New aspects in ammonoid mode of life and their distribution

The intensively debated functional morphology and mode of distribution of ammonites can be clarified and explained when ammonoids are regarded as conch-bearing octopods. The terminal body chambers of some ammonites were modified into a floating egg case, widely dispersing the hatchlings along the course of oceanic and long-shore currents. Hatchlings from eggs attached to a substrate lived and bred in the same region, developing indigenous evolutionary lineages. Females became sexually mature after 1–3 years of age, breeding only once, dispatching numerous eggs at a time. This contributed to the high evolutionary rate of ammonoids. Due to ammonoid short longevity, growth was rapid and septa were frequently precipitated. Ammonite internal molds exhibit small scars of adductor muscles, which could rapidly detach and reattach during septa secretion. The resultant weak hold between the conch and the body was compensated by the septal marginal fluting in the form of backward expanding lobes, into which the soft tissue penetrated, stiffening when needed. Increased suture complexity (unrelated to buoyancy regulation or diving ability) reflects a better hold between the body and the buoyant conch, hence a more successful functioning. The complex network of mantle muscle fibers could also form the template for septa precipitation. The high intelligence and learning ability of extant octopods can explain ammonoids’ adaptation to diverse niches, successfully coping with ecological changes and threats (hence evolution) in contrast to the associated nautiloids. Post-mortal drift of the empty conch was minor due to rapid sinking of shells of dead ammonoids, for which ammonites are good biogeographic indicators.     

Sublethal injuries in Middle Jurassic ammonite shells from Poland

Sublethal injuries, the effects of which are seen as regeneration patterns, are described from Late Bajocian and Bathonian (Middle Jurassic) ammonites from Poland (Polish Jura area) for the first time. The total number of ammonite shells bearing signs of sublethal injuries is small (only 11 specimens, which constitute ∼1.2% of all ammonites investigated), and this value is even smaller (∼0.3 to 0.8%) when analysing a large sample of a particular ammonite species. Specimens under consideration represent ten species, belonging to six genera and five families. All the healed injuries are represented by only one type, referred to as the ‘forma verticata’ of Hölder. This type of regeneration, very common in ammonite shells in general, is an effect of a puncture injury of the shell-secreting mantle-epithelia at the apertural margin. Although many different extrinsic (mechanical) factors may be responsible for such healed injuries, here it is most plausible they are an effect of either competitive or predatory activities. Other causes, like collision of the ammonite shells with the substrate in a high-energy environment, are excluded because the sea-bottom was soft and situated below the storm wave-base. From many potential predators inhabiting the Polish Basin during the Bajocian and Bathonian, the most likely to have caused these injuries are other ammonites, belemnites and nautiloids. Crabs, which are cited in the literature as a probable perpetrator of the ‘forma verticata’ injuries, appear unlikely here, as the ammonites under discussion were not purely benthic.     

Taphonomic differentiation of Oxfordian ammonites from the Cracow Upland, Poland

Taphonomic analysis of Lower and Middle Oxfordian ammonites from the Cracow Upland, southern Poland (localities at Pod???e, Zalas, M?ynka) revealed differences in ammonite preservation. The studied ammonites, usually termed as external and internal moulds, show a more complex state of preservation. In the Middle Oxfordian glauconitic marls, ammonites are preserved as internal moulds with neomorphic calcite shells showing relics of the original internal structure. In the Middle Oxfordian platy peloidal limestones, ammonites are preserved mostly as external moulds, without septal suture, however under microscope might show relics of internal whorls and septa and/or subtle differences in sediment filling phragmocone chambers. In sponge–microbial bioherms and biostromes, ammonite internal moulds have shells, which in contrast to ammonites from glauconitic marls are not strictly neomorphic ones, but originated by shell dissolution and subsequent filling of moldic porosity by calcite cement. In sponge–microbial nodular limestones, the ammonites are strongly deformed and the outer wall is usually removed by dissolution under pressure. Other important taphonomic differences include the rate of compaction (highest in platy limestones), sedimentary infillings, microborings, encrustations and preservation of siphuncular tubes. The majority of the ammonites appear to be phragmocones; aptychi in all facies are rare. Siphuncular tubes are fossilized exclusively in oppeliids, only in specimens from glauconitic marls and platy limestones, although their other taphonomic attributes are different. Tubes seem to have fossilized due to microbially mediated phosphatization that could be favoured by a set of parameters which operated rather at the scale of ammonoid carcasses: closed, poorly oxygenated conditions, and reduced pH. Taphonomic processes were controlled by the sedimentary environment (fragmentation, sedimentary filling, phosphatization of siphuncular tubes), as well as by early and late diagenesis (neomorphic transformation, dissolution, cementation, compaction) influenced by lithology.     

