10.1 Introduction
The molluscs are important to paleontologists for several reasons. First, they have tremendous morphological diversity (only Phylum Arthropoda has more described species). Second, molluscs usually possess well-calcified skeletons that have easily recognized features. Third, the phylum has exploited a wide variety of environments, from terrestrial forests, to freshwater lakes, down to the deepest parts of the ocean. Finally, the molluscs have an excellent fossil record extending back to the earliest Cambrian.
Although diverse in appearance, all molluscs share certain characters
10.2 Classification
Phylum Mollusca is divided into eight classes, all but one of which are extant today. Class Aplacophora forms no hard parts and has no fossil record, so we will ignore it. The other seven classes and their classifications will be discussed in detail separately.
Phylum Mollusca: Cambrian-Recent.
Class Monoplacophora:*
Polyplacophorans, or chitons, are marine molluscs common in intertidal areas today. They have also been found at depths down to 5000 meters, but they are most prevalent on rocks and seaweed in the intertidal zone.
Chitons are generally ovate in outline and have a greatest thickness of about one-fourth their greatest width. The body is bilaterally symmetrical, with a head visible only from the ventral side. A band of muscular tissue, the girdle, runs along the dorsal periphery of the animal. Their girdle commonly has embedded in it small calcareous spines, scales or spicules.
The chiton shell normally consists of eight, articulating calcareous valves with the joints between them running perpendicular to the axis of the body. These valves are constructed in such a way that the animal can often roll up like a pill bug (isopod; Phylum Arthropoda) when disturbed. The surface of the valves is usually smooth, but can be ornamented by ribs, knobs or thin lines. Fossil chiton specimens are almost always loose, disarticulated valves. The lobed nature of the intermediate pieces gives them the common name "angel wings" or butterfly shells. They have been found in rocks of the Cambrian and every succeeding period, but are rare.
10.4 Class Scaphopoda
Scaphopods are semi-infaunal, marine animals and are fairly common today in nearshore and bathyal environments. Their shells are never abundant in the fossil record, except for occassional local concentrations in Cenozoic deposits.
The shell of a modern scaphopod is a tapering tube, open at both ends, with a greatest diameter of little more than 6mm, and a length usuall no greater than 8cm. Because of the position of the mouth, the large end of the shell is anterior, and the small end posterior. The exterior has finely spaced growth lines and usually some type of longitudinal ornamentation. Often the small end has an indented terminal pipe.
Scaphopod soft parts consist of a poorly-defined head, a conical foot, a visceral mass, and the standard molluscan mantle. There are no gills - respiration is accomplished by direct exchange with tissue. The animal feeds by capturing small prey (usually foraminifers) and deposited particles with small tentacles attached to the head. This food is passed to the mouth and processed by a radula.
10.5 Class Bivalvia
The bivalves are characterized by a two-valved shell that protects the soft body completely. These valves are joined by a hinge on the dorsal side of the animal along a line parallel to the length of the body. Thus, unlike the brachiopods, one valve is on the right side of the animal, and the other on the left.
All bivalves are aquatic, with the majority living in shallow marine waters. Most are benthic and quite sluggish, although a few (notably the common scallops, Family Pectinidae) can swim through the water for short distances by clapping the valves powerfully. Many bivalves burrow into various substrata, from sand and mud to wood and rock. Another common life habit among the class is attachment to the sea bottom, or some type of hard substrate (shells, rocks, piers, wharves, boat hulls, etc.).
The bivalve body consists mostly of a muscular foot in the anterior, and a series of gills (ctenidia) and a pair of siphons (if present) in the center and posterior. The viscera include, from front to back, a mouth, stomach, intestines, kidney, anus, and a heart near the dorsal margin. The enclosing mantle produces the hinged shell, which is usually closed by two adductor muscles (anterior and posterior). Bivalves have no diductor muscle system for opening the valves; instead they usually use an elastic ligament that automatically opens the shell when the adductors relax.
Bivalves have been divided in the past into five major groups, based primarily on the method of feeding. These ecological groups are important to the general evolutionary history of the class, and indeed to the history of most of the other benthic marine invertebrates. They are:
10.5.1 Classification
Many diverse schemes have been developed for the classification of bivalves, but in paleontology we use one based on shell morphology. The Treatise on Invertebrate Paleontology employs a system based on general shell shape, microstructure and hinge configuration. These features generally correlate to the various life modes.
A glance at the classification outline will show you that the taxonomy of bivalves is complicated. Remember that you will not be required to identify all the groups listed or classify below the class level.
Subclass Palaeotaxodonta
Rostroconchs are "bivalved' fossils belonging to the only known extinct class of molluscs. A rostroconch resembles a clam superficially, but the shell has no ligament or articulating system at the "hinge"; in fact, layers of calcite extend across this region, so that the hard part of a rostroconch appears much like a taco shell. The rostroconch shell apparently broke periodically at the margin to permit shell growth. The posterior of the shell is usually elongated into a flattened tube. To date, 34 genera have been described. Rostroconchs are usually rather small, often less than two centimeters in length. Some large forms (about 15cm long) can be found in Devonian limestones of the midcontinental United States.
