Morphogenetic significance of the conchal furrow in nautiloids: evidence from early embryonic shell development of Jurassic Nautilida

As reported by many workers over the past two centuries, the inner part of the shell of various straight and coiled Palaeozoic to tertiary nautiloid taxa bears a continuous mid-ventral furrow that extends into the phragmocone and the body chamber nearly to the aperture. Study of the early embryonic shell development of Jurassic Nautilida shows that the most apical part of this so-called conchal furrow originates from the inner part of the initial, calcified shell apex, in line with the inner ventral termination of the central linear depression of the cicatrix, the initial site of shell deposition. The conchal furrow corresponds to a morphological feature arising as a developmental by-product. Rare specimens of scattered ammonoid species (and possibly of bactritoids) display a similar feature, whereas their protoconch lacks a cicatrix. However, the protoconch of recent cuttlefish, Sepia officinalis, often displays a longitudinal fold of the primary shell epithelium. A longitudinal groove or a pair of grooves appears connected with this cicatrix-like structure. Although the mid-ventral ridge in ammonoids must probably be viewed as an incidental 'fabricational noise', whether or not it originates from a so far undocumented optional ridge on the protoconch or from some other structure related to shell development remains an open question.     

Septal implosion in Late Carboniferous coiled nautiloids from Ohio

Mapes, R.H. & McComas, G.A. 2010: Septal implosion in Late Carboniferous coiled nautiloids from Ohio. Lethaia, 10.1111/j.1502–3931.2009.00213.x More than 200 relatively mature coiled nautiloid specimens, assigned to Metacoceras mcchesneyi, were recovered from an Upper Carboniferous shale in northeastern Ohio. Twenty‐seven undistorted specimens reveal that the septa in every specimen were collapsed and/or telescoped. This septal collapse without external shell distortion could only have been accomplished by limited implosion due to excessive pressure. Analysis of the fossils, sediment and the depositional environment indicate that after burial, the nautiloid cameral spaces were probably filled with both liquid and gas, and the body chamber was filled with semi‐solid thixotropic mud. To prevent conch collapse at the time of septal implosion, the thixotropic mud filling the nautiloid body chamber acted as a liquid at the time of stress release during septal failure. The stress was produced by combined lithostatic and hydrostatic pressures, which fluidized the unlithified thixotropic mud that flowed from the body chamber into the phragmocone during septal collapse. After the septal implosion and when flowage ceased, the thixotropic mud quickly resolidified into a solid state providing internal conch support that prevented the collapse of the conch. □Carboniferous, nautiloids, septal implosion, taphonomy, thixotropic mud.     

Early evolutionary trends in ammonoid embryonic development

During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.     

Ventral bite marks in Mesozoic ammonoids

Reports on the predators of ammonoids are rare, although ammonoids were abundant and diverse invertebrates in many Paleozoic and Mesozoic marine ecosystems. Most previous work on lethal ammonoid predation has focused on (sub)circular tooth marks which resulted from fish and mosasaur attacks. In the present study we discuss a relatively common type of bite mark in ammonoid shells, the ‘ventral bite mark’. This typically occurs in a restricted position on the ventral side of the outer body chamber whorl and does not affect either the aperture or the phragmocone. Ammonoid specimens revealing ventral bite marks used in this study were collected from a wide range of strata which range in age from the Lower Jurassic to the uppermost Cretaceous (close to the Cretaceous–Paleogene boundary). These ventral bite marks are absent in the Paleozoic collections studied. The vast majority of ventral bite marks are situated at the end of the body chamber, close to the phragmocone. This is interpreted as the result of predatory attacks on the back or blind side of ammonoids in their living position. The predators aimed for the vital parts and muscle attachments to obtain the edible soft tissues. The agents for most of the ventral bite marks to ammonoids are probably coleoid cephalopods (especially teuthoids) and predatory fishes to a lesser extent.     

The morphospace of Late Permian coiled nautiloids

Occurrences of Late Permian coiled nautiloids are widespread but they have never been analysed in terms of spatial and temporal disparity changes. Morphometric analyses using the cardinal Raupian conch parameters: conch width index, umbilical width index and whorl expansion rate with subsequent analysis by using principal components analysis and non-metric multidimensional scaling, allow the construction of a nautiloid morphospace. The analyses show that there is a stable disparity in the coiled nautiloids from the Wuchiapingian to the Changhsingian. Differences between the three major Late Permian nautiloid occurrences (Salt Range, South China and Transcaucasus-NW Iran) are considerably small; the South Chinese occurrences, however, are characterized by many endemic genera. The most important variation in morphospace occupation is caused by environmental differences such as water depth.     

