Sutural complexity and bathymetry in ammonites: fact or artifact?

The widespread assumption that sutural complexity in ammonites is mainly proportional to water depth is revisited. Fractal analysis has been used for the precise morphometric evaluation of sutural complexity in 131 Upper Jurassic ammonites. Suture lines belonging to twelve families have been analyzed, account being taken of shell structure (coiling, shape of whorl section), sculpture and paleoenvironments. Fractal dimensions obtained in epicontinental and epioceanic ammonites show the unlikelihood of precise relationships between suture complexity and depth, and/or the absence of major differences in habitat depth if bathymetry played any significant role in the configuration of intricate septa. Suture complexity appears to be better related to shell structural types. □ Fractal analysis, ammonite sutures, Upper Jurassic.     

Power scaling of ammonitic suture patterns from Cretaceous Ancyloceratina: constraints on septal/sutural complexity

The spatial scaling of 77 hemisutures from 65 species of Cretaceous heteromorphic ammonites was quantified with the fractal box‐counting method. Fractal dimensions within Baculites compressus did not significantly differ between adult hemisutures; however, the juvenile suture of this species did exhibit a significantly lower fractal dimension. This suggests that variation in sutural complexity between explicitly adult ontogenetic stages may not contribute to significant noise in comparisons between other species/morphotypes. High‐spired, three‐dimensionally coiled heteromorphs with a larger degree of septal asymmetry exhibit higher fractal dimensions in outer whorl hemisutures than inner whorl hemisutures due to their elongation and improved space occupation over a larger whorl surface. Three‐dimensionally coiled ammonites also have higher fractal dimensions on average (mean D_b = 1.45) with respect to their 2‐D coiled counterparts (mean D_b = 1.38). All ammonites in this study exhibit a positive trend between sutural complexity and shell size (proxied by whorl height). These relationships suggest that septal frilling is constrained by shell morphology and whorl section geometry during septal morphogenesis. This, in turn, influences the scaling, space‐filling properties and scaling limits of ammonitic suture patterns. Sutural/septal complexity is also found to positively influence the amount of liquid retained in marginal septal recesses. However, as these septa approach larger scales, less cameral liquid is retained per septal mass. This may further explain the positive relationship between sutural complexity and shell size.     

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.     

Sutural amplitude of ammonite shells as a paleoenvironmental indicator

The Sutural Amplitude Index (SAI), obtained by measuring the maximum height of sutural elements and the length of the suture pattern on ammonoid shells, provides a useful indicator of relative habitat depth for ammonoids with similar shell morphotypes. A higher SAI indicates greater septal support for the shell wall against implosion under increased hydrostatic pressures in deeper waters. Relating the sutural amplitude indices of ammonites found in a well-studied depositional basin, such as the Cretaceous Western Interior Seaway, to morphotype distributions illustrates the utility of this index in bathymetric interpretations. □ Ammonoidea, sutures, Sutural Amplitude Index, Cretaceous, Greenhorn Cyclothem. Western Interior.     

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.     

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.     

Mechanical significance of ammonoid septa with complex sutures

Pre- and post-19th century hypotheses relating hydrostatic pressure to the mechanical function of sutural complexity are compared. The old ideas gave rise to the 19th century 'Buckland hypothesis', which is in turn largely synonymous with the 'Westermann model'. Buckland (1836) postulated that fluted septa buttressed the weak flanks of the phragmocone wall. Two new parameters are introduced to define the covariation between the strength of cylindrical segments of the wall flank bounded by the distance between adjacent lobe and saddle-flutes in transverse sections. The product of the index of wall strength (IWS) and this inverse support angle (ISA) predicts the buckling pressure in a cylinder of infinite length, and it implies that coiled nautiloids were more likely to be imploded via their whorl flanks than the apparently weaker oxyconic ammonoids. The widely used index of sutural complexity (ISC) measures the marginal corrugation which obscures this trend and acts as an elastic bed for both strong and weak walls. However, the ISC is more proportional to habitat depth than the buckling pressure when all other factors are constant. The central thickness of each fluted septum was increased in direct proportion to the distance spanned by the septum and the hydrostatic pressure on it in the 'last septum' position. The marginal thickness was maintained at a more constant value, which permitted the suture to increasingly act like a spring or shock absorber, as the wall thickness was enlarged during ontogeny. Both the ratios, between the central and marginal thicknesses and the closely related ISC, therefore, increased with shell diameter and habitat depth.     

