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Keywords:

  • Late Cambrian;
  • China;
  • plectronocerid nautiloids;
  • siphuncular structures;
  • cephalopod evolution

Abstract

  1. Top of page
  2. Abstract
  3. Previous studies
  4. Material and methods
  5. Results
  6. Discussion
  7. Phylogenetic conclusions
  8. References

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

The chambered shell (phragmocone) in cephalopods functions as a hydrostatic apparatus. The connecting ring in the siphuncular wall has a porous, permeable structure in each chamber that enables the transport of cameral liquid and gas across the siphuncular wall from the chamber to the siphuncular cord. Several structurally different types of connecting rings have been described in Recent and fossil coleoids (Mutvei 1971; Mutvei and Donovan 2006), ammonoids (Mutvei et al. 2004) and nautiloids (Mutvei 1997a, b, 1998, 2002a, b). In nautiloids, each connecting ring is a structurally modified continuation of the septum and septal neck. Two different types of connecting rings have been described hitherto in fossil nautiloids: (1) the Nautilus type, which also occurs in fossil ellesmerocerids, nautilids and tarphycerids, in which the connecting ring is composed of two porous layers: an outer calcified, spherulitic-prismatic (chalky) layer and an inner uncalcified, glycoprotein (conchiolin, horny) one; and (2) the calcified-perforate type that occurs in fossil actinocerids and orthocerids, in which the outer layer is spherulitic-prismatic, and the inner layer is not organic, but fully calcified and perforated by numerous pore canals. During nautiloid evolution, these two structural types seem to have existed in phylogenetically unrelated lineages, reflecting differences in anatomy and mode of life.

The present paper describes the siphuncular structures in some Late Cambrian plectronocerid nautiloids from China, previously noted by Chen and Teichert (1983). No attempt is made to revise the genera and species of these nautiloids. On the basis of the results obtained, the evolution and mode of life in early nautiloids are briefly discussed.

Previous studies

  1. Top of page
  2. Abstract
  3. Previous studies
  4. Material and methods
  5. Results
  6. Discussion
  7. Phylogenetic conclusions
  8. References

The oldest cephalopod, Cyrtoceras cambria, of Franconian (middle Late Cambrian) age, was described by Walcott (1905). In the following decades, additional Cambrian fossils were found and described. Until the 1960s, 50 specimens of undisputed Cambrian cephalopods were known from north-east China, central Siberia, Kazakhstan and Texas. This material was assigned to eight genera in four families (order Ellesmerocerida). After 1962 new discoveries were made; these were summarized by Chen and Teichert (1983). According to these authors, the Late Cambrian nautiloid fauna comprised four orders: Plectronocerida, Ellesmerocerida, Protactinocerida and Yanherida. Only the first two of these are recognized here.

Material and methods

  1. Top of page
  2. Abstract
  3. Previous studies
  4. Material and methods
  5. Results
  6. Discussion
  7. Phylogenetic conclusions
  8. References

The material examined was collected from the upper 7–10 m of the Wanwankou Member of the Fengshan Formation, in Liaoning Province, China. Several of the specimens are holotypes of species described by Chen and Teichert (1983). The material is deposited in the Nanjing Institute of Geology and Palaeontology, China (abbreviation NIG).

The following specimens were restudied (all were previously dealt with by Chen and Teichert 1983):

Order PLECTRONOCERIDA Flower, 1964 Family BALKOCERATIDAE Flower, 1964

Theskeloceras subrectumChen and Teichert, 1983, NIG 73773 (holotype), pl. 3, figs 1, 12; NIG 73774 (paratype), pl. 10, fig. 4.

Theskeloceras benxienseChen and Teichert, 1983, NIG 73772 (holotype), pl. 4, figs 1–2, 4.

Family PROTOACTINOCERATIDAE Chen and Qi, in Chen et al. 1979

Physalactinoceras speciosumChen and Teichert, 1983, NIG 73799 (holotype), pl. 14, fig. 2; pl. 18, fig. 5.

Physalactinoceras papillaChen and Teichert, 1983, NIG 73796 (holotype), pl. 7, figs 5, 8.

Physalactinoceras globosum Chen and Qi, inChen et al. 1979, NIG 73788, pl. 12, figs 4, 7.

Physalactinoceras cf. globosum Chen and Qi, inChen et al. 1979, NIG 73794, pl. 15, figs 2–3.

