the siphuncle in georginidae and other ordovician actinoceroid cephalopods

The siphuncle in Georginidae and other Ordovician act inocer oid cep halo pods MARY WADE

Wade, Mary 1977 10 15: The siphuncle in Georginidae and other Ordovician actinoceroid cephalopods. Lerhoio. Vol. 10, pp. 303-315. Oslo. ISSN 0024-1164.

The lower Middle Ordovician carbonate sediments of the Georgina Basin in Australia contain many and varied Actinoceratida. including an endemic family. Georginidae Wade (1977). with siphuncular calcification consisting of radial lamellae separated by spaces (now sediment-filled) enclosed within calcified annulus walls. In each segment a distinct series of more massive calcareous engrafts grows inward from the inside of the connecting ring and is engrafted into adjacent annuli across the interannulus; this divides the perispatium into longitudinal perispatial sinuses. At each extremity each perispatial sinus is connected with passages leading through the segments to the axial space and, nearer to, or at the interannulus, with radial canals. Axial canals are few, but more than one is normal. Good preservation of Armenoceras and Acfinoceras allows recognition of similar structures in the annuli of normal Actinoceratida.

Mary Wade, Queenslond Museum. Fortitude Volley. Brisbone. 4006. Queenrland. Austrolio; 21sf Moy. 1976 (revised ISfh April, 1977).

The lower Middle Ordovician of the Lower Palaeozoic Georgina Basin, Queensland and Northern Territory of Australia, contains shallow water carbonate sediments with a rich nautiloid fauna. A summary of its stratigraphy and references to previous work may be found in Smith (1972). Actinoceroids are common in the Coolibah and Nora Formations of the Toko Syncline which outcrop around Toko and Toomba Ranges. The Coolibah Formation is the lower; it consists of dense, fine-grained limestone and dolomite, with many beds of algal nodules and a rich and varied fauna dominated by molluscs. Its nautiloid fauna enters above the base and has a patchy lateral distribution, though widespread toward the top. I t is greatly enriched 4-5 m from the top, and again within about a metre of the top, by the addition of new forms, partly endemic. The conformably overlying Nora Formation varies both laterally and vertically from somewhat glauconitic sandstone to coquinite, nautiloids being virtually confined t o coquinites. Its richest actinoceroid fauna is widespread in lenses toward the base; a relict of it persists in the middle part of the formation as a second

widespread fauna. A third fauna enters above the second and in turn looses variety upward; the area in which this fauna is known is more restricted, possibly mainly due to extensive concealment by debris from the scarp of overlying Carlo Sandstone.

As yet the faunas are substantially unde- scribed, but the actinoceroids include a group with longitudinally arranged, canoe-shaped, calcareous bodies called engrafts attached inside the bulge of the connecting rings. As engrafts grow, they interlock with adjacent developing annular siphonal deposits. Thid group is described as the family Georginidae Wade (1977), consisting exclusively of new taxa. Approximately 150 specimens group into six named species, forming the genera Georgina and Mesuktoceras (Fig. lA, B). A residue of rarities and small or poorly preserved specimens is yet to be placed.

Thanks are due to Mr. D. J. Taylor of Sydney for generously suggesting study of the nautiloid fauna of the Coolibah Formation, despite a lingering interest of his own in the fauna which he discovered and sampled in 1958. The specimens he collected passed into other hands and appear to have been lost. All of the

304 Mary Wade LETHAIA 10 (1977)

LETHAIA 10 (1977) Actinoceroid siphuncles 305

Coolibah material is newly collected, but some addi- tional material of the upper and lower faunas of the Nora Formation were loaned by the University of Queensland through the kindness of Dr. J. S. Jell, Mr. F. S. Colliver and Mr. B. Cameron. Dr. Rousseau H. Flower gave constructive criticism, and at a later date Dr. Curt Teichert kindly criticized the manuscript; the result owes much to the study of references that he suggested, and subsequent reading. Mrs. Y. Evans re-drew Fig. 1.

The majority of the specimens (Figs. 2 4 , 6) discussed here are deposited in the Palaeontological Collection of the Queensland Museum, Brisbane, Australia, 4006. Specimen No. F67153 (Fig. 7) is in the collection of the Geology Department, University of Queensland, St. Lucia, Brisbane. Australia, 4067. The original of Fig. 5 was loaned by Rousseau H. Flower.

Preservation The nautiloids have been badly battered during deposition in both these shallow-water formations. Phragmocones of structurally strong species, and portions of only two Georgina sp. survived the battering. Besides pre-depositional destruction of the shells post-depositional stripping often sliced off the upper surfaces of siphuncles at bedding planes, and, sometimes after re-positioning, sliced again. After final burial the preservational history was different in each formation, principally because of different permeability in the sediments.

