New plesiosaur specimens from the Maastrichtian Phosphates of Morocco and their implications for the ecology of the latest Cretaceous marine apex predators

Abstract

Several clades of marine tetrapods, including the apex predators mosasaurs and plesiosaurs, disappeared during the mass extinction at the end of the Cretaceous, about 65My ago. The extreme fossil richness of the Maastrichtian Phosphates of Morocco provides insights into the systematic diversity of the latest mosasaurs where about ten species are known. However, data of the coeval plesiosaurs are comparatively scarce. Up to now, only one species, the elasmosaurid Zarafasaura oceanis, is known. Here we describe new elasmosaurid plesiosaur post-cranial material from the Maastrichtian of the Oulad Abdoun Basin (Morocco) that provides new data about the taxonomical and morphological diversity of plesiosaurs in this area. Most of the new material consists of vertebrae that likely belong to a unique elasmosaurid taxon and differ from all other elasmosaurids documented so far. As Zarafasaura is known only from cranial material, it cannot be determined whether the new material may be assigned to this taxon. The new material shows that the latest Cretaceous plesiosaurs in this low latitude area (about 20°N) were rather gracile, most likely piscivorous taxa that occupied ecological niches similar to those of rather small mosasaurs (e.g., Halisaurus and “Platecarpus” ptychodon) but distinct from those of most coeval large mosasaur taxa. These plesiosaur fossils are also remarkable in that they consist of both juvenile and adult specimens, suggesting limited segregation between individuals of different ontogenetic stages, a feature that might be attributed to upwelling-related, high nutrient input and food availability in this area during the Maastrichtian. Moreover, the possible occurrence, with older specimens, of a neonate specimen – one among the very few known worldwide – suggests a possible social structure organization. The new data contribute to increase our understanding of Late Cretaceous plesiosaur biodiversity and provide new insights into the ecology of latest Cretaceous marine apex predators.