Mastoceras qiushugouenseChen and Teichert, 1983, NIG 73775 (holotype), pl. 9, figs 1–2, 7, text-fig. 25; NIG 73776 (paratype), pl. 13, figs 1, 3, text-fig. 26.

Sinoeremoceras taiziheenseChen and Teichert, 1983, NIG 73782 (paratype), pl. 13, fig. 4.

All shells are embedded in a hard limestone from which they could not be extracted without destroying the specimens. Because the shells are more or less curved, each longitudinal section cuts the shell wall and siphuncle in different planes. All specimens were photographed using a reflected light binocular microscope, Nikon SMZ 1500, equipped with an AxioCam HRc Zeiss ccd camera. Elemental composition of the shell and surrounding limestone was analysed with an energy-dispersive X-ray microprobe (EDAX) at the Swedish Museum of Natural History, Stockholm.

Results

  1. Top of page
  2. Abstract
  3. Previous studies
  4. Material and methods
  5. Results
  6. Discussion
  7. Phylogenetic conclusions
  8. References

Preservation

The shell wall and septa are recrystallized and have a coarse-crystalline structure. EDAX analysis of the holotype of Physalactinoceras speciosum showed that the shell wall, septa and connecting rings, as well as the matrix within the chambers and siphuncular cavity, are of calcium carbonate with subtle differences in elemental composition. The outer, originally porous, spherulitic-prismatic layer of the connecting ring has a higher Mg content than the inner layer. The results of the analysis are presented in Table 1.

Table 1.   Results of EDAX analysis of the holotype of Physalactinoceras speciosum (NIG 73774).
 Calcium (at. wt%)Carbon (at. wt%)Oxygen (at. wt%)Magnesium (at. wt%)
Septa (3)1216–2664–710
Matrix in chambers (1) 631630
Matrix in siphuncular cavity (2) 8–1125–27630–0·2
Connecting ring (7) 7–1520–2661–660·2–2·3

Siphuncular wall

The shells could not be sectioned medially because they are more or less curved and their orientation is oblique to bedding. The cutting plane therefore extends through the siphuncular wall at different levels. The longitudinal sections are usually oblique in both dorsoventral and longitudinal directions (Text-figs 1–2, 4). Study of siphuncular morphology is therefore quite difficult, and taxonomic differences, if they exist, cannot be clearly defined.

image

Figure TEXT-FIG. 1..  A, Mastoceras qiushugouense, paratype, NIG 73776, longitudinal section of the siphuncle close to the median plane; note a calcareous deposit in the lower portion of the siphuncular cavity. B, Theskeloceras subrectum, holotype, NIG 73773, similar section as A; note an incompletely preserved, calcareous deposit in the middle portion of the siphuncular cavity. C, Theskeloceras benxiense, holotype, NIG 73772, paramedian longitudinal section of the siphuncle to show the outline of the siphuncular segments and diaphragms (= adapical portion of the section in Chen and Teichert 1983, pl. 4, fig. 1). D, dorsal side; V, ventral side. Scale bars represent 1 mm.

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image

Figure TEXT-FIG. 2..  A, Mastoceras qiushugouense, holotype, NIG 73775, longitudinal section of the siphuncle close to the lateral side (= adapical portion of the section in Chen and Teichert 1983, pl. 9, fig. 1); note the strongly oblique direction of the siphuncular segments, the diaphragms, and the crystalline matrix within the chambers and siphuncular segments. B, Theskeloceras benxiense, holotype, NIG 73772, longitudinal section of the siphuncle along the dorsolateral side (= adoral portion of the section in Chen and Teichert 1983, pl. 4, fig. 1) to show cyrtochoanitic septal necks and laterally expanded connecting rings. C, Bathmoceras linnarsoni, holotype, MO 150046 (Swedish Museum Natural History, Stockholm), a Middle Ordovician nautiloid from Sweden to show the ventral side of the shell with high, adorally directed lobes of the suture lines as a result of highly oblique siphuncular segments; dorsal side (D), ventral side (V). Scale bars represent 1 mm.

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image

Figure TEXT-FIG. 4.. Physalactinoceras speciosum, holotype, NIG 73799 (= in Chen and Teichert 1983, pl. 18, fig. 5). A, oblique longitudinal section of the shell: the adapical portion is close to the lateral side of the siphuncular wall, the adoral portion is paramedian. B, detail of the adoral portion of the section to show holochoanitic septal necks and pear-shaped protrusions of the connecting rings. C, detail of the middle portion of the section to show laterally expanded connecting rings. D, detail of the adapical portion of the section to show highly oblique direction of the siphuncular segments. Scale bars represent 1 mm.

