Marine vertebrate faunas from the Maastrichtian phosphates of Benguérir (Ganntour Basin, Morocco): Biostratigraphy, palaeobiogeography and palaeoecology
Résumé
The Maastrichtian of Benguérir (eastern part of the Ganntour Basin, Morocco) consists of about 20 m ofphosphates displaying an alternance of soft phosphate levels, marly horizons and hard phosphatic limestones.Isolated teeth of selachians, actinopterygians and marine reptiles are extremely numerous in these phosphaticdeposits and have been used for biostratigraphical, palaeodiversity and palaeoecological purposes.Detailedfield work allowed to establish an exhaustive list of the Benguérir marine vertebrate faunas with theirbiostratigraphical distribution throughfive main fossiliferous levels (L6 to L2) spanning all the Maastrichtian.Their importance for biochronological purposes and correlations with other Maastrichtian phosphate depositsworldwide appears noteworthy.The selachians are currently represented by 60 species belonging to 32 genera and 7 orders. Among them, thegenusSqualicoraxis one of the most interesting concerning high-resolution biostratigraphy and correlationswith other phosphate basins because of important rates of change noted between the 5 species recoveredfrom base (e.g. occurrence ofS. africanus) to top (e.g. strong representation ofS. pristodontus)oftheMaastrichtian. The marine reptiles include mainly mosasaurids but also scarcer plesiosaurs, chelonians andcrocodyliforms, representing at least 14 taxa. The mosasaurid squamates are the most abundant and diversifiedwith at least 8 species ranging all along the succession. The actinopterygians include mainly teleosts but alsopycnodonts, also common in all levels and representing at least 7 taxa.Selachians and reptiles show the same trends, in terms of species richness per level, even if the reptiles are lessinformative due to a less diversified assemblage. For sharks, L6 and L2 show a high percentage of genera andspecies occurring only in the layer concerned. The evolution of diversity in actinopterygianfishes is less clear be-cause of their low diversity. The use of dissimilarity indices and agglomerative method underscores two distinctassociations: a lower one including the levels L6 and L5, and an upper one comprising the levels L4 to L2. Thesetwo associations allow to separate a lower and an upper Maastrichtian level and are important for correlations allaround the southern and eastern margins of the Tethys. Another clear faunal turnover occurs between L3 and L2,because of a high appearance rate in L2 (at least in sharks) suggesting an increase in prey abundance, as testifiedby the rapid increase of marine predator density.Indeed, and through L6toL2, apossiblesignalofanenvironmentaldamage affectingthepredator community canbe noted by faunal turnovers, even if no significant change in prey association was clearly detected.From a palaeobiogeographical point of view, the faunal associations of Benguérir appear typical of the southernand eastern margins of the Tethys, with several typical species not occurring in the northern Tethys.