Recent Nautilus Research Articles

Endocerids: suspension feeding nautiloids?

ABSTRACTFor a long time all extinct cephalopods of the subclass Nautiloidea were considered as ecological analogues of the Recent Nautilus. Recently this view has been rejected: it is now known that among the nautiloids there were not only demersal predators but also epipelagic animals whose life-style and reproduction differed from those of the Nautilus. However, the habits of some nautiloid orders is still poorly understood. One of the most enigmatic cephalopods is the Early Paleozoic nautiloid order Endocerida. Endocerids differ from other nautiloids: they reached gigantic sizes (up to 9 meters), had a wide siphuncle tube and were widespread and numerous during the Ordovician. Since they were an important component of many Ordovician ecosystems, without the understanding of their habits and feeding strategies a correct reconstruction of these ecosystems is impossible. Until now, endocerids have been considered as dominant apex predators, however, this assumption is based on an analogy with the Nautilus mode of life, while the features of the structure of endocerid shells do not confirm this idea and furthermore contradict it. In this article, a new hypothesis is proposed and debated: according to it, the endocerids were planktotrophic cephalopods and the largest of them were giant suspension feeders.

SIPHUNCLE SOFT-PARTS IN THE UPPER JURASSIC AMMONITE KACHPURITES FULGENS

Ammonoid soft tissues are rarely preserved and such findings are of considerable interest. Extremely little is known about the structure of ammonoid siphuncle soft-parts. Until now indisputable fossilized siphuncle soft tissues were known only in a few ammonoid genera. This article describes phosphatized soft tissues inside the siphuncle tubes of the Upper Jurassic (upper Volgian) ammonite Kachpurites fulgens from Central Russia. Cross-sectional views of several Kachpurites siphuncles show a large blood vessel in the center surrounded by four smaller ones. In several specimens, the siphuncle epithelium is preserved. The size and arrangement of the blood vessels in Kachpurites is nearly identical to previously described blood vessels in the siphuncles of Recent Nautilus and the Permian prolecanitid ammonoid Akmilleria electraensis. This similarity indicates a remarkable conservatism of ammonoid siphuncles and provides grounds for the assumption that the last common ancestors of Ammonoidea and Nautilida also had the same structure of the siphuncle soft tissues.

Intraspecific variation of phragmocone chamber volumes throughout ontogeny in the modern nautilid Nautilus and the Jurassic ammonite Normannites.

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.

A new approach for the determination of ammonite and nautilid habitats.

Externally shelled cephalopods were important elements in open marine habitats throughout Earth history. Paleotemperatures calculated on the basis of the oxygen isotope composition of their shells can provide insights into ancient marine systems as well as the ecology of this important group of organisms. In some sedimentary deposits, however, the aragonitic shell of the ammonite or nautilid is poorly or not preserved at all, while the calcitic structures belonging to the jaws are present. This study tests for the first time if the calcitic jaw structures in fossil cephalopods can be used as a proxy for paleotemperature. We first analyzed the calcitic structures on the jaws of Recent Nautilus and compared the calculated temperatures of precipitation with those from the aragonitic shell in the same individuals. Our results indicate that the jaws of Recent Nautilus are secreted in isotopic equilibrium, and the calculated temperatures approximately match those of the shell. We then extended our study to ammonites from the Upper Cretaceous (Campanian) Pierre Shale of the U.S. Western Interior and the age-equivalent Mooreville Chalk of the Gulf Coastal Plain. In the Pierre Shale, jaws occur in situ inside the body chambers of well-preserved Baculites while in the Mooreville Chalk, the jaw elements appear as isolated occurrences in the sediment and the aragonitic shell material is not preserved. For the Pierre Shale specimens, the calculated temperatures of well-preserved jaw material match those of well-preserved shell material in the same individual. Analyses of the jaw elements in the Mooreville Chalk permit a comparison of the paleotemperatures between the two sites, and show that the Western Interior is warmer than the Gulf Coast at that time. In summary, our data indicate that the calcitic jaw elements of cephalopods can provide a reliable geochemical archive of the habitat of fossil forms.

