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Ontogenetic trajectories of septal spacing in Early Jurassic belemnites from Germany and France, and their palaeobiological implications
... For comparisons, with early coleoids, we additionally interpolated the septal distances of Carboniferous Mutveiconites milleri based on Figure 1B provided by Doguzhaeva et al. (2006). Finally, we expanded our matrix data with equivalent measures of belemnites Wani et al. (2018) conditionally determined as Passaloteuthis laevigata, Parapassaloteuthis zieteni, and Pseudohastites longiformis (respectively from the Pliensbachian of Germany and France). These identifications should all be considered with reservation. ...
... *Measurements based on Fig. 1B in Doguzhaeva et al. (2006). **Medial measurements taken from Wani et al. (2018 between embryonic and post-ES parts. Nevertheless, a range of potential hatching markers has been proposed for at least some species. ...
... IC Equivalent to ES.-The absence of an unambiguous border between embryonic and postembryonic stages, with the only constriction being between the IC and the phragmocone (Figs. 2-4), leads to the suggestion that the belemnite ES consisted only of the IC (Wani et al. 2018). In the majority of extinct coleoids, the IC is separated from the phragmocone through a constriction with no further constriction to define much larger ES . ...
Coleoid cephalopods exhibited two distinct reproductive strategies, resulting in small pelagic and large demersal hatchlings, both in the geologic past and recently. In ectocochleate cephalopods, the hatching event is recorded in shell structures (e.g., nepionic constrictions, ultrastructural shifts, or ornamentation differences). In contrast, well-defined hatching markers do not exist on coleoid shells. Changes in septal spacing may be evidence of hatching (e.g., some extant sepiids), but not in all fossil groups. In the present study, we subdivide the early ontogenetic shells of phragmocone-bearing coleoids (belemnoids, spirulids, and sepiids) into key architectural stages and describe their reference to the hatching event. Belemnoids exhibit three key stages, the second of which is here considered to occur shortly before or after hatching. In spirulids and sepiids, there is only one key stage. In Mesozoic belemnoids, spirulids, and sepiids, hatching accordingly occurred with a total shell length of less than 2 mm, which corresponds to mantle lengths of small planktonic hatchlings. Production of small pelagic hatchlings and thus small eggs was therefore the dominant reproductive strategy within the Coleoidea. The first evidence of enlarged hatchlings appeared during the Maastrichtian in Groenlandibelus. During the Eocene, the large-egg strategy apparently became more widespread, particularly in belosaepiids.
... Cross sections were produced to calculate WER and WWI. In addition, intraspecific variation of septal spacing in the Jurassic belemnite Passaloteuthis laevigata, which was examined in Wani et al. (2018), was compared as an example of extinct phragmocone-bearing coleoids. ...
... The structure, which contains the most easily readable information on belemnite ontogeny, is the phragmocone, even though we know very little about its intraspecific variation. New data from Wani et al. (2018) give some insight into belemnite intraspecific variation. The authors measured septal spacing of 23 specimens of the Jurassic belemnite Passaloteuthis laevigata from Buttenheim, Germany. ...
... Even though septal spacing may not be directly comparable to WER and WWI of Nautilus and ammonoids, it shows an ontogenetic pattern similar to PCV . Thus, we reproduced the coefficient of variation of the septal spacing calculated in Wani et al. (2018) through ontogeny (Fig. 4C). It should be noted that a problem in comparing phragmocones of belemnites to those of nautilids is that one cannot obtain information on all ontogenetic stages, because the very last septum (which indicates the end of growth by septal crowding) is usually missing in belemnites. ...
Intraspecific variation of organisms is of great importance to correctly carry out taxonomic work, which is a prerequisite for key disciplines in paleontology such as community paleoecology, biostratigraphy, and biogeography. However, intraspecific variation is rarely studied in ectocochleate cephalopods (ammonoids and nautiloids), for which an excessive number of taxa was established during the past centuries. Because intraspecific variation of fossilized organisms suffers from various biases (time averaging and taphonomy), an extant example is needed for actualistic comparison. We applied 3D morphometry to 93 specimens of Nautilus pompilius from three different geographic populations. This data set was used to examine the intraspecific variation throughout ontogeny in detail. Although there are slight differences between the populations as well as some measurement biases, a common pattern of intraspecific variation appears to be present. High variation in morphometric variables appears early in ontogeny and then decreases gradually in the following ontogenetic stages. Subsequently, the variation shows an increase again before maturity until a sharp increase or decrease occurs toward the end of ontogeny. Comparison with intraspecific variation of ammonoids and belemnites illustrated that some groups have ontogenetic patterns of intraspecific variation that are similar to that of N. pompilius . This implies that the abovementioned ontogenetic pattern of intraspecific variation might be common in some major cephalopod clades.
... (3) The septal interspace L s (corresponds to the terms chamber height, chamber length or septal distance, as used in other coleoid studies; e.g. Jeletzky, 1966;Fuchs et al., 2013;Wani et al., 2018), measured as the distance between consecutive septa along the external (dorsal) spiral; the septal expansion rate (SER) was calculated as the relative increase of L s from septum to septum. (4) The whorl interspace ratio (WIR), calculated continuously from the protoconch to the last chamber based on the extracted outline data (this is shown for one point in Fig. 3). ...
Spirula spirula (Coleoidea: Decabrachia) is a unique deep-sea squid with an uncertain taxonomic status. Here, we apply geometric morphometric analyses to precisely describe changes in conch morphology during the course of ontogeny of 21 specimens collected from 12 localities worldwide. These data were used to explore whether the genus is monospecific or comprises several species. Different 2D and 3D conch parameters are presented based on micro-computed tomography data, combining noninvasive imaging techniques with a range of morphometric analyses. Our data imply that Atlantic and Indo-Pacific specimens form two distinct morphological clusters, potentially representing two pseudocryptic species or two populations undergoing divergence (i.e. in the process of speciation). Given the evolutionary trend from straight to more coiled forms, we suggest that S. spirula represents a neotenous form that migrated from the Indo-Pacific towards the Atlantic via the Agulhas leakage, which has been active since the closure of the Strait of Panama (10–3 Ma).
... Fossil cephalopods are mainly known from preserved mineralized parts such as aragonitic phragmocones (e.g., nautilids: Tajika et al. 2020;ammonoids: Hoffmann et al. 2019, coleoids: Klug et al. 2016a, b, 2019Iba et al. 2012;Wani et al. 2018;Hoffmann and Stevens 2020), calcitic jaws (e.g., nautilids: Saunders et al. 1978;Klug 2001;ammonites: Lehmann 1972;Morton & Nixon 1987;Engeser and Keupp 2002;Keupp and Mitta 2015;Tanabe et al. 2015), and calcitic rostra (e.g., belemnites; Hoffmann et al. 2016Hoffmann et al. , 2019Hoffmann and Stevens 2020;Iba et al. 2012Iba et al. , 2014. While soft parts are rarely preserved (Klug et al. 2015Donovan and Fuchs 2016;Clements et al. 2016), originally chitinous body parts such as jaws, arm hooks, and radulae are occasionally found (Matern 1931;Mapes 1987;Fuchs 2006a;Landman et al. 2010;Kruta et al. 2011Kruta et al. , 2020Klug et al. 2005Klug et al. , 2010aKlug et al. , 2016aKlug et al. , b, 2017Klug et al. , 2020Keupp et al. 2016;Fuchs and Hoffmann 2017;Mitta et al. 2018). ...
