[Palaeontology, Vol. 54, Part 3, 2011, pp. 705–710]

SIPHUNCULAR STRUCTURE IN THE ORDERS TARPHYCERIDA AND BARRANDEOCERIDA (CEPHALOPODA: NAUTILOIDEA) by HARRY MUTVEI and ELENA DUNCA Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden; e-mails: [email protected]; [email protected]

Typescript received 14 April 2010; accepted in revised form 22 September 2010

Abstract: The siphuncular structure is described in two orthocerids, actinocerids, plectronocerids and now also in Silurian taxa, Boionautilus tyrannus and Cumingsoceras com- barrandeocerids. In this type, the inner layer of the connect- planatus, currently placed in the Tarpycerida. Tarphycerids ing ring is calcified and perforated by pore canals. Boionauti- have the Nautilus type of connecting ring that is composed lus and Cumingsoceras are therefore classified with of an outer, thick, spherulitic-prismatic layer and an inner barrandeocerids and not with tarphycerids. glycoprotein layer, the latter was destroyed by diagenesis. However, both Silurian specimens have the connecting ring Key words: siphuncular structure, connecting ring, Tar- of the calcified-perforate type, previously known to occur in phycerida, Barrandeocerida, nautiloid classification.

H itherto, two structural types of connecting rings tarphycerids both have loosely coiled or evolute, planospi- have been distinguished in nautiloids: the Nautilus type ral shells, and were therefore considered closely related. and the calcified-perforated type. In the Nautilus type the However, Flower (1941) pointed out that tarphycerids connecting ring is composed of an outer spherulitic-pris- had a thick connecting ring similar to that in ellesmero- matic layer and an inner, glycoprotein (conchiolin) layer. cerids and endocerids. In his phylogenetic system of The spherulitic-prismatic layer is a direct continuation of nautiloids Flower (in Flower and Kummel 1950) charac- that layer in the septal neck, whereas the glycoprotein terized the order Tarphyceratida by thick connecting rings layer is an uncalcified, structurally modified continuation and the order Barrandeoceratida by thin connecting rings. of the nacreous layer of the septal neck. In the fossil taxa This classification was followed by Furnish and Glenister the inner glycoprotein layer has been destroyed during (1964) and Sweet (1964). In contrast, some later authors fossilization. In addition to recent Nautilus, this type of (e.g. Dzik 1984; Turek 2008) have deviated from this connecting ring occurs in fossil nautilids, tarphycerids, classification and included barrandeocerids with tarphy- ellesmerocerids, oncocerids and endocerids (Mutvei and cerids. Stumbur 1971; Stumbur and Mutvei 1983; Mutvei 2002a; The siphuncular structure in barrandeocerids is often Kro¨ger and Mutvei 2005; Kro¨ger and Landing 2008; Kro¨- imperfectly preserved and therefore still inadequately ger et al. 2009). In the calcified-perforated type the con- known. However, Dr V. Turek found relatively well- necting ring has a two-layered structure. However, the preserved Silurian specimens from Sahara, Algeria col- inner layer, which forms a continuation of the nacreous lected by the senior author, and identified them as layer of the septal neck, is calcified, structurally modified Boionautilus tyrannus (Barrande) and Cumingsoceras and perforated by pore canals. The outer spherulitic-pris- complanatus (Hisinger). These species were classified by matic layer of the connecting ring is usually thin. This type Turek (2008) as tarphycerids. The siphuncular structure is only known in the fossil plectronocerids, orthocerids and in B. tyrannus and C. complanatus is herein described actinocerids (Mutvei 1997, 1998, 2002b; Kro¨ger 2006; and compared with that in C. complanatus, collected in Mutvei et al. 2007). Both types of the connecting rings the Island of Gotland, Sweden. In order to determine seem to have originated in the Upper Cambrian nautiloids the siphuncular type in Boionautilus and Cumingsoceras, (Mutvei et al. 2007; see also Chen and Teichert 1983). a short description of both the Nautilus type and the The structural type of connecting ring in barrandeocer- calcified-perforate type of the siphuncular structures is ids has been a matter of controversy. Barrandeocerids and given.

