Taksonomija, paleoekologija in paleobiogeografija spodnjejurskih radiolarijev Severnih Apneniških Alp

Cifer, Tim (author), Goričan, Špela (mentor), Gale, Luka (comentor)

V zaporedju kremeničnih apnencev in laporjev na Rettensteinu v Severnih Apneniških Alpah je bila najdena ena najbolje ohranjenih in raznolikih spodnjejurskih radiolarijskih združb zahodne Tetide. V 13 radiolarijskih vzorcih je določenih 200 vrst, ki pripadajo 74 rodovom. Dva rodova in 14 vrst je opisanih prvič. To sta rodova Tetractoma n. gen. in Tipiforma n. gen., ter vrste Arcanicapsa spinosa n. sp., Ares rettensteinensis Cifer, Charlottalum austriacum n. sp., Crucella optima n. sp., Hexapyramis ? sphaericus n. sp., Katroma hasta n. sp., Loupanus pliensbachicus Cifer, Pseudoheliodiscus dispinosus n. sp., Pseudoheliodiscus rotaformis n. sp., Tetractoma tollmanni n. gen. n. sp., Thurstonia ? robusta Cifer, Tipiforma missoniae n. gen. n. sp., Tozerium filzmoosense Cifer, and Trexus rotundus n. sp. Zgornjesinemurijska do spodnjepliensbachijska starost je določena z radiolariji in kalibrirana z amoniti, kalcitnim nanoplanktonom in stabilnimi ogljikovimi izotopi. Podana je primerjava z drugimi dobro ohranjenimi in raznolikimi spodnjejurskimi združbami iz območja zahodne Tetide in Pantalase. Obdobje prehoda sinemurija v pliensbachij je obdobje občutnih sprememb v okolju, ki se odražajo tudi v padcu diverzitete in spremenjeni taksonomski sestavi radiolarijskih združb z Rettensteina. Spremembe so bile verjetno posledica zmanjšane produktivnosti radiolarijev v zgornjem delu vodnega stolpca zaradi aridnega podnebja in zato zmanjšane količine nutrientov. Ob koncu sinemurija je bila verjetno povišana slanost, ker je imel bazen zaradi padca morske gladine slabšo povezavo z odprtim morjem. Glede na povečan ali zmanjšan delež v združbah na prehodu med sinemurijem in pliensbachijem se taksoni delijo na občutljive in na neobčutljive na ta okoljski stres. Naselariji katerih delež se v pliensbachiju zmanjša, so monocirtidni, dicirtidni in multicirditni naselariji, medtem ko se delež tricirtidnih naselarijev poveča. Pri spumelarijih se zmanjša delež družin Pantanelliidae, Emiluviidae in Angulobracchiidae, medtem ko se število primerkov v družinah Xiphostylidae in Conocaryommidae poveča. Saturnalidae se množično pojavijo v najvišjem delu sinemurija, kar lahko pomeni, da uspevajo v bolj zaprtih bazenih in so mogoče odporne na povišano slanost. S primerjavo favne z Rettensteina z združbami nahajališč v Mehiki (Baja Californija), Kanadi (Haida Gwaii) in na Japonskem je bilo mogoče prepoznati razlike med združbami različnih biogeografskih provinc. Razlike se izražajo v prisotnosti ali odsotnosti nekaterih taksonov, razlikah v morfologiji radiolarijev nekaterih rodov in diverziteti združb. Moč je prepoznati razlike med območji nizkih in visokih geografskih širin in razlike med Tetido in Pantalaso. Rezultati potrjujejo obstoj tetidine biogeografske province in pantalasine biogeografske province v spodnji juri.

• Language: Slovenian
• Keywords: Radiolaria, Polycystina, spodnja jura, taksonomija, biostratigrafija, paleoekologija, paleobiogeografija, Severne Apneniške Alpe
• Work type: Doctoral dissertation
• Organization: FGG - Faculty of Civil and Geodetic Engineering
• Year: 2021
• PID: 20.500.12556/RUL-128836
• Publication date in RUL: 07.08.2021

Secondary language

• Language: English
• Title: Taxonomy, palaeoecology and palaeobiogeography of Early Jurassic radiolarians from the Northern Calcareous Alps

Abstract

A hemipelagic succession of siliceous limestone and marl on Mount Rettenstein in the Northern Calcareous Alps yielded one of the best-preserved Early Jurassic radiolarian assemblages of the Western Tethys. Thirteen radiolarian samples yielded 200 species belonging to 74 genera. Two genera and 14 species are newly described: Tetractoma n. gen., Tipiforma n. gen., Arcanicapsa spinosa n. sp., Ares rettensteinensis Cifer, Charlottalum austriacum n. sp., Crucella optima n. sp., Hexapyramis ? sphaericus n. sp., Katroma hasta n. sp., Loupanus pliensbachicus Cifer, Pseudoheliodiscus dispinosus n. sp., Pseudoheliodiscus rotaformis n. sp., Tetractoma tollmanni n. gen. n. sp., Thurstonia ? robusta Cifer, Tipiforma missoniae n. gen. n. sp., Tozerium filzmoosense Cifer, and Trexus rotundus n. sp. A late Sinemurian to Early Pliensbachian age of the radiolarian assemblages is determined with radiolarians and calibrated with ammonoid data, calcareous nannofossils and stable carbon isotopes. The studied assemblages are compared with other rich and well-preserved Early Jurassic radiolarian assemblages of the Western Tethys and Panthalassa. The Sinemurian-Pliensbachian transition was a period of great environmental changes that are reflected in a considerable diversity drop and changes in relative abundances of the studied radiolarian assemblages. The observed faunal break across the boundary indicates lower surface productivity of radiolarians due to decreased nutrient supply induced by arid climate. An increased salinity connected to sea-level drop is inferred at the end of the Sinemurian. Based on their response, radiolarians are classified into sensitive and insensitive to this environmental stress. Nassellaria whose relative abundances decrease above the Sinemurian-Pliensbachian boundary, are monocyrtid, dicyrtid and multicyrtid Nassellaria, whereas the number of tricyrtid Nassellaria increases. Spumellaria with decreased relative abundances are Pantanelliidae, Emiluviidae and Angulobracchiidae, whereas the number of Xiphostylidae and Conocaryommidae increases. Shortly below the Sinemurian Pliensbachian boundary Saturnalidae are notably abundant; they appear to be resistant to or even thrive under increased salinity. With comparing the studied fauna with diverse faunas from Baja California Sur (Mexico), Haida Gwaii (Canada) and Japan, significant differences between biogeographic provinces are recognized. These differences are reflected in the presence or absence of certain taxa, total diversity and by morphological characteristics within some genera. Differences can be observed between low-latitude and mid- to high latitude assemblages, and between the Tethys and Panthalassa. The present results support the existence of the Tethyan biogeographic province and a Panthalassan biogeographic province in the Early Jurassic.