Accurately assign fossil droppings to vertebrate spiral intestine types: case of the coprolites from the Kem-Kem beds (Morocco; Cretaceous)

The Kem Kem Geological Group (Morocco) is renowned for its emblematic mid-Cretaceous vertebrate fauna and the high abundance of predators represented among its fossil assemblage. In such predators-dominated context, analysing associated fossil faeces may yield valuable insight on the trophic interactions and biotic dynamics existing at that time in this ecosystem. Here, we analysed the first reported Kem Kem coproassemblage (ca. 200, relatively large, coprolites) in order to assess how their external and internal morphological features allow separating groups assignable to different vertebrate producers. Recognising a majority of spiral-shaped coprolites, we first produce a phylogeny with robust ancestral character state reconstruction for the evolution of the spiral intestine among vertebrates based on our review of the fossil and modern occurrences of this organ. Applying microtomography and thin-sectioning to the dataset, we compare the relevance of different morphological features of the faeces to differentiate among producing taxa, depending on if they are externally or internally observed. Finally, we use faeces size, and inclusions content, to achieve discriminating between producers, providing a relevant framework to enable producer assignation. Our results reveal that both tetrapod and non-tetrapod faeces were similarly deposited and preserved at burial and deposit, but that it was mostly four different types of non-tetrapod producers (with funnel-type spiral guts and one another with a scroll-type spiral gut representing an extinct convergence) which were the most prompt to deposit faeces in the system. Globally, in the sample locality, spiral and non-spiral types were produced by intestine of comparable diameters. The assemblage reveals that external features can be indicative of interspecific differences of the producers, but that equivalent internally measured features are more efficient at indicating groups of producers. The absence of specific types of valves in the assemblage corresponding to likely abundant taxa (e.g. holosteans) could evidence some ancestral state of the spiral valve in the mid-Cretaceous, relative to its inferred evolution among vertebrates.