The role of suture complexity in diminishing strain and stress in ammonoid phragmocones

Several hypotheses have been put forward to explain the sinuosity and complexity of suture lines in Ammonoidea. At present, the two principal opponent views maintain either that high complexity was a requisite to reinforce the shell in response to hydrostatic pressure, or that complexity augmented the attachment area for muscles. By using finite element calculations and analytical estimates of simplified ammonoid shell geometries, it is shown that complex suture lines reduced dramatically the strain and the stress in the phragmocone. The calculations lend support to the hypothesis that high sinuosity is an evolutionary response to external pressure. Additionally, it is found that without complex septa, the inward deformation of an ammonoid with thin shell would cause it to shrink in response to pressure and to lose buoyancy by a non-negligible amount.     

Soft‐tissue imprints in fossil and Recent cephalopod septa and septum formation

Several soft‐tissue imprints and attachment sites have been discovered on the inside of the shell wall and on the apertural side of the septum of various fossil and Recent ectocochleate cephalopods. In addition to the scars of the cephalic retractors, steinkerns of the body chambers of bactritoids and some ammonoids from the Moroccan and the German Emsian (Early Devonian) display various kinds of striations; some of these striations are restricted to the mural part of the septum, some start at the suture and terminate at the anterior limit of the annular elevation. Several of these features were also discovered in specimens of Mesozoic and Recent nautilids. These structures are here interpreted as imprints of muscle fibre bundles of the posterior and especially the septal mantle, blood vessels as well as the septal furrow. Most of these structures were not found in ammonoids younger than Middle Devonian. We suggest that newly formed, not yet mineralized (or only slightly), septa were more tightly stayed between the more numerous lobes and saddles in more strongly folded septa of more derived ammonoids and that the higher tension in these septa did not permit soft‐parts to leave imprints on the organic preseptum. It is conceivable that this permitted more derived ammonoids to replace the chamber liquid faster by gas and consequently, new chambers could be used earlier than in other ectocochleate cephalopods, perhaps this process began even prior to mineralization. This would have allowed faster growth rates in derived ammonoids.     

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.     

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.     

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.     

Internal shell destruction in some Silurian nautiloids

Internal destruction of septa in Silurian brevicone cephalopods has taken place during post-mortem descent. The breakage was caused by implosions due to increasing water pressure. A pre-vious interpretation involving instantaneous internal collapse of septa is discussed and opposed. Due to shell construction, the surviving parts of broken septa, and broken septal fragments contained within the shell, the breakage of septa must have taken place in one chamber at a time, with a speed controlled by the water seeping in through the partly blocked siphuncle. The destruction could occasionally leave isolated septa unbroken, which confirms an interpretation of repeated breakage. As the destruction of septa has taken some time it can not be excluded that shells have undergone post-mortem drift.     

AMMONOIDS ACROSS THE PERMIAN/TRIASSIC BOUNDARY: A CLADISTIC PERSPECTIVE

Abstract:  The rapid diversification of ceratitid ammonoids during the earliest Mesozoic has been taken at face value as an example of explosive radiation following the Permian/Triassic mass extinction. However, the validity of this interpretation has never been tested within a phylogenetic framework. A total evidence cladistic analysis of Mid–Late Permian and Induan (earliest Triassic) ammonoids confirms the monophyly of the Ceratitida. Partitioned phylogenetic analysis of suture line characters vs. shell shape and ornament characters confirms the importance of suture-line characters for resolving the higher taxonomy of ammonoids. When the cladogram is compared with the observed fossil record, the resultant tree implies that the divergence of a number of early Triassic lineages actually occurred during the latest Permian. If these range extensions are taken into account the ammonoid per-genus extinction rate across the Permian/Triassic boundary drops from c. 85 per cent to c. 60 per cent.     

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.     

Empirical 3D model of the conch of the Middle Jurassic ammonite microconch Normannites: its buoyancy,the physical effects of its mature modifications and speculations on their function

A 3D model of the Middle Jurassic ammonoid Normannites with an apertural modification from Thürnen, Switzerland, was constructed using physical–optical tomography. It was tested to determine whether the formation of the apertural modification affected shell orientation, to estimate buoyancy regulation and to reconstruct the mode of life of this ammonoid. No drastic postural changes occurred between the 3D models that excluded and included lappets, suggesting that the lappets were not formed to change the syn vivo shell orientation and, in turn, locomotion. We speculate that these adult shell modifications served to protect the soft parts during the reproduction period. Buoyancy calculations based on the model assume that ammonoids were positively buoyant when the phragmocone was devoid of liquid. When 31% of the entire phragmocone was filled with liquid, the living animal would have reached neutral buoyancy in contrast to 27% of cameral liquid filling when the weight of the aptychi is included. Provided that smaller ammonoids had more cameral liquid than bigger ammonoids, such as the modern Nautilus, Normannites examined in this study would have been able to maintain neutral buoyancy and might have had a demersal, nektobenthic or nektonic habitat somewhere in the water column.     

