First direct evidence of ammonoid ovoviviparity

Ammonoids had high evolutionary rates and diversity throughout their entire history and played an important role in the high‐resolution sub‐division of the Mesozoic, but much of their palaeobiology remains unclear, including the brooding habitat. We present our study of the first recorded ammonite embryonic shell clusters preserved with calcified embryonic aptychi in situ within the body chambers of mature macroconch shells of the Early Aptian (Early Cretaceous) ammonite Sinzovia sazonovae. The following support the idea that the clusters are egg masses, which developed inside ammonite body chambers: the absence of post‐embryonic shells and any other fossils in these clusters, the presence of the aptychi in all embryonic shell apertures and peculiarities of adult shells preservation. These facts confirm earlier speculations that at least some ammonoids could have been ovoviviparous and that, like many modern cephalopods, they could have reproduced in mass spawning events. The aptychi of ammonite embryonic shells are observed here for the first time, indicating that they were already formed and calcified before hatching. Our results are fully congruent with the peculiar modes of ammonoid evolution: quick recovery after extinctions, distinct evolutionary rates, pronounced sexual dimorphism and the nearly constant size of embryonic shells through ammonoid history. We assume that adaptation to ovoviviparity may be the reason for the presence of these features in all post‐Middle Devonian ammonoids.     

Early evolutionary trends in ammonoid embryonic development

During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.     

Rhythms of ammonoid shell secretion

From measurements of the thickness of successive growth rings shell growth curves were constructed for seven ammonoid genera of different taxonomic position and geological age. All the curves have 'sinusoidal' aspect due to the alternation of maximum and minimum growth increments of the shell tunc. Growth curves made for two ammonoid shells belonging to the genera Proloxyclymenia (D₃fm) and Euphylloceras (K₁a) show regularly recurring minima and maxima at intervals of 14–16 growth rings. The fact that the minima and maxima of the growth curves recur exactly at intervals of 14–16 growth rings seems to reflect the relationship between the shell tube secretion and the formation of septa. This fact also seems to support the views about fortnightly or 28-day cycles in the construction of ammonoid hydrostatic chambers. By counting the total number of septa in the phragmocones, the duration of phragmocone secretion for 23 shells was estimated. Preliminary inferences on the presence in ammonoid shells of daily, bidaily and weekly growth rings are made. □ Ammonoidea, growth rhythms.     

Extensive intraspecific variation in a Triassic ammonoid from Siberia

The ammonoid species Czekanowskites rieberin.sp . from the Lower Anisian of Arctic Siberia (River Olenek) shows an enormous range of intraspecific variability. The 700 investigated specimens are packed in one single concretion. They are all adult with preserved living chambers and apertural structures. Morphologically they range from keeled smooth suboxycones with narrow umbilicus, through feebly ribbed platycones with a little wider umbilicus, to subcadicones with relatively wide umbilicus and straight ribs with bullae. The morphological variation is continuous; all specimmh belong to a single genetically linked population, variants of one biospecies. This phenomenon, which is quite common in boreal Triassic ammonoid faunas, has in the past led to serious taxonomic oversplitting. It presents a serious challenge to current ideas about a close correlation between modeof life and shell morphology in arnmonoids.□ Amntonoidea, boreal Triassic, ecology. Siberia, variability .     

Protoconch morphology and peculiarities of the early ontogeny of the Cambrian helcionelloid mollusks

Morphology of the initial parts of shells of the ancient Cambrian mollusks is studied. New original data on size of the embryonic shell of the Cambrian gastropods are analyzed. It is supposed that mollusks had larvae of both direct (benthic) and planktic lecitotrophic types of development even as early as the Cambrian. The wide geographic distribution of some of the Cambrian mollusk species in the distant paleobasins of the world is a convincing evidence for the existence of planktic stage of development in the mollusk ontogeny of that period.     

