diversification occurred due to environmental changes in the Pleistocene period which allowed for thicker vegetation to appear in the Amazonian rainforest. Fossils Several different fossils have been linked to the genus Saimiri through examinations of their dental and cranial morphology, including the Early Miocene fossil Dolichocebus which was discovered in Gaiman, Argentina and dated to around 20.5 million years ago, and the Middle Miocene fossil of the genus Neosaimiri discovered in La Venta, Colombia, which has been dated to between 12.1 and 12.5 million years ago. Taxonomic classification Originally all squirrel monkeys were considered to be of the same species; they were first divided into two different ‘types’ - ‘Roman’ and ‘Gothic’ - in Paul D. Maclean's article ‘Mirror Display in the Squirrel Monkey, Saimiri sciureus’, published in 1964. Many different opinions on the taxonomic classification of Saimiri boliviensis as a separate species have been published, however various studies conducted by several researchers have concluded that Saimiri boliviensis is one of at most seven different species of Saimiri. Based on the geographical distribution and the morphological and behavioural characteristics of the specimens studied, Hershkovitz proposed the existence of Saimiri boliviensis as a distinct species with two sub-species, Saimiri boliviensis boliviensis and Saimiri boliviensis peruviensis. Based on cranial measurements and coat colouring, Thorington proposed Saimiri sciureus boliviensis as a sub-species of Saimiri sciureus. Another prominent report published in 1993 supports Thorington's classification, however after further investigation into the genetic characteristics of the monkey a more recently published report concludes that Hershkovitz's description is the most accurate. The dispute over the taxonomic and genetic classification of the black-capped squirrel monkey has become increasingly relevant with regards to their use in biomedical research, due to the fact that hybridisation may have an effect on the reproductive capabilities of the monkey, and has the potential to cause a differentiation in its susceptibility to certain pathogens and infections. Alternative or previously proposed taxonomic names include: Saimiri sciureus boliviensis Callithrix boliviensis (monkeys from Bolivia) Callithrix entomophagus (monkeys from Tefé, Brazil) Saimiri boliviensis nigriceps (monkeys from Cosnipata, Peru) Saimiri boliviensis pluvialis (monkeys from the Rio Jurua in Brazil) Distribution and habitat Saimiri displaying the characteristic ‘Roman arch’ facial pattern of the black-capped squirrel monkey have been documented throughout most of Bolivia, northern Peru, and between the Jurua and Purus Rivers in Brazil. They are found in lowland tropical rainforests near water in densely forested and swampy regions.