The Coolibah Formation is relatively im- permeable. Originally calcified fossils enclosed in it are still calcified, while. surface silicification has affected exposed fossils, hardening their surfaces so that they etch out. The effects of pre- and post-depositional wear and styloliti- zation are often disguised by this partial silicification which supplied a relatively smooth surface to portions that are found to lack original

Fig. 1. Approx. X2. 0 A:1-3. Georgina faylori Wade, the type species, ranging through the upper third of Coolibah Formation. 0 B:1-3, Mesakroceras arachne Wade, ranging through the lower half of Nora Forma- tion. l - sections through septal necks viewed from above; 2 - longitudinal sections 1% segments long; 3 - sections through interannuli, a plane in fully developed Georgina and a three-dimensional zone in Mesakroceras. a - annulus wall; b - radial lamellae. growth-lies shown at one side; c - engraft; d - engraft core membrane; e - perispatial sinus; f - connecting ring; g - engraft core membrane crossing perispatium to fuse with connecting ring; h - segmental sinuses; i - endocones in Mesakroceras; j - septum; k - contact layer; I - interannulus; m - radial canal; n - canal distributary; o - axial space. Stipple indicates former spaces filled by sediment.

surfaces, when compared in cut sections with rock-covered calcareous surfaces. Plastic de- formation affected a limited number of individuals. Clear silica, which was apparently emplaced early in diagenesis, fills the centres of some siphuncles and can preserve the organic walls of the axial and radial canals. Indeed some specimens still show organic walls of canals in calcitic material. A thin layer of dark, fine-grained calcite covers the bounding surfaces of each annulus of the siphuncular deposits, and forms central membranes in the radial lamellae. The connecting rings are similar, in contrast to the septal necks of clearer, crystallized calcite. It is believed that the original structure of the dark layers was minute crystals of carbonate in a conchiolin matrix, for the appearance is similar to the organic-rich ‘dark layer’ frequently seen in sections of molluscan shells (Gregoire 1962; Mutvei 1972) and also in bilamellar Forami- nifera. At all events, the dark layers - or membranes - served bases for elongate crystal growth, normal to their surfaces.

Preferential silicification picks out not only the surfaces of the annuli but more rarely the cores of the radial lamellae (Figs. 2, 3). In general, portions show preferential silicification that, by preserved texture in calcified material and comparative distribution of similar textures, might be expected to have been laid down in association with organic matter.

The Nora Formation is much more permeable than the Coolibah. Its high proportion of coquinites, often containing phosphatic nodules, bespeaks even more nondeposition andl or erosion than is evidenced in the Coolibah Formation. Bivalves and gastropods - ara- gonitic fossils - are frequently reduced to moulds by deep surface weathering, but as the coquinites were largely calcite brachiopod r e mains, they are mainly still limestone. The fragile Georginidae apparently lacked cameral deposits and were almost always reduced to siphuncles only, but some other nautiloids are represented by carbonate cameral deposits and siphuncle fillings. These cameral deposits could indicate either extremely fast reversion of fine-grained aragonite to calcite, which has then been preserved while the more solid lamellar aragonite of the molluscan shells was wholly or partly dissolved, or initially calcitic siphuncular and cameral deposits. The query on the initial composition indicated by Teichert


Fig. 2. Armmoceras sp. 2, natural weathering of surface silicification; one third of the thickness above the base of the Coolibah Formation. 0 A, B. X1.6. Viewed from broken adapical end, white line follows intersect of interannulus: 0 C. X3.6. Dark pointers show radial canals in section, white pointer shows axial space and axial canal.

(1967) remains since Grtgoire (1962) and Mutvei (1972) agree that the pellicle of the septum and connecting ring separates the spherulitic-prismatic layer at the outer edge of the septum from the cameral space, so it cannot be equated to cameral deposits. Grt- goire also reminded us that it was formed at the time of chamber formation. not at the time of siphuncle calcification. Rarely, a calcified shell wall is present.

Fig. 3. Georgina taylori Wade, F7091, paratype. X2, in slightly oblique view to show the engrafts tucking under the edges of the annulus and radial lamellae in section. Natural weathering of partly silicified material.

Calcification of siphuncles in Georginidae and A rmenoceras The Georginidae have annular siphuncle deposits apparently formed by a large number of tightly-packed, laterally flattened blisters that grew inward from the septa1 neck constriction, forming annuli that expanded anteriorly and posteriorly as well as inward. (Exceptionally, growth commences from the edge of the previous annulus instead of the neck, as also happens in other Actinoceratida.) In early stages many blisters are overstepped by their neighbours and cease growth, but relatively few are overstepped in later stages of growth (Fig. 3). The walls of these ‘blisters’ form the radial lamellae and the annulus walls, both of a ‘membrane’ of fine-grained ‘dark layer’ calcite (the colour and texture of conchiolin-rich layers of shell material) and associated calcification consisting of small, elongate, spicular crystals. These formed normal to the inner surface of each membraneous blister, progressively recording membrane expansion by dark and light growth-lines. Resultant evidence points to the axial crests of the blisters as the main growth zones. Material of Armenoceras spp. and Actinoceras sp. from the same deposits allows the extension of these observa- tions to conventional Actinoceratida, as radial lamellar ‘core’ membranes can occasionally be observed in them (Figs. 2, 4). These core membranes have relatively long spicular crystals normal to them, those from adjacent


