Showing papers in "Lethaia in 1975"

The origin of the psychrosphere as recorded in changes of deep-sea ostracode assemblages

Abstract: Cretaceous and Cenozoic benthic ostracodes and their species assemblage structure have been analyzed by several different association and distribution form techniques. 425 core samples were obtained by the Deep Sea Drilling Project from 77 sites of the first 14 Legs of its survey of the world ocean floor. These analyses of 80 species from 27 composite planktic-foraminiferal time zones representing the last 75 million years of deep-sea faunal development revealed that a major change took place at about 40X106 BP in Late Eocene or Early Oligocene, marking the time of origin of the present cold, bottom water-mass system (psychrosphere). Prior to this world-wide event another important, but previously unknown, ostracode fauna had been present in a wanner deep sea for at least 25 million years without significant change.

Trace fossils as environmental indicators in the Corallian of England and Normandy

Abstract: The distribution of trace fossils in Corallian (Upper Jurassic) rocks has been studied in three coastal areas in Yorkshire, Dorset, and Normandy. Three trace fossil associations, primarily related to the hydrodynamic conditions and only indirectly to depth, can be distinguished: the high-energy Diplocraterion association, and the low-energy Rhizocorallium and Teichichnus associations. The characteristic members of these associations can be used as environmental indicators throughout the Jurassic.

Provinces, communities, and the proposed nektobenthic habit of Ordovician conodontophorids

Abstract: Ordovician conodontophorid distribution, paleogeography, paleoclimate, and global tectonics strongly support the existence of one biogeographic conodontophorid province, largely restricted to equatorial regions and characterized by raised salinity and temperature. This Midcontinent Province is recognized in North America, Australia, and Siberia. The same data indicate that in other areas the North Atlantic Province represents a normal-marine, virtually cosmopolitan fauna. In both provinces, communities are recognized that show a lateral segregation, strongly indicating that the majority of Ordovician conodontophorids were benthic or nektobenthic. The only Ordovician pelagic forms we recognize are the simple cone genera that either lack, or have a simple, symmetry transition series. It appears possible to correlate certain evolutionary changes in the conodont apparatus with changes in preferred habitat.

Biology of the Hyolitha

Abstract: Hyoliths are Paleozoic fossils that have a calcareous exoskeleton consisting of an elongate, usually bilaterally symmetrical cone, a close fitting operculum, and a pair of curved appendages. Their skeletal ultrastructure resembles the crossed-lamellar shell layers of some molluscs. Several specimens from the Ordovician of France and the Cambrian of Antarctica have parts of the gut preserved by infilling matrix, showing that both mouth ad anus were located near the cone aperture. Muscle scars in other hyolith shells indicate that the animal had a series of dorsoventral and longitudinal, or longitudinal and circular muscles, which operated through a hydrostatic skeleton to protract and retract the head, to open and close the operculum, and to move the appendages. Although the shell form and skeletal ultra-structure of hyoliths are of a molluscan type, the muscle insertions suggest that the hyolith cone is not homologous with the dorsal exoskeleton of primitive molluscs. Hyoliths probably constitute a small extinct branch of phylum size, related to the Mollusca and the Sipunculoidea. All three groups may have had common ancestors in the late Precambrian.

Morphology and genesis of nodular phosphates in the Cenomanian Glauconitic Marl of south‐east England

Abstract: Phosphatic nodules are abundant in the Glauconitic Marl (Cretaceous, Cenomanian) of south-east England, particularly where the sequence is condensed. Some of the nodules are derived from the underlying Upper Greensand, and are phosphatic fossil fragments, fossil moulds, and calcareous concretions. Concretions in particular show signs of a complex history of multiple phases of boring, encrustation, phosphatisation, and glauconitisation. Phosphate and glauconite are both replacements after fine-grained carbonate sediment and cement. The majority of the phosphates are whole and fragmentary moulds of fossils. The origin of theses nodules involved: (1) infilling of shells, (2) burial, (3) prefossilisation-cementation of fossil infillings. probably by high magnesian calcite, (4) dissolution of aragonitic shell material, (5) disinterment and exposure of moulds on the sea floor, followed by (6) phosphatisation. boring. and enerustation by various organisms, and sometimes glauconitisation. Many nodules bear evidence of several cycles of cementation, exposure, mineralisation, boring and enerustation. The closest Recent analogues to the Glauconitic Marl phosphates appear to be the phosphatic crust and nodules forming today off the coast of southern California. The features described and processes inferred from the Glauconitic Marl occurrences appear to have been widespread in nodular phosphatic facies.