Appearance of morphological novelty in a hybrid zone between two species of land snail

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.     

Sutural complexity in artificially deformed human (Homo sapiens) crania.

The pattern of complexity of cranial sutures is highly variable both among and within species. Intentional cranial vault deformation in human populations provides a controlled natural experiment by which we were able to quantify aspects of sutural complexity and examine the relationship between sutural patterns and mechanical loading. Measures of sutural complexity (interdigitation, number, and size of sutural bones) were quantified from digitized tracings of 13 sutures and compared among three groups of crania (n = 70) from pre-European contact Peru. These groups represent sample populations deformed in 1) anteroposterior (AP) and 2) circumferential (C) directions and 3) an undeformed population. Intergroup comparisons show few differences in degree or asymmetry of sutural interdigitation. In the few comparisons which show differences, the C group is always more interdigitated than the other two while the AP group has more sutural bones. The sutures surrounding the temporal bone (sphenotemporal, occipitotemporal, and temporoparietal) most frequently show significant differences among groups. These differences are related to the more extreme binding of C type deformation and are consistent with hypothesized increases in tension at coronally oriented sutures in this group. The larger number of sutural bones in the AP group is consistent with the general broadening of the cranium in this group and with experimental evidence indicating the development of ossicles in areas of tension. We suggest that so few changes in sutural complexity occurred either because the magnitude of the growth vectors, unlike their direction, is not substantially altered or because mechanisms other than sutural growth modification are responsible for producing the altered vault shapes. In addition, the presence of fontanelles in the infant skulls during binding and the static nature of the binding may have contributed to the similarity in complexity among groups.     

ADAPTATION FROM RESTRICTED GEOMETRIES: THE SHELL INCLINATION OF TERRESTRIAL GASTROPODS

The adaptations that occur for support and protection can be studied with regard to the optimal structure that balances these objectives with any imposed constraints. The shell inclination of terrestrial gastropods is an appropriate model to address this problem. In this study, we examined how gastropods improve shell angles to well‐balanced ones from geometrically constrained shapes. Our geometric analysis and physical analysis showed that constantly coiled shells are constrained from adopting a well‐balanced angle; the shell angle of such basic shells tends to increase as the spire index (shell height/width) increases, although the optimum angle for stability is 90° for flat shells and 0° for tall shells. Furthermore, we estimated the influences of the geometric rule and the functional demands on actual shells by measuring the shell angles of both resting and active snails. We found that terrestrial gastropods have shell angles that are suited for balance. The growth lines of the shells indicated that this adaptation depends on the deflection of the last whorl: the apertures of flat shells are deflected downward, whereas those of tall shells are deflected upward. Our observations of active snails demonstrated that the animals hold their shells at better balanced angles than inactive snails.     

Can ribs and septa provide an alternative standard for age in ammonite ontogenetic studies?

In ammonites, the time between two successive septal secretions is assumed to be almost stable during growth within homogeneous sets with regard to phylogeny and morphology. The number of septa (included in the adult phragmocone) is proposed as an alternative standard for age. At each moment of growth, the rate of shell secretion can be directly estimated by the measure of shell length between two septa. Equivalent results are obtained for ribs whose formation is also dependent on an almost stable rhythmical process. Results obtained from septa and size used as age standards are compared in the example of Liparoceratidae-Amaltheidae evolution. The two standards provide estimations of age which diverge from more than 1 to 2. For example, some small species previously seen as progenetic are now classified as dwarfed. These results emphasize the risk of using size as the only standard of age. Alternative standards not only in ammonites but also in other animal groups should be investigated.     