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Septal necks

The septal necks are long, straight and holochoanitic on the mid-dorsal side, each extending adapically close to the previously secreted septal neck (D in Text-figs 1–2). Only a narrow gap separates two successive necks. Already on the dorsolateral sides, the septal necks become rapidly shorter and strongly cyrtochoanitic. The necks have a similar cyrtochoanitic shape on the lateral and ventral sides (Text-figs 1C, 2B, 3, 4C).

image

Figure TEXT-FIG. 3.. Physalactinoceras cf. globosum, NIG 73794, section of the siphuncular wall in the dorsolateral side to show details of a completely recrystallized septum (s) and septal neck (sn), and two layers in the connecting ring: the outer spherulitic-prismatic layer (sph) and the inner compact calcareous layer (c) perforated by numerous pore canals (p). Scale bar represents 0·1 mm.

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The septal necks on the dorsal side were described by Chen and Teichert (1983, p. 52) as ‘changing gradually from holochoanitic to very short cyrtochoanitic and strongly recumbent’ in Theskeloceras, as ‘variable in length from holochoanitic to orthochoanitid in different species and at different growth stages of the same individual’ in Physalactinoceras (Chen and Teichert 1983, p. 75), and as ‘straight, gradually changing adorally from holochoanitic to hemichoanitic’ in Mastoceras (Chen and Teichert 1983, p. 92).

However, as far as can be judged from the longitudinal sections, the septal necks on the dorsal side show a similar holochoanitic shape in at least two of the three genera (compare Text-fig. 1A and B). The differences described by Chen and Teichert (1983) seem to be a result of different obliquity of the sections, both longitudinally and dorsoventrally.

Longitudinal sections close to the lateral side of the siphuncular wall reveal that the siphuncular segments are highly oblique, sloping adorally from the dorsal to the ventral side at an angle of between 30 and 40 degrees (Text-figs 2A, 4D). Although the ventral side of the shell is not exposed, the suture lines certainly formed conspicuous, adorally directed lobes, the height of which corresponds to that of 2–3 chambers. Similar, highly oblique, siphuncular segments occur in bathmoceratid nautiloids, which are unrelated to plectronocerids (Text-fig. 2C).

Connecting rings

The morphology of the connecting rings is difficult to reconstruct in detail because of the obliquity of the longitudinal sections of the siphuncle. On the mid-dorsal side, they are straight and exposed in the narrow gap between every two consecutive septal necks (dorsal side D, Text-fig. 1A–B). On the dorsolateral sides, where the gap between the consecutive necks gradually widens, each connecting ring initially forms an extensive, pear-shaped protrusion that is narrow between the two consecutive necks but widens within the chamber (Text-fig. 4B). On the lateral sides, where the septal necks are short and separated by long interspaces, each connecting ring strongly expands outwards, protruding into the chamber with a semicircular outline (Text-figs 2B, 3, 4C). The ventral side of the connecting ring also expands into the chamber, but to a lesser extent than the lateral sides (V in Text-figs 1A–C, 2A–B). The differences in shape of the connecting rings in the different taxa described by Chen and Teichert (1983) and considered to be systematically important, can probably be explained by the highly variable orientation of the longitudinal sections through the siphuncular wall.

The microscopic structure of this wall is comparatively well preserved in a specimen of Physalactinoceras globosum (NIG 73788; Chen and Teichert 1983, pl. 12, figs 4, 7). The septal necks, sectioned on the dorsolateral side, are completely recrystallized into coarse calcite crystals and their outlines are indistinct (s, Text-fig. 3). The connecting rings are much better preserved than the septal necks, probably because they are porous. Each ring consists of two layers: the inner layer is light-coloured, completely calcified, about 500 µm thick (c, Text-fig. 3), and traversed by numerous, narrow pore canals about 200 µm apart (p, Text-fig. 3). This layer seems to be a direct continuation of the principal layer of the adjacent septal neck. The outer layer is about half as thick as the inner layer. It is porous, dark-coloured, and seems to have a spherulitic-prismatic structure (sph, Text-fig. 3). It originates from the outer surface of the adjacent septal neck. The connecting ring structure is nearly identical to that in post-Cambrian actinoceratid nautiloids in which the inner layer is wholly calcified and perforated by numerous pore canals, and the outer layer is calcified and has a porous, spherulitic-prismatic structure (Mutvei 1997a, 2002a, b).

