Unbalanced food web in a Late Cretaceous dinosaur assemblage

• Kem Kem fossil vertebrate assemblages are analysed considering stratigraphic data.
• Collecting bias weakly affects the fossil record.
• These assemblages are homogeneous geographically but not stratigraphically.
• The observed unbalanced dinosaur fauna, regardless the stratigraphy, is discussed.
• This disequilibrium is explained partially by the peculiar trophic chain.

The rich assemblage of continental vertebrates from the Cenomanian Kem Kem Beds (Morocco) is one of the best known and most diversified for the mid Cretaceous period (Cenomanian). This assemblage, however, shows apparent ecological oddities, in particular the overabundance of theropod dinosaurs versus plant-eating dinosaurs. Several hypotheses have been proposed to explain this unbalanced ratio, including a peculiar ecosystem, non-systematic collecting, taphonomic factors, stratigraphic uncertainties and/or behavioral aspects of the dinosaur groups concerned. Except the comparison of proportions of taxa between field and shop data ( McGowan and Dyke, 2009), the other hypotheses have not been tested so far because of the lack of accurately measured sections and systematically collected field data. Based on new field data, we test the above-mentioned hypotheses. The analysis is focused on the ratio of plant-eating versus carnivorous dinosaurs.This dataset confirms the unbalanced ratio; moreover, the stratigraphic distribution of fossils is quantitatively not homogeneous and consequently important to avoid time-averaging, i.e. the mixing of fossils of different ages together into a single unit. The origin of the unbalanced food web among dinosaurs is related neither to non-systematic collecting, nor to stratigraphic biases. The palaeoenvironment seems to be the only likely factor to explain the significantly high proportion of carnivorous versus plant-eating dinosaurs. Indeed, the deltaic palaeoenvironment offered unfavourable conditions for the setting of stable terrestrial vegetation but favouring aquatic life. This aquatic life formed the basic level of an aquatic or semi-aquatic food web, which directly fed top predators, such as theropods in general and spinosaurs in particular.