New insights into the buccal apparatus of the Goniatitina: palaeobiological and phylogenetic implications

Exceptionally well-preserved radulae and lower jaws found both inside the body chamber of Cravenoceras fayettevillae (Mississippian) and in isolated nodules (Mississippian and Pennsylvanian) were analysed using CT scan and synchrotron propagation phase-contrast tomography. This approach reveals new anatomical details allowing us to investigate wear, preservation and muscle insertion in the buccal mass of the Goniatitina. For comparison, Recent cephalopod radulae were also investigated. The radula in the Goniatitina consists of nine elements per row, and its morphology is conservative in the group. The shape of the teeth is similar to that in some Recent coleoids. The lower jaw is morphologically closer to that of Recent Nautilus, consisting of a wide outer lamella and a reduced inner lamella. The morphology of the inner lamella reflects the outer lamella in smaller dimensions and does not protrude posteroventrally. The space between the inner and outer lamellae allows for muscle insertion. Our morphological data indicate that a mosaic of characters is present in the buccal mass of Goniatitina with some parts of the buccal mass being more primitive than others. This implies that different parts of the buccal mass may have followed different evolutionary histories.

Sutural variation in the ammonites Oxynoticeras and Cheltonia from the Lower Jurassic of Bishop's Cleeve, Gloucestershire, England and its significance for ammonite growth

Large samples of pyritized ammonites of Oxynoticeras and Cheltonia from the Lower Jurassic of Bishop's Cleeve, Gloucestershire, England, reveal that a majority retain parts of the body chamber and died as juveniles. Both genera exhibit similar high variation in sutural angle, chamber size and numbers of chambers per whorl, suggesting they are closely related, perhaps sexual dimorphs. Many examples preserve impressions of paired dorsal and unpaired ventral muscle scars, as well as drag bands and (more rarely) pseudosutures. All stages of the chamber formation cycle described in Recent Nautilus can be recognised in both genera. Juvenile ammonites apparently died at random times in the chamber formation cycle. Frequency of incomplete final septa suggests that septal secretion took about 30% of the full cycle. Rare examples from a restricted horizon preserve narrow, radiating darker bands on the internal moulds. Mean chamber angle and mean angle between dark bands correlate strongly in Cheltonia, suggesting formation of dark bands was related to septal secretion. The dark bands are interpreted as false colour bands secreted on the inside of the aperture during pauses in growth, but there is no other evidence for episodic growth in either genus. Spacing of growth lines is similar to that reported for Recent Nautilus and to spacing of pseudosutures. Both genera may have moved their bodies forward slowly during chamber formation, unlike the rapid movement in Nautilus. This may be one way in which ammonite growth differed from that of Nautilus. Irregular chamber sizes in both ammonites suggest that the period of chamber formation varied, perhaps due to variable pauses in apertural growth. At least Oxynoticeras and Cheltonia grew much more irregularly than modern Nautilus.

Siphuncular structure in the Recent Nautilus, compared with that in Mesozoic nautilids and ammonoids from Madagascar

The connecting ring in the Recent Nautilus is composed of two layers: a fibrous, organic inner layer, and a porous, spherulitic-prismatic outer layer. The organic layer is an uncalcified continuation from the nacreous layer of the septal neck. In the Mesozoic nautilids Cymatoceras and Paracenoceras from Madagascar, that have the Nautilus type of siphuncles, the inner fibrous-organic layer of the connecting ring has been destroyed by diagenesis. The outer spherulitic-prismatic layer is twice as thick as in Nautilus, which indicates that, in addition to jet-powered swimming, they also used short-term buoyancy changes for vertical migrations, and that the mode of life of these nautilids was somewhat different from that of the Recent Nautilus. Moreover, the Madagascar nautilids lack the auxiliary ridge by which the connecting ring in Nautilus is firmly attached to the inner surface of the preceding septal neck. The siphuncle in Nautilus can therefore withstand a much higher hydrostatic pressure than that in the fossil nautilids that lacked this deposit. In ammonoids the fibrous-organic layer of the connecting ring has a solid, semi elastic structure. In all 11 ammonoid taxa from Madagascar here studied, this layer is preserved whereas the nautilids from the same stratigraphic horizons have lost this layer. This clearly indicates that the fibrous-organic layer of the connecting ring in nautilids is chemically different from that in ammonoids.