lamellae abutting on either side and thus filling the annulus; this allows easy recrystallization. In Georgina Basin material the exclu- sion of sediment fill from the annuli speaks more strongly than any other argument for original total calcification of the Armenocerar and Actinoceras annulus interiors, for even the radial canals are quite commonly sediment- filled in these deposits. A generalization that enclosure by sediment seems to have inhibited recrystallization from fibrous to granular calcite seems valid for all the Coolibah Formation actinoceroids. If the axial cavity is part-filled with sediment and part with coarsely-crystallized calcite, the calcite within the annulus walls of the filled portion is more likely to be finely crystalline than it is in the portions adjacent to coarsely crystalline calcite. This is probabry due ,to permeability differences. The common presence of sediment between radial lamellae may be sufficient to explain the greater proportion of preserved original structure in the Georginidae. The organic matter in the annulus walls also may have inhibited recrystallization to granular calcite. The silicified and naturally etched fragments of normal actinoceroids in this deposit show mainly annulus walls, with a limited number of specimens showing the junctions with radial lamellae, and sometimes radial canals (Fig. 2). It is in the recognition of radial lamellae as a normal feature of actinoceroids that this study differs most strongly from the interpretation of growth of siphuncular calcification given by Mutvei (1965). Both of us regard the calcification as formed by re-activated siphuncular epithelium, because the period of calcareous siphonal deposition follows the period of septum and connecting ring formation after the formation of about 8-20 chambers. Mutvei has envisaged the deposition of calcareous ridges which merge to form fluted, cylindrical layers of calcareous material as re-activation spreads in the epithelium. Each successive layer reached further anteriorly and posteriorly until the space available for the annulus was filled. This explanation would naturally occur to anyone who studied the growth lines, if they had not seen identical growth lines revealed by cuts parallel to radial lamellae that are separated from their neighbours by sediment, as in Georgina; having seen these, one finds it natural to seek for evidence of radial lamellae in long-known actinoceroids. Forms

with relatively thin radial lamellae and spaces between them are closest to the Georginidae in this aspect of morphology, e.g. Kochoceras tyrelfi (Fig. 5).

A few individuals of the species of Georgina with the largest axial cavity (G. dwyeri Wade) have a limited amount of layered secondary carbonate deposited around the base of the axial cavity, but it scarcely impinges on the space. The slightly older genus Mesaktoceras is characterized in part by extremely reduced annulus calcification. A wide, irregular axial cavity would have resulted from annulus re- duction but for the development of a layered deposit of endocones that match the irregular shape of the cavity (Fig. 1B:2). This also is characteristic of the genus. Because of the endocones, the functional axial cavity is about the same size as in 'Georgina (Fig. 1A:2, B:2). The size-range of the axial cavity in Georgina can be a specific character but the funnel- shaped anterior ends: passed through similar developmental stages in all species, and are much less characteristic. Therefore the separate name 'anterior cavity' is used for the anterior portion. Original calcification in Georginidae has been pieced together from patches of detailed structure (growth lines, crystal growth, variable textures and compositions) found relict among recrystallized structures. Progressive growth has been traced in anterior growth stages and sometimes by concentric growth- lines seen in cuts parallel to lamellae (Fig. 1A:2, B:2). Etched anterior cavities show that the axial wall of each annulus is formed of narrowly convex, more or less longitudinal ridges, their sides folded into deep creases which separate adjacent ridges. This is a pic- ture familiar to every actinoceroid worker since Barrande. Sections relate the creases to the dark, core membranes of the radial lamellae. Similarly, sections show the outer end of each dark layer passing through the wall to fuse with the dark layer of the outer surface, the annulus wall membrane. The wall membranes (of whatever complex shape) are calcified only on their inner surfaces, but the radial lamellae, unless very crowded, are calcified on both sides (i.e. the radial lamellae are double, or different in structure from the adjacent walls). Their regular growth lines show the addition of concentric bands at their peripheries except where they abut other annuli (Fig. 1A:2, B:2). Any possible hypothesis about growth zones


