Ontogenetic and macroevolutionary patterns of Anisian (Middle Triassic) ammonoids from Nevada, USA

Abstract

The aim of the PhD thesis is to increase our understanding of processes influencing morphologic changes of ammonoids, which are marine, shelled organisms related to modern squids, octopuses, and cuttlefish. To untangle morphologic relationships, ontogenetic and macroevolutionary patterns of Anisian ammonoids of the Fossil Hill Member in NW Nevada, USA were investigated. Therefore, three main localities in two different regions were studied: (1) The Fossil Hill in the Humboldt Range, (2) the Muller Canyon and (3) the Favret Canyon in the Augusta Mountains. The studied sequences are amongst the world’s most famous bearing Anisian ammonoids assemblages. For the palaeoenvironmental reconstruction we measured lithostratigraphic sections in high-resolution (cm-scale) and collected rock samples at all localities. The rock samples provided 186 thin sections. In total we collected more than 8000 fossils, of which more than 7800 are ammonoids. In order to get a better understanding of the morphologic changes of those species, morphometric analyses on ammonoids were paired with palaeoenvironmental observations. The major research aim can therefore be broken down to three objectives: 1) Palaeoenvironmental reconstruction and geochemical analysis of stratigraphic sequences, (2) ammonoid species associations and analysis of alphataxonomy, and (3) ammonoid morphology including the analysis of ontogenetic cross-sections morphology (Traditional and Geometric Morphometric analysis).