New data on the microvertebrate fauna from the Upper Jurassic or lowest Cretaceous of Ksar Metlili (Anoual Syncline, eastern Morocco)

Abstract

The Middle Jurassic – Early Cretaceous period witnessed the emergence of some major representatives of modern continental vertebrate groups (stem lissamphibians, squamates, therian mammals and birds) and angiosperms, at a time when fragmentation of Pangaea was underway. The successive Moroccan microvertebrate faunas of Ksar Metlili (?Berriasian) and Guelb el Ahmar (Bathonian) from the Anoual Syncline significantly improve our poor knowledge of Gondwanan and especially African palaeobiodiversity at this time. They are among the richest known from the Mesozoic of Gondwana, and are well placed in northwestern Africa to record faunal interchanges with Laurasia. Here we focus on the Ksar Metlili fauna, first documented in the 1980s and most recently resampled in 2010, which produced 24 541 microremains representing 47 species of 8 main groups (Chondrichthyes, Actinopterygii, Sarcopterygii, Lissamphibia, Lepidosauromorpha, Testudinata, Archosauromorpha and Synapsida). It includes remarkable taxa: the oldest stem boreosphenidan mammals from Gondwana, probably some of the last non-mammaliaform cynodonts, a basal ornithischian, possibly freshwater teleosaurid crocodylomorphs, and some of the rare occurrences of choristoderes and albanerpetontids in Gondwana. Comparison of the Ksar Metlili fauna with that of Guimarota (Kimmeridgian, Portugal) further provides evidence of numerous shared taxa of Laurasian affinities, in contrast to the occurrence of few taxa with Gondwanan affinities. This suggests complex palaeobiogeographical relationships – implying both vicariance and dispersal events – of North Africa within Gondwana at the Jurassic–Cretaceous transition. Finally, the faunal similarities with the Guelb el Ahmar fauna question the Cretaceous age of the Ksar Metlili fauna, suggesting an alternative possible Late Jurassic age.

Geology and paleontology of the Upper Cretaceous Kem Kem Group of eastern Morocco

“…Far north Kem Kem equivalents. Red bed units that may also correspond to the Kem Kem beds are situated some 50 km north in the Anoual Syncline in the eastern High Atlas, a basin known for yielding a diverse Lower Cretaceous (Berriasian) vertebrate assemblage including mammals and dinosaurs (Hahn and Hahn 2003, Knoll and Ruiz-Omeñaca 2009, Lasseron et al 2019. The section extends upwards from these fossiliferous horizons to units immediately below a capping Cenomanian-Turonian platform.…”

“…The absence of any body fossils of these two clades is extraordinary for five reasons. First, eutherian mammals are present in earlier deposits on Africa including localities in Morocco (e.g., Sigogneau-Russell 1995, Heinrich 1998, Lasseron et al 2019. Second, scores of fragile pterosaur bones are preserved in both formations of the Kem Kem Group, many of these were as fragile as avian remains.…”

An African Radiation of ‘Dryolestoidea’ (Donodontidae, Cladotheria) and its Significance for Mammalian Evolution

“…The best known and most intensively studied component of the fossil assemblage from the KM site are the mammaliaforms, especially the mammals. At least 19 species have been identified (Lasseron 2020), belonging to haramiyidans, eutriconodontans, non-cladotherian trechnotherians, 'dryolestoids', stem-zatherians ('peramurans') and tribosphenidans (Table 1). It is the richest mammalian fauna known in Africa, comparable to the richest contemporary mammalian faunas from Laurasia.…”

“…Since the time of its discovery in 1983 and the field expeditions that followed in 1986 and 1988, the microvertebrate assemblage from the Upper Jurassic -lowest Cretaceous of the Ksar Metlili site (KM), Anoual Syncline, eastern Morocco (Fig. 1), has emerged as one of the richest and taxonomically most diverse vertebrate assemblages from the Mesozoic of Africa and Gondwana (Sigogneau-Russell et al 1990, 1998Lasseron 2020;Lasseron et al 2020).…”

“…2). Following the new field campaigns, the complete KM vertebrate assemblage has been revised, leading to an update of its faunal list, now including at least 53 distinct species (Lasseron 2019(Lasseron , 2020Lasseron et al 2020).…”