Evolution of calcareous nannoplankton in the Late Triassic: Insights at Eiberg Basin (Austria)

Abstract (Swedish)

Kalkbildande nannoplankton anses vara de mest produktiva kalkproducerande organismerna på jorden och spelar en avgörande roll i den marina kolcykeln. Dessa organismer uppträdde först i västra Neo-Tetishavet under Karn (första etagen i Yngre trias) och har sedan dess varit viktiga bidragare till ackumuleringen av kalciumkarbonat i marina sediment. Sen rät (sista etagen i Yngre trias) representerar ett kritiskt tidsintervall för att undersöka utvecklingen av kalkhaltiga nannofossil. Denna fas är dessutom särskilt betydelsefull eftersom den omges av två stora händelser: framväxten av kalkhaltiga nannoplankton och det första massutdöende de genomgick – det sen-triasiska massutdöendet. Trots denna periods betydelse är många aspekter av de evolutionära utvecklingsvägarna, paleoekologiska och paleomiljömässiga preferenserna samt amt reaktionerna på utdöendet hos kalkhaltiga nannofossil fortfarande otillräckligt förstådda. Denna studie belyser dessa kunskapsluckor genom att undersöka mångfald och förekomst av kalkhaltiga nannofossil i den Yngre triasiska Eiberg-sektionen (Österrike). Med hjälp av svepelektronmikroskopi (SEM) och ljusmikroskopi (LM) genomfördes en detaljerad identifiering och kvantifiering av nannofossil på totalt 59 prover.
Här visar jag att förekomsten nannofossil under Yngre rät i Eiberg-sektionen uppvisar en gradvis ökning. Denna trend blir tydligare mot toppen av sektionen. Ökningen avbryts dock abrupt av en kollaps vid det sen-triasiska massutdöendet (ETE). Tillsammans med ökningen av förekomsten mäts en måttlig ökning i mångfald, kännetecknad av uppkomsten av nya morfotypiska former. Dessutom har två grundläggande frågor rörande fylogeni och paleoekologi besvarats. På fylogenetisk nivå presenteras två signifikanta fynd: 1) Bekräftelsen och klassificeringen av Eoconusphaeraceae som coccoliter, vilka tidigare betraktades som nannoliter. Denna familj representerar sannolikt det ursprungliga släktledet till de jurassiska koniska coccoliterna; 2) Bekräftelsen av Prinsiosphaera triassica crenulata som en distinkt underart till Prinsiosphaera triassica, som först föreslogs av Jafar (1983) men senare ifrågasattes.
När det gäller paleoekologi definierar våra resultat den ekologiska fördelningen av nyckeltaxa med större säkerhet: 1) Eoconusphaera zlambachensis anses nu vara begränsad till nedre delen av den fotiska zonen; 2) Prinsiosphaera triassica triassica frodades i ytvattnet. Detta senare taxon uppvisar en tydlig känslighet för variationer i cystförekomst, där dess förekomst minskar när cystorna ökar och ökar när cystorna minskar. Detta mönster tyder på ett ekologiskt beroende av ytvattnets förhållanden. Denna känslighet stärker tolkningen att det var en ytlevande art som höll sig till den övre delen av den fotiska zonen.
Slutligen tillhandahåller vi nya insikter i biostratigrafin under detta intervall. Särskilt dokumenteras det första uppträdandet (FO) av den jurassiska arten Schizosphaerella punctulata tidigare än vad som tidigare antagits.
Dessa fynd understryker Eiberg-sektionens betydelse för förståelsen av nannofossilen utveckling under sen trias och belyser klimatets och miljöns påverkan under stora utdöendehändelser.

Abstract

Calcareous nannoplankton are considered the most productive calcifying organisms on Earth and play a critical role in the marine carbon cycle. These organisms first appeared in the Western Neo-Tethys Ocean during the Carnian (early Late Triassic) and have since been key contributors to calcium carbonate accumulation in marine sediments. The Late Rhaetian represents a critical interval for investigating the evolution of calcareous nannofossils. Moreover, this stage is particularly significant as it is bounded by two major events: the emergence of calcareous nannoplankton and the first mass extinction they would undergo – the end-Triassic mass extinction. Despite the importance of this period, many aspects of the evolutionary pathways, palaeoecological and palaeoenvironmental preferences, as well as extinction responses of calcareous nannofossils remain unclear. This study addresses these gaps by examining the diversity and abundance of calcareous nannofossils at the classic Upper Triassic Eiberg section (Austria). Using Scanning Electron Microscopy (SEM) and Light Microscopy (LM), detailed nannofossil identification and quantification were performed on 59 samples.