Cephalopod embryonic shells as a tool to reconstruct reproductive strategies in extinct taxa

An exhaustive study of existing data on the relationship between egg size and maximum size of embryonic shells in 42 species of extant cephalopods demonstrated that these values are approximately equal regardless of taxonomy and shell morphology. Egg size is also approximately equal to mantle length of hatchlings in 45 cephalopod species with rudimentary shells. Paired data on the size of the initial chamber versus embryonic shell in 235 species of Ammonoidea, 46 Bactritida, 13 Nautilida, 22 Orthocerida, 8 Tarphycerida, 4 Oncocerida, 1 Belemnoidea, 4 Sepiida and 1 Spirulida demonstrated that, although there is a positive relationship between these parameters in some taxa, initial chamber size cannot be used to predict egg size in extinct cephalopods; the size of the embryonic shell may be more appropriate for this task. The evolution of reproductive strategies in cephalopods in the geological past was marked by an increasing significance of small‐egged taxa, as is also seen in simultaneously evolving fish taxa.     

Ecological aspects of a silicified bivalve fauna from the Silurian of Gotland

The silicified Wenlockian (Silurian) bivalve fauna from MÖllbos, Gotland, is part of a life assemblage. The vast number of shells show unusual phenomena, e.g. shell repair, pearl and tumour formation, etc. A number of shells contain epibionts and bored, round holes. Presumptive predators of the bivalve community are discussed. Size-frequency distribution of the two most abundant species possibly reflects age classes. The fauna, comprising eleven species, is dominated by deposit-feeders (90 %). They exhibit niche diversification, including at least three different feeding levels within the sediment.     

Alternative methods of measuring competition applied to two Australian hermit crabs

Summary This article describes the intertidal hermit crab species assemblage at One Tree Island, Great Barrier Reef, Australia. Competition for shells between the two most abundant species, Clibanarius virescens and Calcinus latens, is studied in more detail. Competition appears to be primarily exploitative. The relative intensities of inter- and intra-specific competition between this pair of species are estimated using two different methods. The first is based on habitat overlap data in conjunction with a mathematical model of shell population dynamics described in Abrams (1980). The second method is more direct, and is based on following the fate of marked empty shells. Results of the two methods are very similar. This supports the validity of assumptions made in applying the first method. The relative amount of interspecific competition between these two species appears to be greater than that for other hermit crab species pairs studied previously.     

Eggshell structure, mode of development and growth rate in birds

The shell of an egg contributes to successful embryogenesis in many ways, such as through protection, respiration and water exchange. The shell is also the major source of calcium for the development of high-calcium consuming organs, e.g. the skeleton, muscles and brain. Some studies show, moreover, that growth rate may play a fundamental role in the pattern of skeletal development in birds: the faster the growth the less ossified the skeleton is at hatching. We predicted, therefore, that slow (precocial) and fast (altricial) growing bird species should lay eggs encased in shells with different structures adapted to support different rates of calcium removal by developing embryos. We tested this prediction by comparing the fine structure of the inner eggshell surface (mammillary layer) from 36 bird species belonging to 18 orders ranging from Struthioniformes to Passeriformes. Using scanning electron microscopy, we compared the mammillary layer of both non-incubated eggs and eggs at the time of hatching, i.e., before and after embryonic development and the accompanying calcium removal. The results were consistent with the prediction, i.e., the number of mammillary tips per unit of surface area was associated with mode of development and growth rate. The number was higher, and calcium removal was also more extensive, in shells from precocial bird species than in shells from altricial bird species.     

Resource partitioning and competition for shells between intertidal hermit crabs on the outer coast of Washington

Summary Resource partitioning was quantified for 6 species of intertidal hermit crabs in the genus Pagurus, that occur on the outer coast of Washington. This, together with field evidence of shell shortage and with laboratory experiments to quantify the mechanism of interactions for shells, allowed estimation of the relative intensities of inter-and intraspecific competition between these species. The findings were that: (1) the magnitude of intraspecific competition was greater than any single interspecific competitive effect for all of the species; and (2) the relative proportion of intraspecific competition was greater for the middle and upper intertidal species than for the lower intertidal species. Studies at several outer coast sites supported these generalizations. Both of these findings are consistent with the hypothesis that competitive divergence has occurred in the past. The structure of the outer coast hermit crab assemblage is compared with that of the San Juan Archipelago hermit crab assemblage. Differences between the two do not seem to be the result of adaptive responses to the presence of more competing species in the former group.     

