On the origin of bactritoids (Cephalopoda)

There is a high probability that bactritoids represent a paraphylum or polyphylum. The initial chambers or protoconchs of the Early-Middle DevonianBactrites Sandberger,Devonobactrites Shimansky, andLobobactrites Schindewolf are elongated spheres with a diameter of 0.3–1.0 mm. The initial chambers are larger in diameter than the slender, smooth shaft located adorally to the initial chamber. Similar apices occur in a number of Late Silurian sphaerorthoceridans with central siphuncles. Sphaerorthoceridans with a bactritoid-like apex and an eccentric siphuncle are known from the Early Devonian. The earliest questionableBactrites occurs in the Pragian (middle Early Devonian). By Emsian time bactritoids are common elements of cephalopod faunas.Bactrites-like orthocones of the Middle Ordovician and Late Silurian are homeomorphs with clearly different early growth stages. Thus, the time interval between the first appearance ofBactrites and the origin of ammonoids can be narrowed down to the Pragian to Early Emsian. The placement of the siphuncle in a ventral marginal position has been used as one of the critical morphologic features in defining the bactritoids. However, the displacement of the siphuncle from subcentral or eccentric positions toward the conch margin occurred at least three times during the Ordovician — Early Devonian evolution of the Orthocerida. Thus, there is a high probability that a marginal shift of the orthocerid siphuncle occurred in post-Emsian times, too.     

The bearing of the new Late Cambrian monoplacophoran genus Knightoconus upon the origin of the Cephalopoda

Yochelson, E. L., Flower, R. H. & Webers, G. F.: The hearing of the new Late Cambrian monoplacophoran genus Knightoconus upon the origin of the Cephalopoda.
Knightoconus , a new genus of the Hypseloconidae (Mollusca: Monoplacophora) from rocks of early Franconian age in Antarctica, is multiseptate. The multiple septa are a criticàl feature to be expected in a form ancestral to cephalopods. Fossil cephalopods, however, invariably have a siphuncle as well as septa; some gastropods, some hyolithids, and some monoplacophorans also have septa but lack a siphuncle. Therefore, only the siphuncle can be considered a unique and particularly significant feature of the cephalopod shell. Hypothetical reconstructions of molluscan anatomy support the notion that cephalopods may have been derived directly from a hypseloconid having a high, slightly curved, multiseptate, bilaterally symmetrical shell, by the subsequent development of a siphuncle.     

The Origin and Initial Rise of Pelagic Cephalopods in the Ordovician

Background

During the Ordovician the global diversity increased dramatically at family, genus and species levels. Partially the diversification is explained by an increased nutrient, and phytoplankton availability in the open water. Cephalopods are among the top predators of todays open oceans. Their Ordovician occurrences, diversity evolution and abundance pattern potentially provides information on the evolution of the pelagic food chain.

Methodology/Principal Findings

We reconstructed the cephalopod departure from originally exclusively neritic habitats into the pelagic zone by the compilation of occurrence data in offshore paleoenvironments from the Paleobiology Database, and from own data, by evidence of the functional morphology, and the taphonomy of selected cephalopod faunas. The occurrence data show, that cephalopod associations in offshore depositional settings and black shales are characterized by a specific composition, often dominated by orthocerids and lituitids. The siphuncle and conch form of these cephalopods indicate a dominant lifestyle as pelagic, vertical migrants. The frequency distribution of conch sizes and the pattern of epibionts indicate an autochthonous origin of the majority of orthocerid and lituitid shells. The consistent concentration of these cephalopods in deep subtidal sediments, starting from the middle Tremadocian indicates the occupation of the pelagic zone early in the Early Ordovician and a subsequent diversification which peaked during the Darriwilian.

Conclusions/Significance

The exploitation of the pelagic realm started synchronously in several independent invertebrate clades during the latest Cambrian to Middle Ordovician. The initial rise and diversification of pelagic cephalopods during the Early and Middle Ordovician indicates the establishment of a pelagic food chain sustainable enough for the development of a diverse fauna of large predators. The earliest pelagic cephalopods were slowly swimming vertical migrants. The appearance and early diversification of pelagic cephalopods is interpreted as a consequence of the increased food availability in the open water since the latest Cambrian.     