Due to the lower fossilization potential of chitin, non-mineralized cephalopod jaws and arm hooks are much more rarely preserved as fossils than the calcitic lower jaws of ammonites or the calcitized jaw apparatuses of nautilids. Here, we report such non-mineralized fossil jaws and arm hooks from pelagic marly limestones of continental Greece. Two of the specimens lie on the same slab and are assigned to the Ammonitina; they represent upper jaws of the aptychus type, which is corroborated by finds of aptychi. Additionally, one intermediate type and one anaptychus type are documented here. The morphology of all ammonite jaws suggest a desmoceratoid affinity. The other jaws
are identified as coleoid jaws. They share the overall U-shape and proportions of the outer and inner lamellae with Jurassic lower jaws of Trachyteuthis (Teudopseina). We also document the first belemnoid arm hooks from the Tethyan Maastrichtian. The fossils described here document the presence of a typical Mesozoic cephalopod assemblage until the end of the Cretaceous in the eastern Tethys.
... Belemnites are extinct coleoid cephalopods, i.e., relatives of modern squids, cuttlefishes and octopuses (Fuchs 2006;Kröger et al. 2011;Iba et al. 2012Iba et al. , 2014Klug et al. 2016;Hoffmann et al. 2016. With the ten-armed coleoids, the decabrachians, they share an internal skeleton largely surrounded by a muscular mantle, a large brain compared to other invertebrates, ten arms, chitinous jaws, large lateral eyes, and a predatory mode of life (e.g., Naef 1922;Reitner and Urlichs 1983;Doguzhaeva et al. 2002Doguzhaeva et al. , 2003Weis and Delsate 2006;Klug and Fuchs 2010;Klug et al. 2010aKlug et al. , b, 2016Keupp and Mitta 2015;Clements et al. 2016;Donovan & Fuchs 2016;Klug and Tajika 2018;Wani et al. 2018;Jenny et al. 2019;. In contrast to other hard parts, the low magnesium calcite rostra of belemnites represent abundant fossils in the Jurassic and Cretaceous, sometimes occurring in rock-forming numbers (Doyle and Macdonald 1993;Rita et al. 2018). ...
Although belemnite rostra can be quite abundant in Jurassic and Cretaceous strata, the record of belemnite jaws was limited to a few specimens from Germany and Russia. Here, we describe and figure three cephalopod jaws from the Middle Jurassic Opalinus Clay of northern Switzerland. Although flattened, the carbonaceous fossils display enough
morphological information to rule out an ammonoid, nautiloid or octobrachian origin of the two larger jaws. Their similarities to belemnite jaws from Germany and Russia conforms with our interpretation of these specimens as belemnite jaws. Based on their rather large size, we tentatively assign these two jaws to the megateuthidid Acrocoelites conoideus. The third jaw is a rather small upper jaw of an ammonoid. Since Leioceras opalinum is by far the most common ammonite in this unit in northern Switzerland, we tentatively suggest that the upper jaw belongs to this species.
... Similar relationships between hatching and decreased septal spacing followed by increased septal spacing during the earliest ontogenetic stage have been recognized in other cephalopods (modern spirula and modern and fossil nautiloids (the order Nautilida); Landman et al. 1983;Tanabe & Tsukahara 1987;Arnold et al. 1987;Chirat & Rioult 1998;Wani & Ayyasami 2009;Wani & Mapes 2010;Yamaguchi et al. 2015). These facts suggest that such a relationship is one common characteristic among cephalopods, except for ammonoids and belemnites (both are assumed to hatch with a protoconch and no chamber; Landman et al. 1996;Arai & Wani 2012;De Baets et al. 2015;Wani et al. 2018). ...
In this study, the ontogenetic trajectories of septal spacing between succeeding chambers of five modern cuttlefishes, Sepia esculenta, S. lycidas, S. latimanus, S. pharaonic and Sepiella japonica, which were all wild‐caught around Japan, were analysed. The ontogenetic trajectories of septal spacing of all examined cuttlefishes demonstrate a decrease in septal spacing followed by an increase during the earliest ontogenetic stage. This trend is assumed to be related to hatching. After the rapid decrease and increase in septal spacing, species‐dependent trends occur irrespective of sex and locality. Based on cluster analyses of general trends recognized in each species, the five examined species can be categorized into two groups: (1) a group by S. pharaonis, S. esculenta and Sepiella japonica; and (2) S. latimanus and S. lycidas as more distant branching groups within the five examined species. This classification is concordant with some phylogenetic clades determined from DNA analyses. Therefore, we hypothesized that the ontogenetic trajectories of septal spacing among modern cuttlefishes are phylogenetically dependent. If this hypothesis holds in fossil cuttlefishes, the examination of ontogenetic trajectories of septal spacing would give new insight into the recognition not only of the life history but also of the phylogeny of fossil cuttlefishes.
... nautilids, and belemnites is of great interest because septa and chambers are constructed by the soft tissues of the animal, and may, therefore, provide information about key aspects of life history such as hatching, growth changes, and mode of life [16][17][18][19] . In addition to the conventional 2D-analyses of septal spacing through ontogeny (i.e., measuring septal rotational angles 16,[20][21][22], recent destructive and non-destructive methods to three-dimensionally reconstruct chamber volume have been developed [23][24][25][26] . Obtaining 2D-data is advantageous because of the simple preparation of fossils (grinding and polishing), requiring minimal lab time and post-processing; however, the changes of septal angle through ontogeny are sometimes very subtle, and thus this method may mask some important details. ...
Reconstructing the physiology of extinct organisms is key to understanding mechanisms of selective extinction during biotic crises. Soft tissues of extinct organisms are rarely preserved and, therefore, a proxy for physiological aspects is needed. Here, we examine whether cephalopod conchs yield information about their physiology by assessing how the formation of chambers respond to external stimuli such as environmental changes. We measured chamber volume through ontogeny to detect differences in the pattern of chamber volume development in nautilids, coleoids, and ammonoids. Results reveal that the differences between ontogenetic trajectories of these cephalopods involve the presence or absence of abrupt decreases of chamber volume. Accepting the link between metabolic rate and growth, we assume that this difference is rooted in metabolic rates that differ between cephalopod clades. High metabolic rates combined with small hatching size in ammonoids as opposed to lower metabolic rates and much larger hatchlings in most nautilids may explain the selective extinction of ammonoids as a consequence of low food availability at the end of the cretaceous.
... mm long. Different scenarios for the embryonic development of shell elements have been summarized by Bandel, Engeser & Reitner (1984) and Wani et al. (2018) suggesting that belemnite hatchlings possessed only a protoconch with no or a few chambers. The assumed active nektopelagic lifestyle of the belemnite hatchlings could have contributed significantly to their wide geographic distribution and evolutionary success (see e.g. ...