On the efficiency of the buoyancy apparatus in ammonoids: evidences from sublethal shell injuries

The five greatest sublethal injuries were selected from a collection of more than 12,000 predominantly Mesozoic injured or otherwise pathological ammonoids. The loss of shell mass from these survived injuries was calculated and compared with comparable tolerances in the recent Nautilus . These ammonoids tolerated a shell loss up to four times greater than in Nautilus . The maximum tolerated shell loss indicates an unexpected buoyancy compensation mechanism. The buoyancy of the selected specimens was calculated. The results show that the buoyancy of all the observed ammonoid shells was positive. In order to maintain neutral buoyancy after injury, these ammonoids had to fill the phragmocone with a volume of mass. Nautilus compensated a maximum mass loss requiring a liquid refill of 3% of the cameral capacity, the ammonoids compensated a maximum of observed mass loss requiring a liquid refill of more than 10% of cameral capacity. The ratio of chamber volume/siphuncular surface area in the ammonoid Lithacoceras is 0.043, indicating that the relative area of the siphuncular epithelium in Lithacoceras is significantly higher when compared with a ratio of 0.12-0.14 in the adult Nautilus . The phragmocone in ammonoids offered the ability of a much more active buoyancy regulation than in Nautilus .     

Buoyancy of some Palaeozoic ammonoids and their hydrostatic properties based on empirical 3D‐models

The interpretation of the function of the ammonoid phragmocone as a buoyancy device is now widely accepted among ammonoid researchers. During the 20^th century, several theoretical models were proposed for the role of the chambered shell (phragmocone); accordingly, the phragmocone had hydrostatic properties, which enabled it to attain neutral buoyancy, presuming it was partially filled with gas. With new three‐dimensional reconstructions of ammonoid shells, we are now able to test these hypothetical models using empirical volume data of actual ammonoid shells. We investigated three Palaeozoic ammonoids (Devonian and Carboniferous), namely Fidelites clariondi, Diallagites lenticulifer and Goniatites multiliratus, to reconstruct their hydrostatic properties, their syn vivo shell orientation and their buoyancy. According to our models, measurements and calculations, these specimens had aperture orientations of 19°, 64° and 125° during their lives. Although none of our results coincide with the aperture orientation of the living Nautilus, they do verify the predictions for shell orientations based on published theoretical models. Our calculations also show that the shorter the body chamber, the poorer was the hydrodynamic stability of the animal. This finding corroborates the results of theoretical models from the 1990s. With these results, which are based on actual specimens, we favour the rejection of hypotheses suggesting a purely benthonic mode of life of ammonoids. Additionally, it is now possible to assess hydrodynamic properties of the shells through ontogeny and phylogeny, leading to insights to validate theoretical modes of life and habitat through the animal's life.     

Post-hatching early life history of Cretaceous Ammonoidea

Post-hatching early life histories in Cretaceous Ammonoidea are discussed on the basis of density calculations of the shells in 71 species belonging to four separate suborders. The calculation was made under the assumption that a newly hatched ammonoid had a gas-filled chamber and a succeeding body-filled whorl terminating at the primary constriction. The results show that the density of the species examined at the hatching stage is almost constant and is relatively smaller than that of seawater, i.e. the animals are positively buoyant. This fact strongly suggests a planktic mode of life. In all species, the density increases gradually with growth and attains neutral buoyancy at 2.C2.5 mm in shell diameter. Thus, most ammonoids probably changed their mode of life from planktic to nektoplanktic or nektobenthic at this critical point. The rare occurrence of newly hatched specimens (ammonitellas) in many ammonoid assemblages may also support this interpretation. Planktic duration of a newly hatched ammonoid might be regulated by the animal's density at hatching, shell growth pattern, cameral volume (or hatching size), and rate of cameralliquid removal (or siphuncle diameter). The latter two seem to be very important factors in determining the biogeographical framework of species, as demonstrated in the Tetragonitaceae.□ Cretaceous, Ammonoidea, density calculation, early life history .     

