133845.pdf

(Tuis.) #1

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.

Free download pdf