130 MARTIN ABERHAN
Fig. 3. Per-species extinction rates of bivalves per million years through Early Jurassic time, (a) Andean basins,
(b) NW Europe. Key for time units as in Figure 1.
Within the temporal accuracy of the present
study, several species (11 in South America and
14 in NW Europe) seem to have their first
appearance simultaneously in different regions.
For the regions concerned it remains unclear
whether these species belong to the category of
originating or that of immigrating species. When
counting the numbers of originating and of
immigrating species these taxa were omitted.
Since their percentages in the various time
intervals are relatively low (0% to 8.5% in South
America and 1.6% to 6.1% in NW Europe), it is
unlikely that this procedure strongly distorted
the comparison of origination rates and
immigration rates.
Patterns of diversity, extinction and
immigration
Figure 1 shows the diversity of bivalves over the
stages and substages of the Early Jurassic for the
Andean basins and for NW Europe. Both
regions show similar diversity trends with
increasing values from the earliest Jurassic to a
peak in the late Pliensbachian, followed by a
sharp drop in the early Toarcian. In South
America, this decline is continued, albeit less
markedly, into the middle Toarcian and finally
the late Toarcian, where the diversity reaches
a relative minimum. Recovery from this
Pliensbachian-Toarcian extinction did not take
