Tarphycerida

Abstract: Well preserved nautiloid cephalopods dominate diverse macrofossil assemblages in early and mid Ordovician platform carbonates across Sweden (the so-called ‘Orthoceratite Limestone’ lithofacies). Until now these faunas have been little studied apart from pioneering works by Angelin and Holm during the last century and a few more recent papers by Troedsson, Jaanusson and Mutvei. This paper provides an updated review of Swedish Volkhovian and Kundan nautiloids assigned to the Ellesmerocerida, Endocerida, Actinocerida, Dissidocerida, Orthocerida, Tarphycerida and Oncocerida. Several taxa are key in deciphering the evolution of Ordovician cephalopods, especially early Oncocerida, Endocerida and Orthocerida, and provide further evidence for the systematic position of the lituitid nautiloids.

Abstract: The siphuncular structure is described in twoSilurian taxa, Boionautilus tyrannus and Cumingsoceras com-planatus, currently placed in the Tarpycerida. Tarphyceridshave the Nautilus type of connecting ring that is composedof an outer, thick, spherulitic-prismatic layer and an innerglycoprotein layer, the latter was destroyed by diagenesis.However, both Silurian specimens have the connecting ringof the calcified-perforate type, previously known to occur inorthocerids, actinocerids, plectronocerids and now also inbarrandeocerids. In this type, the inner layer of the connect-ing ring is calcified and perforated by pore canals. Boionauti-lus and Cumingsoceras are therefore classified withbarrandeocerids and not with tarphycerids. Key words: siphuncular structure, connecting ring, Tar-phycerida, Barrandeocerida, nautiloid classification.H itherto , two structural types of connecting ringshave been distinguished in nautiloids: the Nautilus typeand the calcified-perforated type. In the Nautilus type theconnecting ring is composed of an outer spherulitic-pris-matic layer and an inner, glycoprotein (conchiolin) layer.The spherulitic-prismatic layer is a direct continuation ofthat layer in the septal neck, whereas the glycoproteinlayer is an uncalcified, structurally modified continuationof the nacreous layer of the septal neck. In the fossil taxathe inner glycoprotein layer has been destroyed duringfossilization. In addition to recent Nautilus, this type ofconnecting ring occurs in fossil nautilids, tarphycerids,ellesmerocerids, oncocerids and endocerids (Mutvei andStumbur 1971; Stumbur and Mutvei 1983; Mutvei 2002a;Kro¨ger and Mutvei 2005; Kro¨ger and Landing 2008; Kro¨-ger et al. 2009). In the calcified-perforated type the con-necting ring has a two-layered structure. However, theinner layer, which forms a continuation of the nacreouslayer of the septal neck, is calcified, structurally modifiedand perforated by pore canals. The outer spherulitic-pris-matic layer of the connecting ring is usually thin. This typeis only known in the fossil plectronocerids, orthocerids andactinocerids (Mutvei 1997, 1998, 2002b; Kro¨ger 2006;Mutvei et al. 2007). Both types of the connecting ringsseem to have originated in the Upper Cambrian nautiloids(Mutvei et al. 2007; see also Chen and Teichert 1983).The structural type of connecting ring in barrandeocer-ids has been a matter of controversy. Barrandeocerids andtarphycerids both have loosely coiled or evolute, planospi-ral shells, and were therefore considered closely related.However, Flower (1941) pointed out that tarphyceridshad a thick connecting ring similar to that in ellesmero-cerids and endocerids. In his phylogenetic system ofnautiloids Flower (in Flower and Kummel 1950) charac-terized the order Tarphyceratida by thick connecting ringsand the order Barrandeoceratida by thin connecting rings.This classification was followed by Furnish and Glenister(1964) and Sweet (1964). In contrast, some later authors(e.g. Dzik 1984; Turek 2008) have deviated from thisclassification and included barrandeocerids with tarphy-cerids.The siphuncular structure in barrandeocerids is oftenimperfectly preserved and therefore still inadequatelyknown. However, Dr V. Turek found relatively well-preserved Silurian specimens from Sahara, Algeria col-lected by the senior author, and identified them asBoionautilus tyrannus (Barrande) and Cumingsocerascomplanatus (Hisinger). These species were classified byTurek (2008) as tarphycerids. The siphuncular structurein B. tyrannus and C. complanatus is herein describedand compared with that in C. complanatus, collected inthe Island of Gotland, Sweden. In order to determinethe siphuncular type in Boionautilus and Cumingsoceras,a short description of both the Nautilus type and thecalcified-perforate type of the siphuncular structures isgiven.

Abstract: Coiled nautiloid cephalopods from the British Silurian are described and their taxonomy revised. The Order Oncocerida is represented by a single specimen of Oxygonioceras oxynotum . Twelve species from the Order Tarphycerida (within which is here subsumed the Order Barrandeocerida) are described: one of Trocholites , two of Ophioceras , two of Catyrephoceras , three of Lechitrochoceras , one of Peismoceras , two of Systrophoceras , and one of Uranoceras . Their distribution, stratigraphical and geographical, is indicated.

Abstract: Tragoceras falcatum (Schlotheim, 1820) is a common, loosely coiled estonioceratid (Tarphycerida, Cephalopoda) occurring in the Kunda Regional Stage (early Darriwilian, Middle Ordovician) of Estonia. Although the species is quite wellknown, we document some features for the first time. For example, one specimen from the Harku quarry (Estonia) with a phosphatized replacement shell exhibits growth halts (megastriae) on the body chamber. As they are not preserved in smaller specimens, we suggest that these megastriae formed at the approach of maturity, possibly also reflecting sexual dimorphism and cycles of reproduction (iteroparity?). Additionally, the specimen shows minute soft-tissue imprints (drag bands and pseudosutures). These imprints are comparable to patterns in other cephalopods such as ammonoids, bactritids and other nautiloids, but have not yet been reported from Palaeozoic nautiloids. However, they might have been misinterpreted as oncomyarian muscle attachment scars previously. Lastly, we discuss the taphonomy of the specimen, which was encrusted by multiple bryozoan colonies post-mortem. Furthermore, it shows questionable traces of bioerosion. Key words: Tarphycerida, Estonioceratidae, Darriwilian, megastriae, bryozoa, Baltoscandia