Stephen J. Gould, les mitrates et les monstres

The interest of analyses based on teratological specimens to understand the mechanisms and rules of ontogenetic development is a central theme in Gould's literature. This paper deals with monsters through two topics focusing on a group of atypical Palaeozoic echinoderms: mitrate stylophorans. The first topic deals with the phyletic position of stylophorans within deuterostomes. Three rival scenarios have been proposed, based on different interpretations of the stylophoran appendage. One of these scenarios implies that mitrates are « hopeful monsters » (calcichordate theory). An analogy with various hypotheses formulated for the disappearance of dinosaurs shows that the calcichordate model is not a scientific hypothesis, unlike the two other interpretations, which can be tested. The second topic is dedicated to the description of an abnormal specimen of the mitrate Mitrocystites mitra, from the Middle Ordovician of Bohemia. The abnormal plating of its lower surface suggests that the insertion of centrals within the posterior region of the marginal thecal frame is an iterative phenomenon in Mitrocystitida.