The Paris Biota (Spathian, Early Triassic): at the dawn of modern marine faunas?

Started in November 2019

Funding: doctoral grant

Supervisors: Arnaud Brayard and Emmanuel Fara

Defended the 16 december 2022

Abstract

The Permian-Triassic crisis (P-T crisis; ~252 Ma) was undoubtedly the most severe of the Phanerozoic, with the extinction of over 80% of marine species, and about 70% of continental vertebrates. This event also constitutes a crucial turning point in the history of life by marking one of the most important faunal shifts of the marine realm: the transition from Sepkoski’s Paleozoic Evolutionary Faunas, to faunas structured and comprising similar main components as those of today, i.e., Sepkoski’s Modern Evolutionary Faunas. This transition was not instantaneous but took place at least throughout the Early Triassic. During this interval, the environment was very unstable and marked by a succession of smaller-scale biotic crises, the most important of them being that of the late Smithian. This prolonged environmental instability is often assumed to have postponed the post-P-T crisis biotic recovery in the marine realm. Indeed, the current paradigm is that the Early Triassic marine recovery was delayed and sluggish, in particular for the benthos. The biotic recovery is thought to have initiated slowly and stepwise from low to high trophic levels only during the late Early Triassic, with complex ecosystems finally re-emerging only by the middle Anisian, i.e., at least ~6-8 myr after the P-T crisis. Such a slow recovery pattern might be exacerbated by the poorly-known fossil record for this time interval. In such context, a highly unexpected, diversified and complex marine fossil assemblage uncovered in the western USA basin and dated from immediately after the late Smithian crisis was reported in 2017. Named the Paris Biota, its components open a stunning new window on Early Triassic marine ecosystems, the immediate aftermath of the late Smithian crisis, and the rise of Modern Evolutionary Faunas.

In this work, I characterize spatiotemporally the Paris Biota and further explore its taxonomical composition. Through the study of four newly uncovered sites, also from the western USA basin, I show that the Paris Biota extended at least region-wide, and persisted at least partially throughout the Spathian. Additionally, following the description and/or revision of the Paris Biota arthropods, bivalves, and a few other echinoids, the Paris Biota appears considerably richer than initially thought. Therefore, it can no longer be regarded as only an isolated exception, consequently refuting the current paradigm of a delayed and sluggish biotic recovery following the P-T crisis at least at a regional scale, and based on other worldwide evidence, most likely also at a global scale.

Subsequently, I explore the taxonomic diversity trends, throughout the whole Triassic, of three major marine groups: bivalves, echinoderms, and osteichthyes (here in the restricted sense of all marine Actinopterygii and Sarcopterygii). After discussing the diversity trends of each of these groups individually, and highlighting the new features of each, I synthetize the whole and propose, accordingly, a new alternative scenario for the post-P-T crisis biotic recovery. In this scenario, the key points are the following: (i) the recovery was not particularly delayed and was well underway as soon as the late Early Triassic; and (ii) although Sepkoski’s Modern Evolutionary Fauna was present during the Anisian, some groups not considered as components of it, despite being key elements of modern ecosystems (e.g., scleractinian corals, dinoflagellates, and coccolithophores), only appeared or took over following the Carnian pluvial event (early Late Triassic).

In addition, I present here a geochemical-composition based taxonomic identification model I developed using the available Paris Biota material. Using the whole morphology of elemental spectra acquired through Synchrotron μ-XRF, I highlight the existence, within a same biota, of a complex taxon-specific geochemical signature that is provenance- and preservation-independent. This signature may therefore be used as a new taxonomic tool, opening the way to a new field of study that is geochemical taxonomy.

Keywords

Palaeontology; Conservation Lagerstätten; Paris Biota; Spathian; Early Triassic; Taxonomy; Macroevolution

Jury

Marie-Béatrice Forel, Museum national d’Histoire naturelle – reviewer
Bertrand Lefebvre (Chargé de recherche à l’Université Claude Bernard, Lyon 1) – reviewer
Emmanuelle Vennin (Professeure des universités à l’Université de Bourgogne) – examiner
Loïc Villier (Professeur des universités à Sorbonne Université) – examiner
Nicolas Goudemand (Professeur à l’ENS de Lyon) – examiner
William J. Foster (Associate Professor at the University of Hamburg) – examiner
Arnaud Brayard (Directeur de recherche CNRS à l’Université de Bourgogne) – supervisor
Emmanuel Fara (Professeur des universités à l’Université de Bourgogne) – cosupervisor

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The Paris Biota (Spathian, Early Triassic): at the dawn of modern marine faunas?

