Opening of the Hispanic Corridor and Early Jurassic bivalve
biodiversity
MARTIN ABERHAN
Museum fur Naturkunde, Institut fur Palaontologie, Invalidenstr. 43, D-10115
Berlin, Germany, (e-mail: [email protected]
Abstract: The Hispanic Corridor is a postulated marine seaway linking the eastern Pacific
and western Tethyan oceans as early as Early Jurassic times. Two existing hypotheses
relate the Pliensbachian-Toarcian bivalve extinction and recovery to immigration of
bivalve species through the Hispanic Corridor. The extinction hypothesis implies that, in
South America, the Pliensbachian-Toarcian extinction can be partly explained by the
immigration of bivalves through the Hispanic Corridor and subsequent competitive
replacement. The recovery hypothesis states that, in NW Europe, the renewed rise in diver-
sity in the late Toarcian/Aalenian was largely a consequence of immigration of taxa from
Andean South America via the Hispanic Corridor.
To test these hypotheses, I calculated immigration and origination rates of bivalves per
million years. In both regions, early Pliensbachian to Aalenian immigration rates remained
at low levels, thus disproving both hypotheses. By comparison, the origination of new
species generally played a much more important role than immigration in controlling
overall diversity of both regions. Future research should investigate if this is a more general
pattern in the recovery of post-extinction biotas.
The apparently global Pliensbachian-Toarcian diversity crisis may be best explained by
a combination of physicochemical factors, invoking intense volcanism, sea-level highstand
and widespread anoxia, as well as biological factors. Recovery from this mass extinction
commenced when origination rates increased again, which, in the Andean basins, was in
the Aalenian and in NW Europe, the late Toarcian.Investigations of the history of diversity and of
evolutionary processes that generate biodiver-
sity not only improve our knowledge of the fossil
past, but may contribute significantly to our
understanding of the course and consequences
of the modern biodiversity crisis. A promising
palaeontological approach is the analysis of the
geographic variation of patterns observed
during extinction episodes and their subsequent
recoveries. In the present study, I analyse the
biogeography of Early Jurassic bivalves with the
aim of testing two hypotheses existing in the
literature. These hypotheses relate the Pliens-
bachian-Toarcian mass extinction to the
opening of a marine connection (the so-called
Hispanic Corridor) between the eastern Pacific
and western Tethyan oceans. The first hypothe-
sis states that, in South America, the preferential
disappearance of endemic bivalves across the
Pliensbachian-Toarcian boundary can be partly
explained by immigration of cosmopolitan
species via the Hispanic Corridor and subse-
quent competitive replacement (Aberhan &
Fursich 1997). The second hypothesis states that,
in NW Europe, the renewed rise in bivalve
diversity in the late Toarcian/Aalenian was
largely a consequence of immigration of taxa
from Andean South America through the
Hispanic Corridor, presumably filling the eco-
space vacated during the extinction event
(Hallam 1983, 1996; Hallam & Wignall 1997). I
demonstrate that neither of the two hypotheses
can be sustained. Whilst the extinction may be
best explained by a combination of global
palaeoenvironmental changes and biological
processes, the recovery appears to be largely
controlled by increasing within-region origina-
tion rates rather than immigration of already
existing species.
Early Jurassic bivalve biodiversity and the
Hispanic Corridor
Their high abundance and taxonomic diversity
since early Mesozoic times, along with an
excellent fossil record, make bivalves an ideal
group for palaeodiversity analysis in the marine
realm. At the species level, Early Jurassic
bivalve diversity is known in detail from NW
European epicontinental seas (Hallam 1986,
1987) and from western South America
(Damborenea 1996; Aberhan & Fiirsich 1997,
2000), where bivalves occur very abundantly in a
series of backarc basins, the so-called Andean
basins. In both regions, bivalves experienced a
From: CRAME, J. A. & OWEN, A. W. (eds) 2002. Palaeobiogeography and Biodiversity Change: the Ordovician
and Mesozoic-Cenozoic Radiations. Geological Society, London, Special Publications, 194,127-139.
0305-8719/02/$15.00 © The Geological Society of London 2002.