Belemnites are an extinct group of Mesozoic coleoid cephalopods with a fossil record ranging from the early Late Triassic [about 240 million years ago (Mya)] to the Cretaceous/Palaeogene boundary (65 Mya). Belemnites were widely distributed, highly abundant and diverse, and an important component of Mesozoic marine food webs. Their internal shells, specifically their low‐Mg calcite rostra, have been used as palaeoenvironmental carbonate archives for the last 70 years. This is primarily due to the assumption that the rostrum calcite formed in equilibrium with the oxygen isotope composition of ambient sea water. Of prime importance for the reliable interpretation of isotope data derived from these biogenic carbonates is a robust reconstruction of the palaeobiology of their producers. Here we provide a critical assessment of published reconstructions of belemnite soft‐body organization and their lifestyle and habitats. Different lines of evidence, including sedimentological, geochemical, morphological, and biomechanical data, point towards an outer shelf habitat of belemnites, for some taxa also including the littoral area. Belemnite habitat temperatures, oxygen content, salinities, and life span are constrained based on observations of the ecology and life history of modern coleoids. Belemnite habitat depth might have been largely controlled by food and temperature, with a temperature optimum between 10°C and 30°C. The distribution of modern coleoids is for most species restricted to well‐oxygenated water masses and a salinity between 27 and 37 psu. The trophic position of belemnites as both predators and prey is documented by unique fossil finds of stomach contents and soft tissue preservation, such as jaws, hooks, and ink sacs. Belemnites were medium‐sized predators in the epipelagic zone (not deeper than ∼200 m) hunting for crustaceans, other cephalopods, and fishes. Taxa with elongated rostra probably were fast and highly manoeuvrable swimmers. Forms with conical rostra represent slow but highly manoeuvrable swimmers, and forms with depressed rostra likely had a bottom‐related life habit. Predators of adult belemnites were sharks, bony fishes, and marine reptiles. Belemnites, like most of the modern coleoids, were relatively short lived, most likely living only for 1–2 years. Understanding the biomineralization of belemnite rostra is highly relevant for an improved interpretation of their geochemistry. Here we confirm that belemnite rostra are composed of low Mg‐calcite fibres, but they do not contain distinct types of laminae. These fibres are composed of two distinct calcite phases. One phase is a filigree network of tetrahedral organic‐rich calcite and the second phase is represented by organic‐poor calcite.
The ontogenetic trajectories of septal spacing between succeeding chambers of two phylloceratid ammonoids, Hypophylloceras subramosum and Phyllopachyceras ezoense, from the Haboro and Kotanbetsu areas, north‐western Hokkaido, Japan, were analysed. The ontogenetic trajectories of septal spacing of H. subramosum demonstrate a general trend with large intraspecific variation: two cycles of increasing to decreasing spacing followed by almost constant spacing. The large intraspecific variation can be subdivided into three types, making this species polymorphic. The ontogenetic trajectories of septal spacing of P. ezoense also show intraspecific variation, but with a different trend: one cycle of increasing to decreasing spacing and an increasing trend after that. The intraspecific variation can be subdivided into two types, which suggests that this species is dimorphic, possibly sexually dimorphic. Dimorphism is supported by two observations: (1) the difference in the ontogenetic trajectories of septal spacing is only seen in the later ontogenetic stages; and (2) the two types co‐occur with comparable abundance throughout all stratigraphic horizons. Detailed analyses of ontogenetic trajectories of septal spacing may reveal polymorphism in other ammonoid clades.
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, 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.
The structure of pro-ostracum and primordial rostrum is presented at early ontogenetic stages in Lower Jurassic belemnites temporary assigned to ?Passaloteuthis from north-western Germany. For the first time the pro-ostracum was observed in the first camerae of the phragmocone. The presence of a pro-ostracum in early shell ontogeny supports Naef's opinon (1922) that belemnites had an internal skeleton during their entire ontogeny, starting from the earliest post-hatching stages. This interpretation has been previously questioned by several writers. The outer and inner surfaces of the juvenile pro-ostracum were studied. The gross morphology of these surfaces is similar to that at adult ontogenetic stages. Median sections reveal that the pro-ostracum consists of three thin layers: an inner and an outer prismatic
layers separated by a fine lamellar, predominantly organic layer. These layers extend from the dorsal side of the conotheca to the ventral side. The information obtained herein confirms the idea that pro-ostracum represents a structure not present in the shell of ectocochleate cephalopods (Doguzhaeva, 1999; Doguzhaeva et al. 1999b, 2002a).
The primordial rostrum surrounds the protoconch and continues as a distinct prismatic layer along the phragmocone, at least along the first 25-30 camerae. The conotheca proper is absent. The phragmocone wall (= conotheca) is composed of an inner prismatic layer that continues from the mural part of the first septum, and three thin layers, which belong to the pro-ostracum. The latter layers are externally covered by the prismatic layer of the primordial rostrum. This shows significant differences in the shell wall structure between ectocochleates and belemnites.
Nautilus remains of great interest to palaeontologists after a long history of actualistic comparisons and speculations on aspects of the palaeoecology of fossil cephalopods, which are otherwise impossible to assess. Although a large amount of work has been dedicated to Nautilus ecology, conch geometry and volumes of shell parts and chambers have been studied less frequently. In addition, although the focus on volumetric analyses for ammonites has been increasing recently with the development of computed tomographic technology, the intraspecific variation of volumetric parameters has never been examined. To investigate the intraspecific variation of the phragmocone chamber volumes throughout ontogeny, 30 specimens of Recent Nautilus pompilius and two Middle Jurassic ammonites (Normannites mitis) were reconstructed using computed tomography and grinding tomography, respectively. Both of the ontogenetic growth trajectories from the two Normannites demonstrate logistic increase. However, a considerable difference in Normannites has been observed between their entire phragmocone volumes (cumulative chamber volumes), in spite of their similar morphology and size. Ontogenetic growth trajectories from Nautilus also show a high variation. Sexual dimorphism appears to contribute significantly to this variation. Finally, covariation between chamber widths and volumes was examined. The results illustrate the strategic difference in chamber construction between Nautilus and Normannites. The former genus persists to construct a certain conch shape, whereas the conch of the latter genus can change its shape flexibly under some constraints.
In this chapter we discuss the mode and rate of growth in ammonoids, focusing primarily on postembryonic growth. We first discuss the general mode of growth and then describe the ontogenetic sequence of growth stages. These stages are recognized on the basis of changes in morphology. For example, a gràph of the increase in size of whorl width versus shell diameter in an individual reveals changes through ontogeny that pinpoint the end of one growth stage and the beginning of another. We next discuss the overall rate of growth through ontogeny and establish a generalized growth curve. In this discussion, we refer to other cephalopods whose rate of growth is known. Fluctuations in the rate of growth that are superimposed on this growth curve are indicated in ammonoids by the presence of such shell features as varices and constrictions.
Spirula spirula is a unique deep-sea squid with
unknown taxonomic status. Precise description of shell
morphology may help to decide whether the genus contains
one or more species. Here, a straight forward description of
ontogenetic changes of shell parameters is presented for a
single shell of Spirula spirula. Using micro-computed tomography,
surface and volumetric data, e.g., chamber
volumes and surface areas, as well as siphuncle volumes
and surface areas were collected and used for the description.