A NEWLY HATCHED COILED NAUTILOID FROM THE PERMIAN OF ITALY

Well-preserved, newly hatched coiled nautiloids from the late Paleozoic are relatively unusual. Thus, the recovery of a hatchling specimen from the Sosio Limestone (Permian, probably Wordian) of Italy that can be assigned to Sholakoceras cf. S. pleuronautiloides (Gemmellaro, 1890) is notable. The embryonic and posthatchling shell of S. cf. S. pleuronautiloides shows some morphological similarities to Recent Nautilus Linnaeus, 1758; however, some dissimilarities are significant, and these features have implications on the overall phylogenetic pattern exhibited by these organisms through time and the evolutionary processes that led to that pattern. The specimen comes from the extremely fossiliferous Pietra di Salomone megablock southwest of Palazzo Adriano, Sosio Valley (Monti Sicani, western Sicily). This and a couple of similar occurrences (blocks) are informally known as the “Sosio Limestone.” The geographic situation and geological setting were given in detail by Flugel et al. (1991), who also discussed the facies and age of the Pietra di Salomone megablock. These authors recognized carbonate breccias and calcarenites deposited by debris flows and turbidity currents in a base-of-slope position. The calcareous material was originally deposited on carbonate platform margins prior to gravity-induced transport. According to Flugel et al. (1991), most of the Pietra di Salomone megablock has a Wordian (Murghabian) age. However, for some of the pebbles derived from reef facies, a Capitanian (Midian) or even Wuchiapingian (Dzhulfian) age is also possible. Jenny-Deshusses et al. (2000) gave a Capitanian to Wuchiapingian and probably even a Changhsingian age for the Pietra di Salomone megablock. Therefore, the studied nautiloid specimen has a Wordian to latest Permian age. The Sosio Limestone is famous for its well-preserved invertebrate fauna. Overall, more than 300 species of invertebrates have been described from the Pietra di Salomone, and more than 520 species have been reported from all Sosio blocks (Mascle, …

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 black layer in cephalopods from the German Muschelkalk (Triassic)

Thin, dark, probably phosphatic coatings were found on the dorsum in front of and sometimes behind the aperture of 50 specimens of Paraceratites and Ceratites (Ammonoidea) belonging to 14 species and subspecies and in three specimens of Germanonautilus, all from the Middle Triassic of Germany. The proportions, occurrences, position, outline, and preservation in fossil Nautiloidea and Ammonoidea (originally organic matter) of this structure support the hypothesis that it is homologous with the black layer in Recent Nautilus and Allonautilus. It is not yet possible to test whether these cephalopods show homologous styles of the development of these structures or whether the black layer can be identified in a common ancestor. In contrast to many ammonoids, Ceratites and Paraceratites, most Palaeozoic ammonoids, and some Mesozoic ammonoids probably did not have lower mandibles that were suitable for the closure of the aperture. They probably possessed a dorsally extending mantle (supracephalic mantle fold) and a hood, as in Recent Nautilus and Allonautilus, that was attached to the black layer. This interpretation is corroborated by a similar morphology of the black layer in an adult specimen of the nautilid Cenoceras from the South German Middle Jurassic and three specimens of Germanonautilus from the South German Middle Triassic (both Nautiloidea).

Large shell injuries in Middle Ordovician Orthocerida (Nautiloidea, Cephalopoda)

Sublethal injuries are described from six fragments of orthocerids, which belong to Orthoceras regulare Schlotheim, 1820, Orthoceras scabridum Angelin, 1880, Nilssonoceras nilssoni (Boll 1857) and Plagiostomoceras laevigatum (Boll 1857) from the Baltic Orthoceratite Limestone (Arenig-Llanvirn, Middle Ordovician) of Sweden, and of northern Germany. The injuries represent shell breakages with an exceptionally large absolute size. The largest observed injury measures more than 60 mm from the aperture to its distal rim. Injuries of that magnitude have previously never been described from Ordovician molluscs. All breakages represent aperture peelings, exclusively affecting the body-chamber of the living animal. The predators which are responsible for these injuries were most probably nautiloids or eurypterids. A calculation of the relative dimension of the shell loss which resulted in the breakage shows that these breaks were small in comparison with the maximum tolerated shell loss in Recent Nautilus or Mesozoic ammonoids.

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.