河北秦皇岛地区奥陶纪奇壁角石科分子的发现及意义

奇壁角石科（Ａｌｌｏｔｒｉｏｃｅｒａｔｉｄａｅ）分子在我华北东部属首次发现。它产于马家沟组下部。界于Ｄｅｆｏｒｍｏｎ－ｃｅｒａｓ－Ｐｅｒｉｐａｔｏｃｅｒａｓ与Ｐｏｌｙｄｅｓｍｉａ－Ｗｕｔｉｎｏｃｅｒａｓ－Ｅｏｓｉｏｔｅｌｕｓ组合带之间偏下。奇壁角石科是Ｆｌｏｗｅｒ,Ｒ．Ｈ．１９９５年创立的,至今科内已建立６属１３种和３个相似种。但以往所描述标本绝大多数仅保存有部分内体管,对其它特征所知甚少。文中描     

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

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

Biological response to experimental damage of the phragmocone and siphuncle in Nautilus pompilius Linnaeus

Tsujino, Y & Shigeta, Y. 2012: Biological response to experimental damage of the phragmocone and siphuncle in Nautilus pompilius Linnaeus. Lethaia, Vol. 45, pp. 443–449. Three adult specimens of Nautilus pomplilius Linnaeus from the Philippines were experimented on to estimate the biological response to damage of the phragmocone and siphuncle in this cephalopod mollusc. In addition, the data obtained from the experiments were used for discussion of shell damage in ammonoids and in other extinct cephalopods. Specimen’s phragmocone and siphuncle were perforated and severed artificially, followed by observations in the laboratory tank during periods of 75 and 132 days. For at least 2 or 3 months, all individuals survived after damage to the phragmocone and siphuncle despite loss of neutral buoyancy. Based on our observations after completion of the experiments, the severed adoral remaining part of siphuncle healed by the siphunclar epithelium. In addition, perforation of the phragmocone was partly repaired by shell secretion from the dorsally extending mantle due to subsequent volution of shell growth. Our experiments revealed that damage to the phragmocone and siphuncle in Nautilus was not necessarily a lethal injury. It may be possible that such biological response also applies to extinct ammonoids and nautiloids. In a similar case of extinct ammonoids and nautiloids, damage to their phragmocone and siphuncle may also not have been a lethal injury as with Nautilus. However, some factors leading to death are likely to be dependent on the degree of damage to the phragmocone and siphuncle and influence of hydraulic pressure. □Ammonoids, injury, nautiloids, Nautilus, phragmocone, repair, siphuncle.     

Strength of concave septa and depth limits of fossil cephalopods

Westermann, G. E. G.: Strength of concave septa and depth limits of fossil cephalopods.
Simple septa with spherical curvature are present in the shells of all Endocer-oidea, Actinoceroidea, Bactritoidea, and most Nautiloidea and Coleoidea. Such septa act as quasi-hemispherical concave membranes when subjected to hydrostatic pressure. Since the tensile strength of a spherical membrane is directly proportional to the ratio of its thickness and radius of curvature, measurements of these parameters on polished and thin sections of septa can be used to obtain strength of the septum against implosion. Depth limits of fossil cephalopods can be made by calibrating these measurements in terms of recent implosion data on 'living' Spirula and Nautilus . Estimates of septal strength are augmented by strength estimates for long septal necks and cylindrical to globular connecting rings.
Assuming that actual habitats ranged to approximately two-thirds of the mechanical limits of the shells, the following maximum depth ranges are indicated from this preliminary survey: Endoceroidea 100–450 m; Actinoceroidea 50–150 m; Nautiloidea, Ellesmerocerida 50–200 m, Orthocerida 150–500 m, Oncocer-ida <150, Discosorida <100 m, Tarphycerida <150 m, Nautilida 200–600 m; Bactritoidea c. 400 m; Coleoidea, Aulacocerida 200–900 m, Sepiida 200–1000 m, Belemnitida 50–200 exceptionally 350 m.     

