Romain Jattiot PhD thesis

The Smithian (Early Triassic) ammonoid diversification-extinction cycle

Funding: Swiss National Science Fondation

Supervisors: Arnaud Brayard (laboratoire Biogéosciences) and Hugo Bucher

Beginned in september 2013



The Permian-Triassic (PT) boundary mass extinction (~252 Ma) is known as the largest biodiversity crisis in the history of life, with a loss of about 80-90% of marine species. This event led to the replacement of typical Palaeozoic marine faunas by taxa typical of Modern faunas. Nevertheless, some nekto-pelagic organisms such as ammonoids rapidly recovered during the Early Triassic compared to many other marine organisms. Ammonoids reached taxonomic richness levels much higher than during the Permian in the early-middle Smithian, i.e., only ~1 myr after the PT crisis. Then, the late Smithian witnessed the most severe intra-Triassic crisis affecting the nekton. This late Smithian event turns out to be of equal or larger magnitude than that of the PT boundary mass extinction. Thus, the Smithian is a crucial time interval recording the first global, major ammonoid diversification-extinction cycle after the PT boundary mass extinction. Although Smithian ammonoids have received a steadily increasing attention from the earth science community in the last decade, in-depth investigations on Smithian ammonoid taxonomy, biostratigraphy and biogeography are still of paramount importance in the frame of studies on the Early Triassic biotic recovery. This dissertation particularly focuses on material from Timor Island and from the western USA basin, both areas being recognized as including an exceptional record of Smithian ammonoids.

As a first step towards an all-encompassing understanding of the Smithian ammonoid diversification- extinction cycle, we reinvestigate in the first part of this dissertation the late Smithian ammonoid extinction and its emblematic taxon: Anasibirites Mojsisovics. The number of valid species that should be included in this genus is a highly relevant question when analysing this extinction at the highest possible taxonomic resolution. Taking intraspecific and ontogenetic variations into account, our revised taxonomy of Anasibirites leads to a substantial amendment of the magnitude of this late Smithian event. Accordingly, the severity of this extinction is likely greater than previously assumed.

The second part of this dissertation is devoted to Smithian ammonoids from Palomino Ridge, Nevada. This section was initially discovered in 1967 by Norm Silberling, but its stratigraphic succession and associated faunas have never been described till now. More than a thousand ammonoid specimens were collected with a detailed bedrock stratigraphic control, leading to a monographic work on their taxonomy and systematics. One new genus (Palominoceras) and one new species (Pseudosageceras bullatum) are described. We also provide the first quantitative Smithian ammonoid biochronological scheme for the western USA basin, using the Unitary Associations method.

In the third part of this dissertation, the biogeography of Smithian ammonoids is investigated quantitatively within the western USA basin, i.e., at a regional scale finer than all Early Triassic ammonoid biogeographical studies achieved so far. Previous studies identified a subdivision between the northern part and the southern part of this basin in terms of thickness and dominant lithologies of the sedimentary record, as well as subsidence rates. This N/S subdivision is also qualitatively perceptible in the distribution of ammonoid faunas within the basin, but it remained to be quantitatively analysed based on a large dataset covering the entire basin and as many depositional environments as possible. Potential parameters controlling the distribution and abundance of ammonoids at this mesoscale are discussed.

Finally, in a fourth part, a tremendous collection of Smithian ammonoids from Timor Island was examined. This region has long been recognized for yielding extraordinarily well-preserved Early Triassic ammonoids. The considerable amount of specimens, treated into a comprehensive monograph, made it possible to thoroughly revise their taxonomy, with a special emphasis on ontogeny and covariation between shell shape and ornamentation. Based on the unprecedented assessment of intraspecific variation for many species, their synonymy is discussed in details whenever possible. One new genus (Roopnarinites) and five new species (Paraspidites bicarinatus, Flemingites lidakensis, Subflemingites bihatiense, Baidites obesus and Churkites warei) are also described. These results have a direct impact on the accuracy of diversity counts at the species level, which is important for potential future broad-scale diversity analyses. This material also provides highly valuable data for comprehensive biogeographical and phylogenetic studies hopefully to come.



Ammonoids, Early Triassic biotic recovery, paleobiogeography, taxonomy, Smithian, western USA basin, Timor



M. Arnaud Brayard, Director
M. Hugo Bucher, Co-director
M. Emmanuel Fara, Examiner
M. Marcelo Sanchez, Examiner
M. Neil Landman, Reviewer
M. Dieter Korn, Reviewer