Occurrence of early post‐hatching Jurassic Nautilida in Normandy, France: palaeobiologic, palaeoecologic and palaeobiogeographic implications

Nautilida are present and sometimes abundant in various sedimentary environments of the Jurassic series (Sinemurian to Kimmeridgian) in Normandy, but in the majority of cases the individuals are mature or at a late stage of ontogenetic development. Comparative analysis of the palaeobiogeographic distribution of Nautilida according to their ontogenetic stages shows that early post-hatching specimens are present only in particular palaeoenvironments which are rarely visible and laterally limited in outcrop. The sedimentologic and palaeoecologic originality of the Carixian, Aalenian and Bajocian deposits bearing these peculiar specimens is related to their palaeogeographic position along the western border of the Anglo-Paris Basin, on the eastern margin of the Armorican Massif, where Jurassic deposits overlie Palaeozoic rocky reefs or shoals. The egg-laying places of the Jurassic Nautilida show palaeoenvironmental features similar to those of the recent Nautilus, having been controlled by the water temperature and the nature and morphology of the sea bottoms. A hard, anfractuous and stable substratum, low sedimentation rates and warm and well-oxygenated marine waters appear to have been required for egg-laying and for the embryonic development of these cephalopods. The discovery of early post-hatching fossil Nautilida is fundamental to a better understanding of their palaeobiogeographic distribution at local to global scales. The stenotopic nature of their egg-laying places explains the segregation between newly hatched individuals and the rest of the population and leads also to a distinction between the notions of ‘dispersal potential’ and ‘settlement potential’ for populations of Nautilida. Three palaeobiogeographic maps show that Nautilida were absent in the Jurassic Boreal realm. This exclusion could be closely dependent on the thermal requirements that govern their reproduction. By comparison, the contemporary occurrence of ammonoids in this realm appears to reflect differences in reproductive strategies.

Shell ultrastructure, muscle-scars, and buccal apparatus in ammonoids

(1) The Cretaceous genus Aconeceras has large ventro-lateral muscle-scars in addition to smaller dorsal and ventral scars. This indicates that some ammonoid groups at least probably had strong retractor muscles to the head and funnel, and that their muscle system was more complex than that in the Recent Nautilus. (2) Radula of Aconeceras has multicuspid teeth, known among recent cephalopods only in bathypelagic octopods. (3) In the Cretaceous heteromorphic genus Ptychoceras, repetition of shell layers and mode of truncation of initial portion of shell indicate that the shell surface was covered by soft tissue, at least in apertural region. (4) Shape of shell aperture in many ammonoids differs considerably from that in Nautilus by lack of hyponomic and ocular sinuses. This may indicate principal morphological differences in the head-funnel region of the body between ammonoids and Nautilus. (5) Transition from retro- to prochoanitic septal necks, which took place with different evolutionary rates in different ammonoid orders, improved the osmotic pumping system, and made it possible to form a new portion of siphuncle prior to secretion of next septum.

The jaw apparati of ectocochleate cephalopods

A b s t r a c t: The present state of knowledge concerning the function of anaptychi, aptychi, and opercula of ammonoids is reviewed. The jaw apparati found associated with the lytoceratid genera Gaudryceras and Tetragonites by KANI~ et al. (1978) and TANABE et al. (1980) and classified as rhynchaptychus type of ammonoidjaws by L~HMANN (1981) are re-interpreted. Morphologically, they are very similar to Nautilus jaws. Their interpretation as allochthonous nautiloid jaws is discussed and considered probable. However, beccublast cell impressions in Gaudryceras and recent Nautilus differ considerably. Further investigations are deemed necessary concerning a) the nature of the anaptychi in Palaeozoic anaptychus assemblages, whether they are opercula or jaw elements; b) the function of the laevaptychi. Their jaw nature is still open to doubt; c) the autochthony of the rhynchaptychus-type lower jaws found in lytoceratid Upper Cretaceous ammonites in Japan.