A paradox survival – report of a repaired syn vivo perforation in a nautiloid phragmocone

The nautiloid Trocholites depressus (Eichwald, 1840) from the Lasnamägi regional stage (Darrivillian, Middle Ordovician) of Vaike Pakri Island (North-West Estonia) is the only known ectocochleate cephalopod that survived and healed a perforation of the phragmocone. Two chambers of the specimen were broken during its lifetime. The injury is located on the venter of the conch directly above the peristomal opening of the body chamber. It is reconstructed that the peristomal mantle tissue carried out an initial sealing of the injured chambers. The complete calcified sealing and compensation of the irregular shell surface started late with the overgrowth of the septa of the preceding whorl. The position and diameter of the siphuncle were not disturbed by the regeneration showing that these characters allowed a low phenotypic variability. Despite the trauma in the midlife growth history of this specimen, it appears to have reached maturity.     

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.     

LATE CAMBRIAN PLECTRONOCERID NAUTILOIDS AND THEIR ROLE IN CEPHALOPOD EVOLUTION

Abstract:  Numerous plectronocerid nautiloids appear in the Upper Cambrian of China. We have restudied their siphuncular structure, first described some 20 years ago. The siphuncle is characterized by: (1) long and holochoanitic septal necks dorsally but short and recurved necks laterally and ventrally; (2) strongly expanded connecting rings laterally; (3) two calcified layers in each connecting ring, outer spherulitic-prismatic and inner compact, the latter perforated by numerous pore canals; and (4) highly oblique siphuncular segments. The strongly expanded lateral sides of the connecting rings, together with the highly oblique course of the siphuncular segments, considerably enlarged the surface area of the connecting rings in each chamber, thereby increasing the transport capacity of cameral liquid. Thus, from their first appearance, plectronocerid nautiloids had developed a siphuncle for the replacement of cameral liquid with gases, and this system had a better and a more sophisticated design than that seen in stratigraphically younger nautiloids. However, their small orthoconic or slightly cyrtoconic shells were not well adapted for jet-powered swimming.     

CONSTANCY OF RELATIVE VOLUMES OF ZONES IN SHOOT APICAL MERISTEMS IN CACTACEAE: IMPLICATIONS CONCERNING MERISTEM SIZE,SHAPE, AND METABOLISM

The volumes and dimensions of shoot apical meristem zones were determined for 21 species in 10 genera of the Cactaceae. If the volumes of the central-mother-cell zone (CMCZ), the peripheral zone (PZ), and the pith-rib meristem (PRM) are expressed as percentages of the total volume of the corpus, then they are remarkably constant (CMCZ = 11.1% of the volume of the corpus, PZ = 61.2% and PRM = 27.1%) regardless of the relative size of the apical meristems. The relative volume of the tunica decreases, however, as the whole apex increases. The zones have variable shapes, and whereas the PZ and PRM are always trapezoidal in longitudinal section, in apices with large volumes, these zones have a flatter, more flaring shape than they have in smaller apices. Thus large apices are flatter and less hemispherical than are small apices. The CMCZ, however, maintains a constant shape and is always relatively hemispherical regardless of its volume. A hypothesis that relates all of these shape and volume relationships as an integrated growth sequence is proposed.     

Apertural constrictions in some oncocerid cephalopods

Stridsberg, Sven 1981 12 15: Apertural constrictions in some oncocerid cephalopods. Lethaia , Vol. 14, pp. 269–276. Oslo. ISSN 0024–1164.
In some oncocerid cephalopods the shape of the aperture, siphuncle and the general outline of the shell have long served as generic characters. The aperture is mostly elaborated into a certain number of sinuses which take their final shape only in the adult. Therefore, knowledge of the relative age of the animal is required. The last chamber may serve as an indicator of age. A last chamber smaller than the second last indicates a mature specimen. This is because continued growth would have caused the death of the animal as the buoyancy turned negative. Moreover, it is of great importance to study the growth lines along the peristome to observe whether growth has ceased or not. Growth variations have been compared with growth stages. Furthermore, a constricted or contracted aperture can only be determined on specimens with the shell still preserved. Functional parallels are drawn between the Aprychopsis operculum and the restricted aperture. * Cephalopoda, Oncorerida. aperture, ontogeny, growth lines, functional morphologv , Aptychopsis. Silurian, Gotland .     

