Shell Ornamentation Research Articles

MORPHOLOGICAL DIFFERENTIATION AND GENETIC COHESIVENESS OVER A MICROENVIRONMENTAL GRADIENT IN THE MARINE SNAIL LITTORINA SAXATILIS.

The marine gastropod Littorina saxatilis has different ecotypes in shores only a few meters apart. This has both taxonomic and evolutionary implications. Here we report on an extreme type of within-shore dimorphism in shell characters. In the wave-exposed rocky shores in northwestern Spain, we found one form of L. saxatilis in the upper-level barnacle zone. It had a white, ridged shell, with black bands in the grooves. Another form confined to the lower-shore mussel belt had a smooth shell that was either white and tessellated or darkly colored. These two forms cooccured in a narrow midshore zone together with individuals that had combined characters, but were present in low frequencies (11%-29%). We used principal-component analysis of metric shell characters to study variation in shell size and shape. We found that the upper-shore form was larger than the lower-shore form. We also found small but significant differences in shell shape. Experiments in a common laboratory environment suggested the differences in shell ornamentation and color are inherited, but the individuals did not develop the morph-specific characters until a shell height of about 3 mm. The occurrence of mainly two distinct forms may suggest the presence of two species that hybridize. An analysis of five polymorphic enzyme loci in populations of snails from three geographically separated sites indicated, however, that there was no positive correlation between morphological distances and genetic distances among populations on a geographic scale (tens of kilometers). Thus, we rejected the hypothesis of two species. However, on a microgeographic scale (meters), genetic differentiation between groups with the same form was less than differentiation between forms. This indicated a partial barrier to gene flow between the two forms, and preliminary mate choice data suggested this was caused by nonrandom mating in the midshore zone of overlap.

Devonian ancestors ofNautilus

A Famennian nautilid is described that partially fills the morphologic and stratigraphie gap between Givetian and Tournaisian records of the order. A new generic nameDasbergoceras gen. nov. is proposed for it. Its loosely coiled, slender juvenile shell indicates close relationships to earlier Lechritrochoceratidae, rather than the Oncoceratida. The shell ornamentation, with oblique ribs and longitudinal striation, appears to be the primitive feature of the order. Tight coiling of the adult shell progressively developed during the Early Carboniferous while tightly coiled embryonic shells seem to be a post-Permian feature. The septal morphology, which is the most important feature distinguishing RecentNautilus from all the Tertiary nautilids, may be inherited from Cretaceous ancestors of theCimomia Aturia lineage. The nautilids probably originated from the kionoceratid Orthoceratida in late Early Ordovician.

Morphological variations of benthic foraminiferal tests in response to changes in ecological parameters: a review

Some of the relatively recent literature correlating morphological variation in benthic foraminifera with environmental parameters such as temperature, salinity, carbonate solubility, depth, nutrition, substrate, dissolved oxygen, illumination, pollution, water motion, trace elements, and rapid environmental fluctuation is reviewed. It appears some variables (most notably depth) are recorded more frequently, which may affect some conclusions. Although each variable is treated separately, it appears that almost no variables act independently on test morphologies. In reviewing the literature, it becomes clear that there are many individual trends, especially with shell ornamentation, but few broad ones, and that it is almost impossible, with exception of some of the larger reef-dwelling, symbiont-bearing foraminifera, to predict how any species will react to various parameters. The broad trends concern thinning or thickening of carbonate tests with changing carbonate availability, temperature, and salinity. It appears that many observations of morphological changes within species may not be recorded in the literature, perhaps because authors did not recognize the importance of small details that would be of importance at a later time.

Early and Middle Devonian gastropod biogeography

Abstract Early and Middle Devonian gastropods show biogeographic patterns remarkably similar to those of better studied faunal groups of this same interval, notably articulate brachiopods, rugose corals, and trilobites. Three biogeographic realms are recognized: Old World, Eastern Americas, and Malvinokaffric Realms. Gastropod diversity is highest in the Old World Realm and lowest in the Malvinokaffric Realm. The diversity patterns and the high degree of shell ornamentation suggest that the Old World Realm was generally warmer than the Eastern Americas Realm, and that both were considerably warmer than the cool temperate to cold polar waters of the Malvinokaffric Realm. The utility of gastropods for fine-scale delineation of biogeographic units is illustrated for the Eifelian of western North America. At least two subprovincial units (the Alaska-Yukon and Nevada subprovinces) can be recognized. Eifelian gastropods from interior and southeastern Alaska belong to a single unit (the Alaska-Yukon Subprovince), and are most closely related to coeval faunas of northwestern Canada, suggesting little displacement of most of Alaska's so-called 'suspect' terranes. Plotting the data on the Devonian palaeogeographic maps of Scotese results in several suggested emendations: (1) North America should be moved south by 10-20°; (2) Australia is too far south on the Emsian and Givetian reconstructions, it should be in a more palaeotropical position; and (3) Siberia is too far north, it too should also be placed in a palaeotropical position.