Here, I demonstrate that nannofossil abundance during the upper Rhaetian at the Eiberg section shows a progressive increase. This trend becomes more pronounced towards the top of the section. However, this increase is abruptly interrupted by a collapse at the End-Triassic Mass Extinction (ETE). This increase in abundance is accompanied by a moderate rise in diversity, marked by the appearance of new morphotypes. Furthermore, two fundamental questions concerning phylogeny and palaeoecology have been resolved. On the phylogenetic level, two significant findings are presented: 1) The confirmation and classification of Eoconusphaeraceae as coccoliths, until now considered as nannoliths. This family likely represents the ancestral lineage of Jurassic conical coccoliths; 2) The validation of Prinsiosphaera triassica crenulata as a distinct subspecies of Prinsiosphaera triassica as initially proposed by Jafar (1983) and disputed thereafter. In terms of palaeoecology, our results define the ecological distribution of key taxa with greater certainty: 1) Eoconusphaera zlambachensis is now considered to be confined to the lower photic zone; 2) Prinsiosphaera triassica triassica thrived in surface water. This latter taxon exhibits pronounced sensitivity to variations in cyst abundance with its abundance decreasing as cysts increase and rising as they decline. This pattern suggests an ecological dependence on surface ocean conditions. This sensitivity reinforces the interpretation that it was a surface-dwelling species inhabiting the upper part of the photic zone.
Finally, we provide new insights into the biostratification during this interval. Notably, the First Occurrence (FO) of the Jurassic species Schizosphaerella punctulata is documented earlier than previously thought.

These findings emphasise the significance of the Eiberg section in understanding nannofossil evolution during the Late Triassic and highlight the influence of climatic and environmental changes during major extinction events.

Popular Abstract

Calcareous nannoplankton are microscopic marine organisms (≤30 µm) that produce calcium carbonate, through biomineralisation. They are among the most prolific calcifying microorganisms on Earth and play a vital role in the marine carbon cycle. These organisms first appeared in the Western Neo-Tethys during the Carnian (Late Triassic, ~237–227 million years ago) and flourished in both diversity and productivity throughout the Norian and early Rhaetian (Late Triassic, 227–208.5 million years ago). However, they experienced a near-total extinction around the Triassic-Jurassic boundary (~201.4 million years ago). This makes the Late Rhaetian a critical period for studying their evolution and responses to environmental changes. This study investigates the diversity and abundance of calcareous nannofossils at the Eiberg section in Austria, focusing on the time interval before the End-Triassic mass extinction (ETE), i.e., the Late Rhaetian. Using scanning electron microscopy and light microscopy, 59 samples were analysed in detail.

In my study, I observed a significant increase in nannofossil abundance during the upper Rhaetian, peaking just before their collapse at the ETE. This increase in abundance coincides with a moderate rise in diversity, marked by the emergence of new genera and/or subspecies. Based on detailed morphological analysis, two significant findings are presented: 1) The Eoconusphaeraceae are coccoliths produced by planktonic algae, revising their previous classification as nannoliths (of unknown affinity); 2) Prinsiosphaera triassica crenulata is validated as a distinct subspecies of Prinsiosphaera triassica, resolving a long-standing debate. Furthermore, palaeoecological findings associate Eoconusphaera zlambachensis with the lower photic zone, while Prinsiosphaera triassica triassica is identified as a surface-dwelling species sensitive to variations in calcareous dinoflagellate cysts abundance, highlighting its dependence on surface ocean conditions. Finally, this study records the first occurrence of Schizosphaerella punctulata in the late Rhaetian, earlier than previously known from the Jurassic. The occurrence of this species, known to thrive in cooler waters, suggests either a pre-extinction cooling event or a presence driven by the deepening of the Eiberg Basin.

This study at Eiberg section advanced our understanding of nannofossil evolution during the Late Triassic showing an increase in abundance and diversity before a major collapse in species richness associated with the End-Triassic mass extinction. The data provided is essential for redefining the phylogeny of coccolithophores, establishing palaeoecological preferences for Eoconusphaera zlambachensis and Prinsiosphaera triassica triassica, and revising the stratigraphic occurrence of Schizosphaerella punctulata.

• author: Bouhal, Farah
• supervisor: Sylvain Richoz; Isaline Demangel
• organization: Department of Geology
• course: GEOR02 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Nannofossils, Late Triassic, Eoconusphaeraceae, coccoliths, nannoliths
• publication/series: Dissertations in Geology at Lund University
• report number: 701
• language: English
• id: 9187860
• date added to LUP: 2025-05-09 12:56:43
• date last changed: 2025-05-09 12:56:43