Most invertebrate paleontology textbooks classify the rostroconchs as members of the Bivalvia, but the consensus among paleontologists today is that they are sufficiently different enough to rank as their own class of molluscs. It appears that the rostroconchs are the ancestral stock from which the bivalves and scaphopods evolved, making the rostroconchs a paraphyletic clade.
10.7 Class Gastropoda
Gastropods, or snails, are a group of invertebrates familiar to everyone. They are well known in part for their ability to occupy and hold tenaciously to a diversity of living environments, from the sediments of the deep sea to the lettuce heads in our own gardens.
In spite of the wide assortment of life modes among the gastropods, they have a rather simple body plan. Most have a shell, spirally-coiled to some degree, that covers the main organs of the body. The muscular foot forms a base for the typical snail, and is the animal's principal means of locomotion. Many gastropods have tentacles and eyes in the head region, along with some sort of feeding apparatus. Respiration is usually accomplished with a feather-like gill (ctenidium) that extends into the mantle cavity at the anterior of the body. In some gastropods the mantle cavity is posterior, along with the gill. A few terrestrial snails lack gills, having instead either an air-breathing "lung" formed from the mantle cavity, or a mantle surface modified for respiration.
Most snails have an unusual feeding mechanism just inside the mouth that has been used in the classification of living forms. This device, termed the radula, is a long belt of tough, flexible material with rows of minute teeth. It is usually used like a strip of rasping sandpaper to scrape food from a surface or bore into other shells. Some snails have radulae modified into poisoned darts for immobilizing or killing prey.
Another feature that some snails shape is the operculum. The gastropod operculum is similar to operculum in some cnidarians and protista, in that it acts like a door to the shell's opening (aperture), closing in times of stress (e.g. storms or predation).
No general discussion of the Gastropoda is complete unless the process of torsion is mentioned. While in the larval stage, snails twist the viscera in a counterclockwise direction to bring the anus above the head. Ancient snails probably did not undergo torsion, and some advanced forms have reversed the effects secondarily.
Paleontologists of course deal almost exclusively with the shells of snails. Unfortunately, the snail shell often reflects very little of the critical soft-paRt anatomy. Classifications then, of extinct gastropods involve considerable inference.
Most snail shells can be thought of as elongated cones wound into a spiral by varying degrees. Those shells that are planispiral are coiled symmetrical in a single plane. Pseudoplanispiral is a condition where the shell is coiled in a plane, but cannot be divided into symmetrical halves by this plane. Conispiral shells are coiled along an erect cone. Most conispiral shells are coiled clockwise (dextral) down the cone, but a few are coiled in the counterclockwise direction (sinistral). The fundamental patterns form the basis of a variety of shell types.
10.7.1 Classification
Since the characteristic features of gastropods are primarily soft anatomical characters, paleontological classification is difficult. In this lab you will be expected to recognize only certain distinctive groups.
Gastropods have been placed in three subclasses: the Prosobranchia ("forward gills"), the Opistobranchia ("backward gills"), and the Pulmonata ("lung bearers'). All three are abundant in the fossil record, but the first is by far the most common.
Subclass Prosobranchia
The prosobranchs are divided into three orders, which we will spend most of our time with.
The opistobranchs are exclusively marine, rather small snails with a minor fossil record. There is a general trend in the group toward a reduction and internalization of the shell. Pteropods are very small, nektic opistobranchs found in the plankton. They have a thin shell and an impressive, bilaterally symmetrical "winged" foot that enables them to swim. Pteropods are useful warm-water indicators in some Quaternary deep sea sediments, but they are rare as fossils prior to that time.
Subclass Pulmonata
The pulmonate gastropods include over 7800 species in about 50 families, but in this lab we will see very few. Pulmonates are mostly terrestrial and freshwater dwellers, so they appear sporadically in the marine record. These snails are useful for correlating nonmarine sequences with marine deposits. Separation of the pulmonates from the prosobranchs using only shell morphology is truly a job for experts!
10.8 Class Cephalopoda
The cephalopods are a group of molluscs quite different from the rest of the phylum. Along with the standard molluscan body plan, they have a large well developed head, with large well developed eyes, and a set of prehensile arms that usually bear rows of suction cups. Included in this class are the squids, octopuses, and chambered nautiluses. Instead of being sessile or at best sluggish like the other molluscs, cephalopods are quick movers, swimming through the water or crawling over the substrate. They also have a variety of advanced defensive mechanisms, such as camouflaging ability or ink clouds.