An account of the techniques using ostracodes in palaeolimnology in Australia

Ostracodes are common in Australian waters, especially in saline lakes where they are particularly diverse. This makes them indispensable “tools” for Australian palaeolimnological studies, especially since many Australian lakes are saline. The following types of subjective information can be obtained from the study of Australian fossil ostracode faunas without knowledge of the ecology or taxonomy of their living representatives: the search for a life assemblage can lead to detection of the presence of water currents and the effects of reworking in addition to the identification of particular facies, such as a lake shoreline or a desiccation phase; rate of sedimentation can be estimated as well as the amount of sediment compaction of drying; information on water oxygenation and acidification can be obtained; and, finally, presence of seasonality affecting lakes can be identified. With additional reference to the ecology of particular species, permanency of lake water can be assessed and so can the variability of rainfall and, even at times, the detection of whether a lake was close to a summer/winter rainfall boundary. Shell ornamentation can inform on water depth. In addition, salinity data can be obtained as well as some information on water chemistry and temperature. The latter from trace elements (Mg, Sr) and stable isotope ( δ 18O) studies of ostracode shells.

Paleobiology of Early Cretaceous protocardiids, Caribbean Province

The Protocardiinae is represented by 11 valid species of Protocardia and one species of Nemocardium in the Lower Cretaceous of the Gulf of Mexico. These taxa appeared in the Caribbean Province through a combination of immigration and evolution from the local stock. One species group, Protocardia texana-Protocardia salinaensis-Protocardia subspinigera, developed as geographically and environmentally distinct populations that migrated in space and time. Another group, Protocardia roanokensis-Protocardia prosogyrata-Protocardia nutans-Protocardia denisonensis, differentiated through time by dichotomous branching. These species occupied sand to mud substrates. The coarseness of their shell ornament is positively related to substrate grain size. The geographic and stratigraphic distributions of these species illustrate that diversity is positively correlated with the extent of distribution and that endemism develops during transgression.

Differences in morphology in relation to microhabitat in littorinid species from a mangrove in Papua New Guinea (Mollusca: Gastropoda)

Field studies on the north coast of Papua New Guinea show that there are three species in the Littorina scabra complex on mangroves. Two are restricted to root structures, while the third occurs on the leaves. The species differ in shell shape, ornamentation and colour, in penis shape, and in radula morphology. The shells of the leaf species are much weaker than those from the roots. Many of the differences observed may be adaptive on the preferred substratum.

Shell sculpture as a defensive adaptation in ammonoids

Quantification of Paleozoic, Triassic, Jurassic, and Cretaceous ammonoid shell ornamentation shows that commonality and roughness of ornamentation increased throughout the geologic range of the ammonoids. The two major hypotheses concerning the function of ammonoid shell ornamentation are that 1) ornament served a protective (defensive) function against shell breakage by predators, and 2) it increased hydrodynamic efficiency of the shell during swimming. The heavily ribbed and spined ammonoid shells of the late Mesozoic have ornamentation too coarse to have served any hydrodynamic purpose. The increasing proportion of such shells during the Jurassic and Cretaceous may have been in response to increased numbers of late Mesozoic shell crushing predators and “better armed” ammonoid prey. This trend parallels adaptive trends of other invertebrate groups during the “Mesozoic marine revolution” as defined by Vermeij (1977).

