A new limulid (Chelicerata, Xiphosurida) from the Late Cretaceous (Cenomanian–Turonian) of Gara Sbaa, southeast Morocco

Abstract

Two specimens of a new species of horseshoe crab, Mesolimulus tafraoutensis sp. nov., are described from the Late Cretaceous (Cenomanian–Turonian) Gara Sbaa Lagerstätte of southeast Morocco. These most likely represent juveniles, as suggested by their small size and possession of a number of characteristics, such as short genal spines, that are characteristic of modern juvenile horseshoe crabs. Despite this, the development of the prosomal keel into a broader cardiac ridge and the scalloped lateral margins of the cardiac lobe clearly place these specimens within Mesolimulus. A further characteristic, the occurrence of only two tubercles on the thoracetron pleural ridges, marks Mesolimulus tafraoutensis sp. nov. as a distinct species. As Mesolimulus resolves phylogenetically as a total group limulid outside of the crown group, the new discovery indicates that stem-lineage limulids persisted into the Cretaceous and co-existed with crown-limulids as they underwent their major radiation.