).

Per-species extinction rates, analogous to Raup

& Sepkoskrs (1984) per-family extinction rate,

are determined as the total extinction rate

divided by the number of species extant during

the substage. For each region, extinction metrics

are calculated separately for the extinction of

endemic bivalves, the regional disappearance
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