Boionautilus gen. nov. from the Silurian of Europe and North Africa (Nautiloidea, Tarphycerida)

Abstract

Tarphycerids of the suborder Barrandeocerina are striking owing to the large size of the shell and similarity with the recent <i>Allonautilus</i> Ward & Saunders, 1997. They are widely distributed in the Silurian rocks of Europe. They were originally classified within the genus <i>Nautilus</i> Linnaeus, 1758 but generic assignment remained questionable despite the detailed description and excellent illustration provided by Barrande in 1865. Species belonging to this group have been assigned to the genera <i>Barrandeoceras</i> Hyatt, 1884, <i>Heracloceras</i> Teichert, 1940, or <i>Cumingsoceras</i> Flower, 1950 but none of them is appropriate considering their morphology, phylogenetic trends and stratigraphic range. Therefore, a new genus <i>Boionautilus</i> is established here. An evolutionary trend is discerned, expressed especially in the tighter coiling of the shell - a transition from an evolute shell (gyroceracone at the beginning of the second whorl) with subelliptic cross section, to a slightly involute shell with a subtrapezoidal cross section. The dorsoventral diameter of the whorl increased more rapidly, and septa became more densely spaced. <i>Boionautilus tyrannus</i> (Barrande, 1865) is now known to occur in North Africa (Algeria, the Ougarta Range). Previously-mentioned finds in peri-Gondwanan sediments of the Carnic Alps are here rejected. Differences in the colour of septa and connecting rings in one exceptionally well-preserved specimen of this taxon from Algeria and one Bohemian specimen of <i>B. sternbergi</i> (Barrande, 1865) are remarkable and indicate high primary content of organic matter within the connecting rings

Silurian-Devonian boundary events and their influence on cephalopod evolution: evolutionary significance of cephalopod egg size during mass extinctions

“…In adult animals, the aperture was orientated anteriorly. The best-known example represents the genus Boionautilus, having a shell strongly resembling the presentday Nautilus (Turek 2008). Coiled shells also appeared in some lineages of oncocerids.…”

“…However, the majority of these revisions were not based on studies of the actual fossils, but only on a re-evaluation of Barrande's descriptions and figures (see Manda & Turek 2009a). Only a limited number of the published studies were based on direct examinations of Barrande's original materials plus newly collected cephalopod specimens from Bohemia (Horný 1956(Horný , 1965Marek 1971;Turek 1975Turek , 1976Turek , 2008Turek , 2010Kolebaba 1977;Stridsberg & Turek 1997;Manda & Turek 2009c). A general overview of cephalopods from the Přídolí and Lochkovian was published by Turek and Marek in Kříž et al (1986) and by Manda (2001).…”

“…The vast majority of new revisions of Bohemian cephalopods preserved Barrande's species and the number of species that were synonymised is relatively low (e.g. Marek 1971, Stridsberg 1985, Turek 2008, Manda 2008, Manda & Turek 2009b.…”

An Early Silurian (Aeronian) cephalopod fauna from Kopet-Dagh, north-eastern Iran: including the earliest records of non-orthocerid cephalopods from the Silurian of Northern Gondwana

“…The general paucity of these particular taxa is further indicated by the cephalopod fauna of the Seyahou Formation of the Zagros Mountains, Iran, which consists entirely of members of the Orthoceratoidea (unpublished data). By contrast, representatives of the Oncocerida, Discosorida and Barrandeocerida all occur in Silurian strata (Wenlock to Přídolí) associated with the west Mediterranean sector of peri-Gondwana (Gnoli 1990(Gnoli , 2003Gnoli & Serpagli 1991;Gnoli & Serventi 2009;Histon, 2012c;Kröger 2008;Serpagli & Gnoli 1977) and Perunica (Manda 2007(Manda , 2008Manda & Turek 2009;Turek 2008). Such taxa are generally rare, but may be relatively abundant in some areas for limited intervals.…”

Trocholites Conrad, 1838 (Nautiloidea, Tarphycerida) in the Middle Ordovician of the Prague Basin and its palaeobiogeographical significance

“…Lithuitids originally grouped together with tarphyceratids (see Flower 1950, Furnish & Glenister 1964 in fact comprise an independent clade "Lituitina" Dzik, 1984 within the Orthoceratoidea Khun, 1940(see Dzik 1981, 1984. In addition, some authors considered the order Barrandeocerida Flower, 1950 as a suborder within the Tarphycerida (see Turek 2008), but more data concerning early ontogeny and structure of the siphuncle are needed. Tarphyceratids appeared during the latest early Ordovician and their diversity rapidly reached its maximum just before the Middle Ordovician (Frey et al 2004).…”