Chen and Teichert (1983) recognized 1–3 three layers in the connecting rings of plectronocerids and protactinocerids, and they compared the three layers in Physalactinoceras with the horny, chalky and pellicle layers of modern Nautilus (Chen and Teichert 1983, p. 75). However, in the shells at our disposal, usually only two layers can be clearly distinguished, and the inner layer is not ‘horny’ but calcified and traversed by pore canals.

Diaphragms

Usually one diaphragm is formed in each siphuncular segment, and is attached to the siphuncular wall along the septal neck. Because the siphuncular segments are highly oblique, so are the diaphragms, sloping with a slight curvature adorally from the dorsal to the ventral side (Text-figs 1C, 2A). In places, 2–3 diaphragms occur in one segment (Text-fig. 1C). Thus, the shape, number and attachment of diaphragms are influenced by the shape and obliquity of the siphuncular segments.

Chen and Teichert (1983, p. 74) stated that in plectronocerids the spaces between diaphragms were empty, whereas in protactinocerids they were ‘filled with originally deposited calcitic material’. However, according to our observations, no structural differences between the diaphragms occur in the coarse-grained calcitic matrix that was precipitated between the diaphragms in Theskeloceras benxiense (Text-fig. 1C) and T. subrectum (Chen and Teichert 1983, pl. 10, fig. 3) as well as in Mastoceras qiushugouense (Text-fig. 2A), Sinoeremoceras taiziheense and Physalactinoceras speciosum (Chen and Teichert 1983, pl. 2, figs 1, 4; pl. 18, fig. 5). The matrix has the same structure as the calcitic precipitates within the chambers.

‘Endocones’

Chen and Teichert (1983, p. 93) described a calcareous deposit in the siphuncular cavity of Mastoceras qiushugouense that ‘vaguely suggest slightly asymmetrical cones’. It is possible that this deposit represents siphuncular segments and diaphragms that collapsed post-mortem (Text-fig. 1A). A somewhat similar deposit occurs in the median portion of the siphuncular cavity of Theskeloceras subrectum, but it is incompletely preserved (Text-fig. 1B). The deposits in question differ in structure and preservation from the conical endosiphonal deposits in endoceratid nautiloids (e.g. Mutvei 1964, pls 1–6).

Discussion

  1. Top of page
  2. Abstract
  3. Previous studies
  4. Material and methods
  5. Results
  6. Discussion
  7. Phylogenetic conclusions
  8. References

Late Cambrian plectronocerids

According to Chen and Teichert (1983, p. 74), the order Protactinocerida resembles the order Plectronocerida ‘in most features, differing, however, from the latter in its much larger siphuncle with much more strongly expanded segments and its more advanced diaphragms and development of calcite fillings in the spaces between diaphragms’. However, our studies show no differences in the expansion rates of the siphuncular segments in these taxa (compare Text-fig. 4A and C). Also, the shape of the diaphragms and the calcite infill between them are similar in both taxa (compare Text-figs 1C and 2A). We therefore suggest that the family Protactinoceratidae, previously referred to the order Protactinocerida, should be transferred to the order Plectronocerida.

In spite of the early appearance of plectonocerids, their siphuncular structure is the most advanced among nautiloids. The connecting ring is wholly calcified and traversed by numerous pore canals. The latter probably housed extensions from the epithelium of the siphuncular cord, thereby increasing the physiologically active surface area of this epithelium (Mutvei et al. 2004); the connecting ring acquired a considerably enlarged surface area by its extensively expanded lateral sides and strongly oblique course. These features indicate that the siphuncular wall could withstand hydrostatic pressure comparatively well, and that plectronocerids probably would have been capable of vertical migrations. Calcified connecting rings, perforated by numerous pore canals, also occur in orthocerid and actinocerid nautiloids (Mutvei 1997a, 1998, 2002a, b), and in coleoid belemnoids (Mutvei and Donovan 2006).

