EARLY JURASSIC BIVALVE BIODIVERSITY 131
place before the Aalenian, but species richness
remained distinctly below the Pliensbachian
diversity maxima. In NW Europe, the diversity
minimum is situated in the middle Toarcian.
Recovery commenced in the late Toarcian, and
by Aalenian times bivalve diversity surpassed
even Pliensbachian diversity values.
Total extinction rates of endemics and cosmo-
politans, and rates of the regional disappearance
of cosmopolitans, are depicted in Figure 2. The
most prominent features of the various curves
are the marked extinction peaks in the late
Pliensbachian. In NW Europe, they reflect the
worldwide extinction of cosmopolitans and the
extinction of endemics, whereas in the Andean
basins, it was mainly the endemics that went
extinct. Albeit less markedly, elevated extinc-
tion rates continue into the early Toarcian.
Late Pliensbachian extinction peaks also are
evident in the graphs of per-species extinction
rates (Fig. 3). In contrast to Figure 2, however,
the extinction rate of NW European endemics,
and that of cosmopolitans that went extinct in
South America, reached their maxima in the
early Toarcian. This indicates that a protracted
interval of time, spanning the late Pliensbachian
and early Toarcian, showed elevated extinction
levels.
Figure 4 depicts the immigration rates for
both regions, and emphasizes the portion of
species that apparently spread through the His-
panic Corridor. Throughout the Early Jurassic,
immigration rates were relatively low, ranging
from zero to less than four species per million
years. A tendency towards increasing immi-
gration rates through time can be identified in
NW Europe. An opposite, although repeatedly
interrupted trend may be recognized in South
America.
Discussion
Dispersal through the Hispanic Corridor
As can be seen from Figure 4, none of the
bivalve species seems to have migrated through
the Hispanic Corridor before the Pliensbachian,
and from Pliensbachian times onwards migra-
tion through the Corridor apparently was in
both directions. These patterns argue for a
Pliensbachian opening of the Corridor, provid-
ing a restricted faunal exchange between the
eastern Pacific and western Tethys oceans for
the remainder of Early Jurassic time. These
results support the conclusions of Aberhan
(2001), which were obtained by the same
method (see the three criteria mentioned
above). In contrast to Aberhan (2001), who
Fig. 4. Immigration rates of bivalve species per
million years through time (Sinemurian to Aalenian).
(a) Andean basins, (b) NW Europe.
concentrated on the distribution of a single
group of bivalves in five regions, the present
analysis considers the whole bivalve fauna,
although the main focus is on only two regions.
A slightly earlier, Sinemurian time for the
effective operation of the Hispanic Corridor was
proposed by Damborenea (2000), based on
similarity coefficients at the generic level.
However, with this approach it is difficult to
preclude alternative dispersal routes. All in all,
the Hispanic Corridor seems to be a viable
feature of Early Jurassic Pangaea and, as a bio-
geographic link between two major oceans, had
the potential to influence regional diversity.
Test of the extinction hypothesis
The extinction hypothesis implies that immi-
gration of bivalves from Europe through the
Hispanic Corridor contributed to the Pliens-
bachian-Toarcian bivalve extinction in South
America (Aberhan & Fursich 1997). Figure 4
shows that immigration rates into the Andean
basins were extremly low in the early Pliens-
bachian and in the early Toarcian, whereas