Nautiloid shell taphonomy: Interpretations based on water pressure

Nautiloid taphonomy is a difficult and widely neglected topic acting as a test of many of the assumptions made in nautiloid paleobiology, phylogenetic interpretations, and geological facies analysis. The Eocene nautiloids from the offshore London Clay Formation of England are considered from the perspective of flooding and implosion experiments on Recent Nautilus. The deeper water facies are often characterized by body chambers in approximate life position, one or two complete septa, and various types of shell debris. Intermediate depth facies show intact and often vertically embedded phragmocone walls, with zones of internally broken and spar cemented septa. Taphnomic characteristics of the former facies probably result from the implosion of shells at depths of 200 or 300 m on the seafloor, while those of intermediate depth facies result from the sinking of gas-filled shells to the seafloor and subsequent bending stress failure of septa left unsupported by sediment in the last whorl. Evidence for the removal of the body from the largely unbroken shell by octopus illustrates the potential for a biological influence on the taphonomic history of nautiloids through the Phanerozoic. Tertiary nautiloids were probably unusually prone to extensive post-mortem transport, a process which tends to remove shells from the fossil record under most geological circumstances.

Functional interpretation of inner shell layers in Triassic ceratid ammonites

The wrinkle layer the inner prismatic layer are described in three Triassic ceratid genera: Phyllocludiscites. Megaphyllites Proarcestes. Both layers have their counterparts in the shell wall of the recent Nautilus: the wrinkle layer corresponds to the mantle-adhesive layer the inner prismatic layer to the myostracal layer in Nautilus. A detailed structural functional comparison between these layers is given. The wrinkle layer is also compared with the oblique prismatic layer in recent gastropods.

Phosphatic connecting rings and ecology of an Ordovician ellesmerocerid nautiloid

The phosphatic siphuncle of Bactroceras latisiphonatum Glenister, a Late Ordovician member of the ancestral order of nautiloid cephalopods, displays the outer ‘chalky’ and inner ‘horny’ tubes seen in the aragonitic connecting rings of Recent Nautilus and Carboniferous Orthocerida. The specimens were collected from the Malongulli Formation of central New South Wales. Bactroceras resembles Mesozoic ammonoids in having a marginal phosphatic connecting ring that is partly a product of diagenetic phosphatization. Both the septa and connecting rings were apparently able to resist greater hydrostatic water pressures than other ellesmerocerids and had a potential implosion depth comparable to that of mature Nautilus. The septal necks and siphuncle position resemble the morphology of Bactrites; but it is unlikely that Bactroceras and ‘Eobactrites’ were directly ancestral to bactritoids and ammonoids.

Flexible nacre in the nautiloid Isorthoceras , with remarks on the evolution of cephalopod nacre

Secondarily phosphatized shells of Isorthoceras sociale (Hall) from the Upper Ordovician Maquokcta shale, Iowa, U.S.A., often have their original ultrastructurc preserved. Scanning Electron Microscope study reveals that the nacreous layer is similar to that in the shells of Recent Nautilus, except that the nacreous tablets usually have a much larger diameter, and that numerous vertical interspaces arc left between adjacent stacks of tablets. Because of the latter condition, the nacre in A sociale is porous, whereas it is compact in Nautilus. The pores in the nacre were probably filled with a non-mincralizcd organic matrix. The high porosity and high content of organic matrix made the nacreous layer of I. sociale much more flexible and considerably less strong than that in Nautilus. As a consequence, the shells of I. sociale, and probably also of other orthoconic cephalopods, were more flexible and could not withstand such high hydrostatic pressures as the shells of Nautilus. During evolution, the porosity of the nacreous layer became completely reduced. As a result, the mechanical properties of the nacre changed towards a lower flexibility and a higher tensile strength. Cephalopoda, Nautiloidea, nacre, ultrastructurc, phosphatization, Upper Ordovician.

A new Middle Ordovician torticonic nautiloid

Abstract A new, dextrally coiled torticonic nautiloid, Pakrioceras holmi, n.gen., n.sp., is described from the lowermost Middle Ordovician (Viruan) Aseri Stage of northern Estonia. This is the next to oldest torticonic cephalopod hitherto described. On the basis of the posterior (‘ventral’) position of the siphonal tube and the occurrence of the thick spherulitic-prismatic layer of the connecting rings, Pakrioceras is assigned to the family Estonioceratidae of the order Tarphycerida. Studies with scanning electron microscopy and energy-dispersive analysis of X-rays (EDAX) show that aragonite in the spherulitic-prismatic layer was substituted by calcium phosphate during early diagenesis. This layer is therefore ultrastructurally well preserved and consists of bundles of acicular crystalline elements, separated by interspaces. Its ultrastructure is identical to that in the corresponding layer of the Recent Nautilus.