The ability of the siphuncular epithelium to transport osmotically the cameral liquid across the connecting ring is difficult to estimate. In living Nautilus, this epithelium has a limited osmotic capacity and the emptying of cameral liquid from newly formed chambers is a slow process (Ward 1987), although the permeability of the connecting ring is considerably higher (Chamberlain and Moore 1982). It can be assumed that during the early stages of nautiloid evolution, the siphuncular epithelium had a limited osmotic capacity, but that this was compensated for in plectronocerids by the enlarged surface of the connecting ring and numerous pore canals. The closely spaced septa, characteristic of Late Cambrian nautiloids, were probably also related to the limited osmotic capacity of the siphuncular epithelium because small chambers contained small volumes of cameral liquid to be replaced by gas.

Plectronocerids had small orthoconic or cyrtoconic shells, which were certainly not adapted for jet-powered swimming. Lateral movement was probably limited to crawling or moving slowly on the sea-floor; however, vertical movements were probably common and could perhaps have been extensive.

Late Cambrian ellesmerocerids

The order Ellesmerocerida was erected by Flower (in Flower and Kummel 1950). Most ellesmerocerids are imperfectly preserved and their diagnostic characters are still poorly understood. Several taxa, originally included by Flower and Kummel (1950), Flower (1964) and Furnish and Glenister (1964), have subsequently been removed from this order (Chen et al. 1979; Chen and Teichert 1983; Frey 1995; Mutvei 2002a, b; Kröger and Mutvei 2005). As pointed out by Chen and Teichert (1983), ellesmerocerids constitute more than half of Late Cambrian nautiloids from China. These authors characterized the Cambrian ellesmerocerids by the following features: orthoconic to strongly cyrtoconic shells, mostly endogastrically curved; tubular siphuncles of variable diameter often containing diaphragms, subcentral or marginal in position; and connecting rings of variable thickness.

Ellesmerocerids were not included in our study. A revised definition of the stratigraphically younger ellesmerocerids has recently been given by Kröger and Mutvei (2005). The connecting ring is characterized by an outer, porous, calcified, spherulitic-prismatic layer, whereas the inner layer, usually not preserved, was probably made up of a fibrous, glycoprotein (conchiolin) layer as in living Nautilus. A similar preservation of the connecting rings also occurs in fossil tarphycerids and nautilids (Stumbur and Mutvei 1983; Mutvei 2002b). To judge from the illustrations in Chen and Teichert (1983), it is probable that the connecting ring structure in Late Cambrian ellesmerocerids from China was similar to that in two Early and Middle Ordovician taxa: Pictetoceras (Cyclostomiceratidae; Mutvei and Stumbur 1971, text-fig. 1b, pl. 1, fig. 3; pl. 2, fig. 2; Kröger and Mutvei 2005, text-fig. 1B), and Oelandoceras (Ellesmeroceratidae; Kröger and Mutvei 2005, text-fig. 1A).

The mode of life of ellesmerocerids is still difficult to determine with confidence. As in plectronocerids, their small orthoconic and cyrtoconic shells were not adapted for jet-powered swimming. Their connecting ring structure is likely to have been mechanically weaker than that in plectronocerids, indicating that they probably lived in shallow-water environments.

Phylogenetic conclusions

  1. Top of page
  2. Abstract
  3. Previous studies
  4. Material and methods
  5. Results
  6. Discussion
  7. Phylogenetic conclusions
  8. References

Our study shows that the earliest known nautiloids comprised at least two distinct phylogenetic lineages, the ellemerocerids and plectronocerids, which were characterized by different types of connecting rings. Plectronocerids had structurally advanced, calcified connecting rings perforated by numerous pore canals. This indicates that they could have been capable of vertical, probably diurnal, migrations. Similar types of connecting rings occur in orthocerid and actinocerid nautiloids (Mutvei 1997a, 1998, 2002a, b). Ellesmerocerids had connecting rings of the Nautilus type characterized by an outer, porous, calcified, spherulitic-prismatic layer, and an inner organic glycoprotein (conchiolin) layer. They were probably less capable of vertical migrations because of their mechanically weaker connecting rings, which were probably also less permeable to cameral liquid/gas transport than the connecting rings in plectronocerids. The latter structural type also occurs in fossil tarphycerids and nautilids (Mutvei 2002b).

Ellesmerocerids and plectronocerids both have small orthoconic or cyrtoconic shells that were not adapted for jet-powered swimming. Thus, since the beginning of their evolution, nautiloids had highly advanced structures for buoyancy regulation. Despite this advanced hydrostatic ability to neutralize the combined weight of the calcareous shell and body mass, their ability for jet-powered swimming probably appeared later in the course of nautiloid evolution.