Post-mortem descent with septal implosion in Silurian nautiloids

Late Silurian nautiloids from Bohemia have either (1) thin and densely spaced septa of which many are broken and internally accumulated, or (2) thick and widely spaced septa wnich are all intact. Since the latest chamber (s) and the shell wall are undamaged, septal fragmentation occurred by implosion during postmortem sinking, with sea water rushing in through the siphuncle. The latest connecting ring(s) were more permeable than the immature ones, permitting pressure compensation in the latest chamber(s). Septal debris accumulated adapically indicating the (ultimate) sinking orientation. Depth (maxima) of the nautiloid habitats and of the Bohemian basin are estimated from the strength parameters of the broken and intact septa: brevicones — epipelagic, weak longicones — moderately shallow pelagic, strong longicones — nektobenthic, sea floor depth — several 100 m. Silurian-Nautiloids-Shell-Connecting rings-Bathymetry.     

Clonal Analysis Provides Evidence for Transient Initial Cells in Shoot Apical Meristems of Seed Plants

Abstract Drift of mutated sectors in sectorial or mericlinal plant chimeras has been interpreted as indirect evidence of initial impermanence at the apex. However, the same effect may result from mutation in noninitial cells positioned close to the vertex of the apical dome. Clonal analysis of the cell packets present in the superficial layer of spruce and magnolia apices provided the library of patterns suggesting that the position and the number of initial cells, and in some cases also the meristem axis inclination, may change over time. Multicellular clones originating from a single cell have been found in the geometric center of some apices, whereas in other apices the cellular center (where three or four clonal borders meet) did not correspond to the geometric center of the apex. Such effects may result only from initial impermanence.     

Body size of orthoconic cephalopods from the late Silurian and Devonian of the Anti‐Atlas (Morocco)

Fluctuations in body size of orthoconic cephalopods are reported from late Silurian to Late Devonian sediments at several locations in the Tafilalt and in the Dra‐Valley (Anti‐Atlas, Morocco). The combination of measurements of diameters and apical angles allows the reconstruction of their total conch size (length and volume), which revealed a strongly right‐skewed size distribution with an average length of 278 mm, while the largest Devonian actinocerids exceeded 2 m. Within the examined groups (Actinocerida, Orthocerida, Pseudorthocerida), there is no uniform trend, but rather frequent fluctuations with maximum sizes in the late Lochkovian and early Emsian. Body size decreased in times of extinction events, while stable periods are mostly associated with a size increase. Additionally, conch size correlates well with gamma diversity and global δ¹³C values. Furthermore, the apical angle and septal diameter of orthocones appear to correlate, but only when their mean values are compared across beds.     

Co-estimation of phylogeny and divergence times of Argonautoidea using relaxed phylogenetics

This is the first study to investigate molecular phylogenetic relationships among all four genera of the superfamily Argonautoidea, a clade of diverse pelagic cephalopods with extraordinary characters such as ovoviviparity, dwarf males and secondary “shell” development. Phylogenetic relationships and divergence times within the superfamily were co-estimated using relaxed phylogenetic techniques. A sister-taxon relationship was recovered between Argonauta and Ocythoe and between Tremoctopus and Haliphron. The most recent common ancestor of Argonautoidea was estimated to date from the early Tertiary under the scenario that a lack of a “shell” in the ocythoid lineage is a primary characteristic. In contrast, a later Tertiary most recent common ancestor was estimated under the scenario that a “shell” was present in the early ocythoid lineage and was subsequently lost.     