Girvanella and other algae as depth indicators

Abstract: Girvarzella is interpreted to be the calcified sheath of a variety of filamentous blue-green algae. At present there are no clear limits which can be set on its depth of occurrence. The usefulness of benthic calcareous algae as depth indicators in palaeoenvironmental studies is dependent upon the level of identification. At high taxonomic levels it is very limited. Algae are not restricted to the euphotic zone (ka. 150 m). Calcareous green algae are restricted to shallower water (0-ca. 100 m) than are calcareous red algae (0-ca. 400 m) but they are prone to fragmentation and subsequent transportation. Narrower depth limits for lower taxonomic levels have been demonstrated in Recent green and red algae, but have yet to be recognized in ancient environments. In the Palaeozoic, problems of correctly identifying calcareous algae further restrict their use as depth indicators.

Burrowing of the lingulid brachiopod Glottidia pyramidata: its ecologic and paleoecologic significance

Abstract: Contrary to popular assumption, the pedicle of Glottidia is not its principal burrowing organ. The brachiopod props itself up with the pedicle and enters the sediment with the valves leading, anterior end first. The pedicle trails behind. Burrowing is accomplished by cyclical valve motions: rotary, sliding, and gaping movements are used. Rapid valve closure ejects water from the mantle cavity to loosen the sediment. The lateral setae convey mucus-bound sand posteriorly (upwards). X-radiography shows that the burrows are U-shaped: in a few hours, the animal reappears in feeding position with the anterior end pointing out of the sediment and the pedicle extending down into the burrow. This burrowing process explains the substrate preferences of lingulids. The thick. closely spaced setae are adapted for burrowing. The spacing between individuals is increased, suggesting competition for food.

Lower Cambrian trace fossil evidence for predation on trilobites

Abstract: Predation upon trilobites previously has been inferred from large coprolites containing trilobite fragments, and from specimens of trilobites with healed wounds. The discovery of large burrows (Dolopichnus gulosus, n. ichnogen., n. ichnosp.) in micritic quartz arenite of the Lower Cambrian Poleta Formation in Esmeralda County, Nevada, suggests that sea anemones preyed upon trilobites. Dolopichnus n. ichnogen., vertical cylindrical burrows with a central cylindrical core, is interpreted as dwelling burrows of sea anemones. In the specimens studied, the core contains coarser-grained material, and in one series of burrows, is composed of trilobitc fragments and micrite pellets. cemented with sparite. The central cylinder is interpreted to be a cast of the sea anemone's coelenteron, which in some specimens contains stomach contents.

The production and preservation of trilobite resting and furrowing traces

Abstract: The production and preservation of trilobite resting and furrowing traces is discussed with special reference to specimens from Upper Cambrian strata in North Walcs, Spain, and Poland. Resting excavations are attributed largely or completely to animal activity, and do not normally rcsult from currcnt scouring. Resting excavations, furrows, and many other trace fossils are formed at the sediment-water interface and not bencath a thin sand layer or by burrowing between beds. The factors which allow the initial production of the traces on the ocean floor muds (e.g. cohesion, shear strength, low pore water pressures) will also favour their preservation during deposition of the casting sand.

Calcification in the bivalve periostracum

Abstract: The periostracum in certain bivalves is imbedded with calcified, spiculelike structures analogous if not homologous to cuticular spicules found in the Aplacophora and Polyplacophora (chitons). Although rare or absent in most living bivalves, calcified periostracal structures are apparently an ancestral feature in some bivalve groups, i.e. the Mytilacea, Permophoridae, Myoida. and Anomalodesmata. Ancestors of the Bivalvia and Polyplacophora may have been covered with a flexible, spiculestudded cuticle. Shell plates in these two classes may have originated through a modification of the mechanism of spiculelike cuticular calcification. resulting in a primordial shell with simple prismatic structure.

The stromatoporoid animal

Abstract: Stromatoporoids were a subphylum of the Porifera whose soft parts can be reconstructed by comparisons with the living sclerosponges Merlia and Astrosclera. The living tissue was confined to the upper surface and penetrated only short distances into the coenosteum. Astrorhizae are traces of an excurrent water canal system that interfered with the secretion of the skeleton in some stromatoporoids but was entirely above the hard tissue in others. The stromatoporoid skeleton was composed of trabecular or spherulitic aragonite. Calcitization and dissolution of the aragonite proceeding from the centers of calcification outward account for the microstructures (fibrous, compact, tripartite, ordinicellular, cellular, melanospheric) commonly observed in the calcite skeletons of fossil stromatoporoids. Reconstructions showing the proposed relationship of the soft tissue to the hard tissue of Labechia, Stictostroma, Actinostroma and Stromatopora are presented.

