Because the first Paleontological Society short course covered the Mollusca, it may be appropriate to record a few words concerning that occasion and give belated thanks to those who participated. After being president of the Society, it occurred to me that most of the membership associated the Journal of Paleontology with it, but did not seriously consider the organization as a focal point for other activities. By giving a short course annually, I felt we could inform the membership on scientific matters, and also build up additional interest in society activities. The annual short courses on the various fossil groups and phyla have more than fulfilled that expectation. They tell the world that paleontology is alive and well.

Mollusca is a phylogenetic concept, not a morphological/anatomical concept (Pojeta, 1980; Runnegar and Pojeta, in press). In order for a taxon to qualify for placement in the Mollusca, the taxon must be shown to be phylogenetically related to other taxa in the phylum. In point of fact, Mollusca cannot be defined morphologically/anatomically. The only acceptable definition of Mollusca is that the phylum is made up of its contained taxa.

Definition and classification probably rank second only to marriage as a source of controversy, but it is not in that spirit in which I want to write; I only want to show that there may be more than one viewpoint about Mollusca. Problems of definition and classification are different. Workers may agree on the defintions of phyla and classes, yet disagree on their arrangement. Definition and classification each have two separate and distinct phases: the work itself on developing a concept; and the general acceptance of this concept. New ideas by their very nature go against prevailing opinion; it is not a new idea itself which is the critical feature as is its acceptance. Acceptance of definition and/or classification tends to be a passive process and most teachers are content to quote uncritically that which is in the latest textbook.

Achieving consensus on the definition of the Mollusca is fortunately not prerequisite to the exercise of taxonomic classification and phylogenetic analysis. Definitions emerge from analysis, and it is therefore germane to consider the ways in which analytic procedures and underlying assumptions may affect growth of knowledge of the Mollusca.

Fossil mollusks are common and conspicuous objects in shallow marine strata of Ordovician and younger ages, and so the broad features of the post-Cambrian history of the Mollusca have been known for many years. However, as all but one of the eight classes of the Mollusca had evolved by the beginning of the Ordovician, it is obvious that the pre-Ordovician history of the Mollusca holds the key to an understanding of the primary radiation of the phylum. It was with this view in mind, that John Pojeta, Jr. and I began our joint studies of Cambrian mollusks some 13 years ago.

Early ontogeny is still the least well-known part of the molluscan life cycle, despite its unarguable significance in ecology, biogeography, and evolution. The past decade has seen an acceleration in biological research on molluscan embryonic and larval development, and paleontologists have recently begun to explore this field as well. Although the very earliest phases of ontogeny are inaccessible to the paleontologist, a remarkable amount of information can be derived from the preserved details of larval shell morphology. The fossil record thus affords rich opportunities to study directly the development of extinct species, both from a phylogenetic standpoint and for tracing the evolutionary and biogeographic effects of these early stages in life history. Access to long-extinct clades and an ability to document rates and patterns of evolution allow paleontologists to test hypotheses that can be addressed only indirectly using Recent data. Here I will emphasize these paleontological aspects of molluscan development (see also Jablonski and Lutz, 1983); for the more strictly biological aspects of reproduction and embryology, see Giese and Pearse (1977, 1979), Verdonk et al. (1983) and Tompa et al. (1984).

Like most classifications of molluscan shell microstructure published during the past 25 years (e.g., MacClintock, 1967; Kobayashi, 1964, 1971; Taylor, Kennedy and Hall, 1969, 1973; Grégoire, 1972a), the present one is based largely on Bøggild's (1930) monographic work, redefined from a modern perspective of combined light and scanning electron microscopy. However, this is the first attempt to integrate shell microstructure terminology for mollusks with that employed by students of bryozoan and brachiopod shell microstructure (e.g., Williams, 1968a,b, 1970, 1973; Williams and Wright, 1970; Armstrong 1968, 1969; Sandberg, 1971, 1977; Brunton, 1972; MacKinnon, 1974, 1977; MacKinnon and Williams, 1974; Iwata, 1981, 1982). An integration of nomenclatorial schemes is desirable for purposes of interphylum comparison, and is presently needed because there is considerable overlap and inconsistency in the application of microstructural terminology even within single molluscan classes. The present synthesis of shell microstructure nomenclature is possible primarily because of the extensive data base of invertebrate shell mineralogy, microstructure and especially ultrastructure published in more than 300 references in the past 15 years. To these data, the authors have contributed original information of shell mineralogy and microstructure for scores of Recent and fossil mollusks, brachiopods and bryozoans, with a clear emphasis on bivalved mollusks. Many inadequately described microstructure terms have been reanalyzed during the course of this study, either by examining species cited in the literature, or by using closely related species. Perhaps because they are better studied, but probably for other reasons as well, the diversity of molluscan shell microstructures is considerably greater than that of brachiopods and bryozoans combined (Carter, 1979). Consequently, most of the present nomenclature is based on mollusks, and only three of the major microstructural arrangements described in this guide (crossed bladed, semi-nacreous and semi-foliated) were known first in brachiopods or bryozoans and later recognized in molluscs.

