New paleoecological interpretations of the South American Miocene Mourasuchus (Alligatoroidea, Caimaninae)

Abstract

The South American Caimaninae Mourasuchus has an unusual skull morphology, characterized by a long, broad, flat skull along with long, narrow mandibles with short symphyses and numerous, but small teeth. This peculiar morphology claims for similarly peculiar paleoecological theories to be made for this lineage. Namely, the morphology seen in both the skull and jaw ? which differs from the trade-off proposed for crocodilian food capturing strategies based on cranial morphology by providing neither speed nor strength, but an increase in area to the skull ? precluded in Mourasuchus the capacity to capture and hold large prey in the fashion of modern crocodiles. Some post-cranial structures, such as the shortness of the neck, corroborate this view as they would preclude Mourasuchus to perform strong neck movements, which are also necessary to the ingestion of large prey in modern crocodiles. As such, one of the speculated theories for the alternative feeding habits of Mourasuchus regarded these animals as aquatic, passive filter feeding crocodiles. However, recent studies show that the shortness of the neck also indicates that Mourasuchus had a less hydrodynamic body than what is seen, for example, in an extant semi-aquatic crocodile. In its turn, this indicates not only that Mourasuchus was not capable to hold and dismember large prey, but also that they could have inhabited preferentially shallow water or semi-terrestrial environments, such as a swamp. The diet items could have consisted of fishes ? either small or even large, but slow-moving ones such as Dipnoi (Lepidosiren) ? crustaceans and other small animals or even plants, as suggested in previous works. Swampy environments were abundant during the Miocene in South America, a time and space in which the crocodile fauna was very diverse, and niche and habitat partitioning were essential to the biodiversity and morphological disparity seen in such fauna. In this context, Mourasuchus would contribute to it by occupying not only a different ecological niche, but also a different, specialized habitat ? swampy environments ? leaving other niches and habitats for other crocodilians known for the Miocene of South America such as Purussaurus (big-sized predator), Gryposuchus and Hesperogavialis (longirostrine piscivorous) and Caiman (opportunistic, medium-sized predators). The interpretation of the reconstructions of cranial soft structures in Mourasuchus, as vasculature, sense organs, brain and nerves, together with the musculature of the head and neck will certainly increase the knowledge about the paleoecology of this South-American Miocene bizarre group of Crocodylia.