New evidence of xiphosurids from the Middle Jurassic of Morocco: Palaeoenvironmental, palaeoecological and palaeobiogeographical implications

Abstract

© 2018 Elsevier B.V. Horseshoe crabs (Chelicerata, Xiphosura) from Mesozoic deposits are extremely rare in the fossil record of the African continent. Here we report new evidence of the occurrence of xiphosurans in North Africa. These are horseshoe crab traces, including the ichnogenera Kouphichnium (repichnia) and Selenichnites (fodinichnia and/or domichnia), which have been discovered in Middle Jurassic strata of the Imilchil area (Central High Atlas, Morocco). They are preserved on upper and lower bed surfaces of sandy limestones and marls in the upper parts of the Tislit and Imilchil formations (late Bajocian-early Bathonian). The Selenichnites traces, measuring up to 30 cm in width, co-occur with theropod and birdlike tracks. The reported Kouphichnium is the first record from Mesozoic deposits of Morocco, while Selenichnites is the second occurrence in Jurassic strata of Africa. These ichnogenera evidence different behaviour of horseshoe crabs, i.e. Selenichnites is the result of burrowing activity when searching for food in the sediment, and Kouphichnium is a regular locomotion trace left on the sediment surface, occasionally leaving an impression of the telson. The combination of sedimentological data and ichnological analysis indicates a shallow-water subtidal depositional environment preceding the Bathonian regression of the Atlas domain. Palaeobiogeographically, the discoveries indicate the presence of horseshoe crabs at the southern margin of the Tethys. Furthermore, they enhance our knowledge of their previously scarcely documented distribution in Gondwana during the Mesozoic.