Advantage of the method, combining non-invasive
imaging techniques with classical morphometry, is
discussed.
Analyses of distances between succeeding septa throughout ontogeny of modern coleoids Sepiella japonica and Spirula spirula reveal that the first several septa (until ca. 6th-8th septa in Sepiella japonica and until ca. 2nd-3rd septa in Spirula spirula) have relatively larger septal distances than the following septa and then the distances between succeeding septa abruptly decrease at the time of hatching for both species, which are known from previous rearing observations and stable isotopic analyses. If this relationship holds in fossil cephalopods preserving internal shells, their paleo-hatching timings could be reconstructed through the ontogenetic analyses of their septal distances.
A great number of new studies have been carried out on ammonoid embryonic development in the last two decades. We focus here on novel developments and interpretations in the description of the embryonic shell (including terminology, shape, size, ornamentation, microstructure, septa, siphuncle and muscle scars), the sequence of embryonic development, reproductive strategy and post-hatching mode of life, followed by conclusions and possible future areas of research.
Belemnites (order Belemnitida), a very successful group of Mesozoic cephalopods, provide an important clue for understanding Mesozoic marine ecosystems and the origin of modern cephalopods. Following current hypotheses, belemnites originated in the earliest Jurassic (Hettangian, 201.6-197 Ma) with very small forms. According to this view their paleobiogeographic distribution was restricted to northern Europe until the Pliensbachian (190-183 Ma). The fossil record is, however, biased by the fact that all the previous studies on belemnites focused on Europe. Here we report two belemnite taxa from the Hettangian of Japan: a new species of the Sinobelemnitidae and a large taxon of the suborder Belemnitina. The Sinobelemnitidae, which may be included in the future in a new suborder, have also been recorded from the Triassic of China, specimens so far poorly understood. The presence of a very large rostrum attributed to the Belemnitina suggests in addition that a diverse belemnite fauna evolved earlier than previously thought. Our new findings therefore (1) extend the origin of the belemnites back by similar to 33 m.y. into the Triassic, (2) suggest that this group did not necessarily originate in northern Europe, and (3) imply that belemnites survived the Triassic-Jurassic extinction, one of the five big mass extinctions in the Phanerozoic. Since belemnites provided a considerable amount of food as prey, the origination of belemnites is probably an important event also for the evolution of their predators, such as marine reptiles and sharks.
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).
The pro‐ostracum of the early Sinemurian belemnite Nannobelus from the Belgian Province of Luxembourg is preserved as a thin, irregularly mineralized (phosphatized and pyritized), finely laminated structure, which is situated dorsally between the calcified rostrum and phragmocone. It has a median field with a criss‐cross pattern of bluntly pointed, curved growth lines and fine longitudinal ridges, as well as two lateral fields characterized by a fine ornament of closely spaced, longitudinal striae, each lateral field showing a narrow anterior belt‐like portion, the width of which equals about one‐third of that of the median field. Their posterior portion is remarkably asymmetrical, because its free margin (which does not about the median field) curves ventrally and the interspace between striae gradually increases here. The striation of the lateral field is formed by the longitudinally exposed narrow portions of succeeding, overlapping sublayers of the pro‐ostracum. Additionally, an internal sublayer with a silicified, honeycomb‐like structure is demonstrated in the pro‐ostracum. Based on microlamination that is comparable to that of the chitinous gladius in extant squids and on the irregular mineralization (unlike the rest of the shell), the pro‐ostracum is considered to have been originally mainly organic, containing an intermediate cartilaginous sublayer with a typical honeycomb‐like structure. The cartilaginous sublayer supposedly provided protection of the pro‐ostracum against fractures which might have resulted from regular contractions together with the muscular mantle during jet‐propulsion. Ultrastructural and chemical data on Nannobelus favour the interpretation of the pro‐ostracum as a novelty of the skeleton in coleoids rather than as a dorsal projection of the phragmocone wall.
The investigation of the outcroped sections of the day pits Unterstürmig and
Buttenheim along the western margin of the Northern Franconian Jurassie
Mountains (Oberfranken, Bavaria) allow to correlate the both sections. Similar to
KRUMBECK (1931), we propose to divide the Spinatum Zone of Franconia into a
lower part characterized by the typical am mo ni te species of the apyremumSubzone
(Pleuroceras transiens, P solare, P apyrenum and P salebrosum) and an
upper part defined the first occurrence of the ammonite species group of Pleuroceras
spinatum (BRUGUIERE).
Ammonoids retain a record of growth in their shells, and, therefore, material is readily available for studies of early ontogeny. Such studies were performed first in the mid-19th century and have been pursued with vigor ever since. Using optical and scanning electron microscopy, ammonoid workers have described the morphology of the early whorls and have attempted to reconstruct the sequence of early ontogenetic development and to identify the embryonic shell.
A review of fossil evidence supports a pelagic mode of life (in the water
column) of ammonoids, but they may have spent their life close to the seabottom
(demersal), planktonically, or nektonically depending upon the ontogenetic stage
and taxon. There are good indications for a planktonic mode of life of ammonoid
hatchlings, but a broad range of reproductive strategies might have existed (egglaying,
fecundity). Isotope and biogeographical studies indicate that some forms
migrated or swam for considerable distances, whereas others may have been
primarily transported by oceanic currents during early and/or late ontogeny.
Diverse ammonoid habitats are also supported by evidence from predator–prey
relationships derived from characteristic injuries and exceptional fossil finds,
which indicate chiefly predatory or scavenging lifestyles. Sublethal injuries preserved
in some ammonoid shells, as well as rare stomach and coprolite contents,
provide evidence of predation by other cephalopods, arthropods and various
jawed vertebrates. Various lines of evidence suggest that different groups of
ammonoids had quite different ecologies, but shell shape alone can only give
upper constraints on ammonoid capabilities, a matter that needs to be considered
when interpreting their diversity and evolutionary history.
Belemnites, a very successful group of Mesozoic cephalopods, flourished in Cretaceous
oceans until the Cretaceous−Paleogene event, when they became globally extinct. Following this event the modern types of cephalopods (squids, cuttlefish, octopus) radiated in the Ceno- zoic in all oceans. In the North Pacific, however, a turnover from belemnites to the modern types of cephalopods about 35 m.y. before the Cretaceous−Paleogene event documents a more complex evolutionary history of cephalopods than previously thought. Here we show that the modern types of cephalopods originated and prospered throughout the Late Cretaceous in the North Pacific. The mid-Cretaceous cephalopod turnover was caused by cooling and the closure of the Bering Strait, which led to a subsequent faunal isolation of this area. In the Late Cretaceous the former niches of the fast-swimming belemnites were taken over by the modern types of cephalopods, which evolved endemically. The Cretaceous−Paleogene event only allowed the modern types of cephalopods to spread globally and to take over the niches previously held by belemnites.
