In the late 1980s two distinct muriqui species were recognized: the northern muriqui (Brachyteles hypoxanthus) and the southern muriqui (B. arachnoides) [2,3]. Note the spider reference in the scientific name of the southern species. Steve Kemper summarizes the current classification and conservation status :
Once called woolly spider monkeys, muriquis occur in two closely related species that scientists didn't officially split until 2000: northern (Brachyteles hypoxanthus) and southern (Brachyteles arachnoides). Both species live only in Brazil, in scattered remnants of the once-vast Atlantic coastal forest, now greatly reduced by clearing for pasture and agricultural land. Because of extensive habitat fragmentation, both muriqui species are classified as endangered, the northern one critically: Only 1,000 of them survive, spread across about a dozen patches of forest [...]
Muriquis are of particular interest in biology and anthropology, since they are capable of great behavioral plasticity. Muriquis display a social and sexual “life style,” that differs from the ruffian behavior observed with many Old World primates. Karen Strier has been studying the critically endangered northern muriqui since 1982. She applies a comparative approach to understand the behavioral ecology of primates, gaining new insights into population viability and making significant contributions to the competition versus cooperation debate in the science of evolution [1,4].
Keywords: primatology, biological anthropology, taxonomy, nomenclature; Animalia > Chordata > Mammalia > Primates > Atelidae.
References and more to explore
 Steve Kemper: No Alpha Males Allowed. Smithsonian September 2013, 44 (6), 38-43. [www.smithsonianmag.com/science-nature/Humans-Would-Be-Better-Off-If-They-Monkeyed-Around-Like-the-Muriquis-220566921.html].
 Encyclopedia of Life: Brachyteles hypoxanthus [eol.org/pages/4472389/hierarchy_entries/24921143/details].
 Encyclopedia of Life: Brachyteles arachnoides [eol.org/pages/323939/overview].
 Department of Anthropology, University of Wisconsin-Madison: Karen B. Strier [www.anthropology.wisc.edu/people_strier.php].