Why so many dipnoans? A multidisciplinary approach on the Lower Cretaceous lungfish record from Tunisia

The Lower Cretaceous record of vertebrates from Africa is problematic as the majority of fossil localities lack adequate stratigraphic and paleoecological data when compared with coeval Laurasian deposits. Thereby, our comprehension of paleocommunities and paleobiogeographic patterns may be affected by the lack of multidisciplinary approach. Among taxonomically and paleoecological significant clades, lungfishes (Sarcopterygii, Dipnoi) are commonly found in the Cretaceous fresh water, brackish and marginal-marine deposits of Gondwana, although identifiable elements are limited to isolated tooth plates. We provide the first taxonomic identification of dipnoans from the Ain el Guettar Formation of southern Tunisia (Oum ed Diab Member, Albian). Identification of tooth plates based on morphological parameters and phylogenetic analyses indicate the co-occurrence in a discrete stratigraphic unit of at least five lineages referable to Equinoxiodus, Neoceratodus, Asiatoceratodus and/or Ferganoceratodus, Ceratodus, and Lavocatodus. This unusually high diversity is unparalleled in the fossil record and is also challenged by an actualistic comparison with extant taxa. We suggest that a series of taphonomic factors significantly inflated observed lungfish diversity in the estuarine and marginal-marine deposits of the Oum ed Diab Member. Therefore, we recognize the fossil fauna as representative of a larger, inland paleo-hydrographic system. This study confirms the paleoecological scenario resulted from the analyses on terrestrial reptiles from the Oum ed Diab Member.