Started in November 2019

Funding: doctoral grant

Supervisors: Arnaud Brayard and Emmanuel Fara

Defended the 16 december 2022

 

Abstract

The Permian-Triassic crisis (P-T crisis; ~252 Ma) was undoubtedly the most severe of the Phanerozoic, with the extinction of over 80% of marine species, and about 70% of continental vertebrates. This event also constitutes a crucial turning point in the history of life by marking one of the most important faunal shifts of the marine realm: the transition from Sepkoski’s Paleozoic Evolutionary Faunas, to faunas structured and comprising similar main components as those of today, i.e., Sepkoski’s Modern Evolutionary Faunas. This transition was not instantaneous but took place at least throughout the Early Triassic. During this interval, the environment was very unstable and marked by a succession of smaller-scale biotic crises, the most important of them being that of the late Smithian. This prolonged environmental instability is often assumed to have postponed the post-P-T crisis biotic recovery in the marine realm. Indeed, the current paradigm is that the Early Triassic marine recovery was delayed and sluggish, in particular for the benthos. The biotic recovery is thought to have initiated slowly and stepwise from low to high trophic levels only during the late Early Triassic, with complex ecosystems finally re-emerging only by the middle Anisian, i.e., at least ~6-8 myr after the P-T crisis. Such a slow recovery pattern might be exacerbated by the poorly-known fossil record for this time interval. In such context, a highly unexpected, diversified and complex marine fossil assemblage uncovered in the western USA basin and dated from immediately after the late Smithian crisis was reported in 2017. Named the Paris Biota, its components open a stunning new window on Early Triassic marine ecosystems, the immediate aftermath of the late Smithian crisis, and the rise of Modern Evolutionary Faunas.

In this work, I characterize spatiotemporally the Paris Biota and further explore its taxonomical composition. Through the study of four newly uncovered sites, also from the western USA basin, I show that the Paris Biota extended at least region-wide, and persisted at least partially throughout the Spathian. Additionally, following the description and/or revision of the Paris Biota arthropods, bivalves, and a few other echinoids, the Paris Biota appears considerably richer than initially thought. Therefore, it can no longer be regarded as only an isolated exception, consequently refuting the current paradigm of a delayed and sluggish biotic recovery following the P-T crisis at least at a regional scale, and based on other worldwide evidence, most likely also at a global scale.

Subsequently, I explore the taxonomic diversity trends, throughout the whole Triassic, of three major marine groups: bivalves, echinoderms, and osteichthyes (here in the restricted sense of all marine Actinopterygii and Sarcopterygii). After discussing the diversity trends of each of these groups individually, and highlighting the new features of each, I synthetize the whole and propose, accordingly, a new alternative scenario for the post-P-T crisis biotic recovery. In this scenario, the key points are the following: (i) the recovery was not particularly delayed and was well underway as soon as the late Early Triassic; and (ii) although Sepkoski’s Modern Evolutionary Fauna was present during the Anisian, some groups not considered as components of it, despite being key elements of modern ecosystems (e.g., scleractinian corals, dinoflagellates, and coccolithophores), only appeared or took over following the Carnian pluvial event (early Late Triassic).

In addition, I present here a geochemical-composition based taxonomic identification model I developed using the available Paris Biota material. Using the whole morphology of elemental spectra acquired through Synchrotron μ-XRF, I highlight the existence, within a same biota, of a complex taxon-specific geochemical signature that is provenance- and preservation-independent. This signature may therefore be used as a new taxonomic tool, opening the way to a new field of study that is geochemical taxonomy.

 

Keywords

Palaeontology; Conservation Lagerstätten; Paris Biota; Spathian; Early Triassic; Taxonomy; Macroevolution

 

Jury

Marie-Béatrice Forel, Museum national d’Histoire naturelle - reviewer
Bertrand Lefebvre (Chargé de recherche à l’Université Claude Bernard, Lyon 1) - reviewer
Emmanuelle Vennin (Professeure des universités à l’Université de Bourgogne) - examiner
Loïc Villier (Professeur des universités à Sorbonne Université) - examiner
Nicolas Goudemand (Professeur à l’ENS de Lyon) - examiner
William J. Foster (Associate Professor at the University of Hamburg) - examiner
Arnaud Brayard (Directeur de recherche CNRS à l’Université de Bourgogne) - supervisor
Emmanuel Fara (Professeur des universités à l’Université de Bourgogne) - cosupervisor

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