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The Structure and Formation of the Siphuncular Tube of Quenstedtoceras Compared with That of Nautilus (Cephalopoda)

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The Structure and Formation of the Siphuncular Tube of Quenstedtoceras Compared with That of Nautilus (Cephalopoda) N. Jb. Gcol. Palaont. Abh. 161 2 153—171 j Stuttgart, Marz 1981 The Structure and Formation of the Siphuncular Tube of Quenstedtoceras compared with that of Nautilus (Cephalopoda) By Klaus Bandel, Erlangen With 9 figures in the text Bandel, K. (1981): The Structure and Formation of the Siphuncular Tube of Quenstedtoceras compared with that of Nautilus (Cephalopoda). — N. Jb. Geol. Palaont, Abh., 161: 153—171; Stuttgart. Abstract: The mode of formation of new chambers of the ammonite Quen­ stedtoceras was analysed in detail and compared with that of Nautilus. The siphuncular tube formed independently from the septum and was attached to the ventral shell wall. The septum was fixed in shape by rapid mineralization and the nacreous septum formed after stabilization of septum morphology. Porous pillar zones that connected chamber liquid with the tissue of the living siphuncle were formed between mineral ends of the organic siphuncular tube within septal necks. K e y words: Perisphinctida (Quenstedtoceras), Nautiloidea (Nautilus), siphun­ cular tube, septum, skeleton, aragonite, organic microstructure. Zusammenfassung: Bei der Neubildung einer Gehausekammer schied Quenstedto­ ceras das Siphonalrohr ab, bevor sich ein neues Septum bildete. Das Rohr wurde der ventralen, inneren Schalenoberfliiche und dem vorherigen Septum mit orga- nischen Lamellen verheftet. Letztere setzen sich in spharolithisch-prismatische Ara- gonitwiilste fort, die auf der mincralischen Schalenoberflache aufgewachsen sind. Das Septum mit seiner komplex verfalteten, randlichen Sutur entstand rasch aus mineralischen und organischen Ausscheidungcn im Schleim der vom Mantelgewebe ausgeschiedenen, extrapallialcn Fliissigkeit. Hierbei wurde die Oberflache des unter Zug stehenden Mantelepithels in seiner Form nachgepriigt nicht aber selbst mineralisiert, wobei der Aufwuchs spharolithisch-prismatischer Aragonit-Kristall- rasen nach hinten erfolgte. Nach der mineralischen Versteifung des Septums und der vollstandigen Abscheidung des organischen Siphonalrohres lostc sich der rtick- wiirtige Mantel von den geregelt verteilten Muskelanheftungspunkten und -linien an der Schaleninnenseitc ab, und erst danach erfolgte die Ausscheidung des Perl- mutterseptums. Nach Beendigung der Perlmutter-Abscheidung wurde die neue Kammer funktionell. Der neue Abschnitt des Siphonalrohres greift in den nach 154 Klaus Bandel vorne gebogenen Septenkragen des vorhergehenden Septums hinein. Hierbei liegt zwischcn spharolithisch-prismatischer Anheftung des hinteren Rohrsegments und der glcichartig strukturierten Anheftung des vorderen Rohrsegmentes eine ausge- dchnte, porose Pfeilerzone, in der eine von der Kammerflussigkeit abgetrennte kleine Fliissigkeitsmenge Platz fand, und die das Gewebe des Siphostranges mit dem Innenraum der abgeschlossenen Rammer in Verbindung brachte. Die Kammer- bildung bei Ammoniten wird mit der von Nautilus verglichen und die Struktur, Bildung und Funktion des Septalapparats diskutiert. Introduction The siphuncular tube is the most characteristic feature of shell bearing cephalopods. It differentiates them from all other members of the phylum Mollusca. In regard to all other shell features, such as planispiral coiling, shell of aragonitic nacreous and prismatic structure, septation of apical shell portions, other mollusc groups may show one or more of these characters, but none are known that have a tube connecting sealed off portions of the shell with the living tissue as is present in the chambered cephalopods. This very ancient acquisition of a septate, benthic limpet-like mollusc that lived in Cambrian times (Y ochelson et ah, 1973) enabled the cephalopod branch of the molluscs to conquer the water above its original benthic habitat and adopt a free swimming mode of life. The tube connecting the soft body situated in the anterior portion of the shell (living chamber) with the sealed off apical portion of the shell and its chambers is filled with living tissue. The sealed off chambers themselves no longer contain living tissue and after formation are filled with water. D enton (1974), D enton & G il pin -B rown (1961, 1966, 1971) and D en­ t o n , G il p in -B rown & H o w a rth (1961, 1967) have shown conclusively that living cephalopods with chambered shell can pump water in and out of the chambers quite easily and can thus adapt their weight to the desired depth of water. They attain neutral buoyancy by changing the amount of liquid held within the chambers by interaction of the cells of the living siphuncle with the liquid held within the chambers. The siphuncular tube of Nautilus is permeable to sea water, imperme­ able to air and able to withstand pressures equivalent to well above 450 m of water depth (C ollins 8c M in to n, 1967). According to these authors a Nautilus shell would fill in about 2 hours if the living siphuncle did not prevent it, since the siphuncular tube