Sheltered preservation: a peculiar mode of ammonite occurrence in the Cretaceous Yezo Group, Hokkaido, north Japan

Maeda, H. 1991 01 15: Sheltered preservation: a peculiar mode of ammonitc occurrence in the Cretaceous Yeto Group, Hokkaido. north Japan. Lethaia , Vol. 24, pp. 69–82. Oslo. ISSN 0024–1164.
'Sheltered preservation', in which many small ammonites ('refugees') arc preserved in the inside and/or the lower umbilical void of a large ammonite shell ('shelter'), is a common preseivational style in the Cretaceous Yezo Group in Hokkaido, north Japan. The best example is large Calycoceras of Middle Cenomanian age, attaining 300 to 500 mm diameter, whose shell usually hours more than 100 individuals of Desmoceras comprising mainly juveniles less than 12 mm in diameter. In contrdst with other preservational styles of Desmoceras from the same horizon. the shells of refugee ammonites have not suffered phragmoeone collapse, and the inner whorls and a protoconch are intact. Many refugee ammonites are allochthonous, as indicated by the incomplete preservation of their living chamber and the absence of a jaw apparatus. The sheltered preservation is, therefore, a kind of 'Konzcntrat Lagerstatten' completed by both pre- and post-burial processes. Acting as a 'concentration trap' on the sea floor, a large empty ammonite shell probably triggered off such accumulation of the small ammonite shells in post-mortem transport across the sea floor. This biostratinomical process biased the original thanatococnosis of ammonites, so that its immature specimens were selectively accumulated and preserved in the shelter. Ammonoidea, taphonomy, Konzentrat Lagerstätten'. differenrial preservation, Cretuceous. Hokkaido .     

Shell features, main colonized environments, and fractal analysis of sutures in Late Jurassic ammonites

A precise approach to the quantification of relationships between suture complexity, as measured by fractal analysis (step-line procedure), the architecture of shells, and the main colonized environments, has been made in a set of Late Jurassic ammonites ( N =507). Statistically significant differences between fractal-dimension ( D _f) mean values of evolute and involute shells are interpreted as caused by differences in the surface:volume ( S:V ) ratio. Suture complexity is also related to the shape of whorl section. The lowest D _f values correspond to subcircular whorl sections (low S:V ratio) and the highest ones to acute sections (high S:V ratio). The shape of flanks shows correlation with suture complexity. The highest values of D _f are found in planulate shells and the lowest ones in whorl cross sections with convex flanks. Highly significant differences appear between D _f mean values from unsculptured shells and those from ammonites with ribs and/or tubercles of medium to large size. Multivariate analysis shows a combined variation of shell features and suture complexity, resulting in a heterogeneous distribution of D _f values within the ammonite morphospace, mainly according to structural (shell architecture) and ornamental (sculpture strength rather than density) factors. Finally, the data obtained on relationships between suture complexity and the colonized environments (epicontinental vs. epioceanic inhabitants) suggest that suture complexity is not primarily related to bathymetry, and/or that no major differences in habitat depths existed between epicontinental and epioceanic ammonites.     

