EARLY JURASSIC BIVALVE BIODIVERSITY^133
Test of the recovery hypothesis
In NW Europe immigration seems to have
played a certain role in the recovery of the
bivalve fauna (Fig. 4). Following the Pliens-
bachian-Toarcian extinction, immigration rates
were moderately high in the middle Toarcian and
reached a peak in the Aalenian. However, immi-
gration from other regions, in particular from
southern Europe, was more important than
immigration from the eastern Pacific ocean
through the Hispanic Corridor. In the middle
Toarcian, all six species that immigrated into NW
Europe (Ctenostreon rugosum (Smith)*,
Entolium corneolum (Young & Bird)*, Eopecten
spondyloides (Roemer), Nuculana ovum
(J. de C. Sowerby)*, Palaeonucula hammeri
(Defranee) and Pseudopecten barbatus
(J. Sowerby)) also occurred in early Toarcian or
older sedimentary rocks of southern Europe.
Only three of them (those marked with an
asterisk) are known from eastern Pacific areas,
thus reducing the potential candidates that might
have utilized the Hispanic Corridor for migration
to a relatively low number. Moreover, the middle
Toarcian increase in immigration rates did not set
off a recovery since bivalve diversity fell to a
minimum in the middle Toarcian (Fig. 1). Simi-
larly, of the seven species immigrating into NW
Europe during the late Toarcian (Dacryomya
lacryma (J. de C. Sowerby), Inoperna sower-
byana (d'Orbigny), Modiolus imbricatus (J.
Sowerby), Nicaniella voltzii (Honighaus), Paral-
lelodon hirsonensis (d'Archiac)*, Plagiostoma
rodburgensis (Whidborne) and Pseudolimea
duplicata (J. de C. Sowerby)*), only the two
marked with an asterisk fulfil the criteria to rank
as an immigrant through the Hispanic Corridor.
In Aalenian times, immigration again was
largely from southern Europe rather than from
eastern Pacific areas. From nine species immi-
grating into NW Europe (Actinostreon
gregareum (J. Sowerby)*, Arcomytilus pectina-
tus (J. Sowerby), Camptonectes laminatus
(J. Sowerby), Ceratomya concentrica (J. de C.
Sowerby), Gervillaria alaeformis (J. Sowerby),
Gresslya peregrina (Phillips)*, Osteomya dilata
(Phillips), Parainoceramus obliquus (Morris &
Lycett) and Pholadomya fidicula J. de
C. Sowerby*), only three (those marked with an
asterisk) apparently immigrated through the
Hispanic Corridor. So there is little supporting
evidence for the hypothesis that immigration of
species through the Hispanic Corridor was
responsible for the diversity increase observed
in the late Toarcian and Aalenian. However,
from Figure 4 alone the relative importance of
immigration for recovery is not apparent.
Fig. 5. Immigration and origination rates of bivalve
species per million years for various Jurassic stages
and substages. (a) Andean basins, (b) NW Europe.
The importance of immigration versus
radiation for recovery
To put the contribution of immigration for
biodiversity into perspective, I compared
immigration rates of bivalve species to origina-
tion rates (Fig. 5). In both regions, Early Jurassic
immigration rates usually remained distinctly
below origination rates. Only in the time interval
immediately following the Pliensbachian-
Toarcian extinction was this pattern reversed,
and the number of immigrant species exceeded
that of newly originating species. However,
middle Toarcian immigration rates were not
significantly higher than at other times in the
Early Jurassic and certainly were not high
enough to cause an increase in total bivalve
diversity (Fig. 1).
It remains possible that immigrants per se did
not add significantly to the recovery in both
regions, but that, after immigration, they
evolved into new species and thus promoted a
diversity increase. The latter group would then
be counted in the category of originating species,
and would have blurred the relative importance
of immigrants. Precise knowledge of the
phylogeny of a group is a prerequisite for
recognizing such relationships, but this is rarely
known for Jurassic bivalves. Nevertheless, some
tentative statements can be made. Species
immigrating into NW Europe from the middle