Influence of setting,sieve size,and sediment depth on multivariate and univariate assemblage attributes of coral reef-associated mollusc death assemblages from the Gulf of Aqaba

Measures of diversity and ecology of marine invertebrate assemblages depend on a variety of factors including environmental conditions and methodological decisions. In this study, the influence of such factors on multi- and univariate assemblage parameters of molluscan death assemblages from the Gulf of Aqaba (Red Sea, Jordan) was evaluated. Sediment samples were collected at two coral reef types, a patch reef at 13 m of water depth characterized by fine-grained sediments and a Millepora-fringing reef with coarse-grained sediments at 5 m of water depth. The upper and lower 10 cm of the sediment column were separately removed and sieved with mesh sizes of 1 and 2 mm. A large dataset of 6400 bivalve and gastropod shells was compiled to evaluate how setting, sediment depth, and sieve size influenced taxonomic composition and species richness, species-abundance patterns and the Shannon–Wiener index, the number of drilled shells per species and drilling frequency (DF) of the assemblage. Setting had the strongest impact on all aspects, followed by sieve size, but sediment depth was insignificant, probably due to complete homogenization of the sediments by reworking and bioturbation. Multivariate assemblage parameters distinguished much better between categories (setting, sieve size) than univariate measures. Sieve size-related disagreements recognized between the two higher taxa are mostly due to the underlying difference in body-size distribution of bivalve and gastropod assemblages. We conclude that species richness and other ecological characteristics of molluscan death assemblages in coral reef-associated sediments will most strongly reflect habitat complexity of the sites chosen, are significantly influenced by methodological decisions (i.e., sieve size), will only poorly preserve temporal patterns, and the results will differ between bivalves and gastropods.     

The ammonoid genus <Emphasis Type="Italic">Spiroceras</Emphasis> (Spiroceratidae,Ammonoidea) from the Upper Bajocian of the Northern Caucasus

Ammonites of the genus Spiroceras are described from the Upper Bajocian of the Kyafar River Basin (Karachay-Cherkessia), including macroconchs with preserved apertures and (for the first time in the entire history of the study of Middle Jurassic heteromorphs) microconchs with lateral lappets. The microconchs of S. bispinatum (Baugier et Sauzé), mainly represented by cyrtocones with one ventral row of nodes, correspond to microconchs with a gyroconic shell and a single row of nodes. Macroconchs and microconchs of S. annulatum (Deshayes), the ribs of which lack prominent nodes or spines, are found together. The assemblage also contains small-sized slightly uncoiled shells of Spiroceras aff. S. fourneti Roman et Pétouraud with widely spaced ribs and two prominent rows of large nodes, presumably macro- and microconchs. The ornamentation of this species resembles that of the monomorph Bajocia rarinoda Sturani from the upper part of the Lower Bajocian; perhaps the latter was ancestral to the Middle Jurassic heteromorph. The above species of Spiroceras, as well as S. obliquecostatum (Quenstedt), are figured.     

Preferred orientation of calcite in the ratite and tinamou eggshells

Using X-ray diffractometry, the development of preferred orientation was studied in one species from each of the extant ratite families, from two moas and two tinamous and, as a preliminary comparison, from representative species from 16 of the other carinate orders. A preferred orientation begins to develop immediately after the start of shell deposition, and in all shells studied was one in which the (001) planes tend to lie parallel to the shell surface. The degree of (001) texture that develops in the ratite and tinamou shells is greater than that developed in the other carinate orders. Development in the ostrich, kiwi and tinamou is uninterrupted so that there is a very high degree of (001) preferred orientation at the exteriors of the shells. In the Emu, cassowary, rhea and the two moa species, a very high degree of (001) texture reverses gradually within the central layer of the shell, and in some species this is accompanied by the development of a very weak (104) preferred orientation. The weak (001) textures observed in the shells of the 20 species from the carinate orders other than the Tinamiformes develop slowly and without interruption through the shell; in four species a weak (104) texture develops simultaneously to that of the (001), forming a double texture similar to that already reported in the majority of shells of the domestic fowl.     