for the axial wall of each annulus has to allow for the merging, at the bottoms of the creases, of the axial wall and the expanding edges of the corresponding radial lamellae. An over-all structure of a ring of tightly-appressed, slowly- expanding blisters best fits the growth-lines, the observed over-stepping of some lamellae during growth, and the bilateral calcification of both short and long radial lamellae. On this theory the main growth zones for all expansion were the crests of each of the axial ridges, and contiguity with adjacent ridges and axial body-structures were mechanisms by which single wall membranes gradually merged at the bottoms of the creases, to become double radial membranes. Surfaces of adjacent annuli (particularly those roofing and flooring interannuli and axial walls) are often fluted where the wall membranes have turned in to form the core membranes of radial lamellae. In the interannuli the radial canals most frequently follow these flutes and shape themselves per- manent furrows on the surfaces as the annuli approach maturity (Fig. 1A:3; Teichert 1933, PI. 1949). The geometric term interannulus has been coined to cover every siphuncle part between adjacent annuli, or the space left by the removal of organic material from this area. In the early stages of calcareous deposition each interannulus contained a great deal of siphuncular tissue, which was reduced with growth, usually to a sliver of tissue, necessarily bulged over the radial canals and sometimes enclaing segmental sinuses as well (p. 311). Preservation failures reduced the organic content sometimes to nothing, but as degrees of alteration are well known, there is no need to change the geometric term with the preservational aspect of the interannuli. This term covers both ‘horizontal lamellae of soft tissues of siphonal cord‘ and ‘transverse interspace between two consecutive calcareous rings’ (Mutvei 1965), with the exception of the tissues of the perispatia which Mutvei also included in the ‘horizontal lamellae . . .’.

In section interannuli normally appear as thin, dark lines where annuli have been tightly appressed. They have very frequently been

Fig. 4. Armen0cern.r sp. 4, F7229. 0 A, ~ 1 . 8 . Oblique called ‘radial canals’, but this term should be dorsoventral section showing radial lamellae radiating reserved for tubular structures carried in the from axial cavity. Recrystallization masks most by interannulus. In forms where both radial canals mid-radius, but two of them can be traced to margin and segmental enter the interannulus, at bottom indicators. White lines=interannulus. B, X1.8. Longitudina\ section of same, after removal of radial cannot be distinguished unless another slice. their organic walls, or the furrows they once


occupied, can be observed (Fig. 1A:2; h, m). On the other hand, if the canal and sinus structures occupy different areas, as in the thick interannulus of Mesakroceras (Fig. 1B:2; m, h), the canals and sinuses can be identified by the positions of their apertures.

The growth of calcification and enclosure of canals are substantially alike in all actinoceroids, although comparison of Georgina with Mesaktoceras (in which the growing surface of the axial ridges was transmuted to a sheet forming endocones) suggests another possibility: if the roof and floor of the interannulus did not contact each other early, lateral fusion of this growing space would also have been possible. This appears to have occurred in Jeholoceras, from a preliminary examination of rather rare specimens from the Nora Forma- tion. These- have extremely irregular inter- annular surfaces that structurally compare very close19 with two figures of Adamsoceras holrni (Troedsson) (Mutvei 1965, Fig. 13A, B; not Fig. 13C). Radial lamellae are shown in Armenoceras spps. 2, 4 and Kochocertar tyrelli (Figs. 2, 4, 5 ) and many published figures of Actinoceratida, e.g. Nybyoceras multicubicu- latum Teichert & Glenister, Wutinoceras logani Flower, Jeholoceras robustum Kobayahsi & Matsumoto, Rayonnoceras bassleri Foerste & Teichert, R. girtyi Foerste & Teichert, and (see also Teichert & Crick 1974) Huroniella severnense (Foerste & Savage) and Huronia vertebralis Stokes. Teiched & Crick have com- piled a short review of past descriptions of such lamellae or ‘blades’ as they have sometimes been termed. The family Georginidae is further characterized by the possession of engrafts, strongly calcified, canoe-shaped structures formed by long, fibrous crystal growth normal to a thin, dark-coloured core membrane which merges with the inside of the connecting ring (Wade 1977, PI. 4:6, 7; this paper Figs. 1A:2, 3, B:2, 3; 6, 7). Anterior growth stages show that calcification began to be deposited on these engraft core membranes at approximately the same time as the annuli commenced to grow, and growth lines show that length was added to aligned crystal fibres so that the completed engrafts are massive, fibrous structures which became engrafted across the interannuli into the edges of adjacent annuli as growth proceeded; their own growth was checked by this enclosure, rather later in Mesaktoceras than in Georgina. While

Fig. 5. Kochoceras tyrelli, Carson Mine, near Winni- peg, Manitoba, X 1.5. Facing sides of a saw-cut, venters inward; interannulus cut at top left, dark calcite, rich in organic matter, forms cores to radial lamellae (many of which are broken) and to interannulus.

the connecting rings have rarely been preserved, and the connection between connecting rings and engrafts even less often, the core of each engraft is discernible on the etched surfaces as a narrow, longitudinal slit (Fig. 6), and in calcified material as a dark, fine-grained layer similar in composition to the connecting rings, annulus wall membranes and radial lamellar core membranes. The only previous mention of a structure similar to engraft core membranes appears to be Teichert & Crick (1974), describing ‘a set of longitudinal blades that radiate inward from the connecting ring’ of H . severnense. Their figures appear to represent the annulus wall rather than the connecting ring for their P1. 3:2 and probably PI. 3 5 seem to show fragments of thin connecting rings which tend to camouflage the furrows on the outside of the walls of the annuli. They are tightly applied to the walls


