Mollusks |
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Mollusks are an amazingly
diverse group of animals that live in a wide variety of environments. They can
be found inhabiting trees (Figure 1:
D),
gardens, freshwater ponds and streams, estuaries, tidal pools, beaches, the
continental shelf, and the deep ocean. Some mollusks are excellent swimmers,
others crawl or burrow in mud and sand. Others remain stationary by attaching
themselves to rocks, other shells, or plants; or by boring into hard surfaces,
such as wood or rocks. Adult mollusks can range in size from a few mm (0.1 in.)
to over 22 m (>70 ft.) in length as documented for some giant deep-sea
squids. Their weight can vary from a few mg (a fraction of an ounce) to over
227 kg (500 lb.) as recorded for the giant south Pacific Tridacna
clams.
The number of living species of mollusks has been
estimated to range from 50,000 to 130,000. Everyone is probably familiar with
some type of mollusk. They are the slugs and shelled pests in your backyard
garden; the scallops, clams, mussels or oysters on your dinner plate; the
pretty shells you see washed up on the beach (Figure 2:
D);
the pearls or other treasures in your jewelry box; the octopus or squid at an
aquarium.
The word Mollusca is translated from Latin as soft-bodied but
few physical characteristics are unique to all mollusks. The mollusks are
invertebrates and therefore lack a backbone; they are unsegmented and most
exhibit bilateral symmetry. Most mollusks can be described as free-living,
multicellular animals that possess a true heart, and that have a calcareous
exterior skeleton that covers at least the back or upper surface of the body.
This exterior skeleton provides support for a muscular foot and the internal
body organs, including the stomach mass. A thin flap of tissue called the
mantle surrounds the internal organs of most mollusks, and it is this mantle
that secretes the animal's shell. The nervous system of mollusks varies greatly
from group to group; the clams and tusk shells have very simple nervous
systems, while the squids, octopi and some other mollusks have concentrated
complex nerve centers and eyes (Figure 3:
D)
equivalent to vertebrates.
Mollusks first appear in the fossil record about 545 million years ago in earliest Cambrian time, but the record of their origin and early evolution has not been discovered in the fossil record. By late Cambrian time (~520-505 million years ago) most of the modern groups of mollusks can be found in some primitive form as fossils occurring in marine deposits. During the Ordovician (~505-438 million years ago) a major radiation of mollusks occurred, with thousands of species of mollusks appearing in the fossil record of that time.
During the Mesozoic (~245-65 million years ago) the ammonites, a relative of the modern chambered nautilus, flourished and are an important part of the fossil record, but they became extinct at the end of the Cretaceous at the same time as the dinosaurs. Many types of clams and snails also disappeared at the end of the Cretaceous, including the rudists, a group of bivalved mollusks that lived much as modern reef-building organisms do. The disappearance of these marine animals opened up environmental niches to be filled by a radiation of new species of all types of animals at the beginning of the Cenozoic.
The Cenozoic (beginning around 65 million years ago to the present) marks the time period when the modern groups of mollusks evolved, beginning with the marine clams and snails following the end of the Mesozoic. During the last million years land and fresh-water mollusks have evolved rapidly, occupying the terrestrial realm to an extent never seen in their fossil record.
Biostratigraphy - The appearance and disappearance of species of fossils of any type at specific points in time can serve as age indicators for the rocks or sediments that contain them. A good index fossil or guide fossil for a particular period of time should have the following characteristics:
Mollusks have many of the properties of good index fossils: The hard shells of many mollusks means they are generally well-preserved, and they are often one of the few fossils found in certain environments. Sometimes they are only preserved as molds or casts in ancient limestones, but they can still be identified and used to establish the age of the rocks.
The general families of mollusks are easy to recognize, although it usually takes someone who has studied the taxonomy and morphology of mollusks to identify the species. Macro-mollusks make excellent biostratigraphic tools for geologists in the field because they are large enough to be seen at the collection locality and allow the geologist to make a quick determination of the age or stratigraphic position of his or her sample. At some localities, such as exposures of the Pliocene Pinecrest Beds in Florida, the mollusks are so abundant that they are the primary component of the sediment or rock, and very little sand is present in between the mass of shells. Micro-mollusks are often well-preserved and abundant in cores, and can be used to establish the age of the subsurface rocks.
Mollusks in general do evolve slower than many microfossils, such as the foraminifera or calcareous nannofossils, and they rarely have world-wide geographic distribution, so for Cenozoic rocks, they are used primarily for regional or provincial correlation. However, for Upper Paleozoic and Mesozoic rocks, ammonites are used as world-wide age indicators because they evolved very rapidly and were distributed throughout the seas during those time periods.
Establishing the age and
equivalence of sedimentary deposits (Figure 4:
D)
is crucial to determining the distribution, path of flow, and potential sources
of pollution for ground water; to identifying and predicting the distribution
of economically important minerals and fossil fuels; and to determining the
location of faults, meteorite impact sites, and other catastrophic prehistoric
events. Fossil mollusks, for example, have been used to determine the extent
and degree of disruption of an
Late Eocene impact crater
in the Chesapeake Bay; to map the distribution of subsurface geologic and
hydrologic units in Florida; and to determine the position of an extensive
deltaic system in the North Carolina Coastal Plain during the Mesozoic.
Chronostratigraphy - In addition to contributing relative age information about sedimentary deposits, mollusks provide the raw material for isotopic dating of Cenozoic lithologic units. The calcareous skeletons of mollusks contain the element Strontium (Sr). As they secrete their shells they absorb both isotopes of Sr (86Sr and 87Sr) from seawater and thus record the 87Sr/86Sr ratio present in seawater at that point in time. Measurement of this ratio allows scientists to determine an absolute numerical age for the deposit containing the mollusks' skeletons.
Paleoceanography and
Paleoecology - Mollusks can be used to determine the general climate
regime and the depositional environment of the rocks and sediments in which
they are found. We can determine the environment or climate of ancient deposits
by examining where and under what conditions the living relatives of the
fossils exist. For example, today the snail genus Fasciolaria (Figure
5:
D)
is an extremely aggressive carnivore that lives in
tropical
and subtropical
lagoonal or bay environments. When we find Fasciolaria in fossil
deposits we can infer that the sediments or rock were formed in a tropical or
sub-tropical shallow-water setting. In addition, by studying the worldwide
distribution patterns of mollusks, inferences can be made about paleocean
currents and
paleoclimate
on a global scale.
Ecosystem History and Reconstruction - Mollusks, along with other calcareous animals, can be used to determine the history of ecosystems and to aide in their reconstruction. For example mollusks provide clues about the bottom conditions and salinity in marine environments. Generally, we are looking at very recent time-scales, perhaps the last 100 or 200 years. By using basic paleoecology tools, and examining the changes that occur in faunal and floral assemblages over time, we can determine changes in the biotic, chemical and physical parameters of an ecosystem. These changes can be compared to historical records of alteration of the environment, major storms, etc. to determine if there is some correspondence. Knowledge of how the system has changed over time and possible causes of the change allows ecosystem managers and policy makers to determine the possible extent to which human-induced changes have altered the natural system. Some changes that have been blamed on human activity may actually be part of a natural cycle. By understanding the natural range of variability that exists within any ecosystem, wise and economical decisions about restoration can be made.
