A Silurian oncocerid with preserved colour pattern and muscle scars (Nautiloidea)

Abstract

A new genus Euryrizocerina (Nautiloidea, Oncocerida, Oonoceratidae) from the late Silurian strata of Bohemia (Prague Basin) is established. Two species are included within the genus: the type species E. normata (Barrande, 1877) and E. pulchra sp. nov. The genus is striking for two features -the preservation of shell colouration and muscle attachment scars in the type species. These features are reliable for taxonomic and palaeoecological analysis of nautiloids, but due to their only occasional preservation in fossils, the evolutionary trends of these characters remain poorly understood. The colour pattern of Euryrizocerina consists of irregular pulsed transverse bands that clearly differ from the colour patterns known in other oncocerids where rather regular wave-like or zig-zag bands have been observed. It is questionable whether the new type of colouration represents a taxonomic or morphotypic feature, as Euryrizocerina is the first oncocerid to be recognised with a highly curved shell that retains colouration. Oncocerids exhibit multiple paired muscle scars and there is uncertainty whether these represent an original metamery, inherited from a cephalopod ancestor, or a derived feature. Euryrizocerina possesses four pairs of retractor muscle scars, but its ancestor "Oonoceras" fraternum and allied forms, possess multiple pairs of muscle scars around the base of the body-chamber, as do the majority of oncocerids. A similar reduction in the number of muscle scar pairs has been documented in Devonian rutoceratoids. The high evolutionary plasticity of muscle attachments patterns, as indicated by these examples, suggests that the muscle complex and its attachment to the shell was under adaptive control and is more likely to reflect the form of the shell and the mode of life of the organism. It seems likely that shell colouration and the pattern of attachment of the retractor muscles have a much more complex evolutionary history (including several convergences) than was previously supposed. •

A long-lasting steady period of isotopically heavy carbon in the late Silurian ocean: evolution of the δ13 record and its significance for an integrated δ13, graptolite and conodont stratigraphy

“…The first appearence of the index graptolite Pristiograptus fragmentalis is documented at all known sections above the appearance of the Prionopeltis archiaci-Atrypoidea modesta Community (see Přibyl 1983, Kříž et al 1986, Manda et al 2012. The Prionopeltis archiaci-Atrypoidea modesta Community, being the first post-carbon isotope excursion community, was highly diversified and it bears several new faunal groups (see Havlíček & Štorch 1990;Šnajdr 1980;Kříž 1998bKříž , 1999bManda & Turek 2009). For example, during this period the origin of the long-lasting archaeogastropod clade Porcellioidea Koken in Zittel, 1895 is documented which forms part of marine communities for more than 350 Ma (late Silurian-Late Cretaceous; Bandel 1993, Frýda & Blodgett 1998, Frýda & Ferrová 2011.…”

“…The rich cephalopod fauna of this bed had already been studied by Barrande (1865Barrande ( -1877 and elements of several cephalopod lineages are described from this level (Turek 1975(Turek , 1976Kříž et al 1986;Kříž 1998b;Manda & Turek 2009;Manda & Frýda 2010).…”

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

“…Coiled exogastric shells with few whorls (1-4) and a thin marginal siphuncle were also described in some Oonoceratidae (e.g. Eurizoscerina; Manda & Turek 2009c).…”

“…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).…”

Phenotypic plasticity of colour-patterned Plectodonta (Strophomenida, Brachiopoda) from the Lower Devonian of the Barrandian area (Czech Republic)

“…When observed, these finds are often published in particular contributions. In the Barrandian area of the Czech Republic, shells retaining colour pattern are known mostly among the Silurian and Devonian cephalopods (Barrande 1868(Barrande -1870Turek 2009;Manda & Turek 2009a, b, 2011, 2015Turek & Manda 2010, 2012 and gastropods (Perner 1907(Perner , 1911Kříž & Lukeš 1974;Horný & Henry 1999;Jankovský 2003). Colour markings interpreted as the original colour patterns were described also in the hyolith Ottomarites (Valent & Malinky 2008) and the athyridid brachiopod Merista (Kříž & Lukeš 1974), both also sampled in the early Devonian limestone of the Barrandian area.…”