Fig. 6. Georgina roylori Wade, F7135, X3.5. Previously unfigured paratype, showing engrafts inserted across interannuli; gashes near centre of segment mark exits of radial canal distributaries, probably enlarged by solution; paired pores at apices of engrafts lead to segmental sinuses. Fragments of engraft core membranes are preserved at two apices but mostly the core membranes are represented by narrow furrows.

of the annuli, but this is a common diagenetic effect and does not imply the absence of a perispatium. The spaces between lamellae in their P1. 2:3 and P1. 3:l and 3-5 appear to be within the annulus walls and therefore are not perispatia but natural spaces between radial lamellae with short calcareous deposits on their core membranes (i.e. with crystal deposits which do not abut across the blisters but leave empty spaces within them) as the central half of P1. 2:3 shows at the lower right and hints elsewhere. The question of whether H . severnense has ordinary open perispatia or peri-

spatial sinuses like Georginidae awaits re-study. It would be premature to ally it to the georginids. Short calcification upon radial lamellar membranes occurs in some other conventional Actinoceratida, e.g. Fig. 5. While it is good to see the long-overdue recognition of radial lamellae in Actinoceratida, it is not possible to assert their presence in the originals of all illustrations appearing to show them. In some specimens there is staining along the surfaces where calcification from neighbouring radial lamellae mutually abuts, and this staining may be clearer and less easily destroyed by diagenesis than the radial lamellar membranes. It is necessary to relate any such structures to furrows in the annulus wall membranes, as, indeed, Teichert & Crick have done.

The circulatory system of G eorginidae Almost every specimen with axial canals preserved has more than one, but as a rule it is clear that canals did not occupy all the axial space. Radial canals diverged from axial canals in a graceful sweep, but the angle of their departure from the axial cavity is decided mainly by the shape of the interannulus, whether flat, convex, oblique-convex, or even strongly produced near the axis. Radial canals may branch sparsely near the axis or be dendroid (Mg. 1, A:3). They are quite thick- walled and of relatively narrow bore. They impinge on the inner ends of engrafts and commonly divide, passing around the engrafts to the perispatia. External openings of radial canal distributaries are commonly gashes rather than pores on silicified specimens (Fig. 6). The term ‘canal distributaries’ is used because it is not often known whether a maze of small passages spread between the doubled membranes between the annulus wall and the engrafts, as appears to occur in Georgina linda Wade, or whether the distributaries were sinuses. Silicified specimens often show large spaces here, but parts (at least) of these spaces are due to solution of the membrane material. Sections of calcified specimens show relatively thick ‘dark layer’ material, and very rarely, possible sinuses. One example of rock fill - glauwnitic sand - in strips wider than the similarly filled radial canals is known from Mesaktoceras and portrayed in Fig. 1B:2.

LETHAIA 10 (1977) Actinoceroid siphuncles 3 11

The core membranes of the engrafts define longitudinal perispatial sinuses in place of a single perispatium. It is not known whether the separation broke down if the engrafts were short or few, but in the majority of species there are pores at top and bottom of each perispatial sinus (Fig. 6); these lead into internal passages best seen in Mesaktoceras where they are surrounded by calcareous endocone material (Figs. lB:l, 2). These passages lead to the axial cavity. They are interpreted as venous sinuses but have been referred to simply as segmental sinuses. Their position and rela- tionships seem to be quite variable, sometimes intraspecifically. In some specimens of G. taylori they appear to run between the engrafts and annulus wall and via the interannulus, in others th? pores lead through the annulus wall into the annuTiis, probably passing to the axial space via a radial lamella (Fig. 1A:2 shows both those possibilities). More strongly cyr- tochoanitic older individuals of G. taylori often have steeply oblique passages leading from the pores to the central cavity, apparently within the radial lamellae (Wade 1977, PI. 4:3). Pores occur regularly in pairs, one each side of the engraft core pembranes, at both extremities of the engrafts (Fig. 6), but they may occur more than four times as frequently as engrafts. In M . arachne segmental sinuses may have wider and less sharply defined passages than radial canals.

The least known structures in the whole circulatory system are the perispatial sinuses. They may be much more strongly tubular than is suggested by Fig. 1A:3, B:3 (compare G. dwyeri, Fig. 7). This does not often show such deep grooves, although grooves are a repeated feature of large individuals (Wade 1977, PI. 6: 1-3). Its perispatial sinuses frequently, probably regularly, end in pores to segmental sinuses anteriorly and posteriorly and receive canal distributaries. In spite of being the largest species it has relatively narrow engrafts, quite closely spaced, and each of the grooves attributed to the perkpatial sinuses occupies more than half the space between adjacent engraft cores. Wade (1977, P1. 4:1, 2) illus- trated near-transverse sections of G. taylori cut very close to the contact layer platform edge; these show narrow tubes under the edge of the septa1 neck. They do not necessarily give a sample of the perispatial structure as a whole, even though the representation of frequent

Fig. 7. Georgina dwyeri Wade, U.Q. F67153. Paratype, X 1, showing exceptionally well-developed grooves of perispatial sinuses.

contact between the wall membrane and the connecting ring is definitely not an artifact. In summary, though the perispatial sinuses of G. dwyeri were probably single between any pair of engraft cores, this is not evidence for any other species.

