Catalogue of the Living Bivalvia of the Eastern Pacific Ocean ...

Geographic ranges were obtained by computer processing
records compiled by one degree blocks of the coastline segregated
in several depth categories. All end points were confirmed
by examination of material: in the few situations
where this was not possible, the literature records were
accepted only if there is only a small possibility of error of
identification. Even so, terminal ranges may frequently be
collecting artifacts , rather than true end points, though the
records from the central American region are fairly reliable
thanks to good coverage in the Los Angeles County
Museum, the Allan Hancock Foundation and the Stanford
(now in the California Academy of Science) collections. The
situation is less stable in the poorly collected Bering Sea and
the Chilean fauna.
The median latitude, the arithmetic mean of the north and
south limits of distribution, may be a useful comparative
index. Schenck & Keen (1936) proposed a faunistic analysis
based on mid-points on the assumption that 'provinces' are
indicated by clustering of mid-points at their centre. It is now
accepted that centres of distribution do not necessarily coincide
with the geographical centre (Newell 1948). The value
of end-point analysis is not as much a pioneer attempt to use
objective criteria for biogeographic province delimitation, as
a tool for comparing faunas, particularly Pleistocene assemblages
(Schenck & Keen 1940, largely repeated by Schenck
1945), when bathymetrical and ecological conditions are
taken into account. The use of mid-point analysis and correction
factors for extant faunal displacement, will be fully
discussed in a forthcoming paper (Bernard & McKinnell
MS).
The temperature range given for each species is an attempt
to supply another index useful for estimating paleotemperatures
of Pleistocene bivalve faunas. The shown values were
generated by computer from data entered by depth. The
calculated value is the mean of the closest adjoining records,
ignoring sharp thermoclines or micro-regional climates, so at
best it is a compromise, subject to revision as more precise
data become available. Data were obtained from Love (1971,
I 972a, b, 1973), Gorshov (1974), Muromtsev (1963),
Stevenson et al. (1970), Kuksa (1978) and the U.S. Department
of Commerce (1970). Averaging the temperatures of
the species occurring probably will not yield a defendable
estimate of temperature, but the proportion of thermophilic
or cryophilic species will indicate the minimal temperature
range.
The geological range for each species is noted using standard
Pacific coast provincial megafossil chronology and refers
to the eastern Pacific only. Critical comparisons were
made where possible between fossil and living material,
particularly with fossils from the Atlantic and the northwestern
Pacific Oceans. This brought to light the frequently
diverging views of the species concept held by neontologists
and their palaeontologist colleagues (Newell and Boyd
1978). Differentiating characters change over time, but morphological
features used to cluster populations into species
must be accorded comparable importance over space and
time. If characters fall into the expected variability of the
species, then temporal separation is insufficient to maintain
specific status.
2
ORIGIN AND CLASSIfiCATION OF BIVALVIA.
Various organisms with apparently bivalved exterior
shells are known from the earliest fossil strata, but undoubted
Bivalvia first appear in the Ordovician, already diversified
into major groups from which all living forms can be derived
(Pojeta 1971). The antiquity of these major lines has resulted
in the proposal of subclasses, and even subphyla (Scarlato &
Starobogatov 1978), for a group long thought to be more
uniform than many other invertebrate phyla. Nevesskaya et
al. (1971) recognised the underlying uniformity of the Bivalvia
and used only superorders for primary divisions. However,
increasing information on comparative anatomy, , shell
ultrastructure, systematic biochemistry, and palaeontology,
require high-level taxa neccessary to express diverse relationships
of a group which may be less cohesive than suggested
by superficial study.
Numerous classificatory schema have been proposed during
the past two centuries, initially based solely on the shell,
then on the soft anatomy or ecology. The growth and
rationale of the various systems have been fully reviewed by
Cox (1960) and again by Newell (1965), expanded and
lightly revised by Newell in Moore (1969). As emphasis
moved from conchology to malacology and attempts were
made to draw in palaeontological data, it was soon evident
that systems based on single characters were illogical and
frequently contrary to obvious relationships. Recent efforts
at classification qattempt to acknowlege the entire spectrum of
information, but as characters have evolved at different rates
and complicated by convergence, mosaic evolution, and
parallelism, evidence frequently is conflicting and cannot be
accorded equal importance. This subjective weighing that is
the ultimate task of the systematists and becomes more controversial
and personal the higher the taxon. Furthermore, as
the shell is the most accessible structure, and the only available
one to the palaeontologist, ideally taxonomy should be
workable at the conchological level.
Attempts to construct early phylogenies and origins for the
Mollusca are an entertaining and harmless intellectual diversion,
well summarised by Vagvolgyi (1967). There exists no
comprehensive and persuasive theory of descent for the
Bivalvia, but mention should be made of recent interesting
speculations. Pojeta et al. (1972) advanced the view that
rostroconchs, a heterogeneous group of fossils, variously
assigned to the Arthropoda, Brachiopoda and Mollusca, in
fact represent an extinct class of Mollusca, ancestral to the
Bivalvia (Runnegar & Pojeta 1974). Heraultipegma, a Cambrian
fossil of uncertain affinity is considered the earliest
rostroconch by Pojeta & Runnegar (1976). Earlier, Pojeta et
al. (1973) announced that Fordilla, hereto considered a
Lower Cambrian arthropod (Müller 1975; Missarzhevskiy
1974), represented the earliest known Bivalvia. Finally,
Runnegar (1978) derived the bivalved Fordilla from the
saddle-like single valve of Heraultipegma. This controversial
contention that rostroconchs are ancestral to Bivalvia,
is, at best, conjectural, as rostroconchs never developed
a true bivalve shell, ligament, or adductor muscles. An
explanation is neccessary to account for the absence of Bivalvia
from the Middle Cambrian to the Ordovician if Fordilla is
accepted as the first true Bivalvia. It may be more prudent to