(^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.