Echinoids from the Callovian of Wallücke (Wiehen Hills, W Germany): convergent adaptations to challenging conditions

Abstract Fossils of sea urchins (class Echinoidea) are generally rare in European shallow-marine deposits from the Callovian (late Middle Jurassic), which are dominated by fine-grained, siliciclastic sedimentation. Taphonomic biases make it difficult to determine whether this rarity reflects genuinely low diversity. Specimens found in obrution deposits help overcome these biases and provide valuable insights into diversity and paleobiology of Jurassic echinoids. This study describes a well-preserved echinoid fauna from the Callovian Ornatenton Formation of the Wiehen Hills. Four species are recorded, with three recognized as new. Procidaris relicta sp. nov. is based on a single specimen and stands as the youngest miocidarid known to date. Polycidaris vadeti sp. nov. is an unusually small polycidarid with distinct spine morphology, represented by hundreds of specimens. Diademopsis wallueckensis sp. nov. is a small pedinid with thin test, also represented by several hundred specimens. The early irregular echinoid Disaster moeschi is recorded from a single layer of the sequence. Both P. vadeti and D. wallueckensis occur in large monospecific accumulations that are interpreted as short-lived mass occurrences preserved by obrution events. The species are convergent in being very small and delicate, yet apparently already sexually mature, indicating adaptations to only temporarily favorable conditions. The Wallücke echinoid fauna shares similarities to faunas preserved under comparable conditions, particularly from the Lower Jurassic, showing that a small number of regular echinoid species were able to successfully adapt to these environments.