Early Carboniferous ammonoids from Oued Temertasset in the Mouydir of Algeria frequently show conspicuous septal crowding at various stages in ontogeny, but particularly immediately at the end of the phragmocone. The analysis of this phenomenon suggests that closer septal spacing is probably not related to maturity in these specimens. Septal crowding occurs in different sizes, and interim septal crowding in juvenile growth stages with subsequent recovery to normal septum distances is common. Four groups of species with different modes of septal crowding can be separated. Close septal spacing is interpreted as a response to adverse ecological conditions such as depletion of oxygen, which caused deceleration of growth at the aperture.
The species of the genera Wocklumeria and Parawocklumeria are investigated in terms of their conch geometry, intraspecific variability, and ontogenetic trajectories using a rich material. Both genera are closely related, as indicated by a number of characters regarding conch geometry, ornament, and suture line. Some of the peculiar conch characters, including adult modifications, such as opening of the umbilicus, a terminal constriction, lateral narrowing of the mature aperture, and extreme septal crowding that is paralleled by the loss of suture characters can be seen in Wocklumeria. Wocklumeria sphaeroides (RICHTER, 1848) and Wocklumeria denckmanni (WEDEKIND, 1918) are distinguished on the base of degree of triangular coiling in intermediate whorls. Wocklumeria oblivia n. sp. and Wocklumeria boulmanensis n. sp. are newly described from the Anti-Atlas of Morocco.
Tajika, A. & Wani, R. 2011: Intraspecific variation of hatchling size in Late Cretaceous ammonoids from Hokkaido, Japan: implication for planktic duration at early ontogenetic stage. Lethaia, Vol. 44, pp. 287–298.
Intraspecific variations of the early shell dimensions (ammonitella and protoconch diameters) of two Late Cretaceous (earliest Campanian) ammonoid species (Gaudryceras tenuiliratum and Hypophylloceras subramosum) from the Haboro and Ikushumbetsu areas, Hokkaido, Japan, show no significant difference between these areas that are approximately 110 km apart. The geographic distributions of G. tenuiliratum and H. subramosum are supposed to be mainly controlled by the flotation and transportation during the embryonic stage within floating egg masses and/or post-embryonic stage because of their small hatchling sizes (1.18–1.46 mm in diameter for G. tenuiliratum, and 0.91–1.13 mm in diameter for H. subramosum), suggesting these two ammonite species at the embryonic and/or post-embryonic stages were transported at least 110 km. Postulating that the velocity of palaeocurrent around the Haboro and Ikushumbetsu areas during the Cretaceous Period was 0.25 m/s, similar to those in the modern ocean current flowing off the eastern Pacific coast of Hokkaido, the egg masses and/or hatchlings of G. tenuiliratum and H. subramosum were buoyant and transported more than 5 days. The preliminary comparison of hatchling size through time suggests that the hatching sizes of H. subramosum in Hokkaido increased slightly from the Middle Turonian until the earliest Campanian (during about 7 Myr). □ammonoid, hatchling, paleoecology, variation, Cretaceous.
Belemniten (Donnerkeile, Teufelsfinger) waren räuberische Kopffüßler, die im Jura die Weltmeere besiedelt haben. Von ihnen sind heute fossil meist nur die pfeilförmiger hinteren Teile der Schale übrig. Dieser Band beschreibt die aus süddeutschen Juraablagerungen bekannten Belemniten in derzeit gültiger Nomenklatur und Systematik. Zoologen und Geologen, aber auch engagierte Freizeitpaläontologen werden die übersichtlichen Bestimmungstabellen und die vielen Fotografien zu schätzen wissen.
Observations on the morphology of the early whorls of Eutrephoceras dekayi (Morton), a widespread Cretaceous nautilid, are supplemented with oxygen and carbon isotopic analyses (δ ¹⁸ O and δ ¹³ C) of the early septa of five well-preserved specimens to help identify the point of hatching on the shell. Septa 4 and 5 are more closely spaced than preceding septa and probably correspond in time of formation with a constriction or first broken aperture on the outer shell one-third whorl forward of the fourth septum. In modern Nautilus , morphologic, isotopic, and observational data suggest that similar features mark hatching. Between the fourth and fifth septa in E. dekayi , δ ¹⁸ O values show a shift of variable magnitude from heavy to lighter values followed by a return to heavier values over the next one to three septa. This isotopic shift is compatible with a hatching interpretation and may be explained as the result of kinetic and equilibrium effects on emergence from an egg capsule.
Eutrephoceras dekayi hatched at about 9 mm in diameter, one-third the hatching size of modern Nautilus. Like Nautilus, E. dekayi probably produced few young, all of which were active swimmers at hatching. In contrast, Mesozoic ammonoids produced numerous offspring ranging from 0.5 to 1.5 mm in diameter which may have spent some time in the plankton. These differences in life history may correlate with differences in ecologic specialization, environmental tolerance, and habitat between ammonoids and nautilids and may have contributed to their disparate rates of evolution during the Mesozoic.
Carbonate skeletons of fossil marine organisms are widely used to reconstruct palaeoceanographic parameters. Specifically, the geochemistry of Jurassic and Cretaceous belemnite rostra is traditionally interpreted to represent near sea-surface seawater properties. More recently, an increasing number of workers, have reported significant scatter in geochemical data (e.g., δ18O, δ13C, element/Ca ratio) when comparing rostra from the same stratigraphic level or within a single belemnite rostrum. This scatter is not explained by differential diagenetic overprint alone. Here we report petrographic evidence on the primary ultrastructure of rostra of Megateuthis (Middle Jurassic) and Belemnitella and Gonioteuthis (Late Cretaceous). The biogenic ultrastructure consists of a filigree framework of triaxial branches and tetrahedrons of variable size forming a honeycomb-like network. Data presented here suggest that these rostra yielded as much as 50 to 90% primary pore space. On the level of a working hypothesis - and in analogy with modern cephalopods - we propose that the pore space was formerly filled with body fluid and/or organic compounds during the life time of these organisms. Intra-rostral porosity was post mortem occluded by earliest diagenetic isopachous calcite cements of a non-biogenic origin. These may have been precipitated due to increased alkalinity related to the decay of organic matter. If this holds true, then the resulting fabric represents a composite biogenic/abiogenic structure. In order to optically separate the two calcite phases forming a single calcite fibre, we employed a wide range of state-of-the-art analytical tools to thin sections and ultra-thin sections of well-preserved specimens. Pending a verification of these well-supported ultrastructural data by means of high-resolution geochemical analyses from biogenic and abiogenic phases, we suggest that these findings have significance for those using belemnite rostra as archives of their palaeoenvironment.
This chapter reviews the ammonoid taphonomy, especially focusing on recent advances in various methodologies (including new data on necrosis and fossil diagenesis of organic components and experimental techniques with modern analogues). Aspects on necrosis and fossil diagenesis of organic components in ammonoids are discussed based on the comparison of the jaw compositions between two modern cephalopods and four fossil cephalopods from the Upper Cretaceous of North America and Japan. Ammonoid taphonomy, especially biostratinomic processes (waterlogging, implosion, post-mortem surfacing, transport on the seafloor, deformation after sedimentation in soft sediments, shell fragmentation, transportation of jaws), is reviewed from various experimental analyses with modern analogues (e.g., experimental approaches with modern nautilus shells or plastic models, hydrostatic calculations). This chapter also reviews the diagenetic processes (compaction and dissolution), the exceptional preservation, and preservation styles of ammonoids in various rocks (clastic and carbonate rocks).
