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John Flynn, André Wyss, John Finarelli, and Karen Sears have been studying the brains of the ancestors of modern Neotropical primates and their early Old World fossil simian counterparts. They were able to determine that the brains of platyrrhines of the Americas were as small as those of the catarrhines of Africa and Eurasia, which suggests that there was independent evolution of brain size increase in New and Old World anthropoids.

The published their results in the American Museum of Natural History’s journal, Novitates. The title of the paper is, “Estimating body mass in New World “monkeys” (Platyrrhini, Primates), with a consideration of the Miocene platyrrhine, Chilecebus carrascoensis,” and it is open access. Flynn and colleagues made 80 measurements from the skulls, jaws, and teeth of 17 different species of living New World monkeys that ranged across the full spectrum of body sizes and calculated the encephalization quotients (E.Q.’s) — the ratio of brain size relative to their body size.

They took this matrix of E.Q.’s and applied it on the skull of a primitive primate dating to 20 million years ago, the early platyrrhine Chilecebus carrascoensis. Chilecebus is the oldest and most complete well-dated primate skull from the New World. It was described by Flynn and collaborators in 1995 in this Nature paper, “An Early Miocene anthropoid skull from the Chilean Andes.” Their results of the new paper showed that Chilecebus weighed around 583 grams and had an E.Q. ratio of only 1.11—a much smaller relative brain size than any living New or Old World anthropoid, which have E.Q.’s ranging from 1.39-2.44. This suggests that the larger brain sizes seen in both groups today evolved independently.

    Sears, K.E., Finarelli, J.A., Flynn, J.J., Wyss, A.R. (2008). Estimating body mass in New World “monkeys” (Platyrrhini, Primates), with a consideration of the Miocene platyrrhine, Chilecebus carrascoensis. Novitates, 3617, 1-32.
    Flynn, J.J., Wyss, A.R., Charrier, R., Swisher, C.C. (1995). An Early Miocene anthropoid skull from the Chilean Andes. Nature, 373(6515), 603-607. DOI: 10.1038/373603a0
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