acts as a permeable membrane. Gas can enter the chambers by diffusion only. C ham berlain (1978) estimated the permeability of the siphuncular tube of Nautilus and fossil ectocochleate cephalopods using previously published data on the siphuncle composition for his calculations. Thus he used the assumption that liquid must move through the horny tube of the Nautilus siphuncle only. He therefore came to results which were controversial to C ollins & M in to n ’s experiments. C ham berlain concluded that the rate of fluid removal from the chambers Structure and Formation of the Siphuncular Tube of Quenstedtoceras 155 is low, lower than suggested by C ollins & M inton (1967) and D fn ton Sc G il p in -B row n (1966). The living siphuncle and the chambered portion of the shell that has been sealed off from the living tissue in Sepia and Spirula remains through­ out life the most important apparatus for buoyancy control. The contact between living tissue and the liquid held in the chambers thus lies within the siphuncular tube. This tube is not permeable throughout, but only at certain places, as Bandel Sc B oletzky (1979) have shown conclusively for the living members of chambered, shell-bearing cephalopods (Sepia, Spirula, Nautilus). According to these authors the function of the siphun­ cular system in general has most probably never changed in the chambered shells of recent and fossil cephalopods. Bandel Sc Boletzky (1979, Figs. 25, 26) also demonstrated that the siphuncular tube of the Ammonoidea and its attachment to the septum differs in several respects from that of recent Nautilus and Spirula. The data for their reconstruction of the siphuncular tube of ammonites had been extracted from the available literature. Thanks to Prof. Dr. A. Z eiss (Inst, fiir Palaontologie, Erlangen) it was now pos­ sible to study extremely well-preserved Jurassic material of the ammonite Quenstedtoceras from the Polish locality Lukow, and to verify and correct this reconstruction. Quenstedtoceras Previous data: Bandel Sc B oletzky (1979, Figs. 23, 24, 25, 26) combining literature data of G ra nd jean (1910), M iller Sc U nklesbay (1943), M utvei (1967), E rben Sc R eid (1971), B ayer (1975) and others came to the following reconstruction of the relationship of siphuncular tube and septum in ammonoids. The septal neck of semi-adult and adult ammonoids always points towards the living chamber (apertural). The walls of the siphuncular tube are not continuous with this septal neck, but are made of organic material similar to the horny tube of the Nautilus siphuncle. The siphuncular tube consists of portions, each of which has the same length as the corresponding chamber. These individual portions are not fused with one another so as to form one continuous organic tube; instead they are connected to each other by calcareous material. The apical part of each tube segment is narrower than the apertural part in the seg­ ment formed before. The siphuncular tube of the ammonoids is covered by an organic pellicle that separates from the tube near its ends. With these organic sheets the siphuncular tube is attached to the ventral shell wall. The siphuncular tube in most ammonoids becomes independent from the septum during ontogenesis and M utvf.i (1975) suggested that if formed after the septum is completed. K ulicki (1979), in contrast, presented convincing evidence that a new tube section forms before the septum 156 Klaus Bandel Fig. 1—6 (Legend see p. 157) Structure and Formation of the Siphuncular Tube of Quenstedtoceras 157 is completed. The siphuncular tube is attached to the septal neck by sur­ rounding calcareous deposits. In the course of their ontogeny the aramo- noids develop a new porous zone in their siphuncular tube, when the septal necks changed from a retro- to prosiphonate arrangement. B a n d e l Sc B o l e t z k y thought that it might be possible that chamber liquid could be drained on both ends of the tube segment crossing the chamber. N e w data: The septum of Quenstedtoceras in construction is mainly nacreous. At the place where the siphuncle penetrates through the septum, a round aperture is bent towards the living chamber forming the so-called prochoanitic septal neck. The position of the siphuncular tube is close to the outside of each whorl (ventral) in the individuals that have grown after hatching from the egg case. Thus the siphuncular tube runs marginally, almost in contact with the inner side of the outer shell (Fig. 1). The nacreous layer composing most of the outer shell is overlain by a thin inner prismatic layer throughout. Along the insertion of the nacreous septum onto the inner side of the outer shell this prismatic layer increases in thickness, attaining greatest thickness just posterior of the attachment of the septum (Fig. 4). This increased thickness is due to the fact that a prismatic ramp attaches a thick organic pellicle here, which further back­ wards (apically) merges with the organic siphuncular tube (Fig. 1), forming its outermost layer. There may be not only one such sheet, but several, in which case each of these pellicles shows an individual attachment to the inner prismatic layer of the mineral shell.
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