Paleoecology of Middle Ordovician stromatoporoid mounds in Vermont

Abstract: Fossiliferous mounds of carbonate mud are a distinctive facies in the middle Chazy Group (Crown Point Formation) at Isle La Motte, Lake Champlain. The mounds are surrounded by bedded calcarenite of spar-cemented pelmatozoan debris. Channels which cut into the mounds during mound growth are filled with the same calcarenite. The mud-free intermound rocks and the mound biota suggest agitated, normal marine shallow-water environments. The principal lime-secreting organisms within the mounds are stromatoporoids, calcareous algae, tabulate corals, sponges, and bryozoans. Each mound is dominated in terms of biomass by one of three groups: stromatoporoids, calcareous algae, and bryozoans. Most of the mound biota first appear at the base of the Crown Point Formation. In the lower Crown Point Formation the organisms increase in number and species. Both changes in the biota are related to periods of shallowing of the Chazy sea which are also reflected in the character of the carbonate sands.

Cladostrobus and the systematics of cordaitalean leaves

Abstract: Cladostrobus Zalessky, 1918, is a male strobilus with spirally arranged microsporophylls, having a rhomboid lamina and a long stalk. The sporangia occur in groups. Microspores are of monosaccate type and are without a haptotypic mark. The name Cladaitina dibnerae gen. et sp. nov. is introduced for isolated spores of this kind. Cladostrobus may be a male strobilus of plants that bore Rufloria leaves. The group Cordaitales, as known now, comprises a heterogeneous assemblage of incompletely known plants, mostly sterile leaves. An artificial classification is proposed for sterile leaves, similar to the one already in vogue for dispersed spores and pollen. This classification is in fact a key for identification and cataloguing, and is not meant to show natural affinities.

Towards a theoretical basis of paleoecology: concepts of community dynamics

Abstract: The science of paleoecology suffers from a lack of conceptual frameworks. Paleoecologists have been overconcerned with the inadequacies of the fossil record: as a result, community palmecology has historically developed very slowly. At the community ecosystem level, the need for a theoretical framework is so great that paleoecology must ‘borrow’ the hypotheses of modem ecology. Consideration of the stability-time hypothesis of Sanders in conjunction with the physical setting of transgression and regression has permitted the structuring of three community types and the interpretation of their behavior under variations in the physical environment. These community types (opportunistic. stable mature, relict mature) are recognizable in the fossil record and examples are given from the Upper Pennsylvanian of the Appalachian Basin.

Structure and habits of the ‘living fossil’ pelecypod Neotrigonia

Abstract: Neotrigonia (Molluscs, Pelecypoda). the only surviving genus of the once diverse and widespread Trigoniacea, was collected live from depths of 6–80 m on sandy mud bottoms off southeastern Australia. The two species collected. Neotrigonia gemma Iredale, 1924 and Neotrigonia margaritacea (Lamarck), 1804, are structurally similar and differ mainly in size and coloration. Neotrigonia has such primitive features as an unfused mantle and filibranchiate ctenidia. Its foot is large, active and muscular and the ctenidial ciliary pattern resembles that of the Unionacea. The shell is non-streamlined and has pronounced radial omamentation. Neotrigonia lives infaunally near the sediment surface, filter feeds. and is capable of fairly rapid burrowing. The burrowing facility, conferred by the foot, is unexpected in light of shell morphology. This suggests that shell morphology alone is not necessarily a reliable guide to the life habits of fossil trigoniaceans.

Evolutionary models of lineage‐zones

Abstract: Among the various kinds of biostratigraphic units, lineage-zones are especially interesting and meaningful, not only in stratigraphy but also in evolutionary biology. The chronological shift of morphology in evolution is fundamentally uninterrupted, but sometimes it seems discontinuous or even saltational. Such breaks have been commonly attributed to gaps in sedimentation, but could be explained, at least in some cases, by more essential mechanisms of evolution. The difference in the patterns of morphological change may be primarily related to the nature of non-sex-associated genetic variation. continuous or discontinuous, as well as the strength of selection pressure. Some theoretical models for phenotypic substitution are presented here in terms of population genetics, though these are not the only possible explanation for breaks in fossil records.

