132 MARTIN ABERHAN
Table 1. Relative abundance of selected Early Jurassic bivalve species in northern Chile.
Species
Camptonectes auritus (Schlotheim)
Psendopecten equivalvis (J. Sowerby)
Modiolus ventricosns (Roemer)
Plagiostoma giganteum J. Sowerby
Propeamusium pumilum (Lamarck)
Wevla alala (von Buch)
Nsanplcs
7
4
4
_
(5)
158
% samples
3.5
2.0
2.0
Relative
abundance ( % )
0.5-3.7
0.5-3.5
0.8-3.0
Rel. abundance
(mean)
2.0
1.4
1.8
Samples • number of samples in which species occurs (total number of samples = 202); %samples. percentage of
Early Jurassic samples in which species occurs; relative abundance (%). range of relative abundances of
species in samples; rel. abundance (mean), mean of relative abundances of species. Numbers in
parentheses indicate data that include information from samples with less than 70 individuals. For
comparison, the abundant bivalve Weyla alaia has been included. Data based on Aberhan (1992) and
unpublished information. For discussion see text.
moderately high values characterize the late
Pliensbachian. However, during the critical
time interval, from the early Pliensbachian to
the early Toarcian, only five species possibly
immigrated through the Corridor (Table 1).
Compared to 57 species that went extinct during
the same time interval, this group of five species
seems to be too small to contribute significantly
to the extinction.
It could be argued that a single, eurytopic and
abundant species might have the potential to
cause extinction of many other forms. Table 1
summarizes relative abundance data of species,
including the five species in question. These
data were obtained through detailed palaeo-
ecological analysis of benthic associations from
northern Chile (Aberhan 1992,1993). It becomes
evident that the first three species of Table 1
occurred in only a few samples (less than 4% of
all quantitative samples) and in low abundances
(on average 2% or less of the total fauna of a
sample). Another species, Plagiostoma gigan-
teum, occurred in very low numbers in a few
samples, which were too small to be included in
the statistical analysis of Aberhan (1992, 1993).
For comparison, the ubiquitous bivalve species
Weyla alata has been included in Table 1. It
occurred in nearly 80% of the samples and is one
order of magnitude more common than the other
faunal elements (Table 1). In the Andean basins,
the four species mentioned above can therefore
be classified as rare and geographically restricted
elements of Early Jurassic benthic communities,
and it is indeed very unlikely that they caused
competitive replacement.
An exception is the pectinoid bivalve
Propeamitssium pumilum, which becomes
locally abundant (up to nearly 30%) in
low-diversity, oxygen-controlled environments
(Table 1; Aberhan 1993). Its occurrence in the
early Toarcian coincided with the disappearance
of the morphologically related species Posi-
donotis semiplicata (Hyatt), another low-
oxygen-tolerant flat clam. P. semiplicata first
appeared in the upper Sinemurian of several
allochthonous terranes of the North American
Cordillera and by late Pliensbachian times had
spread to the Andean basins, where it persisted
as a dominant benthic element of the dysaerobic
biofacies into the early Toarcian (Aberhan &
Palfy 1996). The disappearance of P. semiplicata
in the Andean basins, followed by the appear-
ance of P. pumilum possibly represents a case
of competitive replacement, but the South
American age ranges of both species suggest
that competition was pre-emptive rather than
displacive in the sense of Hallam (1990). Also,
competition with P. pumilum cannot explain the
early Toarcian extinction of P. semiplicata in
North America, since P. pumilum seems to be
absent from that region.
It is beyond the scope of the present study to
assess the immigration of predators through an
opening Hispanic Corridor and their potential
impact on the regional diversity of bivalves.
Nevertheless it is worth mentioning that preda-
tion pressure apparently increased strikingly in
the Late Cretaceous and Cenozoic following
adaptive radiation of neogastropods, crabs and
teleost fishes (Vermeij 1977), and in this respect
post-dates the Early Jurassic diversity crisis by
100 million years and more. In summary, immi-
gration into the Andean basins of bivalve
species through the Hispanic Corridor, followed
by competitive replacement, obviously did not
contribute to the observed drop in diversity.