The Evolution and Development of Cephalopod Chambers and Their Shape

The Ammonoidea is a group of extinct cephalopods ideal to study evolution through deep time. The evolution of the planispiral shell and complexly folded septa in ammonoids has been thought to have increased the functional surface area of the chambers permitting enhanced metabolic functions such as: chamber emptying, rate of mineralization and increased growth rates throughout ontogeny. Using nano-computed tomography and synchrotron radiation based micro-computed tomography, we present the first study of ontogenetic changes in surface area to volume ratios in the phragmocone chambers of several phylogenetically distant ammonoids and extant cephalopods. Contrary to the initial hypothesis, ammonoids do not possess a persistently high relative chamber surface area. Instead, the functional surface area of the chambers is higher in earliest ontogeny when compared to Spirula spirula. The higher the functional surface area the quicker the potential emptying rate of the chamber; quicker chamber emptying rates would theoretically permit faster growth. This is supported by the persistently higher siphuncular surface area to chamber volume ratio we collected for the ammonite Amauroceras sp. compared to either S. spirula or nautilids. We demonstrate that the curvature of the surface of the chamber increases with greater septal complexity increasing the potential refilling rates. We further show a unique relationship between ammonoid chamber shape and size that does not exist in S. spirula or nautilids. This view of chamber function also has implications for the evolution of the internal shell of coleoids, relating this event to the decoupling of soft-body growth and shell growth.     

Model for origin,function and fabrication of fluted cephalopod septa

The model supposes that sub-hemispheric septa of deep-water cephalopods evolved transverse pillars by meridional fluting in order to support flat and therefore weak areas of the shell wall. Several trends toward reduction of sutural spacing for improved wall support resulted in rise of fluting and finally, marginal crenulation. During ontogeny, the function of the ammonite septum as complex vault system against ”normal“ pressure from the body was succeeded by compound-pillar function for wall support. Fabrication of ammonitic septa probably involved positive pressure differential of new ”cameral“ liquid, orientation of mantle fibers along stress lines, and successively affixed tie-points which lay on an aponeurosislike mantle structure.     

Untersuchungen zur Frühontogenie vonNautilus pompilius (Linné)

A series of initial shells ofNautilus pompilius was investigated for morphologic and isotopic evidence of the early ontogenetic development. Shell morphologic features, such as curvature of early shell, cancellate sculpture, suture and grouping of septa, as well as very early shell injuries, color banding and shell/egg dimensions, seem to indicate an interpretation of early ontogeny differing from that which is now generally accepted. The O¹⁸/O¹⁶ and C¹³/C¹² ratios in the early shell and septa of twoNautilus specimens are given. Changes in the carbon isotope content are tentatively correlated with the end of the embryonic period and with environmental changes. Variations in the oxygen isotope content are in part ascribed to migrations from warm to cooler water after a certain stage of development. The size of the body chamber of the young animals can be determined by comparison of the isotope contents in outer shell and septa. Existing ideas concerning early ontogeny ofNautilus are critically discussed.     

Mechanische Simulation und funktionelle Evolution des Ammoniten-Septums

Simulation of nautiloid and ammonoid septa and suture lines illustrates the fabricational constraints and possibilities that controlled their evolution towards the paradigm of a pressure resistant structure.     

THE HETEROMORPHS AND AMMONOID EXTINCTION

The heteromorph ammonoids are quoted as a favourite example of degeneration and the decline of a Bauplan‘condemned’ to extinction. With astonishing tenacity this view of the heteromorphs as ‘phylogenetic end-forms’ has embedded itself in the palaeontological literature and is still current. This is contradicted by the most recent investigations, directed especially at the Cretaceous heteromorphs, which necessitate correction of the typolysis concept as well as modification of the most uncontested of the phylogenetic ‘laws’, Dollo's ‘law of irreversibility’. Contrary to the usual textbook hypothesis, the heteromorphs return in several evolutionary lineages to normal coiling of the shell and, in general, to a phylogenetically older type of suture line. At the same time these results encourage fresh reflexion on possible exogenous causes of phylogenetic extinction of the ammonoids. A clear causal connexion exists between this extinction and the far-reaching epirogenic changes in sea level in the late Cretaceous; cosmic explanations are unnecessary. In conclusion it may be added that the precipitate formulation of phylogenetic ‘laws’ and ‘principles’ based on too little basic information has encumbered this branch of palaeontology with a stifling set of prejudices rather than providing it with guide lines crystallized from long experience and observation. It is vitally necessary in the interests of palaeontology that interpretation and observation be separated far more than has been the case in the past.     