Late Palaeozoic mollusc reproduction: cephalopod egg-laying behavior and gastropod larval palaeobiology

Faunal analysis of an oxygen‐depleted marine Lower Carboniferous succession (Late Mississippian Ruddle Shale) suggests how some cephalopod taxa laid their eggs during the Late Palaeozoic. At the Ruddle Shale collecting site in Arkansas, USA, the facies and overall fauna suggest severe oxygen depletion at the sediment/water interface. The Ammonoidea, with their small egg size, were probably laid in suspended gelatinous egg‐filled masses in the water column above the bottom or by attachment of the egg masses to floating debris. The ammonitella embryos developed within the suspended or attached egg mass; hatched individuals became part of the free‐swimming plankton biota. Based on shell morphology the Bactritoidea probably followed the same reproductive pattern. Coiled nautiloids (the Nautilida) and most orthoconic nautiloids (mostly the Pseudorthocerida) probably did not lay their eggs in the mid water column or as floatant attachments. This conclusion is based on the fact that, with one exception, all shells recovered of these two nautiloid orders are well past hatching. Gastropods in the Ruddle Shale are very small and cannot be visually detected in the field. However, microgastropods are abundant in washed residues. Most specimens are much smaller than 1 mm. The largest caenogastropod specimen is 1.3 mm high. These caenogastropods represent isolated larval shells and a successful metamorphosis was impossible because of oxygen depletion on the bottom. Allegations that a size of more than 1 mm is too large for pelagic larvae are refuted by examples of planktotrophic larval shells of modern gastropods (more than 1 mm high) and Triassic caenogastropods (up to 2 mm high) from the Cassian Formation (Northern Italy, South Alps). Repository information is given for the type‐material of the gastropod species Nuetzelina striata  Bandel, 2002 and Anozyga arkansasensis  Bandel, 2002 which were both erected based on specimens from the Ruddle Shale that were illustrated by Nützel & Mapes in 2001.     

The mode of life in ammonoids

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

Desmoceras (Pseudouhligella) intrapunctatum n. sp. (Ammonoidea) aus dem Unter-Albium von NW-Madagaskar mit Ritzstreifen

The new ammonoid speciesDesmoceras (Pseudouhligella) intrapunctatum (DesmoceratoideaZittel, 1895) is described and figured from the dark glauconitic marls of the Lower Albian of Ambatolafia (Mahajanga Basin of northwestern Madagascar). On the molds of its body chamber a well developed system of dotted lines is visible originally described from Palaeozoic ammonoids and nautiloids (= ?Ritzstreifen“Sandberger & Sandberger 1850 sensuTozer 1972). These internal shell structures have been originated by small ridges of the inner prismatic layer as the result of an incomplete mineralization. “Ritzstreifen” could not have observed in any other ammonoid species accompanied withDesmoceras (Pseudouhligella) intrapunctatum, despite of the same excellent aragonitic shell preservation. Therefore, this unique character is interpreted as taxonomically significant on species level.     

First direct evidence of ammonoid ovoviviparity

Ammonoids had high evolutionary rates and diversity throughout their entire history and played an important role in the high‐resolution sub‐division of the Mesozoic, but much of their palaeobiology remains unclear, including the brooding habitat. We present our study of the first recorded ammonite embryonic shell clusters preserved with calcified embryonic aptychi in situ within the body chambers of mature macroconch shells of the Early Aptian (Early Cretaceous) ammonite Sinzovia sazonovae. The following support the idea that the clusters are egg masses, which developed inside ammonite body chambers: the absence of post‐embryonic shells and any other fossils in these clusters, the presence of the aptychi in all embryonic shell apertures and peculiarities of adult shells preservation. These facts confirm earlier speculations that at least some ammonoids could have been ovoviviparous and that, like many modern cephalopods, they could have reproduced in mass spawning events. The aptychi of ammonite embryonic shells are observed here for the first time, indicating that they were already formed and calcified before hatching. Our results are fully congruent with the peculiar modes of ammonoid evolution: quick recovery after extinctions, distinct evolutionary rates, pronounced sexual dimorphism and the nearly constant size of embryonic shells through ammonoid history. We assume that adaptation to ovoviviparity may be the reason for the presence of these features in all post‐Middle Devonian ammonoids.     

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.     