ª The Palaeontological Association doi: 10.1111/j.1475-4983.2011.01041.x 705 706 PALAEONTOLOGY, VOLUME 54

MATERIAL AND METHODS by Turek (2008, figs 3A–C, 7). Cumingsoceras complanatus (Hisinger), collected in Hamra, Island of Gotland, The material includes the following nautiloids Sweden, from the Silurian (Ludlowian).

Barrandeocerids. Boionautilus tyrannus (Barrande) and Orthocerid. Orthoceras regulare (Schlotheim), collected Cumingsoceras complanatus (Hisinger) collected in Sahara, in Harku Quarry, Estonia from the Middle Ordovician NE Ougarta, Algeria, from the Silurian (middle Ludlow- Kunda Stage. ian), Oued Ali Formation, and described and illustrated

ABC

sn sn

conn

conn sn conn

DE F

sn sn

sn

conn conn

conn

TEXT-FIG. 1. A–E, Boionautilus tyrannus, Sahara, Algeria; Oued Ali Formation (Ludlowian). A, median section of the siphuncle to show the septal neck (sn) and connecting ring (conn), ·15. B–E, septal necks (sn) and connecting rings (conn) in higher magnification to show that the septal neck continues into the connecting ring, ·40. F, Cumingsoceras complanatus, Island of Gotland, Sweden; Upper Silurian (Ludlowian); median section of the septal neck (sn) and connecting ring (conn) in higher magnification, ·10. MUTVEI AND DUNCA: SIPHUNCULAR STRUCTURE IN TARPHYCERIDA AND BARRANDEOCERIDA 707

Tarphycerids. Estonioceras sp., collected in Ha¨lludden, The siphuncular structure was studied in polished med- Island of O¨ land, Sweden, and Tragoceras falcatus (Schlot- ian section with a Wild Photomakroskop M 400 at the heim) collected in Kandel, Estonia, both from the Middle Swedish Museum of Natural History, Stockholm. Analysis Ordovician Kunda Stage. of elemental distribution in the shell was made with an

A B

sn

sn nac

conn

spr x

conn

C D

sn

sn nac

x

conn spr conn

TEXT-FIG. 2. A–B, Estonioceras sp., Ha¨lludden, Island of O¨ land, Sweden; Middle Ordovician Kunda Stage. A, median section of the siphuncle to show septal necks (sn) and connecting rings (conn), ·15. B, septal neck (sn) and connecting ring (conn) in higher magnification; note that the connecting ring is composed of a thick spherulitic-prismatic layer (spr); the inner glycoprotein layer is destroyed by diagenesis; its position is marked by x; the nacreous layer (nac) of the septal neck therefore has no continuation into the connecting ring, ·45. C–D, Tragoceras falcatus. Kandel, Estonia; Middle Ordovician Kunda Stage. The septal neck (sn) and connecting ring (conn) have similar structure as those in Estonioceras (see A, B), C = ·15, D = ·45. 708 PALAEONTOLOGY, VOLUME 54

A B

nac

sn

spr sn

conn cp conn

CD E

nac

nac sn sn

spr

cp spr conn conn cp

cp

TEXT-FIG. 3. Orthoceras regulare, Kandel, Estonia; Middle Ordovician Kunda Stage. A, median section of the siphuncle to show septal necks (sn) and connecting rings (conn), ·15. B–C, E, septal neck (sn) and connecting ring (conn) in higher magnification; note that the calcified-perforated layer (cp) of the connecting ring is calcified and has a porous structure being perforated by pore canals; this layer is directly continuous to the nacreous layer (nac) of the septal neck; the spherulitic-prismatic layer (spr) is thin, ·45. D, tangential section of the calcified-perforated layer (cp) of the connecting ring to show the outer openings of the pore canals, ·60. MUTVEI AND DUNCA: SIPHUNCULAR STRUCTURE IN TARPHYCERIDA AND BARRANDEOCERIDA 709 energy dispersive apparatus (EDAX) at the same but shows the same characteristic features as that in Boio- Museum. nautilus. Thus, the principal layer of the connecting ring (conn) is continuous with the nacreous layer of the septal neck (sn) and has about the same thickness (Text-fig. 1F). DESCRIPTION Both the septa and shell wall are yellow. It could not be ascertained whether or not the connecting ring had an Siphuncular structure in Boionautilus tyrannus and outer spherulitic-prismatic layer. Cumingsoceras complanatus