Polyptychoceras, a Cretaceous heteromorph ammonite genus, is characterized by a trombone like shell called a "hamitoid" shell. In order to clarify the shell forming mechanism, a large sample, which consists of more than 320 specimens of P. pseudogaultinum (Yokoyama) obtained from the Upper Santonian of Hokkaido, was biometrically analyzed. Besides the shell coiling, cyclic changes of growth pattern are recognized by the analyses of the shell ornamentation, the relative growth rate of shell height, and the distance between septa. Intermittent shell growth, which was also deduced from the ontogenetic stage distribution in the population samples, is probably the cause of such peculiar shell coiling. Also, we carried out some computer simulations to reconstruct hydrostatically the ontogenetic change of the living attitude of P. pseudogaultinum. It is suggested that the rate of absolute shell growth possibly depends on the living attitude of this ammonite in the water column; the shell grows slowly when the shell aperture faces upward, and grows rapidly when the aperture faces in other directions. It is likely that every individual of this ammonite spent most of its life time with an upward facing aperture.
Several exceptionally well-preserved phragmocones of the belemnite Conobelus, with approximately 20 chambers preserved, were collected from the Valanginian of the Crimean Peninsula (Black Sea). The shell structure was studied with SEM. The prismatic wall of the protoconch wedges out near the mural part of the first septum, a short distance after the closing membrane. The prismatic protoconch wall consists of sublayers. In the extremely thin shell wall at the apex, there is a single sublayer. Additional sublayers appear on the inner shell surface at some distance from the apex. The closing membrane, about as thick as the shell wall, is not continuous with wall of the protoconch and conotheca. The primordial rostrum is structurally different from the rostrum, and separated from it by an organic membrane. The first 20 chambers, at least, seem to lack the conotheca proper. The shell wall is here composed of long mural parts of the septa and/or of the prismatic layer of the primordial rostrum. The first septum is prismatic; others are formed by the nacreous layer Type 2 (Mutvei, 1970), which in sections gives an impression of a granular structure; the mural parts of the septa show tabular nacre. At early ontogenetic stages, starting from the sixth septal neck, the transformation from retro- to prochoanitic septal necks occurs. The replacement of retrochoanitic septal necks by prochoanitic (or transitional) necks in Conobelus is similar to that in am- monoids and in some other branches of cephalopods, and, consequently, represents an example of homeomorphy in cephalopod evolution. In addition, the present paper includes an ultrastructural comparison of early shell ontogeny in belemnites and bactritoids.
Isotopic data (C and O) derived from Callovian (Middle Jurassic) mollusks (bivalves, ammonites and belemnoids, including true belemnites and Belemnotheutis) are presented from a narrow stratigraphic interval in the Christian Malford Lagerstätte, UK. The exceptionally well-preserved mollusks include aragonite-calcite pairs precipitated by individual belemnite animals that enable an assessment of possible "vital" effects and the reliability of using belemnite calcite to determine ocean water compositions. The oxygen isotope data derived from the calcitic rostra of the belemnites (Cylindroteuthis) show modest variability, ranging from 21.2 to 0.9%o (V-PDB), while their accompanying aragonitic phragmocones range from 21.4 to 0.0%o. Data derived from the ammonite Kosmoceras show some scatter, with oxygen isotope values varying from 23.6 to 20.2%o. The aragonite data from Cylindroteuthis, Kosmoceras and Belemnotheutis all overlap, suggesting they inhabited similar (surface) water depths. However, the corresponding data from the calcitic rostra of the Cylindroteuthis specimens suggest temperatures, 5°C cooler. As we have analyzed aragonite-calcite pairs, the discrepancy cannot be explained by environmental effects. Though clearly a vital effect, it is difficult to resolve whether the temperatures derived from the aragonite (phragmocone) are too warm or from the calcite (rostrum) are too cool. Consequently, the applicability of standard paleotemperature equations to Cylindroteuthid belemnite rostra remains unproven. Sequentially sampled ontogenetic isotope data derived from Belemnotheutis phragmocones reveal only modest δ18O variation, consistent with limited movement between warmer (shallower) and cooler (deeper) waters. A coincidental systematic pattern of δ13C enrichment may signal changes in metabolic activity associated with a shift in ecology or feeding with age.
We examined the stratigraphic distribution of ammonites at a total of 29 sites around the world in the last 0.5 myr of the Maastrichtian. We demarcated this interval using biostratigraphy, magnetostratigraphy, cyclostratigraphy, and data on fossil occurrences in relation to the K/Pg boundary in sections without any facies change between the highest ammonites and the K/Pg boundary. The ammonites at this time represent all four Mesozoic suborders comprising six superfamilies, 31 (sub)genera, and 57 species. The distribution of ammonites is dependent on the environmental setting. Recent data suggest that ammonites persisted to the boundary and some species may have survived for several tens of thousands of years into the Paleogene. The best explanation for ammonite extinction is a brief episode of ocean acidification immediately following the Chixculub impact, which caused the decimation of the calcareous plankton including the planktic post-hatching stages of ammonites. The geographic distribution of ammonites may also have played a role in the events with more broadly distributed genera being more resistant to extinction.
Diagenetic and morphological studies of transversely sliced rostra of six belemnite species were carried out. Diagenesis has not destroyed the laminar morphology of rostra in most cases, and diagenetic alterations indicate that the original mineralogy was low-Mg calcite. A revised interpretation of the construction of belemnite rostra is given, where composite radial structures accreting periodically around the surface are suggested as the principal elements. Organic matter is distributed throughout the rostrum, and is probably both inter- and intra-crystalline. Variation in the organic content along radial structures gives rise to a concentric growth pattern. -from Author
The Late Cretaceous belemnite family Belemnitellidae Pavlow, 1914, which includes nine genera and two subgenera, occurs only in the Northern Hemisphere, that is in the North European and North American palaeobiogeographical Provinces of the North Temperate Realm, in addition to the northern margin of the Tethyan Realm in Europe. The North European Province is subdivided into the Central European, Central Russian and Baltoscandian Subprovinces. The centre of origin and dispersal of the belemnitellids lay in the North European Province and all known genera and subgenera occur there. The belemnitellids immigrated intermittently into the Tethyan Realm (at least nine times) and the North American Province (at least six times). The majority of the species occurring in the Tethyan Realm are conspecific with those from the North European Province, whereas the species occurring in the North American Province are endemic, with a few exceptions. The endemic species probably evolved by allopatric speciation from initial migrants. More than a score migrations have been recognized within the subprovinces of the North European Province. The palaeogeographical distribution and migration patterns of the belemnitellids were to a certain extent controlled by eustatic sea-level changes, cool or warm climatic phases and competition, although the cause of several migration events cannot be satisfactorily explained at present.
