134 MARTIN ABERHAN
Toarcian to Aalenian belong to 22 genera, and
species originating in the same time interval are
distributed over 35 genera. Since both groups
have only four genera in common (Actinostreon,
Gervillaria, Pholadomya and Plagiostoma) it
seems unlikely that immigrants gave rise to
many new species. Moreover, these four genera
are not unusually rich in species and comprise
only between one and two species each. In the
Andean basins, the corresponding numbers are
seven genera for immigrating species, 15 genera
for originating species, and the two groups do
not share a single genus.
These results demonstrate that in both regions
the origination of new species generally played a
much more important role than immigration in
controlling overall diversity. Recovery from the
Pliensbachian-Toarcian mass extinction com-
menced when origination rates increased
again, which, in the Andean basins, was in the
Aalenian and in NW Europe started in the late
Toarcian. Immigration in general, and immi-
gration through the Hispanic Corridor in
particular, cannot explain the observed recovery
in diversity.
Conclusions
The aim of the study was to test two hypotheses,
which relate extinction and recovery of Early
Jurassic bivalves to immigration of species
through the Hispanic Corridor. Both hypotheses
failed the test: the Pliensbachian-Toarcian
bivalve extinction in South America was not
caused by the immigration of species through
the Hispanic Corridor and subsequent com-
petitive replacement; and recovery from the
Pliensbachian-Toarcian extinction in NW
Europe was not a consequence of the immi-
gration of species from the eastern Pacific
through the Hispanic Corridor.
Concerning the globally increased extinction
rates of bivalves across the Pliensbachian-
Toarcian boundary, the synchrony of the biotic
crisis and voluminous continental flood basalt
eruptions, sea-level highstand, widespread
anoxia and possibly also massive release of
methane from gas hydrates suggests causal links
of these events (Palfy & Smith 2000; Hesselbo et
al. 2000). In addition to physicochemical
changes, the drop in species diversity may have
been amplified by biological factors. Thus, in the
Andean basins, the extinction peak of endemic
bivalves in the late Pliensbachian is preceded by
an origination peak in the early Pliensbachian.
The time span between these peaks (5,7 Ma)
closely corresponds to the longevity peak of
Jurassic endemic bivalves (4-6 Ma), and permits
a causal relationship between these two features
(Aberhan & Fursich 2000).
Both analysed regions differ in the dynamics
of originating and immigrating clades, and
provide evidence for the biogeographic com-
plexity of biotic recoveries. However, a common
feature of both regions is the observation
that recovery patterns of bivalves from the
Pliensbachian-Toarcian mass extinction were
largely controlled by increasing origination rates
rather than immigration. Other studies showed
that immigration intensity during the recovery
phase may vary from basin to basin. For
example, when analysing geographic variations
in the molluscan recovery from the latest-
Cretaceous extinction, Jablonski (1998)
detected a significantly larger proportion of
immigrants in the North American Gulf Coast
biota as compared to three other biogeographic
provinces. Future research should further
investigate the relative importance of immi-
gration versus radiation for the recovery of
post-extinction biotas, and whether or not
patterns differ between first-order and second-
order mass extinctions.
I thank J. Palfy, Berlin, for critically reading the
manuscript and M. Gahr. Wurzburg. for providing
unpublished information on the stratigraphic distri-
bution of Early Jurassic bivalves from Spain and
Portugal. The comments by the journal referees
C. Little and S. Damborenea are greatly appreciated.
This study was financially supported by grants from
the Deutsche Forschungsgemeinschaft (Ab 109/1-1.
Ab 109/2-1), which are acknowledged with gratitude.
Appendix 1
Stratigraphic distribution of Hettangian to Aalenian
bivalves from the Andean basins.
Taxon Range
Palaeonucula cuevitana Aberhan Het
Palaeoneilo elliptica (Goldfuss) E.Plb - L.Plb
Nuculana ovum (J. de C. Sowerby) L.Plb - L.Toa
Parallelodon groeberi Damborenea E.Plb - M.Toa
Parallelodon aff, groeberi Het
Damborenea
Parallelodon hirsonensis (d'Archiac) L.Sin - L.Plb
Paralleodon riccardii Damborenea E.Plb - L.Plb
Parallelodon sp. L.Plb - E.Toa
Grammatodon concinnus (Phillips) E.Toa - L.Toa
Grammatodon costulatus (Leanza) E.Plb - L.Plb
Grammatodon sulcatus Aberhan L.Sin - L.Sin
Grammatodon sparsicosta Aal
(Gottsche)
Grammatodon cf. toyorensis L.Plb
Hayami
Cucullaea jaworskii Leanza E.Plb - L.Plb
Cucullaea rothi Leanza E.Plb - E.Toa
Cucullaeal sp. L.Plb - L.Toa
Parainoceramus apollo (Leanza) L.Sin - L.Plb