The plurennial life cycles of the European Tettigoniidae (Insecta: Orthoptera)

Summary The effect of drought on embryonic development and on hatching was studied in 13 European Tettigoniidae species. Drought can affect development in three different ways: (1) Embryonic development proceeds slower than if the eggs are in contact with water; (2) it stops (for final diapause) in an earlier embryonic stage; (3) it affects maintenance and termination of the initial embryonic diapause.In many Tettigoniidae species, the initial diapause is prolonged, and may last several years. Without draught stress, between 1 and 7 cold treatments in the laboratory, and with eggs of the Tettigonia-species between 1 and 6 winters in the field were necessary to enable all eggs to complete initial diapause. In Central European species, the number of eggs maintaining initial diapause significantly increased when the eggs had no contact with water at the time when they should recover from diapause. In contrast, termination of initial diapause in Tettigonia caudata from Greece, when the environment became favorable for growth again, was highest in that group of eggs that had lost most water in a preceding period of drought. The importance of the prolonged initial diapause for the survival of unpredictable adverse conditions is discussed.     

Larval ecology and morphology in fossil gastropods

The shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.     

Resource partitioning and competition for shells in a subtidal hermit crab species assemblage

Summary Shell and habitat utilization are quantified for 12 hermit crab species occurring subtidally in the San Juan Archipelago, Washington. The mechanism of competition for shells between these species is investigated using laboratory experiments to determine shell preferences, shell acquisition rates, and rates of exchange of shells via shell fighting. This information is used to estimate relative intensities of inter- and intra-specific competition for shells between the species in this assemblage. In contrast to earlier findings on intertidal hermit crab assemblages, a significant number (5) of the species in this assemblage appear to experience a greater reduction in their shell supply due to members of other species than due to other members of their own species. The relative amounts of inter- and intra-specific competition differ greatly for different species in the community. The high average figures for interspecific/intraspecific competition are largely a result of the presence of three abundant and very generalized species. In spite of the large number of species and relatively high ratios of interspecific to intraspecific competition, the species in this group are not close to a limiting similarity in resource use. There is suggestive evidence that greater selection pressures for divergence in habitat use may have resulted in the lower amounts of overlap observed in intertidal hermit crab assemblages in previous studies.     

Growth trajectories of some major ammonoid sub‐clades revealed by serial grinding tomography data

Molluscs such as ammonoids record their growth in their accretionary shells, making them ideal for the study of evolutionary changes in ontogeny through time. Standard methods usually focus on two‐dimensional data and do not quantify empirical changes in shell and chamber volumes through ontogeny, which can possibly be important to disentangle phylogeny, interspecific variation and palaeobiology of these extinct cephalopods. Tomographic and computational methods offer the opportunity to empirically study volumetric changes in shell and chamber volumes through ontogeny of major ammonoid sub‐clades in three dimensions (3‐D). Here, we document (1) the growth of chamber and septal volumes through ontogeny and (2) differences in ontogenetic changes between species from each of three major sub‐clades of Palaeozoic ammonoids throughout their early phylogeny. The data used are three‐dimensional reconstructions of specimens that have been subjected to grinding tomography. The following species were studied: the agoniatitid Fidelites clariondi and anarcestid Diallagites lenticulifer (Middle Devonian) and the Early Carboniferous goniatitid Goniatites multiliratus. Chamber and septum volumes were plotted against the septum number and the shell diameter (proxies for growth) in the three species; although differences are small, the trajectories are more similar among the most derived Diallagites and Goniatites compared with the more widely umbilicate Fidelites. Our comparisons show a good correlation between the 3‐D and the 2‐D measurements. In all three species, both volumes follow exponential trends with deviations in very early ontogeny (resolution artefacts) and near maturity (mature modifications in shell growth). Additionally, we analyse the intraspecific differences in the volume data between two specimens of Normannites (Middle Jurassic).     