Ammonite habitats in the Early Tithonian of Western Tethys

Cccca, F. 1992 07 15: Ammonite habitats in the Early Tithonian of Western Tethys.
Habitats of Early Tithonian ammonites of Western Tethys are inferred from analysis of faunal spectra, functional morphology. facies characteristics and palaeogeography. Phylloceratina and Lytoceratina are included in a pelagic group characterized by taxa which could reach bathyal depths. This group is very abundant and dominant in some localities. A neritic nektobenthic group, relatively abundant in the Cmbria-Marche Apennines only. is composed of taxa characterired by species bearing simple sutures (some Simoceratidae and Pseudolissoceras ). probably living at shallower depth. The habitat of Haplploceras and some Oppeliidae. is interpreted to be close to the shelf margin/continental slope. Perisphinctids. Aspidoceratidae and some other Oppeliidae are provisionally included in an 'indeterminate' group including deep and shallow taxa. Quantitative studies of assemblage composition and comparison of faunal spectra successions from different pelagic regions allow the characterization of different pelagic environments. Bathymetric hypotheses by means of ammonites are discussed. Z Ammonoidea. habitats. facies, faunal spectra. Jurassic, Tithonian. Tethys .     

Reappraisal of the latest Albian (Mortoniceras fallax Zone) cephalopod fauna from the classical Salazac locality (Gard,southeastern France)

The Salazac locality (Gard, southeastern France) is renowned for the richness of its cephalopod fauna (especially ammonites) from the Mortoniceras fallax Zone (uppermost Albian, Lower Cretaceous). However, most ammonite species have paradoxically been scarcely illustrated up to now. Furthermore, the rare assessments of ammonite taxonomic diversity are presumably inaccurate, as they either result from a drastic typological approach or could not benefit from the most recent advances in modern concepts of ammonoid intraspecific and ontogenetic variations. In this work, we document an original cephalopod assemblage from Salazac represented by abundant, well-preserved specimens (564 studied ammonites, including ca. 50% of heteromorphs) constituting a high diversity. The sample size and quality of this new material allowed some extensive morphological and biometric analyses whenever possible (i.e., for subdatasets in which a significant number of measurements is available) and to thoroughly assess intraspecific and ontogenetic variations for a few species. This led to determine the diagnostic value of various shell characters commonly used to characterize both well-documented and poorly-defined species. In particular, we show that a significant part of the variation in the number of ribs per whorl within the heteromorphic genera Mariella and Turrilitoides is directly related to shell size and that the strength of tubercles covaries negatively with the number of ribs in Mariella, therefore challenging the diagnostic value of these characters. We further suggest that rib density covaries with whorl shape in Mariella, following Buckman’s first rule of covariation. In total, one species of nautilid and 38 taxa (25 genera) of ammonites are described, among which five species are documented from Salazac for the first time. In turn, this study provides valuable data for future phylogenetic, biostratigraphical, biogeographical and broad-scale diversity studies.     

Gehäusespirale und septenform bei ammoniten unter der annahme vagil benthischer lebensweise

The formation of the shell spiral in ammonites is traced back to the effect of weight and buoyancy forces in conjunction with a gastropod-like position of the shell, supposing a vagile benthic mode of life. The downward pull required to maintain this shell position leads to a differentiated attachment of the adhesive muscles to the shell wall as a reaction to the steady tensile stress caused by the weight and buoyancy forces. The form of the stressed subephithelial muscles and of the rear section of the mantle becomes manifest in the form of septa.     