Das filter‐feeding‐System bei Spiriferida

Abstract: SEM studies of the brachidial microstructure of brachiopods of the Spirifer group, Atrypa group, and A thyris suggest that the lophophore of each group was fixed to the basal side of the lamella. On this basis, there are four possible constructions of the spiriferoid lophophore, but the most probable one is a doubled lophophore acting as a filter tube which draws water into the mantle cavity laterally and ejects it through a median exhalant aperture. Rasterelektronenmikroskopische Untersuchungen der Brachidien von Spiriferen, Atrypen und Athyris erlauben die Schlusfolgerung, das bei allen drei Brachiopodengruppen der Lophophor an der proximalen bzw. basalen Seite der kalkigen Spirallamelle gehangen hat. Von den dann moglichen vier Modellen des Lophophorensystems erweist sich das eines Filterdoppelkorbs am wahrscheinlichsten, der das Frischwasser seitlich in das Gehause einstromen und in der Mitte ausstromen lies.

Methods and conclusions in functional analysis: a reply

Abstract: This paper is a reply to criticism presented by C. R. C. Paul and R. Cowen in immediately preceding articles in Lethaia. Contrary to the assertion by Paul, the present author did not reject the paradigmatic method but (1) criticized it for its limitations and seemingly erroneous results, (2) applied it to the richthofeniacean and lyttoniacean brachiopods by use of the pump as paradigm, and (3) extended it to include anatomic, metabolic, and phylogenetic considerations that go beyond the structural and mechanistic constraints of the original formulation. The rhythmic flow mechanism was rejected primarily on the basis of evidence for a ptycholophous lophophore in the Productidina. and the relative inefficiency of ‘valve flapping’ in contrast to ciliary action in pumping nutrient-bearing fluid into the shell. A Chinese blast furnace was claimed by Cowen to employ a single oscillating panel as a pump, but instead this panel must act as a valve that admits air in surges while the draft in the furnace maintains unidirectional flow through the system. This is analogous to ciliary feeding in brachiopods, where undirectional flow is maintained by ciliary pumping, and the valve opens to admit water. A richthofeniid with a Composita entrapped beneath the apertural meshwork is introduced as additional evidence against the likelihood of ‘valve flapping’.

The diachronism of the Wenlock Limestone

Abstract: A quick transgressive event occurs near the top of the Wenlock Limestone. The initiation of this is thought to be synchronous, proving the top of the Wenlock Limestone to be diachronous and younging southwards. As a result the top of this rock unit cannot be used satisfactorily as a time line. It is suggested that the initiation of the transgression be taken as the practical means for the correlation of the Wenlock-Ludlow Series boundary. The Birmingham Siltstone Formation is erected for the strata between Dudley and Ludlow which are younger than Wenlock Limestone and older than the transgression. The Wenlock Limestone of Usk was deposited after the transgression and is unrelated to the main Wenlock Limestone. Thus it is treated as a separate entity and named here the Usk Limestone Formation.

Lower Silurian animal communities — three case histories

Abstract: A temporal succession of three marine animal communities of early Silurian age is described from the Midland Valley of Scotland. These formed part of an offshore onshore series of communities developed on the platform of a prograding delta under regressive tectonic conditions. All three communities were trophically structured in the same way as their modern analogues. In the earliest (isorthid-Glassia) community filterers and collectors were dominant; the absence of swallowers is attributable to the nature of the substrate. Soft-bodied swallowers, revealed only by their network of fine burrows, dominated the succeeding (Skenidioides-Dicaelosia) community. Filterers, which so dominated the hard-bodied fauna, were of only secondary importance: they lived clustered together in small aggregations, the sediments around which were reworked by the swallowers. A major fall of volcanic ash followed by an increase in the variability of the physical environment resulted in the abrupt replacement of this community by the Eoplectodonta-Visbyella community. Filterers were again dominant, with collectors subordinate; swallowers were comparatively rare.

A reappraisal of the paradigm method of functional analysis in fossils

Abstract: The paradigm method involves comparing fossil organs with theoretical structures (paradigms) which are 100% efficient in performing proposed functions. Applications of the method have been criticised as being unsystematic and mechanistic, producing conflicting results and ignoring pertinent limitations. Selected interpretations of zigzag slits and spines in brachiopods and bivalve molluscs are re-examined. None invalidate the method. The last step in paradigm analysis, i.e. selecting the most appropriate function, is a major weakness and should be avoided. Repeated paradigm analysis may be used to interpret evolutionary trends in terms of adaptation and functional efficiency.

An environmental study of Silurian atrypid communities from southern Norway

Abstract: Atrypid communities are described from the Silurian succession of southern Norway. These atrypids show striking variation in their mode of occurrence, several features of which are described quantitatively. Mature, well-developed communities occur in calcareous rocks which were apparently deposited in a well-aerated shallow environment. Stunted communities (identified by small shell size despite maturity) occur in carbonate-poor sediments deposited in deeper and less turbulent environments. Other communities are composed of immature shells which were apparently killed by a catastrophic event. Communities are also seen in which catastrophe was not total, and survivors can be recognised. The atrypid niche was taken over by other animal communities when major environmental changes took place.