Perhaps the one structural feature of the molluscan shell which has historically attracted the most attention from biologists and paleobiologists alike is the banding or growth increment variation associated with so many molluscan species. Such growth patterns are often prominently displayed on the external surfaces of shells and have long been the focus of serious biological and paleontological research (see reviews by Clark, 1974; Lutz and Rhoads, 1980). The usefulness of external shell growth patterns in ecological or paleoecological contexts is limited, however, by both the inability to distinguish true periodic features from random disturbance marks and by the extreme crowding of growth lines near the margins of mature shells. In the last two decades these problems have been surmounted with the recognition of periodic growth patterns within molluscan shells. Internal shell growth patterns are known from all classes of mollusks, but those in the Bivalvia have been studied most extensively. This is a result of the relative ease with which a complete ontogenetic growth record can be obtained by sectioning a shell along the axis of maximum growth (Rhoads and Pannella, 1970). Analogous ontogenetic records are very difficult, if not impossible, to obtain from coiled or spiral shells (e.g., gastropods) using current techniques (Lutz and Rhoads, 1980). This chapter, then, aims to review the major types of internal shell growth patterns described within molluscan shells (mainly bivalves) and to discuss their origin and applications in ecology and paleoecology. Also taken up in this chapter is a brief consideration of geochemical variations (stable oxygen and carbon isotopes and trace and minor elements) within molluscan shells. Physical-chemical, environmental, and physiological influences on shell chemistry are discussed in relation to how biogeochemical variations in the shell may be used to reconstruct paleoenvironmental conditions.

This part of our course builds on the assumption that evolutionary change, although being stochastic in principle, is channeled by external and internal constraints to such a degree that it becomes quasi-predictable – or at least understandable. On this basis it makes sense to use the old methods of comparative morphology in the new framework of constructional morphology (Seilacher, 1970) in order to recognize patterns and to interpret them as trends and evolutionary pathways. For such an approach, bivalves are particularly suited:

1. they deviate little from a common design (for instance they never lost their shell).

2. their preservable hard parts adequately reflect the developmental biography of each individual.

3. their shell form expresses the compromise between developmental constraints and functional paradigm with little interference from soft part anatomy, physiology and biotic interactions.

4. they are diversified enough to provide many examples of parallel adaptations for model testing, particularly if we include the fossil record.

The subphylum Diasoma (Runnegar and Pojeta, 1974) is a phylogenetic taxon used to show the commonality of origin, ecology, and gross structure of the classes Rostroconchia, Pelecypoda (=Bivalvia), and Scaphopoda. Diasomes have pseudobivalved, bivalved, or univalved shells, which often gape at the anterior and posterior ends. The mouth and anus open at opposite ends of the shell. The head is poorly differentiated or absent, and the radula is present or absent. The stratigraphic range is lowest Cambrian through Holocene.

Bivalves are one of the major macroinvertebrate fossil groups of the Phanerozoic. Bivalves have occupied many aqueous habitats, and in doing this have undergone a steady, relatively unchecked increase in diversity (Figure 1). Thus, bivalves are one of the most useful fossil groups in paleoecology, both for environmental reconstruction as well as for deciphering patterns and processes of evolutionary paleoecology.

