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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
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