In Boionautilus the septal necks are short and orthocho- The Nautilus type of siphuncular structure in tarphycerid anitic (sn, Text-fig. 1A). Each neck (sn, Text-fig. 1B–E) nautiloids Estonioceras sp. and Tragoceras falcatus consists of an inner nacreous layer and probably also a thin, outer spherulitic-prismatic layer. The nacreous layer As in other fossil nautiloid taxa with the Nautilus type of has a yellow colour similar to that in the septa and shell siphuncle (Mutvei 2002a), the connecting ring in Esto- wall. It contains four percent iron but no phosphorus. nioceras and Tragoceras was composed of an outer spher- The connecting ring (conn, Text-fig. 1A–E) consists ulitic-prismatic layer and an inner glycoprotein layer of a black, calcified layer that contains 18 percent phos- during the lifetime of the animal (sph, gl, Text-fig. 4A). phorus. As seen in Text-figure 1A–E, this layer is a However, the inner glycoprotein layer has been destroyed direct, but structurally modified, continuation of the by diagenesis (Text-fig. 4B). Its original position in Esto- nacreous layer of the septal neck and has the same nioceras and Tragoceras is indicated by x in Text-figs 2B, thickness. It contains several empty spaces of varying D, 4B). sizes that seem to have been remnants of pore canals. It The outer spherulitic-prismatic layer of the connecting extends to the preceding septal neck and covers its ring originates from the outer surface of the septal neck entire inner surface. The black colour does not mean and increases in thickness in the adapical direction (spr, that it has an organic composition because several solid, Text-fig. 2B, D). It is phosphatised and contains 19 per- calcareous shell fragments in the surrounding sediment cent phosphorus. Its structure is well-preserved and have a similar black colour. Several other fragments shows finely granular and prismatic crystalline elements have a yellow colour similar to that of the shell wall without preferred orientation. In the main part of the and septa in our specimen. In addition to the calcified connecting ring this layer is three to four times thicker perforate layer, the connecting ring probably also has a than the adjacent septum (conn, Text-fig. 2A, C). It thin, spherulitic-prismatic outer layer but it is indis- extends adapically to the preceding septal neck and is tinctly visible. attached on its inner surface without forming an auxil- In the shells of Cumingsoceras from Algeria and Sweden iary ridge. the siphuncular structure is diagenetically more altered

TEXT-FIG. 4. Schematic drawings to A BC elucidate differences in the siphuncular structure between tarphycerids and barrandeocerids. A, Nautilus type of the connecting ring in tarphycerids during the lifetime of the animal; note that the connecting ring consisted of the outer nac spherulitic-prismatic layer (spr) and the nac nac inner glycoprotein layer (gl) the latter a continuation from the nacreous layer (nac) of the septal neck. B, the same connecting ring after fossilization; note x that the inner glycoprotein layer (gl in spr A) is destroyed by diagenesis; its spr cp position is indicated by x. C, calcified- spr perforated type of the connecting ring in gl barrandeocerids; note that the inner layer (cp) is calcified and perforated by pores. 710 PALAEONTOLOGY, VOLUME 54

The calcified-perforate type of siphuncular structure in REFERENCES Orthoceras regulare CHEN, J. Y. and TEICHERT, C. 1983. Cambrian cephalo- poda of China. Palaeontographica A, 181, 1–102. In the calcified-perforate type, herein exemplified by DZIK, J. 1984. Phylogeny of the Nautiloidea. Palaeontologia O. regulare, the siphuncular structure differs consider- Polonica, 45, 3–203. ably from that in the tarphycerids Estonioceras and FLOWER, R. H. 1941. Notes on structure and phylogeny of eurys- Tragoceras. In this type the inner layer of the connect- iphonate cephalopods. Palaeontographica Americana, 3, 1–56. ing ring (cp) is a calcified, structurally modified, con- —— and KUMMEL, B. 1950. A classification of the Nautiloi- tinuation from the nacreous layer (nac) of the septal dea. Journal of Paleontology, 24, 604–616. neck (Text-figs 3A–C, E, 4C). This layer is therefore FURNISH, W. M. and GLENISTER, B. F. 1964. 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