133845.pdf

(Tuis.) #1
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


  • (2.4)
    78.2


Relative
abundance ( % )

0.5-3.7
0.5-3.5
0.8-3.0


  • (0.7-28.6)
    0.5-69.4


Rel. abundance
(mean)

2.0
1.4
1.8


  • (15.0)
    10.0


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unpublished information.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.
Free download pdf