The position of the pores from perispatial to segmental sinuses precludes the formation of secondary deposits, comparable to those of other Actinoceratida, in the georginid perispatia.

The soft tissues The only original organic matter is that stored in the calcified membranes of the annuli, engraft cores, connecting rings, and canal walls, so any interpretation of the soft parts of the siphuncle has to rest heavily on Nautilus and to a lesser degree on Spirula and Sepia.

The fleshy siphuncle of Nautilus contains the minimum of structures which enable the

.


animal to control the liquid and gas contents of the phragmocone. Other nautiloids may have had more structures in their siphuncles but cannot have had less than an artery, distributaries, venous lacunae, sinuses or veins, connective tissues, an epithelium and a more or less anterior growing point. Willey (1902) observed that in living Nautilus there is usually slack at the anterior of the siphuncle, and Stenzel (1964:K86) stated unequivocally that as the muscles shift the body adorally in the shell ‘the siphuncular cord grows correspond- ingly in length’. No one Seems to have made a plain statement that the main growing point of the siphuncle is adjacent to the posterior of the palliosiphonal sinus, but Denton & Gilpin- Brown (1966) have described some fully-formed siphuncular tissues from the last siphuncular segment of a specimen of Nautilus in which - the connecting ring was virtually complete, but the last layer of the septum not yet laid down. (Mutvei 1972 showed that the organic layers of the connecting ring equated to organic matter dispersed in calcareous matter in all but the last layer of the septum, the semi- prismatic layer.) Their detailed illustrations and description depicted the differences between the epithelium of the last siphonal segment and an earlier segment. They appear to have assumed that since the change of secretion (from septal neck to connecting ring) coincides with the change from the normal body wall epithelium to siphonal epithelium in all modern shelled cephalopods, it is self- evident where the growing point of the siphuncle is. The palaeontological literature, however, has long abounded in speculations about the content of large siphuncles. Since connecting rings are now known to be formed by siphuncular epithelium, at the same time as the respective septum and neck are formed by body-wall epithelium, there is no longer room for speculation that actinoceratids and endoceratids (or forms with similarly large siphuncles) may have differed from Nautilus in having some of the main body in the siphuncle. Denton & Gilpin-Brown (1961; 1966) and Denton (1974) further developed knowledge of the siphuncle by their study of its effect in buoyancy-control in Sepia, Nau- tilus, Spirula and, by comparison, the endo- ceratid ‘Dideroceras’. In all of these the connecting ring and siphuncular epithelium acted as a ‘pump’ to remove camera1 liquid from

the last-formed chambers. Comparison with Mutvei (1964, Figs. 19, 28) shows that the semi-prismatic layer of the septal neck must also be involved in the ‘pump’ in Spirula and ‘Dideroceras’, but this does not alter the prin- ciple deduced from their functional analysis. Flower (1964) summarized the structures of the best-preserved connecting rings he had encountered, and has continued to figure and describe them as good specimens have come to hand. All are open to interpretation as ‘pumping mechanisms’. Large Sepia specimens, which become more and more top-heavy as they grow, flood the adapical chambers as a counterweight (Denton & Gilpin-Brown 1961). This parallels the apical weighting of many orthocones, and, indeed, shows the possibility of a liquid counterbalance mechanism which has been suggested before, from calculations of weight alone, without any indication of how the animal might control it.

The question of what did occupy the un- calcified portion of the actinoceroid siphuncles has to be approached from study of the calcified portion. The standard plan of the siphuncle circulatory system (axial canals, radial canals, perispatia, segmental sinuses, and presumed axial sinuses) itself suggests the inter- pretations siphonal arteries, segmental arteries, perispatium or perispatial sinuses (allowing at least semipermeable contact with the camerae; in some other nautiloid forms better communication has been indicated by Flower 1964), segmental and axial venous sinuses or veins. The space available to house axial sinuses or veins was restricted in fully calcified portions to about the dimensions of a narrow siphuncle. Whether it enlarged toward the anterior is unknown, but a region of active epithelium usually commands a good blood-flow. A total blood-flow sufficient for every segment had to pass through the venous sinuses of the anterior axial region to the pallio-siphonal sinus at the rear of the body, but the pallio-siphonal sinus itself did not have to be very large, its func- tions were to protect the growing point of the siphuncle and bridge between the siphuncle and the main body. Recurved to recumbent necks presumably indicate that the thin-walled sinus fanned out from the main body to cover the anterior end of a siphuncle wider than the neck opening. Withdrawal of this flexible structure through the neck would have been per- mitted by forward growth initiating the next


siphuncle segment, and accomplished by the normal life-movements of the animal.

