The Tocantins-Maranhão biogeographic unit is one of the most studied in the Amazon, with large mammal samples deposited in museums. However, this region still presents problems with respect to taxonomic identification and geographic distribution of this group. Some authors consider this region to be a single operational biogeographic unit, although it is composed of two segments separated from each other by the Gurupi River. The objective of the present study is to update knowledge on the diversity of medium and large mammals in the study area and to compare the diversity observed in each segment of this area, verifying the suitability of dividing the region into one or more operational biogeographic units. Despite being well studied, the region is the most degraded in the Amazon. This occurs due to the strong anthropic pressure on the area, characterized mainly by industrial agriculture and cattle ranching for export. A list was prepared, based on the most recent manuals, containing the species expected to occur in the region. To confirm the reliability of these records, a survey of medium and large mammals from the collections of the Museu Paraense Emílio Goeldi, Museu de Zoologia da Universidade de São Paulo, and Museu Nacional, Universidade Federal do Rio de Janeiro, was carried out. The results indicated errors in the distribution of some species, which do not occur in the studied area, contrary to the most recent manuals. The list of expected taxa was composed of 58 species, distributed in 45 genera. Of these, 55 were confirmed through the samples of the mastozoological collections examined, confirming most of the list of expected taxa for the region. Two species, Bradypus tridacty/us and Ozotocerus bezoarticus, were added to the list of mammals present in the study area. The results of the present work confirmed the need for further studies of the medium and large mammal fauna of the Tocantins-Maranhão biogeographic unit. The presence of endemic species, such as Cebus kaapori and Chiropotes satanas, is an indication of the high diversity of mammalian fauna in the region, which makes it urgent to take conservationist measures in order to reduce the anthropogenic impact that has been exerted on this area.
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