CONICET | Buscador de Institutos y Recursos Humanos

Cephalopods are important components of many modern and past marine ecosystems. In addition, externally shelled representatives provide an excellent fossil record dating back to the late Cambrian. In the Early Ordovician, cephalopods experienced a rapid diversification with most of the major groups informally referred to as ?nautiloids? appearing for the first time. Nevertheless, the early evolutionary history and the relationships between those groups remain poorly understood and partly controversial. There exist a number of classification schemes that are partly incompatible with each other (e.g. Dzik 1984; Mutvei 2015; King & Evans 2019). This incongruency is explained by profound differences in the underlying phylogenetic hypotheses, which commonly reflect the author?s opinions on the importance of certain characters rather than being based on a thorough cladistic analysis. To date, cladistics have been only scarcely used in nautiloid research and based on rather limited datasets. Here, we present the first comprehensive phylogenetic analysis of Cambrian and Ordovician nautiloid cephalopods based on a large, newly compiled character matrix. We defined 137 characters and scored them for 163 species from all major groups spanning a time interval of about 50 million years from the Jiangshanian (late Cambrian) to the Hirnantian (Late Ordovician). The species were partly scored first hand and from a thorough survey and critical assessment of the existing literature. In some cases, we replaced missing data by data from congeneric species to maximise the information content of the data set. We analysed the data using both parsimony and Bayesian approaches, with an emphasis on the latter. Bayesian methods are widely applied to molecular data, but they have also gained more attention for morphological data recently, with a rapid development of new methods (Wright 2019). We used the fossilized birth-death model, which incorporates stratigraphic ages sampled through time (Heath et al. 2014). In addition, we applied a relaxed clock model to adjust for rate heterogeneities.The results recover many of the previously recognized groups, indicating that the application of phylogenetic methods on fossil nautiloids is well justified. They may serve as a robust base for a modern phylogenetic classification. Where uncertainties persist, future studies with additional data or refined models may increase accuracy.