Those cephalopods that bear hard parts are, of course, the most interesting to paleontologists. The shell of the Nautilus, and some related but extinct forms, is a planispiral, mostly involute, coiled calcium carbonate cone divided by calcareous walls (septa) into chambers. In the living Nautilus these chambers contain various gases. A narrow tube, termed the siphuncle, extends from the final chamber back through the septa to the intitial chamber. This tube consists of living tissue and is thought to function as a regulator of the gas composition in the chambers (hence the buoyancy of the animal). As the animal grows, it builds successive new septa of increasingly larger size, a larger living chamber, and a longer siphuncle for communication with the rest of the shell. As in all molluscs, the calcareous parts are built by the mantle tissue enveloping the body.
Shell-building cephalopods are known from the Cambrian to the present. They are very important to biostratigraphers, especially those working with Mesozoic rocks, so there is at least a utilitarian classification. We will discuss in detail those fossil cephalopods important to paleontology.
10.8.1 Order Nautiloidea
Several cephalopods that seem only distantly related have been included in this group, so there is considerable diversity in form. Nautiloids appeared first in the Cambrian as straight, conic shells with the siphuncle running down the center of the septa. Where the siphuncle pierced a septum, the wall turned along it, forming a septal neck that in nautiloids usually points posteriorly.
Later nautiloids began to coil the tube in a variety of ways. The siphuncle remained central in location, and the suture pattern (the system of lines formed where the septa meet the interior of the shell wall) is simple (usually a set of sinusoidal curves).
The first nautiloids were probably grazers on algal mats of the Silurian and Ordovician. Gradually they seem to have adopted a more carnivorous habit - probably feeding on arthropods, worms and other molluscs.
Nautiloids were the most important cephalopods of the early and middle Paleozoic. They experienced a few episodes of diversification and extinction throughout their history, but have generally been declining in numbers and species since the Ordovician. One genus, Nautilus, lives today.
12.8.2 Order Ammonoidea
These cephalopods arose in the Devonian directly from the nautiloids through an intermediate stock similar to the straight-shelled bactritid nautiloids. They are usually coiled to some extent. The siphuncle of an ammonoid is usually located along the margin of the shell, not in the center like the nautiloids. Also, the septa are folded intricately, forming more complicated suture patterns than those of the nautiloids.
Ammonoid suture patterns, which have some diagnostic value, are of three basic types:
Ammonoids had a complex history; they began to diversify gradually in the Devonian, but met near extinction at the close of the Permian. In the Mesozoic though, they flowered into hundreds of species with a wide array of shell types. At the close of the Cretaceous the entire group went extinct.
Several cephalopod groups, joined in the subclass Coleoida, built only internal shells or had no shells at all.
10.8.3 Order Teuthida
These are the true squids, which have a fossil record beginning in the Jurassic. They are elongate animals with ten arms and a chitinous shell, termed the pen, lying above the visceral mass. Squid fossils consist of the pens and few rare impressions of the soft parts.
10.8.4 Order Belemnitida
These extinct animals resembled the squids in having cylindrical bodies with a head and set of anterior arms. However, they possessed heavy internal shells that look more like .50 caliber bullets than anything else. Well preserved belemnite shells have a chambered area called the phragmocone, a rudimentary siphuncle, and a massive rostrum that apparently acted as a counterweight.
Belemnites have been found in rocks as old as Mississippian, but are most common in middle and late Mesozoic deposits. They appear to have died out at the end of the Cretaceous, but one very questionable Eocene form is known.
10.8.5 Order Sepiida
Sepiids include the cuttlefish, whose shell is well known to parrots everywhere. Most are flattened dorso-ventrally and have ten arms and eyes similar to squid. They usually live on the sea bottom and capture small animals for food. Like the squids, sepiids can discharge clouds of ink to confuse pursuers.
Spirula is a sepiid that drifts through the waters of the deep sea. It is not dorso-ventrally flattened like most other sepiids, and it has an internal shell with chambers.
10.8.6 Order Octopodida
The octopuses have a very scant fossil record, mainly, of course because they rarely produce hard parts. An impression from the Upper Cretaceous is the earliest known fossil of the group. Octopuses have eight arms (hence the name) and a bulbous body. The female of the the genus Argonauta secretes a small shell to transport its eggs safely; this shell resembles some Mesozoic ammonoid shells superficially, but differs in having no septa or siphuncle.
| 10.9 Terminology | ||
| siphon adductor monomyarian nacre muscle scar gape lunule osphradium torsion whorl apical angle aperture operculum arms jaws living chamber endocochlear rostrum saddle and lobe ammonite | foot isomyarian ligament periostracum pallial line byssal notch escutcheon dentition head radula siphon umbilicus dextral planispiral tentacles siphuncle phragmocone orthoconic camerae goniatite venter | gills (ctenidia) anisomyarian byssus myostracum pallial sinus ribs hinge taxodont foot spire protoconch columella sinistral conispiral mantle cavity septum ectocochlear heteromorph suture ceratite |
Chapter 10: Questions
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