Sexual dimorphism in shells of Cochlostoma septemspirale (Caenogastropoda, Cyclophoroidea, Diplommatinidae, Cochlostomatinae)

Sexual dimorphisms in shell-bearing snails expressed by characteristic traits of their respective shells would offer the possibility for a lot of studies about gender distribution in populations, species, etc. In this study, the seven main shell characters of the snail Cochlostoma septemspirale were measured in both sexes: (1) height and (2) width of the shell, (3) height and (4) width of the aperture, (5) width of the last whorl, (6) rib density on the last whorl, and (7) intensity of the reddish or brown pigments forming three bands over the shell. The variation of size and shape was explored with statistical methods adapted to principal components analysis (PCA) and linear discriminant analysis (LDA). In particular, we applied some multivariate morphometric tools for the analysis of ratios that have been developed only recently, that is, the PCA ratio spectrum, allometry ratio spectrum, and LDA ratio extractor. The overall separation of the two sexes was tested with LDA cross validation.The results show that there is a sexual dimorphism in the size and shape of shells. Females are more slender than males and are characterised by larger size, a slightly reduced aperture height but larger shell height and whorl width. Therefore they have a considerable larger shell volume (about one fifth) in the part above the aperture. Furthermore, the last whorl of females is slightly less strongly pigmented and mean rib density slightly higher. All characters overlap quite considerably between sexes. However, by using cross validation based on the 5 continuous shell characters more than 90% of the shells can be correctly assigned to each sex.     

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 .     

Enigmatic septa in shells of some Middle Jurassic Pholadomya (Bivalvia) from Poland

The interior of 36 specimens of Pholadomya Sowerby (Bivalvia) from the Middle Jurassic of Poland reveals the presence of unusual septa that separate sediment‐filled chambers from the shell interior. The septa occupy one or several recurrent loci in shells of various individuals, that is within umbones, in pallial sinuses and along the shell margins. Based on the location and shape of the septa, eight forms grouped into types and varieties are identified. A possible cause for the formation of septa is sediment toxicity, but intrusion of sediment to the shell interior must have been linked to shell breakage or rupture of the free periostracum. One form of septa occurs in perforated umbones, common in Pholadomya; other forms occur in intact shells, which suggest damage to free periostracum. The most likely cause for the latter is the presence of parasites, especially digenean trematodes, for which clams were intermediate hosts. The morphological aberrations presented here were hitherto unknown in both fossil and extant bivalves. This study is also the first report of pathologies in Anomalodesmata.     

Linking form and function of the fibrous joints in the skull: a new quantification scheme for cranial sutures using the extant fish Polypterus endlicherii

The aim of this study is to connect specific sutural morphologies with the specific types of deformation they experience. To meet this goal, we quantified the morphologies of the interfrontal (IF), interparietal (IP), and frontoparietal (FP) sutures in the extant fish Polypterus endlicherii, and used our published measurements of in vivo deformation of these sutures during feeding to infer how suture morphology and function are connected. Specifically, we found that three relatively simple measures of cross-sectional suture complexity (i.e., the ratio of total sutural length to its shortest end-to-end length; amount of sutural overlap; and size of the largest interdigitation) can be used to distinguish between the IF, FP, and IP sutures, which exhibit very different cross-sectional shapes and responses to loading. Interestingly, these differences in cross-sectional morphology are not reflected by the linear traces of these sutures on the surface of the skull, implying that cross-sectional shape of a suture must be known to infer the loading conditions it experiences. Plotting the three cross-sectional metrics against one another to yield a sutural morphospace shows that the IF, IP, and FP sutures define regions that are largely distinct from one another. Our previous measurements of strain across these sutures suggested that the FP region would lie between the IF and IP regions; instead, the FP region is largely set apart from the other two fields. Based on this discovery, and on the locations of cranial muscles, we propose a new model of deformation in the skull of P. endlicherii during feeding, in which rotation parallel to the skull roof is combined with bending, subjecting the FP suture to complex shearing. Finally, although the sutures of P. endlicherii appear to be significantly less complex than those of mammals, these fish sutures show a similar range of morphologies and perform similar functions as do mammalian sutures.