EARLY JURASSIC BIVALVE BIODIVERSITY 129
Fig. 2. Total extinction rates of bivalve species per million years through Early Jurassic time, (a) Andean
basins, (b) NW Europe. Key for time units as in Figure 1.
of cosmopolitans and the global extinction of
cosmopolitans. Taxa are classified as cosmopoli-
tans if they occurred in western South America
and/or NW Europe and in at least one
more region such as southern Europe, North
America, Siberia etc. Origination rates are
expressed as the number of species originating
in a substage divided by the duration of the
substage.
Finally, I calculated the rate at which species
immigrated into NW Europe and the Andean
basins, respectively. Immigration rates are
defined as the number of immigrating species
divided by the duration of the substage. To be
classified as an immigrant that utilized the
Hispanic Corridor for dispersal a species had to
be: (1) present at opposite sides of the Corridor;
(2) simultaneously absent in the western
Pacific/eastern Tethys; and (3) confined to
relatively low palaeolatitudes. Fulfilment of all
three criteria renders alternative dispersal
routes, such as migration around the periphery
of Pangaea or long-range dispersal of larvae
across the palaeo-Pacific, unlikely (see also
Aberhan 2001). For each bivalve species of the
two regions the geographic distribution was
determined by a survey of the relevant litera-
ture. This was greatly facilitated by the Jurassic
bivalve catalogue housed in Wurzburg. It
consists of copies of most figured specimens of
Jurassic bivalves along with locality and age
information.
