Morphological and ontogenetic criteria for defining a trilobite species: The example of Siluro-Devonian Phacopidae

The palaeontological species concept – a rather subjective concept – is based on morphological criteria and carries a notion of time. The delimitation of a species among trilobites does not break this rule and is based on morphological and ontogenetic features. Thus, among phacopid trilobites, characters such as the visual complex and the vincular furrow are diagnostic. Furthermore, quantitative studies of the morphological disparity and ontogenetic trajectories allow us better to define the species and its variability, and to identify the evolutionary patterns established in Phacopidae during 100 Ma of existence.