A new mode of ammonite preservation – implications for dating of condensed phosphorite deposits

Unusual phosphatic casts of the ammonites Mortoniceras (Subschloenbachia) sp. and Stoliczkaia sp. from the upper Albian condensed phosphorite bed at Annopol, Poland, are discussed in terms of their taphonomic history. These specimens are interpreted as ‘secondary’ external casts of ammonite replicas preserved originally as attachment scars on oyster shells. The following genetic history is suggested for this previously undocumented mode of ammonite preservation: (1) settling of shells of dead ammonites on the seafloor; (2) colonization of these shells by oysters and formation of ammonite replicas on left valves of oysters; (3) dissolution of ammonite shells; (4) reworking and fragmentation of oyster shells; (5) casting of ammonite replicas by phosphatic material; and (6) separation of ammonite casts from oyster shells, either through mechanical disintegration or dissolution of the latter. The specimens studied were formed after dissolution of the ammonite conchs, not prior to this event as in the case of typical ammonite steinkerns (internal moulds). Therefore, they are here referred to as ‘pseudo‐steinkerns’. The time interval between loss of the original ammonite shells and the formation of oyster‐mediated pseudo‐steinkerns may be very extensive. Therefore, the pseudo‐steinkerns may potentially mislead in biostratigraphic dating of condensed phosphorite deposits.     

Why are there few algae on snail shells? The effects of grazing,nutrients and shell chemistry on the algae on shells of Helisoma trivolvis

1. Freshwater snails often lack visible growths of algae on their shells. We tested three possible mechanisms that may account for this (grazing, snail-derived nutrients and chemical defences), using the ramshorn snail Helisoma trivolvis .
2. The experiments were carried out in floating plastic enclosures in a pond and comprised seven treatments. Grazing treatments were: a lone snail (ungrazed, as self-grazing does not occur), Helisoma with conspecifics, Helisoma with the co-occurring pond snail Physa sp., empty shells with Helisoma , and ungrazed empty shells. Nutrient effects were possible in all treatments with occupied shells (lone snail; Helisoma with conspecifics, and with Physa ) versus absent in other treatments. Testing for chemical defences compared algae on fresh empty shells, weathered shells (outer organic periostracum layer absent) and boiled fresh shells (with denaturization of susceptible proteins).
3. Diatoms dominated algal assemblages on snail shells. Although the upright diatom Gomphonema gracile was abundant on all shells, it was dominant on the shells of snails housed with other snail grazers (either Helisoma or Physa ).
4. Only the lone snail (nutrients but no grazing) showed higher algal biomass, so presumably any nutrient effect in the treatments with grazers was masked. Both Helisoma and Physa were observed apparently grazing on Helisoma shells, and consequently algal biomass in multi-snail treatments was similar to that on empty shells. Scanning electron microscopy revealed that algal density was highest near the aperture of live snails, but not empty shells; this is consistent with a nutrient addition effect. There was no evidence of chemical defences against algal growth.
5. In soft-bottomed freshwater habitats with abundant snails, shells of living snails provide nutrient-augmented substrata that may indirectly boost overall snail production.     

The occurrence and preservation of ammonites in the Blue Lias Formation (lower Jurassic) of Devon and Dorset, England and their palaeoecological, sedimentological and diagenetic significance

More than two thirds of beds in the lowest Jurassic, Blue Lias Formation lack ammonites, which are commonly preserved in irregular or planar-bedded, bioturbated limestones, very rarely in laminated limestones and almost never in laminated black shales. Ammonites are preserved in 3D in nodular and planar-bedded limestones and at any orientation to bedding. Co-occurrence with macrobenthos and absence from beds without benthos suggest that Blue Lias ammonites were nektobenthonic. Scour structures and imbrication of ammonites in the Best Bed imply presence of traction currents. Lack of epifauna on large cephalopod shells (and other fossils) implies rapid deposition in event beds. Blue Lias deposition was episodic, not slow and continuous as the fine grain size implies. Undistorted trace fossils, uncrushed ammonites and stable isotope values all suggest early cementation of limestone beds from pore waters of a similar composition to contemporary Jurassic sea water. A clear diagenetic trend exists, with limestones having least, and laminated black shales most, modified stable isotope values. Contrast between trace fossil fills and host sediment demonstrates that Blue Lias rhythms are primary, but limestone beds have been diagenetically cemented.     