The thickness ratios of shells (=whorl breadth/shell diameter) in the heteromorphic scaphitid ammonoid Scaphites planus (Yabe, 1910) from the lower middle Turonian in the Oyubari and Kotanbetsu areas of Hokkaido, Japan were examined in order to determine their mode of migration. The thickness ratios of S. planus differ significantly between the two localities, which suggest that these different populations did not frequently migrate between the two areas (currently ∼130 km apart, although the actual distance during Turonian is uncertain due to the presence of faults and folds between the two areas). There is no difference in hatchling diameters between the two areas, suggesting that the thickness ratios became manifested after the post-hatchling stage due to limited migration with in a nektobenthic habitat. This study suggests that scaphitid ammonoids became nektobenthic with limited migration at a stage earlier than previously thought (not during the transition from normal to abnormal coiling). The limited migration in scaphitid ammonoids might relate to their higher evolutionary volatility.
Macrofossil calcite is of great importance for quantitative reconstructions of palaeoenvironment and palaeoseasonality. The calcite rostra of belemnites, Jurassic to Cretaceous marine invertebrates, are especially suited for such investigations, because they are comparatively large and are structured by growth bands. Despite their use in chemostratigraphic and palaeoenvironmental studies, much of the internal variability of geochemical signatures in rostra is poorly understood.
Here, multiple profiles through a belemnite rostrum of Passaloteuthis bisulcata (∼183 Myr old) were analyzed for δ13C and δ18O values as well as Mg/Ca, Sr/Ca and Mn/Ca ratios. Geochemical signatures of the central 1-2 mm of the profiles indicate diagenetic cementation along the apical zone, for which original porosity of up to 40% can be inferred. The overall δ13C and δ18O values of the other, well preserved parts of the belemnite fluctuate by > 1 per mil, but are nearly uniform within single growth bands. In contrast, Sr/Ca and Mg/Ca in the wellpreserved parts show growth-rate and crystal-shape related variability. Close to the central apical zone, strongly bent calcite crystals are enriched in Mg (up to 70%) and Sr (up to 50%). In contrast, though the remainder of the rostrum, higher calcite precipitation rate can account for Mg depletion of ∼15% and Sr enrichment of ∼15% with respect to co-genetic calcite precipitated at a slower rate. No indication for temperature control on Mg/Ca or Sr/Ca is detected in the investigated specimen. Overall, the new findings indicate that δ13C and δ18O analyses of belemnite rostra produce consistent results regardless of the sampling area within the rostrum, and that growth rate effects on element incorporation are minor with respect to the control exerted by secular changes in seawater composition through time. Additionally, the central part of the rostrum, where strong calcite crystal bending is observed, should be avoided for sampling when studying elemental composition of the calcite for palaeoenvironmental reconstructions.
This study reveals the different modes of relationships between taxonomic richness and sea levels within cephalopods (ammonoids and nautiloids), and that these trends were significantly correlated to the evolutionary change of hatchling size of cephalopods. The temporal changes of reconstructed hatchling habits during the Phanerozoic demonstrate planktic habits in ammonoids and a trend from planktic to non-planktic habits in nautiloids. The correlation of ammonoid richness with sea levels and the lack of correlation in nautiloids with nonplanktic hatchlings contradict the general theory that animals with planktic embryos or larvae have lower speciation rates and extinction probabilities. This contradiction in the fossil evidence of cephalopods suggests that sympatric speciation was a plausible dominant process of their macroevolution. This contradiction relative to prevalent theory, that allopatric speciation is the prevalent mode of speciation in modern animals, suggests that speciation is a more complicated process than previously thought.
Among the ten strong and catastrophic extinctions of ammonites and belemnites in geological history of Earth, at least eight of them (mainly for ammonites) occurred during the development of oceanic acidification events. Our analysis shows that the most vulnerable ontogenetic phase of these animals to oceanic acidification was their small and mainly planktonic paralarvae with either outer (ammonites) or inner (belemnites) calcareous shells. The thin walls of the paralarval shell with aragonite phragmocone serving to keep the neutral buoyancy might have prevented them to migrate deeper than the implosion depth (from several meters to first tens meters). Strong short-term ocean acidification events were the strongest in superficial water layers of the open ocean and shallow waters on the shelf, the very habitats of ammonitellas and paralarval belemnites, with catastrophic impact on integrity of their tiny calcareous shells. We suggest that the damage and corresponding malfunction of the fragile phragmocone to keep the neutral buoyancy was crucial during the development of ocean acidification. Disastrous short-term ocean acidification events contributed to extinctions of the calcareous plankton in Mesozoic, regardless whether they were planktonic during the entire life (e.g., coccolithophorids, foraminifers) or just during a relatively short ontogenetic phase as in ammonites and belemnites.
Early Jurassic belemnites are of particular interest to the study of the evolution of skeletal morphology in Lower Carboniferous to the uppermost Cretaceous belemnoids, because they signal the beginning of a global Jurassic–Cretaceous expansion and diversification of belemnitids. We investigated potentially relevant, to this evolutionary pattern, shell features of Sinemurian–Bajocian Nannobelus, Parapassaloteuthis, Holcobelus and Pachybelemnopsis from the Paris Basin. Our analysis of morphological, ultrastructural and chemical traits of the earliest ontogenetic stages of the shell suggests that modified embryonic shell structure of Early–Middle Jurassic belemnites was a factor in their expansion and colonization of the pelagic zone and resulted in remarkable diversification of belemnites. Innovative traits of the embryonic shell of Sinemurian–Bajocian belemnites include: (1) an inorganic–organic primordial rostrum encapsulating the protoconch and the phragmocone, its non-biomineralized component, possibly chitin, is herein detected for the first time; (2) an organic rich closing membrane which was under formation. It was yet perforated and possessed a foramen; and (3) an organic rich pro-ostracum earlier documented in an embryonic shell of Pliensbachian Passaloteuthis. The inorganic–organic primordial rostrum tightly coating the protoconch and phragmocone supposedly enhanced protection, without increase in shell weight, of the Early Jurassic belemnites against explosion in deep-water environment. This may have increased the depth and temperature ranges of hatching eggs, accelerated the adaptation of hatchlings to a nektonic mode of life and promoted increasing diversity of belemnoids. This study supports the hypothesis that belemnite hatchlings were ‘a miniature of the adults’.
Morphometric analyses of shell morphology in the Cretaceous nautiloid Eutrephoceras clementinum (d'Orbigny, 1840) (Cephalopoda, Mollusca) from the Ariyalur area, southern India, reveal ontogenetic change from hatching to maturity as well as intra-specific variation in shell morphology. The shell breadth has a negative allometric relationship with shell diameter and with whorl height, and the umbilicus diameter has a positive allometric relationship with shell diameter. This shows that shell shape became relatively thinner with less variation, and the umbilicus diameter became relatively broader with growth. The siphuncle position moves from a dorso-central to ventro-central position with, growth. A constriction was recognized on the early whorl at 20 rum in shell diameter, and the interval angles of succeeding septa were changed at the 8th septum, indicating that they hatched at this stage. The bending of umbilical walls of apertures toward the center of coiling Suggests that E. clementinum attained maturity at about 115 mm in shell diameter. The comparison of the shell morphology of E. clementinum with that of E. bouchardianum (d'Orbigny, 1840) reported in the literature clarifies their difference in whorl shape and umbilical size, especially in the adult stage. This kind of morphometric study of nautiloids is essential for elucidating their adaptive designs for environment and mode of life, functional shell morphology, taxonomy, phylogeny, and evolution.