Concrete evidence of the speed of development of organs is sparse in georginids, as in other fossil nautiloids. The shaping and se- creting of connecting rings proves only the existence of an epithelial layer and the flesh in immediate contact with it, for support for each newly-formed, inflated segment could have been largely hydrostatic, with the shape restrained by the septal necks. Engraft core membranes could scarcely have preceded the connecting rings however soon they were initiated, but the only observable certainty is that they were fully developed by the time calcification started, since enclosure put a stop to their growth. However, they are so intimate- ly connected with the circulatory system that it also must 6e regarded as complete by that time; perispatia become observable quite early in calcification in many forms. Actually the very early development of the segmental circulatory system seems likely, as a service mechanism to the functional siphonal epithelium and developing engrafts. The radial lamellae, and membranes enclosing the annulus, are an irrehtably late development in all actinoceroids (this paper, pp. 306, 307, Figs. 2-5). The association of calcification with the development of annuli of membraneous-walled blisters, has recorded an otherwise unguessable change from the tissue-distribution in the earlier part of the siphuncles.

Development of actinoceratoid buoyancy controls Denton & Gilpin-Brown (1961; 1966) and Den- ton (1974) have shown that the chambers of large Sepia, flooded when formed (like those of Nautilus and Spirula), are emptied by the action of the connecting ring materials and siphuncle epithelium. Subsequently the adapical chambers are re-flooded as the animals reach a size at which apicad ballast is necessary for them to hang level in the water. Denton & Gilpin-Brown (1968) described and figured in detail longitudinal drainage ducts in the epithelium of Nautilus, and Denton (1974) men- tioned them in Sepia. Somewhat similar structures must be assumed to have been present in nautiloids in general as the need to transfer liquid was present in all. In large Sepia in

which adapical chambers are flooded, the canals and the remainder of the siphuncle epithelium must behave differently from the way they behave in early Sepia and Nautilus, etc. Perhaps the behaviour of Sepia is more comparable with orthocones than that of Nautilus, for problems of balance were similar. Restric- tions on the constant removal of liquid from the adapical chambers would most naturally take effect progressively from older to younger, though it need not involve only the first chamber when it commenced. Calcification normally does seem to start at the apex but (in some species or individuals) may reach maxi- mum strength after several chambers, and in all forms, later diminishes anteriorly. No very extreme adaptations of morphology or function would have beep required to develop the ‘blisters’ of the actinoceratoid annuli from hypertrophied epithelial drainage ducts. The material and position of the structures are virtually identical, and the usual point of origin of the blisters, inside the septal neck, is the place where the ducts were most restricted to the single function of being ducts by less- ened semipermeable contact with camerae. An occasional point of origin (an individual variant in several, possibly all, actinoceroid groups) is from the edge of the next-earlier annulus; this would be bewildering if the origin of the blisters was not from structures with a certain longitude. Further, the extreme variation in the number of blisters to attain significant length, observed in the Georginidae (Wade 1977, P1. 4: 1, 2, 4) becomes more easily under- stood if their origin is from such a frequent structure as the ducts. Were the blisters to be a new structure adapted to the one purpose, a more consistent number of radial lamellar membranes seems likely.

Conclusions The view that the development of the annuli preceded calcification gains credibility from the lack of similarity among the earliest known actinoceroids, Wade (1977) has already suggested that the Georginidae differ so strongly from the remainder of the Actinoceratida that separation prior to calcification of either group is most likely. A unique georginid and two species of Armenoceras are the earliest actinoceroids in the Georgina Basin. Estimates


of the age of this fauna range from Late Canadian to Whiterock, but within every group of fossils there is conflicting evidence. For example, at a generic level the endoceroids have an older aspect than the actinoceroids, throughout both formations (Flower, personal communication). At all events, the strati- graphic evidence does not conflict with the morphologic probability, for even the younger dating makes the Armenoceras-georginid fauna approximately the same age as early Wutino- ceratidae.

The annuli in Actinoceratida are formed by calcification within thin-walled blisters of organic material which are tightly packed together around the septal necks. As they expanded, many were overstepped by their neighbours so that relatively few surrounded $he axial cavity. The inner ends of these few are all that can be recognized in most horizontal sections, although virtually all exposed anterior cavities show ridging and furrowing at anteriorly greater distances from the axis, due to the successive annuli having different stages of development.

The tight packing together of the blisters resulted in adjacent walls forming a unitary structure, a radially directed lamella which was calcified on both sides in contrast to the remaining walls. These abutted the connecting rings (which fully lined the septal necks) or perispatia, roofed or floored the interannuli, or walled the axial cavity. All these are calcified on their inner sides only. Growth-lines show best in cuts parallel to the lamellae, but good preservation is rare. Their successive shapes record the longitudinal profiles pos- sessed by the inner wall of the anterior cavity at successive growth stages. These factors are more easily demonstrated in the family Geor- ginidae (Wade 1977) than in most previously described taxa, because the calcification of the radial lamellae was short. That from adjacent lamellae did not normally contact across a blister, so it allowed sediment to fill the annuli and inhibit recrystallization. Neverthe- less, many scattered references to ‘blades’ and lamellae occur in literature, and even more are figured.