SIPHUNCULAR STRUCTURE IN SOME FOSSIL COLEOIDS AND RECENT SPIRULA

Abstract: In Jurassic Phragmoteuthis huxleyi Donovan (Order Phragmoteuthida) the siphuncular wall shows unique structural and morphological features. The septal neck is short, about one‐eighth of chamber length, but the connecting ring is extremely long, extending through 5–6 chambers. The permeable siphuncular wall in each chamber is, therefore, unusually thick and consists of 5–6 consecutive connecting rings. Each connecting ring is calcified and has a highly porous structure in being composed of bundles of spicular crystallites, orientated more or less at right angles to the siphuncular wall, and separated by smaller or larger interspaces. A restudy of the belemnoid Megateuthis gigantea (Schlotheim) and the aulacoceratid Mojsisovicsteuthis? shows that the connecting rings in these taxa are also calcified. Each ring has a length of two chambers and consists of several calcified lamellae that are traversed by minute pores. The permeable siphuncular wall in each chamber therefore consists of two consecutive connecting rings separated by a porous prismatic layer. In Recent Spirula the connecting ring is composed of two layers: an outer spherulitic‐prismatic layer and an inner glycoprotein layer, of which the latter is not preserved in dry shells. The connecting ring structure is here similar to that in Recent Nautilus. Our study shows that at least three different structural types of siphuncular wall occur in coleoids. The phragmoteuthid connecting ring has a primitive structure, unknown in other cephalopods. This indicates that this taxon has no closer relationship with other coleoid taxa. The belemnitid‐aulacoceratid connecting ring is calcified and traversed by numerous pore canals. It shows a certain structural similarity to that in fossil actinoceratid and orthoceratid nautiloids. The spirulid connecting ring is structurally similar to that in Recent Nautilus and fossil nautilitid and tarphyceratid nautiloids. Thus the connecting ring structure indicates that coleoids include several, phylogenetically clearly separated lineages.     

The cephalopod macrosystem: A historical review, the present state of knowledge, and unsolved problems: 2. Classification of nautiloid cephalopods

Major classifications of nautiloid cephalopods are critically reviewed. It is suggested that this cephalopod group is subdivided into 5 subclasses and 17 orders: Ellesmeroceratoidea (including the orders Plectronocerida, Protactinocerida, Yanhecerida, and Ellesmerocerida), Endoceratoidea (including the orders Endocerida and Intejocerida), Actinoceratoidea (including the order Actinocerida), Nautiloidea (with the orders Basslerocerida, Tarphycerida, Lituitida, Discosorida, Oncocerida, and Nautilida), and Orthoceratoidea (including the orders Orthocerida, Ascocerida, Dissidocerida, and Bajkalocerida). The above orders are briefly described.     

Middle Ordovician cephalopod biofacies and palaeoenvironments of Baltoscandia

During the Middle Ordovician cephalopods became an important part of the macrofauna of the Baltoscandian carbonate platform. The earliest cephalopod abundance peak was reached during the early Darriwilian, within the Kunda Stage Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones. In sediments of this time interval large orthoconic cephalopods often occur in masses with more than one specimen per square‐meter on bedding surfaces. The assemblages are characterized by the strong dominance of often large endocerids. In proximal depositional settings coiled tarphycerids and other cephalopod groups are an important additional component. In the most distal settings orthocerids are the most important secondary component. Correspondence Analysis of assemblages throughout Baltoscandia revealed three distinct biofacies, which here are termed Orthocerid, Proterovaginoceras and Anthoceras Biofacies, respectively. The biofacies reflect differences in depth and proximity to the shoreline and are consistent with the Baltoscandian Confacies Belts. Spatial changes in absolute abundance and taxonomic composition indicate increased original cephalopod population densities and habitat expansion within the Y. crassus and L. pseudoplanus conodont zones. A nearly coeval abundance peak in a similar facies in South China indicates supraregional causes of the mass occurrence, probably reflecting a globally increased nutrient availability in the water column during the Darriwilian.     

Structure of the first-formed shell of the Middle Ordovician orthid-like brachiopods from the Leningrad Region

Shell structure of the first-formed shell of the Middle Ordovician orthid-like brachiopods from the Leningrad Region is described. The 190-μm-wide first-formed shell is composed of finely granular layer while 700-μm-wide first-formed shell is fibrous. Thus the order Orthida in the Early Paleozoic included brachiopods with both planktotrophic and lecithotrophic larvae in the ontogeny.