`Neocomian' (Valanginian–Barremian) and `Gault' (Aptian–Albian) are lithostratigraphic terms used in the last century to divide the sub-Chalk Cretaceous of northern Europe. The division is based on significant palaeoceanographic and palaeogeographic changes in the early Aptian. These changes are linked to a major turnover of marine floras and faunas, which occurred simultaneously in the Boreal Realm, the Tethys and the Indo-Pacific Ocean. Various floral and faunal elements became extinct in the Aptian both in the Tethys and in the Boreal Realm, while on the other hand new genera and species evolved. Marked changes in the diversification rate were observed for organisms with a different mode of life (plankton, nekton, benthos). Apart from calcareous nannofossils, foraminifera and ammonites, this turnover also occurs in belemnites, and the suborder Belemnitina was completely replaced by the suborder Belemnopseina. The northwestern European Barremian is characterised by the very successful belemnite family Oxyteuthididae (Belemnitina), which was completely replaced by the genus Neohibolites (Belemnopseina) in early Aptian times. This Aptian turnover, which is expressed by a homogenisation of marine floras and faunas, is best explained by significant palaeoceanographic changes occurring in the early Aptian. The sea-floor spreading rate, which increased at the Barremian–Aptian boundary significantly, peaked in the late early Aptian. A mantle plume in the Pacific possibly caused an additional production of oceanic crust. Both processes caused a major sea-level rise. This in turn resulted in a major change of the palaeoceanographic setting and restricted marginal seas, like the northwestern European basins in Barremian times, became drowned. The more open oceanic setting of the Aptian enabled cosmopolitan floras and faunas to spread worldwide. The Aptian saw the radiation of planktonic foraminifera, which played only a minor role in pre-Aptian times. These palaeoceanographic changes, from restricted marginal seas to a more homogeneous pelagic setting, are considered to be the main cause for the significant changes in the composition of belemnite faunas.
No egg of any fossil nautiloid has yet been discovered. However, anomalies of embryonic shell growth, described for the first time in several Mesozoic Nautilida, provide important clues on morphology, structure, and size of their egg capsules; on the physical characteristics where egg laying occurred; and on the hatching processes. Roughness inside the inner egg capsule—caused by hard and uneven egg-laying substrate, locally and temporarily slowing down or stopping the apertural shell growth—could cause temporary deformations of growth lines. Such roughness, caused by stone, is described inside an egg capsule of Nautilus, which was fixed obliquely relative to the egg-laying substrate. This reduced the space between the inner and outer capsules, which locally fused together. The lateral-umbilical grooves, furrows, and deformations of growth lines were probably caused by the inner egg capsule during the prehatching stage. In fossil Nautilida, as in Nautilus, the size of this capsule was relatively small compared with the shell diameter at hatching. During the last stages of embryonic development, the shell extended backwards outside the egg capsule before hatching. This prehatching stage, during which the egg capsule continued to press against the shell, can be marked by a prehatching constriction. In fossil species, as in Nautilus, the inner capsule constituted a kind of “straitjacket” during the last stages of embryonic development. The expansion in whorl width at hatching, in normal as well as in abnormal shells, marks release of this straitjacket. Important deformations of the whorl section probably result from an abnormal form and size of the egg capsules mainly caused by the manipulations by the female during the egg laying on a hard and hollow substrate, increasing the straitjacket effect. An alternative explanation could be that the chorion did not expand adequately. From relatively early embryonic stages (approximately 180° adapical of the nepionic constriction) to hatching, both flaps of the hyponome could be turned backward under the shell, jammed between the inner wall of the egg capsule and the mantle margin, resulting in the formation of paired ventral parallel grooves. Many normal features of the embryonic development of nautiloids can be clarified through the study of the anomalies of embryonic shell growth.
The distribution of aragonite in the skeletal parts of living organisms is reviewed, and its distribution in fossils is described on the basis of several hundred new determinations. Aragonitic fossils are extensively preserved in Tertiary sediments, and are common in Mesozoic rocks, particularly where the enclosing lithology is argillaceous. No aragonite was found by the authors in fossils from Palaeozoic rocks. The most important requirement for the preservation of aragonite is the presence of reducing conditions. It is suggested that the preservation of aragonite is due to the protective effect of organic skeletal matrix, which in turn requires reducing conditions for its preservation. The protective action of the organic matrix is attributed to the formation of a hydrophobic monomolecular layer on the crystal surface, composed of amino acids derived from the breakdown of skeletal matrix proteins.
Morphometric analyses of shell morphology in the Pennsylvanian nautiloid Metacoceras mcchesneyi Murphy, 1970 (Cephalopoda, Mollusca) recovered from coal mines in Madison Township, Columbiana County, Ohio, USA, reveal the ontogenetic change from hatching to maturity as well as intra-specific variation of shell morphology. The shell shape of M. mcchesneyi has isometric relationships, and the umbilicus diameter between umbilical shoulders has a positive allometric relationship with shell diameter. These show that the relative whorl shape was constant through the ontogeny, but the umbilicus became relatively broader with growth. The siphuncle position moved from a ventro-central position toward the center with growth until 420° of the total rotational angle had been attained. A constriction was recognized on the early whorl at 9.5 mm in shell diameter, and the interval angles of succeeding septa were changed at the 5th septum, indicating that hatching occurred at this diameter. The ventral apertural wall, the disappearance of ornamentation toward the last preserved aperture, and the last whorl separating from the previous whorl indicate that M. mcchesneyi attained maturity at ca. 70 mm in shell diameter.Most characteristics of shell morphology in M. mcchesneyi (the relative shell shape and ornamentation through ontogeny) are comparable to those in modern and younger fossil nautilids, irrespective of taxonomy and age, supporting the conclusion that evolutionary rates of shell morphology are conservative in nautilid history.
Specimens of the belemnitellid Praeactinocamax Naidin, 1964 are described from the Upper Cretaceous of NW Siberia (Taimyr Region, Lower Agapa River, Russia). The rostra determined as Praeactinocamax aff. plenus consist of an aragonitic anterior part and a calcitic posterior part with a sharp boundary in between. This boundary surface is referred to as the “alveolar fracture”, and it is a typical morphological feature of early belemnitellids and not a result of diagenetic processes. The occurrence of Praeactinocamax in Arctic areas shows a wider palaeobiogeographical distribution of the genus in the Late Cenomanian—Early Turonian interval than previously known. This finding suggests that migration of the late Cenomanian—early Turonian fauna occurred across Turgai channel. The geographic position of these new records may also explain the occurrence of Praeactinocamax in the Turonian of the US Western Interior Seaway, the origin of which has been hitherto unclear.
This paper analyzes the relationship existing between septa and ornamentation in ammonites showing two basic growth patterns: cyclic and uniform. In both CMCS there exists a strong correlation between ornamentation size and interseptal distance, both characters, being related to each other through growth rate. Nevertheless, in the case of cyclic growth, regulation mechanisms appear which prevent the formation of non-adaptative structures induced by extreme deviations in growth rate.