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Radiolarian phylogeny around the Jurassic-Cretaceous boundary and radiolarian occurrences in the Bosso Valley section, central Italy
Atsushi Matsuoka, Luis O'Dogherty, Špela Goričan, Peter O. Baumgartner, Xin Li, Marco Chiari, Angela Bertinelli

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-07 12:00 PM – 12:20 PM
Last modified: 2018-12-02

Abstract


The Global Boundary Stratotype Section and Point (GSSP) of the Jurassic–Cretaceous boundary (JKB) is the last among the GSSPs in the Phanerozoic. It is defined as the base of the Berriasian Stage. The formal definition was decided in 2016 to use the base of the Calpionella alpina Subzone as the primary marker by the Berriasian Working Group in the International Subcommission on Cretaceous Stratigraphy. The definition is satisfactorily applicable for shallow marine deposits within the western Tethys, north Atlantic and central-south America. Unfortunately, the primary marker taxon cannot be found in the Pacific and circum-Pacific regions since the distribution of Calpionellids is limited to the western Tethys, north Atlantic and central-south America. To determine the base of the Berriasian outside of these regions, alternative markers are needed.

Radiolarians are good candidates for defining the JKB because they are wide spread and can be found both shallow and deep sedimentary facies. Pelagic sequences across the JKB have been reported in ODP/IODP sites in the western Pacific and land sections in Japan, the Philippines, southern Tibet, Iran and others. Evolutionary lineages of several radiolarian taxa across the JKB are reviewed and suitable bioevents, which are approximate to the JKB, are presented. These lineages include the radiolarian genera: Alievium, Archaeodictyomitra, Cinguloturris, Complexapora, Crococapsa, Doliocapsa, Emiluvia, Eucyrtidiellum, Hemicryptocapsa, Hsuum, Loopus, Mirifusus, Mesovallupus, Neorelumbra, Pantanellium, Protovallupus, Protunuma, Pseudodictyomitra, Ristola, Spinosicapsa, Tethysetta, ThanarlaVallupusXitus, Zhamoidellum. Among them the LoopusPseudodictyomitra lineage and the ProtovallupusMesovallupusVallupus lineage can be the most important phylogeny for defining the JKB.

As discussed by Goričan et al. (2018), the evolutionary first appearance datums (FADs) within firmly recognized lineages are extremely valuable. The LoopusPseudodictyomitra lineage is more advantageous than the ProtovallupusMesovallupusVallupus lineage because the former has a much wider paleobiogeographic distribution than the latter. Applying to the so far established radiolarian zones, the JKB lies within the Unitary Association Zone 13 of Baumgartner et al. (1995) and within the Pseudodictyomitra carpatica Zone of Matsuoka (1995). The base of the Pseudodictyomitra carpatica Zone is defined by the evolutionary FAD of Pseudodictyomitra carpatica. Detailed morphological analysis of Loopus and Pseudodictyomitra species is presented and the relationship between the JKB and speciation within the lineage is discussed.

The Bosso Valley section in Umbria–Marche, central Italy, is one of potential candidates for GSSP of the JKB. The Maiolica Formation, which crosses the JKB, is characterized by whitish, beige to gray colored, well-bedded micritic limestones with abundant black to gray chert layers and nodules. Calpionellid stratigraphy and magnetostratigraphy have been studied sufficiently in the section (Housa et al., 2004). The base of the Calpionella alpina Subzone, i.e., the JKB, is placed between Beds 77 and 78  (Housa et al., 2004). We carried out detailed field observations and careful sample collections in a 4-m interval across the JKB. Acid-etched examination of rock samples revealed that well-preserved radiolarians are recognized inside the lime part near the chert layers or nodules. Three samples of micritic limestone below the JKB and two samples above the JKB yield moderatly-preserved radiolarians. The result of radiolarian faunal analysis in the Bosso Valley section is presented.