The Class Gastropoda is one of the most morphologically and functionally diverse groups of living organisms. The diversity is even greater when major morphological (and implied functional) departures are added in from the more than 500 million years of fossil record of the group. There are a number of good basic illustrated accounts of the comparative morphology of the shell, external and internal anatomy, and radula (see especially Cox, 1960; Fretter and Graham, 1962; Hyman, 1967). There are no corresponding accounts of function. Accounts of gastropod ecology, behavior, and life habits are traditionally treated separately and outside of the morphological framework. As a result, the terminology and classification systems developed to describe gastropod form are separate from those dealing with gastropod function.

If taxonomic richness or ecological diversity defines evolutionary success, then the gastropods must be placed among the most successful clades of all time. Today, the gastropods are the most speciose class of fossilizable marine invertebrates. They have a rich fossil record extending nearly to the base of the Phanerozoic, documenting an almost uninterrupted taxonomic and ecological diversification (Figure 1). Yet the snails have been often overlooked as a focus of evolutionary or paleoecological study. Classic evolutionary studies of gastropods, for example Fisher et al.'s (1964) study of Athleta or Gould's (1969) work on Poecilozonites, indicate no inherent deficiencies in the taxon and suggest a simple lack of attention. The purpose of this paper is to summarize some of the major features of the history of gastropod evolution, to review problems in the analysis of fossil gastropods, and to reiterate some long-standing questions about the evolution and relationships of ancient snails. The goal is not only to provide a useful guide for non-specialists facing the prospect of lecturing on the beasts, but also to direct prospective students to problems perhaps worthy of their future research effort.

Paleoecology concerns the life processes and patterns of environmental relationships of groups of ancient organisms during their lifetimes. Two assumptions are fundamental to paleoecological theory: observed patterns in populations, associations, communities and ecosystems represented in the fossil record were imposed by contemporaneous physical and biological environmental factors and their interactions, and at least some past environmental conditions can be discerned from fossil assemblages and their rock matrices.

General Organization of Recent Cephalopods. Extant cephalopods with the exception of Nautilus have an internal shell covered by muscular mantle. Only Nautilus is able to withdraw into its shell and close the aperture with a hood. The internal shells of Sepia and Spirula are mineralized and have chambers that function in buoyancy control (Denton & Gilpin-Brown, 1961, 1971; Denton et al., 1961, 1967) while other teuthids and Vampyroteuthis have an unmineralized internal skeleton that acts as an elastic support to the soft body.

The 520 million year evolutionary history of the Cephalopoda is punctuated by a number of severe crises during each of which this class came very close to extinction. These are fully documented only for the externally shelled, or ectocochlian, cephalopods to which my remarks are, therefore, restricted. The word crisis should be defined narrowly in the sense in which it is used in medicine, business, and the social sciences: “a condition…. felt to endanger the continuity of the individual or his group “(Webster's Third International Dictionary). Within species, genus, and family groups evolutionary crises are, of course, commonplace, and even orders may disappear without threatening the extinction of an entire class. It is only when all, or almost all, taxa of family-group and lower hierarchic level disappear more or less simultaneously, or within a geologically very short period of time, that we may speak of a crisis, a situation that threatens the very survival of a class, or higher taxon, of organisms. It is the purpose of this presentation to study in some detail this kind of situation as it has affected the evolutionary history of the Cephalopoda.

Studying the paleoecology of the extinct chambered cephalopods is somewhat analogous to studying a distant landscape through the wrong end of a powerful telescope: we can see very small views of what must have been great panoramic vistas. Two factors have lead to this situation. First, the extant cephalopods still utilizing the phragmocone system of density reduction are at very low standing diversity, while most of the more interesting taxa, such as ammonites and belemnites, are completely extinct. Our functional interpretations of various shell morphologies thus depend heavily on generalizations based on a small number of sometimes distantly related taxa, such as Nautilus and Sepia. Secondly, studies of fossil associations and their relationships to various facies are clouded by the possibility of extensive post-mortal drift of the cephalopod shells. Because of this, we cannot ever be sure that we are studying cephalopod biocoenoses. In spite of these constraints, however, the past two decades have witnessed a great research effort into the study of cephalopod paleoecology. Debate over various issues has been sometimes clamorous, a sign of a healthy and vigorous utilization of the scientific method by many interested students of the fossil record.