Synthesis and Assembly of Simian Virus 40 I. Differential Synthesis of Intact Virions and Empty Shells

Intact virions and empty shells of simian virus 40 may be rapidly separated from each other and from cell contaminants by a procedure employing a CsCl cushion. This approach permits quantitation of their respective syntheses in infected cells labeled with radioactive amino acids. As much as 5 to 10% of the total acid-precipitable radioactive lysine in infected cell extracts was incorporated into viral particles in a two-hour pulse late in infection. Evidence for multiple origins of empty shells is presented. Some of the empty shells result from breakdown of intact virions. However, empty shells can also form independently of intact virions. First, labeling for periods of 15 min to 2 hr late in the course of infection results in preferential incorporation of (3)H-lysine into empty shells. Secondly, treatment with the deoxyribonucleic acid inhibitor cytosine-beta-d-arabinofuranoside late in infection results in a 50% inhibition in the rate of formation of intact virions with minimal reduction in the rate of appearance of empty shells.     

Morphostructural constraints and phylogenetic overprint on sutural frilling in Late Jurassic ammonites

The functional significance of frilled septa and complex sutures in ammonoids has generated ongoing debate. The 'classic' hypothesis envisages ammonoid shells and septa as designed for resisting ambient hydrostatic pressure, complex sutures being the evidence of strength in shells for colonization of deep habitats. Here we address the 'suture problem', focusing on the analysis and interpretation of variables in our database of Late Jurassic ammonites not included in previous studies, such as whorl height ( W h ), whorl shape ( S ), shell coiling ( WD ), taxonomic grouping and basic planispiral shell shape. The results indicate that sutural complexity, as measured by the fractal dimension ( D f ) value of the suture line, is positively correlated with W h , and that the sutures of oceanic shells tend to provide, for a given W h value, lower D f estimates than do those of neritic shells. No general trend of increase in sutural complexity was noted for specimens recovered from swell areas belonging to oceanic fringes with respect to those that inhabited epicontinental shelves. In fact, Perisphinctoidea, the clade best represented in the database analysed, shows a higher D f mean value in neritic species than in epioceanic ones. Significant differences in sutural complexity were detected for groups of ammonites classified according to shell shapes ( WD , S ). Oxycones and discocones, streamlined potential swimmers, show the highest D f mean values, while spherocones and cadicones, which were presumably vertical vagrants, present the lowest ones. This indicates that sutural complexity was more related to shell geometry than to bathymetry.     

Ammonite eggs and ammonitellae from the Kimmeridge Clay Formation (Upper Jurassic) of Dorset, England

Eight clusters of small spherical and subspherical objects, some isolated and some associated with shells of perisphinctid ammonites, have been recovered from the Lower and Upper Kimmeridge Clay (Upper Jurassic) of the Dorset coast, England. They have been interpreted as ammonite egg sacs and represent the freshest and best-preserved examples known so far. Their structures and the ecological framework in which they occur are discussed. The parents are thought to be members of the two eudemic genera Aulacostephanus and Pectinatites that dominate the biostratigraphy of the ammonites in the range of the Kimmeridge Clay in which they occur. Isolated nuclei of ammonitellae have also been recovered.     

