Biodiversity dynamics during the initial Devonian radiation of ammonoids

Abstract

Ammonoids are an extinct group of externally-shelled cephalopods that appeared during the Early Devonian. The early radiation of these marine invertebrates occurred within a backdrop of environmental conditions repeatedly disrupted by more or less severe crises. To better understand the genesis of these biotic events and their potential abiotic triggers, the present work focuses on the analysis of the biodiversity fluctuations characterizing the evolution of ammonoids during the Devonian Period based on the fossil record from Morocco. The Anti-Atlas of Morocco provides the best record of Devonian ammonoids: the succession is complete and the ammonoid assemblages are well documented in the literature. The taxonomic component of the biodiversity was investigated by analyzing species richness and the associated taxonomic changes at high time resolution (i.e., biozone resolution) and using different analytical methods. Species richness variations through time were computed using a traditional incidence-based binning approach (range-through diversity) and two modern occurrence-based resampling methods (i.e., corrected first-order jackknife and shareholder quorum subsampling). Taxonomic changes were assessed by analyzing variations in extinction and origination rates; this taxonomic volatility was also further evaluated by a poly-cohort analysis. The diversity dynamics of ammonoid species during the Devonian were also explored using network analysis in order to evaluate the possible existence of so-called “evolutionary faunas”. The results reveal a highly fluctuating species richness throughoutthe Devonian, with successive phases of diversification and decline. Peaks of diversity are recorded in the late Emsian, early Eifelian, middle Givetian, late Givetian (Devonian maximum), late Frasnian, and late Famennian. More than 15 episodes of extinction of varying intensity and duration have marked the early evolution of ammonoid; some of those constituted progressive diversity declines, while others were very severe losses or involved sudden faunal replacements. Network analysis highlights the successive diversification of six ammonoid evolutionary faunas of varying longevity. The transitions between these faunas define five major turnover intervals, which respectively match with previously documented Devonian bioevents or crises that are considered to be strongly influenced by global environmental changes: 1) Upper Zlíchov and Daleje events (Emsian), 2) Choteč Event (Eifelian), 3) Kačák Event and Taghanic Crisis (end-Eifelian and middle Givetian, respectively), 4) Kellwasser Crisis (end-Frasnian), 5) Condroz and Enkeberg events (early-middle Famennian boundary). The temporal distribution of the evolutionary faunas corresponds with the phases documented by the analysis of the origination and extinction rates (poly-cohorts) highlighting distinct macroevolutionary dynamics.