The Paramyxoviridae form an increasingly diverse viral family, infecting a wide variety of different hosts and have been, in recent years, linked to disease emergence in many different animal populations and in man. Bats and rodents have been identified as major animal populations capable of harboring paramyxoviruses, and host shifting between these animals is likely to be important driving factor in the underlying evolutionary processes that eventually lead to disease emergence. Here, we have studied paramyxovirus circulation within populations of endemic and introduced wild small mammals of the southwestern Indian Ocean region and belonging to four taxonomic orders: Rodentia, Afrosoricida, Soricomorpha and Chiroptera. We report elevated infection levels, as well as widespread paramyxovirus dispersal and frequent host exchange of a newly emerging genus of the Paramyxoviridae, currently referred to as the Unclassified Morbilli-Related Viruses (UMRVs). In contrast with other genera of the Paramyxoviridae, where bats have been shown to be key host species, we show that rodents (and in particular Rattus rattus) are significant spreaders of UMRVs. We predict that the ecological particularities of the southwestern Indian Ocean, where small mammal species often live in densely packed, multi-species communities, in combination with the increasing invasion of R. rattus and perturbations of endemic animal communities by active anthropological development will have a major influence on the dynamics of UMRV infection. IMPORTANCE: Identification of the infectious agents that circulate within wild animal reservoirs is essential for several reasons: i) Infectious disease outbreaks often originate from wild fauna; ii) Anthropological expansion increases the risk of contact between human and animal populations and hence of disease emergence; iii) Evaluation of pathogen reservoirs helps elaborating preventive measures to limit the risk of disease emergence. Many paramyxoviruses for which bats and rodents serve as major reservoirs, have demonstrated their potential to cause disease in humans and animals. In the context of the biodiversity hot spot of southwestern Indian Ocean islands and their rich endemic fauna, we show that highly diverse Unclassified Morbilli-related viruses exchange between various endemic animal species and their dissemination is likely facilitated by the introduced Rattus rattus. Hence, many members of the Paramyxoviridae appear well adapted for the study of the viral phylodynamics that may potentially be associated with disease emergence.