Manda and Turek—Silurian Nautiloidea, Tarphycerida


the genus. This study supports the conclusion that early-hatched tarphycerids possessed minute curved shells, and were plank- tonic, rather than having the coiled shell and life mode of adults. A planktonic habit for Ordovician tarphycerid juveniles was suggested by Holland (1985). This has been documented by Turek and Manda (2016) in Silurian Ophioceras, an evol- utionarily younger tarphycerid, displaying several evolutionary novelties unknown in other tarphycerids, even those from the Silurian. Thus, to expand the hypothesis of a planktonic habit of juveniles to other tarphycerids, more evidence from evolu- tionarily basal tarphycerids was needed.


Geological setting and taphonomy


Studied material comes from Silurian rocks of Central Bohemia (Bohemian Massif, Czech Republic) and Gotland (Baltic Shield, Sweden). The Central Bohemian Paleozoic is a part of the Barrandian


area consisting of Proterozoic and Paleozoic rocks (Teplá- Barrandian Unit, Bohemian Massive), which represents the peri-Gondwanan terrain, Perunica (see Torsvik and Cocks, 2013). Silurian rocks are preserved in the Prague Synform, a structure formed during Variscian Orogeny. Silurian rocks form a part of the marine sedimentary succession of the so-called Prague Basin (Ordovician–Middle Devonian). Five lithostrati- graphic units were established in the Silurian strata (for sum-


mary see Kříž, 1998), of which two yielded Discoceras. The first unit, the Želkovice Formation, is of Rhuddanian and Aeronian age, consists of black laminated graptolitic shales, and has a maximum thickness of ~25m (Kříž, 1998; Štorch, 2006). The shale was deposited under anoxic conditions in an offshore setting; there is no evidence of near-shore conditions (Štorch, 2006). Benthic fauna includes very rare rhynchonelliform and common linguliform brachiopods that were described from the lowermost and uppermost parts of the formation, respectively (Kříž, 1998). Discoceras occurs in black shales that lack benthic fauna. Graptolites indicate the Demirastrites triangulatus, D. simulans, and D. convolutes biozones (as defined by Štorch, 2006). These graptolitic shales contain rare Discinocaris Woodward, 1866 and Peltocaris Salter, 1863 (considered to be cephalopod opercula) (Turek, 1978). Despite over 150 years


of intensive collecting throughout the formation, only two flattened shells and one fragment of Discoceras are available for study. The second unit, the Motol Formation, contains an up


to 300m thick sedimentary succession ranging in age from upper Llandovery to uppermost Wenlock (Kříž, 1998). The formation consists of alternating volcano-sedimentary complex sets and shale facies. The single available flattened specimen of Discoceras comes from shale in the lower part of the formation, which has been referred to the Cyrtograptus murchisoni Biozone as defined by Štorch (1994). Oxygen deficient, offshore graptolite-rich shales dominate the lower part of the formation; the volcanic activity is limited to occasional basalt effusions (Kříž, 1998). Shales and platy muddy limestones with a benthic Niorhinx Community (Havlíček and Štorch, 1990) developed locally in areas of volcanic activity and indicate the presence of low and isolated bottom elevations swept by bottom currents (Kříž, 1998). The community includes


413


rhynchonelliform brachiopods, trilobites, crinoids, gastropods (Havlíček and Štorch, 1990), and cephalopods (straight shelled pelagic forms, the demersal orthocerid Dawsonoceras annulatum [Sowerby, 1818], the discosorid Phragmoceras munthei Hedström, 1917 [Manda, 2008b], and the herein described tarphycerid Discoceras). The Silurian bedrock of Gotland is a remnant of an exten-


sive low-latitude carbonate platform complex that evolved along the margins of the Baltic Basin and extended from the western parts of the present-day Baltic Sea across the East Baltic and farther southeast to Ukraine (for summary see Calner et al., 2002). The majority of the Discoceras specimens were collected by G. Lindström in the nineteenth century; some of his asso- ciated labels indicate his unrealized goal of describing four new species. Precise localities are not specified; rather, the locality is given as the name of the nearest village in the same land registry as the actual locality. Preservational states of the studied speci- mens, however, facilitate identification of their original horizons and locations (Laufeld, 1974; Stridsberg, 1985). All studied specimens were collected from several sites in the lower part of the Slite beds (units d and g), which correspond with the upper Sheinwoodian to lower Homerian, Kockelella walliseri to Ozarkodina sagitta sagitta conodont biozones and the Monograptus belophorus to Cyrtograptus lundgreni graptolite biozones, respectively (Jeppsson et al., 1994; Jeppsson and Calner, 2003). Studied specimens were collected from light- colored stromatoporoid boundstone, crinoidal grainstone, and dolomite—facies that form biohermal, biostromal, and shoal areas of a proximal platform characterized by stromatoporoid- coral reef complexes (for summary see Calner et al., 2002; Calner and Jeppsson, 2003). Most of the studied specimens of Discoceras were removed from their rock matrix, and conse- quently taphonomic information was lost. A few specimens of D. lindstroemi n. sp. were preserved in cephalopod coquina deposited in a reef cavern (Manten, 1971). The coquinas consist of small straight cephalopod shells and rounded fragments of stromatoporoids (Fig. 7.1, 7.4), or a diverse assemblage of cephalopods associated with crinoid, stromatoporoid, and coral fragments (Fig. 7.3, 7.5). It is uncertain whether the coquinas are the result of post-mortem accumulations or represent a natural faunal assemblage. The caverns may have served as hatching grounds (evinced by the accumulation of small shells), a refuge, or hunting grounds for large predators. Based on observations of the collections of the Naturhistoriska Riksmuseet in Stockholm, Discoceras was associated with a diverse cephalopod fauna of orthocerids, discosorids, oncocerids, nautilids, and actinocerids, of which only a small part has been described (for summary see Stridsberg, 1985, 1988a; Stridsberg and Turek 1997). These mostly demersal cephalopods were part of a highly diversified reef and peri-reef fauna including corals, stromatoporoids, bryozoans, crinoids, brachiopods, gastropods, trilobites, and other fauna (for overview see Manten, 1971; Laufeld, 1974; Calner et al., 2002). Several shells of D. lindstroemi n. sp. are infested with microconchid tubes of Annuliconchus sp. (Fig. 6), which are attached to the shell between undulated frills. Apertures of tubes exhibit various orientations, but are usually directed up or down relative to the life orientation of Discoceras. Microconchids attached to both organic and inorganic substrates (Taylor and Vinn, 2006). Their occurrence


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