In 1914 an expedition organized by Theodore Roosevelt and Candido Rondon went to the confluence of the rivers Aripuanã and Roosevelt (then known as the River of Doubt) in Brazilian Amazonia, where three specimens of an unknown marmoset were collected. The skins, sent to the National Museum in Rio de Janeiro, were found in 1993 by Ronaldo Alperin, who described them as a new subspecies, Callithrix argentata marcai. After a taxonomic reassessment of Callithrix the taxon became Mico marcai.

Two expeditions to relocate the species have recently visited the confluence of the rivers Aripuanã and Roosevelt. In January 2012 interviews with local people suggested potential survey sites. Following these leads, we found a group of three M. marcai on the left bank of the Aripuanã river, confirming the continued presence of the species in the type locality. In January 2013, supported by the Conservation Leadership Programme and the Mamirauá Sustainable Development Institute, we made further observations of M. marcai, sighting it 24 times. Mean group size was four, similar to that of other species of Mico.

As elsewhere in Amazonia, habitat loss is the main threat to primates in this region. Forest is being lost through selective logging and expansion of cattle ranching, and now there is a new threat from infrastructure projects. These include the expansion of transport systems and the construction of seven hydroelectric plants in the Aripuanã and Roosevelt basin. These projects will affect eight protected areas and at least five indigenous areas. In the known range of M. marcai about 640 families will be displaced by one of the hydroelectric plants. It has taken almost 20 years to verify that M. marcai still lives in the wild and it is currently categorized as Data Deficient on the IUCN Red List—will it now move immediately into one of the threatened categories?