The effects of gastropod shell availability and habitat characteristics on shell utilization by the intertidal hermit crab Pagurus longicarpus Say

The shell utilization patterns of two intertidal populations of the hermit crab, Pagurus longicarpus Say, were studied. The populations differed with respect to the physical characteristics of their habitats and the availability of empty gastropod shells. The first population was in an estuary connected to Narragansett Bay. This population had few, if any. empty shells available in the area. The second population was in a rock-cobble area directly on Block Island Sound. There were large numbers of empty gastropod shells available for this population. The consistently high gastropod mortality on this shore appears to be caused by the animals being washed up on the beach, where they die due to desiccation.Data from samples collected from June to November of both 1974 and 1975 showed that there were distinct differences in the shell utilization patterns of the two populations. A greater proportion of the animals collected from the estuary population inhabited damaged shells and/or shells with symbionts compared to the individuals from the population with large numbers of empty shells available.The results of shell selection experiments indicated that individuals from both populations inhabited less than preferred shells. The population with large numbers of empty shells available was qualitatively shell-limited because of the effect of physical factors on the morphology of the gastropod species of that area. In addition, the shell selection experiments showed that individuals from the two areas had significantly different shell preferences. This result may be explained by the effects of the environment on the growth rates of the hermit crabs and/or as an indication that shell preferences may vary in response to the physical factors of a given habitat.     

Avoidance of drilled gastropod shells by the hermit crab Pagurus longicarpus at Nahant, Massachusetts

Most hermit crabs depend on empty gastropod shells for shelter; competition for appropriate shells is often severe. This study determined whether shells that have been drilled by naticid gastropods are suitable for occupancy by the hermit crab Pagurus longicarpus. Differences in the characteristics of empty shells and those occupied by hermit crabs were assessed at two adjacent field sites in Nahant, Massachusetts. Drilling damage was far more frequent in empty gastropod shells than in shells occupied by hermit crabs, suggesting that individuals of P. longicarpus avoid drilled shells. They did not appear to avoid shells with other forms of damage. Laboratory experiments confirmed that these hermit crabs preferentially chose intact shells over drilled shells, even when the intact shells offered were most suitable for crabs half the weight of those tested. Final shell choices were generally made within 1 h. The hermit crabs apparently discriminated between intact and drilled shells based on tactile cues, since crabs kept in the dark showed the same preference for intact shells. The hermit crabs strongly avoided, to nearly the same extent, artificially drilled shells, naturally drilled shells, and shells with holes artificially drilled on the opposite side of the shell from where they would normally be located. Possible selective forces causing P. longicarpus to show such strong behavioral avoidance of drilled shells include increased vulnerability of crabs in drilled shells to osmotic stress, predation, and eviction by conspecifics.     

The growth ofTracheleuglypha dentata (Rhizopoda: Testacea) in clonal cultures under different trophic conditions

Twenty-four clones ofTracheleuglypha dentata were cultivated under different trophic conditions. Half were cultivated with soil extract which induced low bacterial growth. The other 12 were cultivated in the same medium with the addition of Cerophyl to enhance bacterial growth. The dynamics of each clone was followed for about 3 months by counting both live animals and empty shells. The rate of decomposition of empty shells was negligible. The number of empty shells are a direct measure for mortality. Parameters measured were densities, intrinsic rate of growth, generation time, production, and mortality. The densities increased in most of the clones until about the third week; it then reached a plateau or decreased slowly until the end of the experiment. After this initial period, the live animals closed their pseudostome with a thin membrane and remained quiescent until fresh food was given. The level of the plateau was significantly six times higher in the well-fed than in the starved clones, a difference due to a higher rate of growth induced by a shorter generation time during the first 10 days. This high growth produced giant shells, teratological forms created by the fusion of several shells. Mortality appeared at about the 20th day. Because it was higher in the well-fed clones than in the starved clones, it brought the densities of both clones to the same level after 3 months. The first empty shells were produced by either precocious mortality or abortion, as they did not contain reserve plates necessary for a new shell prior to reproduction. The mortality of adults came later and was probably linked with the age of the culture. The starved cells had an unusual morphology which disappeared when the medium was renewed.     

Poliovirus empty capsid morphogenesis: evidence for conformational differences between self- and extract-assembled empty capsids.