The Georginidae also had calcareous bodies called engrafts attached to flanges of organic matter (engraft core membranes) projecting from the inside of the connecting rings. These commenced calcification at approximately the

same time as the annuli, and were engrafted into the edges of the annuli, bridging the interannuli, as both grew.

The system of axial canals, radial canals and canal distributaries in Georginidae has been demonstrated to lead into the perispatial sinuses that also, toward the tips of the engrafts, connect to other passages which lead to the interannulus, or through the radial lamellae, and so to the axial cavity. These systems are interpreted as an arterial system, perispatial sinuses, and venous system. Axial venous sinuses or veins cannot be observed but would not have had thick organic walls, or been enclosed in calcareous matter, and are unlikely to be found.

The blisters of organic matter, in which the calcification of the annuli was deposited, might have originated from hypertrophy of longitudinal epithelial ducts earlier used to empty camerae after formation. In the ancestors of the known actinoceratoids this hypertrophy could have been associated with changes in buoyancy-control such as those that cause large Sepia to flood adapical chambers. It presumably indicates at least partial flooding of the adapical camerae at the ontogenetic stage that calcification commenced in Actino- ceratida. Hypertrophy of the ducts at a phyletic stage prior to calcification (or the introduction of a totally new character, if that explanation of the ‘blisters’ is preferred) possibly also explains the localization of the radial canals (segmental arteries). This is as characteristic of even the earliest known members of the group as perispatia (large sinuses girdling the segments, presumably in place of capillary- sized distributary sinuses like Nautilus).

References Denton, E. J. 1974: On buoyancy and the lives of

modern and fossil cephalopods. Proc. R. SOC. Lond.

Denton. E. J. & Gilpin-Brown, J. B. 1961: The dislri- bution of gas and liquid within the cuttlebone. I . Mar. Biol. Ass. U.K. 41. 365-381.

Denton, E. J. & Gilpin-Brown, J. B. 1966: On the buoyancy of the pearly Nautilus. I . Mar. Biol. Ass.

Flower, R. H. 1957: Nautiloids of the Paleozoic. Geol. SOC. Am. Mem. 67, 829-852.

Flower, R. H. 1964: Nautiloid shell morphology. Siare Bur. Min. Mineral. Res. New Mex. Mem. 13, 1-79.

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Huxley, T. H. & Pelseneer, P. 1895: Report on the specimen of the genus Spirula collected by H.M.S. Challenger. Challenger Rep. 2001. 83, 1-33.

Mutvei, H. 1964: On the shells of Nautilus and Spirula with notes on the shell secretion in non-cephalopod molluscs. Arkiv Zool. ser. 2, 16 (14). 221-278.

Mutvei. H. 1965: On the secondary internal calcareous lining of the wall of the siphonal tube in certain fossil ‘nautiloid’ cephalopods. Arkiv. Zool. ser. 2, I6 (21). 375424.

Mutvei. H. 1972: Ultrastructural studies on cephalopod shells. Part I. The septa and siphonal tube in Nautilus. Part 11. Orthoconic cephalopods from the Pennsylvanian Buckhorn asphalt. Bull. Geol. Instn. Univ. Uppsala, N.S. 3 (8, 9). 237-261, 263-272.

Smith, K. G. 1972 Stratigraphy of the Georgina Basin. Bur. Mineral. Res. Aust. Bull. I l l . 1-156.

(49). 1-71. Stenzel, H. B. 1964: Living Nautilus. In Moore, R. C.

(ed.): Treatise on Invertebrate Paleontology. K. Mollusca 3. K59-IC93. Geol. Soc. Am. and Univer- sity of Kansas Press.

Teichert, C. 1933: Der Bau der Actinoceroiden Cepha- lopoden. Palaeontographica Abr. A . 78, 111-230.

Teichert, C. 1967: Major features of cephalopod evolu- tion. In Essays in paleontology and stratigraphy. R. C. Moore Corn. Vol. Univ. Kansas Geol. Dep. Lawrence Spec. Publ. 2, 162-210.

Teichert, C. & Crick, R. E. 1974: Endosiphuncular structures in Ordovician and Silurian cephalopods. Univ. Kansas Paleont. Contr. 71. 1-13.

Wade, Mary 1977: Georginidae, new family of actinoceratoid cephalopods, Ordovician, Australia. Mern. Queensland Mus. 18 ( I ) , 1-15.

Willey, A. 1902: Contribution to the natural history of the pearly Nautilus. Part 11, Special Contribution. A . Willey’s 2001. Results 6, 736-830.

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the siphuncle in georginidae and other ordovician actinoceroid cephalopods

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