Analysis of a Carboniferous embryonic ammonoid assemblage implications for ammonoid embryology

An embryonic ammonoid assemblage was discovered in a carbonate concretion recovered from a dysoxic, relatively offshore marine shale of Virgilian (Upper Pennsylvanian) age in Kansas, USA. The assemblage consists primarily of two species of the Goniatitina, Aristocerassp. and Vidrioceras sp., whose initial chambers (protoconchs) differ in size and shape. Microscopic observations of serial thin sections of specimens at different growth stages reveal the sequence of embryonic shell development starting with the formation of the initial chamber and ending with the synchronous secretion of a prismatic proseptum and nacreous swelliig (primary varix) at the aperture. The mode of occurrence of the embryonic shells of the two species in the concretion suggests that these ammonoids produced numerous small offspring, a reproductive strategy similar to that in many extant coleoids. □ Ammonoids, embryonic shells, development, Carboniferous, Kansas.     

From near extinction to recovery: Late Triassic to Middle Jurassic ammonoid shell geometry

The Triassic–Jurassic extinction resulted in the near demise of the ammonoids. Based on a survey of ammonoid expansion rates, coiling geometry and whorl shape, we use the Raup accretionary growth model to outline a universal morphospace for planispiral shell geometry. We explore the occupation of that planispiral morphospace in terms of both breadth and density of occupation in addition to separately reviewing the occurrence of heteromorphs. Four intervals are recognized: pre‐extinction (Carnian to Rhaetian); aftermath (Hettangian); post‐extinction (Sinemurian to Aalenian) and recovery (Bajocian to Callovian). The pre‐extinction and recovery intervals show maximum disparity. The aftermath is marked by the disappearance of heteromorphs and a dramatic reduction in the range of planispiral morphologies to a core area of the morphospace. It is also characterized by an expansion into an evolute, slowly expanding part of the morphospace that was not occupied prior to the extinction and is soon abandoned during the post‐extinction interval. Aftermath and post‐extinction ammonoid data show a persistent negative correlation whereby rapid expansion rates are associated with narrow umbilical widths and often compressed whorls. The permanently occupied core area of planispiral morphospace represents generalist demersals whose shells were probably optimizing both hydrodynamic efficiency and shell stability. All other parts of the planispiral morphospace, and the pelagic modes of life the shells probably exploited, were gradually reoccupied during the post‐extinction interval. Planispiral adaptation was by diffusion away from the morphospace core rather than by radical jumps. Recovery of disparity was not achieved until some 30 Myr after the extinction event.     

New aspects in ammonoid mode of life and their distribution

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

西藏申扎地区奥陶纪鹦鹉螺类化石的新材料

塔尔玛桥东-扎扛一带是近年发现的藏北地区第二个有可靠生物化石依据的奥陶系出露区。描述的鹦鹉螺化石是该区鹦鹉螺化石的一小部分,包括1新属、2新种和1未定种,分别是Taermaocerasgen.nov.,Taer-maocerasregularegen.etsp.nov.,Taermaocerasovatumgen.etsp.nov.及Taermaocearssp.。     

The connecting rings of Nautilus and Mesozoic ammonoids: implications for ammonoid bathymetry

The fresh conchiolinous ('horny') connecting rings of Nautilus pompilius decrease ontogcnctically in the strength index (100 times; wall thicknessh/inner radius r), i.e. from ≥ 16 to – 12. A similar decrease of 100 h/ r is the rule in Ammonitina. This may reflect depth migration but, more likely and in analogy with Nautilus, is compensated for by increased conchiolin strength. The mature ammonoid siphunclc, therefore, provides the more reliable indicator for depth limits. No marked shrinkage of connecting rings is evident in fossil nautiloids, but Mesozoic nautilid connecting rings are rarely preserved. Post-mortem alteration of the abundantly preserved ammonoid connecting rings entails mechanical distortion and fragmentation by differential filling of the siphunclc and camerac with sediment, and probable microbial decomposition, proceeding from the body chamber and resulting in pitting and perforation; shrinkage occurred rarely. This is probably related to the high phosphate content as here documented for Haplo-phylloceras. If the phosphate prevalent in ammonoid connecting rings is a primary constituent, rather than of early diagenetic origin, calibration of the ammonoid strength index on the non-phosphatic Nautilus connecting rings may not be justified. The strength indices of ammonoid connecting rings would thus provide only a relative scale of depth limits. Another example of a phylloccratid, Ptychophylloceras, has been found with a connecting ring extending well into the body chamber. D Connecting rings, ammonoids. Nautilus, bathymetry.