The mode of life in ammonoids

The following structural features clearly indicate that ammonoid shells were adapted to withstand considerably higher hydrostatic pressures thanNautilus shells: (1) the corrugated and marginally fluted septa gave the shell wall efficient support against implosion; (2) the secondary connecting rings could grow a great deal in thickness; and (3) the last formed chambers remained full of liquid which supported the last septum. On the basis of the following characters it is concluded that ammonoids were incapable of swimming efficiently by jet-propulsion: (1) the retractor muscles were weakly developed; (2) the life position was unstable and highly variable; and (3) in animals with a ventral apertural rostrum the hyponome was probably absent. Ammonoids are considered here as having been pelagic cephalopods which lived in the upper 1000 m of the oceans, and which probably undertook considerable diurnal vertical migrations, similar to those inSpirula. Only some groups may have adopted a life in shallow epicontinental seas. In the late Mesozoic, ammonoids have been replaced by modern oceanic squids which are extremely numerous in the corresponding pelagic environment.     

The genus Luppovia and the phylogeny of Cretaceous heteromorphic ammonoids

The unique preservation of heteromorphic shells of Luppovia Kakabadze, Bogdanova & Mikhailova 1978 from the Aptian deposits of Bolshoj Balkhan (western Turkmenia) made it possible to study the microstructure, internal shell characteristics and suture ontogeny of this genus. Microstructural investigations revealed that the dorsal shell wall in Luppovia is composed of three layers in contrast to that of monomorphic ammonoids, which is single-layered. This ability of secreting different numbers of layers in monomorphs and heteromorphs demonstrates plasticity of secreting activity of the mantle epithelium. Data obtained from the study of internal shell structure and suture ontogeny have been compared. Both these methods of investigation lead to the conclusion that the genus Luppovia belongs to the order Ammonitida. The results obtained confirm the viewpoint that the Cretaceous heteromorphic superfamilies Turrilitaceae and Ancylocerataceae have independent origins and belong to two different orders, Lytoceratida and Ammonitida, respectively. □ Ammonoidea , Luppovia, shell structure, ontogeny, phylogeny, Cretaceous.     

Dark bands on pyritic internal moulds of the Early Jurassic ammonites Oxynoticeras and Cheltonia from Gloucestershire,England: interpretation and significance to ammonite growth analysis

Abstract: Thin, radiating, darker bands occur on pyritic internal moulds of the Early Jurassic ammonites Oxynoticeras and Cheltonia from Bishop’s Cleeve, Gloucestershire. They closely resemble true colour patterns preserved in Early Jurassic Calliphylloceras from Kutch, India, and false colour patterns reported in Carboniferous and Triassic ammonoids. Up to five dark bands occur within the body chamber, suggesting that they do not represent serially repeated anatomical structures, but the same feature repeatedly formed during growth. Dark bands are interpreted as traces of black bands deposited on the inside of the shell at the aperture during pauses in growth. The angles between dark bands and between septa correlate strongly in Cheltonia, suggesting that pauses in growth coincided with septal secretion during the chamber formation cycle. There are, however, no other indications that growth was episodic in either genus.     

Über den Prosipho,die Prosutur und die Ontogenie der Ammonoidea

The earliest stages of two ammonoids are described. In the first, a specimen of the Lower CretaceousEuhoplites gr.tuberculatus (Sow.), it has been possible to dissect out a well preserved prosiphon which is composed of the same light-coloured carbonate material as the proseptum and the shell wall. The prosiphon forms a very thin and short lamella which is connected with the ventral wall of the protoconch without any trace of a cicatrix. The caecum and siphuncle connecting rings are formed of a dark, shiny (conchyolinic?) material. The second specimen belongs to a Middle JurassicNormannites vulgaricostatus Westermann, and shows two prosutures instead of one. Since the position of the second change in growth ratio and the length of the obviously larval part of the shell are the same as in other specimens, it is supposed that the formation of septa has been accelerated so that a second larval septum has been intercalated. In the ontogenetic development of the ammonoids two rather sudden changes occur. These are shown morphologically in the following positions:

1. a)
   First change in growth ratio (in the oral part of the protoconch).     
   
   

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.