Larval and juvenile gastropods from a Carboniferous black shale: palaeoecology and implications for the evolution of the Gastropoda

A horizon in the late Visean Ruddle Shale from Arkansas contains the oldest well-preserved gastropod protoconchs known from the Americas. The gastropod fauna consists of a diverse larval shell assemblage and a low diversity assemblage of juvenile gastropods that probably had a benthic life habit. Gastropod larval shells are always isolated, i.e. the gastropods did not complete their life cycle (no metamorphosis) and were unable to become benthic. This was caused by unfavorable environmental conditions on the soft muddy bottom that was probably due to anaerobic to exaerobic conditions. The absence or scarcity of bioturbation caused by invertebrate detritus or sediment feeders in both shale and concretions (formed before compaction) favored preservation of the delicate larval shells. The lack or scarcity of infauna and bioturbation as well as the low diversity of the presumed benthos supports an interpretation of a quasi-anaerobic to exaerobic benthic environment. The superbly preserved larval shells demonstrate that there are more caenogastropod clades present in the late Palaeozoic than suggested previously. Some larval shell types have an openly coiled first whorl followed by a planktotrophic larval shell; openly coiled initial whorls are unknown from modern caenogastropods. The vetigastropods have a smooth protoconch of two whorls clearly demarked from the following whorls - a pattern unknown in modern vetigastropods which have a protoconch of less than one whorl and build no larval shell during their planktonic stage. This could indicate a link between Palaeozoic vetigastropods and the caenogastropods.     

Patchiness of local species richness and its implication for large-scale diversity patterns: an example from the middle Miocene of the Paratethys

The best hope for understanding global diversity patterns is to compare local assemblages, which are mostly preserved in taphonomically-complex shell beds. The present study investigates the variability in faunal composition and diversity at the scale of a single outcrop. A total of 152 species (3315 shells) occurred in 25 samples from 5 tempestitic shell beds. Although sampling intensity was high, total species richness was not captured by far at the hierarchical levels present (outcrop, shell beds, samples) because the majority of species is rare. In contrast, sampling intensity was sufficient to cover the most abundant species, as indicated by stable evenness values. Four taxa dominate the assemblage, but their rank order differs strongly between individual shell beds and individual samples; significant differences between some shell beds are evident for faunal composition, and one shell bed differs from all others with respect to species accumulation curves. Within shell beds, rarefaction curves are generally characterized by strongly overlapping confidence intervals, but outliers occur in three of five shell beds. Patchiness is additionally indicated by a wide scatter of diversity indices in some shell beds and by a wide scatter of samples of one shell bed in an ordination on faunal composition. Most of the outcrop-scale variability in faunal composition and diversity can be related to differences between shell beds. This suggests that sampling a single shell bed of the outcrop is insufficient to characterize the local fauna and its diversity, even when sampling intensity (i.e. the number of samples and shells) within the shell bed was high. Similarly, a single sample from such a shell bed may not be sufficient to characterize its diversity, even when the number of counted shells was high. It is therefore confirmed that sampling strategy and sampling intensity are crucial to confidently characterize the shelly assemblages at such a small spatial scale and that dispersed sampling effort with many small replicate samples will characterize a local assemblage and its diversity better than a few large samples. Diversity comparisons of individual samples between localities must account for the high variability present at the smaller spatial scale, as observed in our study.     

Habitat use by a Midwestern U.S.A. riverine fish assemblage: effects of season, water temperature and river discharge

The hypothesis that temperate stream fishes alter habitat use in response to changing water temperature and stream discharge was evaluated over a 1 year period in the Neosho River, Kansas, U.S.A. at two spatial scales. Winter patterns differed from those of all other seasons, with shallower water used less frequently, and low‐flow habitat more frequently, than at other times. Non‐random habitat use was more frequent at the point scale (4·5 m²) than at the larger reach scale (20–40 m), although patterns at both scales were similar. Relative to available habitats, assemblages used shallower, swifter‐flowing water as temperature increased, and shallower, slower‐flowing water as river discharge increased. River discharge had a stronger effect on assemblage habitat use than water temperature. Proportion of juveniles in the assemblage did not have a significant effect. This study suggests that many riverine fishes shift habitats in response to changing environmental conditions, and supports, at the assemblage level, the paradigm of lotic fishes switching from shallower, high‐velocity habitats in summer to deeper, low‐velocity habitats in winter, and of using shallower, low‐velocity habitats during periods of high discharge. Results also indicate that different species within temperate river fish assemblages show similar habitat use patterns at multiple scales in response to environmental gradients, but that non‐random use of available habitats is more frequent at small scales.