In this paper we describe the use of specific proteinases, surface-specific radioiodination, and antigenic reactivity in conjunction with isoelectric focusing for probing the conformations of different polioviral empty capsid species. Naturally occurring empty capsids (called procapsids) with an isoelectric point of 6.8 were resistant to proteolytic digestion by trypsin or chymotrypsin, as were empty capsids assembled in vitro in the presence of a cytoplasmic extract prepared from poliovirus-infected HeLa cells. In contrast, self-assembled empty capsids (isoelectric point, 5.0) were sensitive to both proteinases. Capsid proteins VP0 and VP1 were attacked predominantly, whereas VP3 was resistant to cleavage. Unpolymerized 14S particles possessed a trypsin sensitivity which was qualitatively similar to that of self-assembled empty shells. Surface-specific iodination of virions and procapsids labeled VP1 exclusively. In contrast, radioiodination of self-assembled empty capsids labeled predominantly VP0. After radioiodination the sedimentation coefficient corrected to water at 20 degrees C, the isoelectric point, and the trypsin resistance of the procapsids remained unchanged. Procapsids and extract-assembled empty capsids were N antigenic, whereas self-assembled empty capsids were H antigenic. Self-assembled empty capsids were not converted to pH 6.8 trypsin-resistant structures by incubation with a virus-infected cytoplasmic extract. However, 14S particles assembled in the presence of a mock-infected extract formed empty capsids, 20% of which resembled extract-assembled empty shells as determined by the above-described criteria. These and related findings are discussed in terms of empty capsid structure and morphogenesis.     

Size changes in Jurassic liparoceratid ammonites and their stratigraphical and ecological significance

Over 50 species of liparoceratid ammonites have previously been recognized from the Ibex and Davoei Zones (Lower Pliensbachian, Lower Jurassic) of England alone. Such a high specific diversity is an artifact caused by their stratigraphical importance and the complex nature of their shells. Simple bivariate analysis indicates that Aegoceras species are arbitrary subdivisions of an allometric growth trend. Principal component multivariate analysis shows that Liparoceras and Androgynoceras (sensu Callomon) are quite distinct from Aegoceras , and they display a more random pattern of variation. The existence of stratigraphically useful size changes in liparoceratids mirrors precisely size increases in benthic molluscan taxa, such as the bivalve 'Astarte' platymorpha and the archaeogastropod Eucyclus subimbricata. The ammonite size changes, at least, probably result from phenotypic variation analogous to that observed in the living cephalopod Sepia officinalis. This evidence taken in conjunction with the fact that kosmoceratid ammonite size changes correspond somewhat with benthic faunal changes suggests that ammonites maybe less useful for stratigraphy than is believed.     

Shell use by juvenile fiddler crabs Uca pugnax and U. pugilator

The present study documents for the first time shell use by juvenile fiddler crabs in the salt marsh. Twenty visits were made to six salt marsh sites at Tybee Island, Georgia between 2007 and 2009. One hundred empty Littorina irrorata shells were collected at each site on each field trip. Juvenile carapace width was measured, crabs sexed, and species identification completed using RFLP analysis. Shell use of up to 79% was observed. Two species of fiddler crabs were found in empty shells, Uca pugnax and U. pugilator. U. pugnax was the dominant species at all sites representing 62-84% of the juvenile fiddler crab population. Juvenile sex ratios were female-biased (1.7:1) at all six sites. Juvenile size did not vary significantly between species but males of both species were significantly larger than females. Size frequency distribution of carapace width revealed that shell use varied with size and sex. In the 3 to 4 mm size class, juvenile females outnumbered juvenile males in empty L. irrorata shells while in the 5 to 6 mm size class and greater, juvenile males outnumbered juvenile females in shells. Significantly more juvenile fiddler crabs were found in empty shells during flood than ebb tide at 3 of the sites. This discovery illuminates the resourcefulness of juvenile fiddler crabs and provides another mechanism that might enhance survival.     