References

  1. Top of page
  2. Abstract
  3. Previous studies
  4. Material and methods
  5. Results
  6. Discussion
  7. Phylogenetic conclusions
  8. References
  • CHAMBERLAIN, J. and MOORE, W. 1982. Rupture strength and flow rate of Nautilus siphuncular tube. Paleobiology, 8, 408425.
  • CHEN JUN-YUAN and TEICHERT, C. 1983. Cambrian Cephalopoda of China. Palaeontographica, A, 181, 1102.
  • CHEN, J. Y., TSOU, S. P., CHEN, T. E. and QI, D. L. 1979. Late Cambrian cephalopods of North China: Plectronocerida, Protactinocerida (ord. nov.) and Yanhecerida (ord. nov.). Acta Palaeontologica Sinica, 18, 124.
  • FLOWER, R. H. 1964. The nautiloid order Ellesmeroceratida. New Mexico Institute of Mining and Technology, Memoir, 12, 234 pp.
  • FLOWER, R. H. and KUMMEL, B. 1950. A classification of the Nautiloidea. Journal of Paleontology, 24, 604616.
  • FREY, R. C. 1995. Middle and Upper Ordovician cephalopods of the Cincinnati region of Kentucky, Indiana and Ohio. United States Geological Survey, Professional Paper, 1066, P1P119.
  • FURNISH, W. M. and GLENISTER, B. F. 1964. Nautiloidea – Ellesmerocerida. K129–K159. In MOORE, R. C. (ed.). Treatise of invertebrate paleontology, Mollusca (3). Geological Society of America, Boulder, CO, and the University of Kansas Press, Lawrence, KS, 519 pp.
  • KRÖGER, B. and MUTVEI, H. 2005. Nautiloids with multiple paired muscle scars from Lower–Middle Ordovician of Baltoscandia. Palaeontology, 48, 781791.
  • MUTVEI, H. 1964. On the secondary internal calcareous lining of the wall of the siphonal tube in certain fossil ‘nautiloid’ cephalopods. Arkiv för Zoologi, 16, 375424.
  • MUTVEI, H. 1971. The siphonal tube in Jurassic Belemnitida and Aulacocerida (Cephalopoda: Coleoidea). Bulletin of the Geological Institutions of the University of Uppsala, New Series, 3, 2736.
  • MUTVEI, H. 1997a. Characterization of actinoceratoid cephalopods by their siphuncular structure. Lethaia, 29, 339348.
  • MUTVEI, H. 1997b. Siphuncular structure in Ordovician endocerid cephalopods. Acta Palaeontologica Polonica, 42, 375390.
  • MUTVEI, H. 1998. Siphuncular structure in Silurian narthecoceratid nautiloid from the Island of Gotland. Geologiska Föreningens i Stockholm Förhandlingar, 120, 373378.
  • MUTVEI, H. 2002a. Nautiloid systematics based on siphuncular structure and position of muscle scars. Abhandlungen der Geologischen Bundesanstalt, 57, 379392.
  • MUTVEI, H. 2002b. Connecting ring structure and its significance for classification of the orthoceratid cephalopods. Acta Palaeontologica Polonica, 47, 157168.
  • MUTVEI, H. and DONOVAN, D. 2006. Siphuncular structure in some fossil coleoids and Recent Spirula. Palaeontology, 49, 685691.
  • MUTVEI, H. and STUMBUR, H. 1971. Remarks on the genus Pictetoceras (Cephalopoda: Ellesmerocerida). Bulletin of the Geological Institutions of the University of Uppsala, New Series, 2, 117122.
  • MUTVEI, H., WEITSCHAT, W., DOGUZHAEVA, L. A. and DUNCA, E. 2004. Connecting ring with pore canals in two genera of Mesozoic ammonoids. Mitteilungen aus dem Geologisch-Paläontologischen Institut der Universität Hamburg, 87, 135144.
  • STUMBUR, H. and MUTVEI, H. 1983. A new Middle Ordovician torticonic nautiloid. Geologiska Föreningens i Stockholm Förhandlingar, 105, 4347.
  • WALCOTT, C. D. 1905. Cambrian faunas of China. Proceedings of the United States National Museum, 29, 1106.
  • WARD, P. D. 1987. The natural history of Nautilus. Allen and Brown, Boston, MA, 267 pp.