‘Flapping valves’ in brachiopods

Abstract: M. J. S. Rudwick and others postulate ‘rhythmic-flow’ feeding for the Permian richtho-feniacean bi-achiopods, whereas R. E. Grant claims that they fed by normal ciliary action. Suspension-feeding has two components, current generation and food capture; normal brachiopod lophophores do both, but this is neither universal nor compulsory among animals. Opening and closing the richthofeniid shell generated a ‘tidal-flow’ current precisely analogous to respiratory currents in mammals; this is neither inefficient nor ‘self-defeating’, as Grant claims. Grant's analysis fails because he chose the wrong mechanical analogy (a pump). Morphological features of richthofeniids are better explained on a tidal-flow hypothesis than on a ciliary-flow model, and all the data adduced by Grant in rejecting the former is compatible with it or favorable to it. It explains morphological features that are bizarre mysteries on the ciliaiy-current model, and is therefore superior even though it implies that these Permian brachiopods were radically innovative.

Silurian brachiopods in a stromatoporoid bioherm

Abstract: Ludlovian limestone at Ljugarn, Gotland. repreqents a shallow marine environment of stromatoporoid biohenns and flanking areas of mud and stromatoporoid debris. Brachiopods were restricted to surfaces of the debris deposits. Lissatrypa sulcata, a smooth atrypacean, dominated the brachiopod fauna, with individuals living beak downward in high density clusters, developing weaker pedicle attachment and a sulcus during ontogeny. Surrounding low density areas of Lissatrypa included small, pedicleattached, non-sulcate shells. Howellella, Spirigerina, and Morinorhynchus? lived in isolated clusters near low density Lissatrypa populations, and 10 other very rare brachiopod species may have been non-reproducing members of the community. Brachiopcd species diversity, and thus number of niches, was lower in the biohermal enviroment than in bathymetrically equivalent, level-bottom communities.

Données paléobiologiques et paléobiogéographiques sur quelques Strophochonetinae du Dévonien d'Espagne et du Massif Armoricain

Abstract: The lack of hinge spines on the left side of the ventral valve in Chonetacea is demonstrated here for the first time. This morphological peculiarity has a special bearing on the evolutionary trends of the Strophochonetinae. The existence of several species of Chonetacea, both in the Armorican Massif, the Cantabrian Mountains, and Aragon, is a new fact which confirms very close relations between these three areas during Lower Devonian times.

Editor's column: Planktic, nektic, benthic

Abstract: The terms planktic, nektic, and benthic are taken as examples of the construction and malconstruction of biological and geological terms. Both in vernacular terminology and systematic nomenclature the formation of adjectives and compounds follows the same basic rules - keep the three concepts of stem, suffix, and connective vowel clear, and most of the serious malconstructions will be avoided.

The function of the brachial valve septa in plectambonitacean brachiopods

Abstract: Eoplectodonta transversalis (Wahlcnberg) and other sowcrbyellids have a complex arrangement of septa in the brachial valve. It is argued that the function of these is primarily to support the lophophore and to strengthen the shell, and that the septa mirror is almost exactly the shape of the lophophore. In Eoplectodonta transversalis the ontogenetic develop ment of the lophophore varies from a simple to a complcx ptycholophe. Other plectam-bonitaceans have trocholophous and schizolophous lophophores.

An automated coordinate recorder for biometry

Abstract: Many measuring devices used in micropaleontology are unsatisfactory because observations must be recorded manually. Moreover, most use a linear scale that depends on the presence of predetermined morphological loci. An x, y recorder, largely adapted from instruments available in geological laboratories, is discussed. It obviates manual recording, frees data collection from the constraint of homologous loci and. in some applications, permits more shape information to be collected.

Paleoecology of some Carboniferous Pectinacea

Abstract: Pectinacean bivalves in the Carboniferous of northern California, USA, occur in all observed biotic communities except bioherms of corals and productoid brachiopods. They show equal abundance in offshore, brachiopod-dominated habitats and nearshore, bivalve-dominated habitats, and are among the only common invertebrates in a ‘black shale’, restricted marine community. Different morphologic forms of the superfamily show little ecologic division. Pectinacea are least abundant in limestone, and most abundant in terrigenous beds, and bottom type was more important in controlling their distribution than water depth, current activity, or salinity.