Point pattern analysis as a tool for assessing disease spread and population features in remaining sanctuaries of the critically endangered bivalve Pinna nobilis

An emergent disease has relegated populations of the Mediterranean pen shell, Pinna nobilis L. critically endangered to sanctuaries featuring salinities outside the 36.5 to 39 range. Point pattern analysis was used in three areas of the Alfacs Bay (Ebro Delta) still hosting pen shells to assess the possible undergoing of disease spread by comparing the spatial distribution of live individuals vs. empty shells across spatial scales. We also evaluated the importance of other ecological aspects of conservation relevance such as the size distribution of individuals, and the possible association to seagrass habitats. The population assessment showed no recent mortality and a clear dominance of large adults among empty shells (97.3%) pointing to no disease spread during the study period. At the low spatial scale Nearest Neighbor (NN) analyses evidenced significant clustering (NN Ratios of 0.4–0.8), but in one of the zones NN distances were closer in empty shells than in live individuals, suggesting a former localized outbreak. At the larger spatial scale, MDSCA confirmed clustering patterns up to distances of 115 to 190 m, with higher aggregation of empty shells at the same study zone. The bay also featured low juvenile availability (3.2%), which risks the continuity of the population. No evidence for habitat or conspecific selection could be observed from abundance patterns and variation in NN across study regions. Our research provides a tool for assessing population condition in paralic environments, where salinity conditions tend to slow down disease spread, thus allowing a time gap for undertaking conservation decisions.     

Palaeoecological significance of Barremian ammonite assemblages and facies variations from Southwest Mexico

Analyses of ammonite shell forms of two Barremian stratigraphic sections from Southwest Mexico consist of two well-defined morphotypes: (1) Small uncoiled, mostly leptoceratoid ancyloconic shells of the families Ancyloceratidae and Hamulinidae, and (2) middle-sized involute to moderately evolute oxycone to discocone shells of the family Pulchelliidae. Index taxa allow the recognition of standard ammonite biozones for the Barremian, which permit the relative dating of different processes that occurred through the water column in the environment of deposition. The vertical distribution of ammonite morphotypes and facies suggests changes of the palaeoceanographic and sedimentological conditions that prevailed in the area during Barremian time. Petrologic data, analyses of the organic carbon and carbonate contents of the rocks support the idea that oxygen-deficient bottom waters existed within a shallow marine, tectonically active area with little carbonate deposition during the early early Barremian (upper part of the Taveraidiscus hugii Zone through the base of the Nicklesia pulchella Zone). These conditions in the basin caused a proliferation of middle-water depth ammonites of Morphotype 1 but prevented the abundance of nektobenthic forms of Morphotype 2. Oxic conditions on a more calcareous and open normal marine environment seem to have been reestablished progressively during a transgressive episode from late early-early late Barremian (upper part of the Nicklesia pulchella Zone through the Gerhardtia sartousiana Zone). This environmental setting supported more facies dependent nektobenthic ammonites of Morphotype 2 to flourish within the basin.     

Bioerosion of shells by terrestrial gastropods

Empty shells of terrestrial gastropods remain intact and become fossilized only under particular conditions. The usually thin shells are readily dissolved by rainwater, a process starting often during life. Results indicate that with this chemical weathering they may lose some 1% in weight per month. But this is not the only process by which shell biominerals disappear. During field experiments, living terrestrial gastropods have been observed to actively remove calcareous material from empty shells apparently to use for building their own shell. Empty shells lost ∼30% of their weight in 2 months, indicating this process to be much more important than simple dissolution, and explaining the rapid disappearance of empty shells in the field. Previously, mainly anecdotal mention has been made of this shell scraping. Bones of birds were not scraped by terrestrial gastropods; they lost ∼1% in weight per month at the start due to chemical weathering alone, but weight loss decreased with time and was only 6.5% after 16 months.