Aspects of the adaptive morphology and evolution of the Trigoniidae

During the Jurassic and Early Cretaceous, the Trigoniidae were the dominant family of shallow-burrowing bivalves of warm, shallow seas. Neotrigonia is the only genus that survives today. Judging from the biology of Neotrigonia and the functional morphology and occurrence of extinct genera, the Trigoniidae represent an advanced family of burrowing Bivalvia. The family has been characterized by efficient locomotion, owing in large part to the presence of an unusually muscular foot, resembling that of the living Cardiidae. Other unusual features of the Trigoniidae relate to the presence of the foot. Complex hinge teeth with secondary dentition evolved to maintain valve alignment at the wide angles of gape required for extrusion of the foot. Myophorous buttresses evolved to support the large anterior hinge teeth. These anterior features seem to have obstructed the evolution of a prosogyrous shape of the kind that facilitates burrowing within many other bivalve taxa. Alternatively, there evolved in the Trigoniidae various kinds of external shell ornamentation that aided in burrowing. Thus, the many unusual morphological features of the Trigoniidae have been strongly coadaptive. The Mesozoic Trigoniidae seem to have been more advanced animals than any shallow-burrowing, suspension-feeding bivalves of the Palaeozoic and also than certain successful living groups with similar modes of life. Though slightly less advanced than the Cardiidae, they can be regarded as the cockles of the Mesozoic. Had the group not suddenly been decimated by environmental deterioration at the end of the Mesozoic, it would undoubtedly flourish today. Neotrigonia has, in fact, speciated at a high rate in comparison with other living genera of bivalves. A relatively recent origin and lack of primitive features disqualify Neotrigonia from status as a living fossil genus. It is the sole survivor of its family, but an advanced and modern animal nonetheless.

The bivalve mollusc genusLimatula: A list of described species and a review of living and fossil species in the Southwest Pacific

About 150 named fossil and living species of Limidae grouped in the genus Limatula Searles Wood are listed. They range from Jurassic (perhaps Triassic) to Recent and are now distributed in all oceans and seas from the Arctic to the Antarctic. The genus has not previously been subdivided, but its phylogenetic and biogeographic history can only be approached by recognizing a number of species-groups, some of which are formally proposed as subgenera, using characters of shell shape and ornament that appear to be conservative. Four subgenera are recognized (two new) for 40 living and fossil species-group taxa from Australia and adjacent parts of the Pacific and Southern Ocean: Limatula s. str. (with several groups), Limatuletta nov. (type: L. japonica Adams), Stabilima Iredale (3 species groups) and Squamilima nov. (type: Lima hodgsoni E. A. Smith). The following new species and subspecies are proposed: L. trulla arcis (Oligocene, New Zealand and Victoria), L. powelli (Recent, New Zealand, New South Wales and...

Study of surface-shell features in thecosomata (Pteropoda: Mollusca) by means of scanning electron microscopy

The shell ornamentation of 6 species of Limacinidae and 2 species of Peraclididae has been studied by means of scanning electron microscopy. The author analyzes the phylogenetic relationships between the species considered and the morphologic resemblance with protoconchs of other Gastropoda. The function of some of the features observed is considered, and the relationship between adult and larval environments and shell formation discussed. Pteropoda do not exhibit conspicuous differences between veliger and adult shells, but there is quite an important difference between the protoconch and the rest of the shell, the former being very similar in several Gastropoda. The division of the genus Limacina into 3 subgenera, as proposed by Van der Spoel (1967), is also partially confirmed by the present study.

Bivalve Mollusk Burrowing Aided by Discordant Shell Ornamentation

Oblique and chevron-like ridges on the shell surfaces of certain burrowing bivalve mollusks grip the sediment during shell-rocking movements to aid in sediment penetration. These ridges (characterized by steep dorsal slopes and gentle ventral slopes) have evolved through convergence in several families in association with particular behavioral and ecological traits.

Cretaceous lamellibranchs of family Trigonioididae and their classification

The unusual freshwater bivalve family Trigonioididae (Cox) is poorly known and has only recently been sufficiently studied to construct a taxonomy and phylogenetic relationships. Previous work is reviewed and evaluated. Recent accumulation of abundant trigonioids from the Far East, Mongolia, China, West Kazakhstan, Fergana depression, western spurs of the Hissar Range, and the Kyzyl Kum makes possible taxonomic reevaluation of the family, construction of a more complete phylogeny, and recognition of evolutionary trends. Previous ideas that the Trigonioididae had a geographic range restricted to border zones of eastern Asian seas are discarded, as in the single-character taxonomy based predominantly on shell ornamentation. A taxonomic system is proposed employing shell size, shape, convexity, exterior ornament patterns and hinge characters. Evolutionary trends and diversification in one character do not necessarily coincide with changes in other taxonomically important characters. Five genera are described...

Observations sur de jeunes coquilles de mollusques du Pliocene saumatre du Llobregat (Barcelone)

Abstract Detailed study of the initial stages of development of the shell ornamentation of mollusks from Pliocene brackish-water sediments of the Llobregat valley (Barcelona, Spain) was possible because of the great numbers of young or incompletely developed adult forms which could not survive the brackish environment.