Abstract
Analysis of the only complete early cercopithecoid (Old World monkey) endocast currently known, that of 15-million-year (Myr)-old Victoriapithecus, reveals an unexpectedly small endocranial volume (ECV) relative to body size and a large olfactory bulb volume relative to ECV, similar to extant lemurs and Oligocene anthropoids. However, the Victoriapithecus brain has principal and arcuate sulci of the frontal lobe not seen in the stem catarrhine Aegyptopithecus, as well as a distinctive cercopithecoid pattern of gyrification, indicating that cerebral complexity preceded encephalization in cercopithecoids. Since larger ECVs, expanded frontal lobes, and reduced olfactory bulbs are already present in the 17- to 18-Myr-old ape Proconsul these features evolved independently in hominoids (apes) and cercopithecoids and much earlier in the former. Moreover, the order of encephalization and brain reorganization was apparently different in hominoids and cercopithecoids, showing that brain size and cerebral organization evolve independently.
Highlights
Analysis of the only complete early cercopithecoid (Old World monkey) endocast currently known, that of 15-million-year (Myr)-old Victoriapithecus, reveals an unexpectedly small endocranial volume (ECV) relative to body size and a large olfactory bulb volume relative to ECV, similar to extant lemurs and Oligocene anthropoids
Using highresolution computed tomography (CT) we digitally extracted and reconstructed the endocast of KNM-MB 29100 to assess its bearing on the evolutionary relationship between brain size and complexity in the cercopithecoid lineage, and catarrhines in general
The largest postcranial estimates of body mass for Victoriapithecus do not extend higher than 5.0–5.5 kg
Summary
Analysis of the only complete early cercopithecoid (Old World monkey) endocast currently known, that of 15-million-year (Myr)-old Victoriapithecus, reveals an unexpectedly small endocranial volume (ECV) relative to body size and a large olfactory bulb volume relative to ECV, similar to extant lemurs and Oligocene anthropoids. The relationship between the external morphology, cytoarchitecture and function of the brain is better understood for macaques than for other non-human primates because of their extensive use in neuroscience research[1,2,3,4] It is not known when and in what order cercopithecoids evolved their distinctive pattern of cerebral sulci, brain size, relative size of major brain structures such as the olfactory bulbs, and inferred sensory and behavioural adaptations. The absence of complete hominoid and cercopithecoid cranial fossils from between 32 and 7 Myr ago necessitated a reliance on phylogenetic comparative studies of living primates, fossils outside this time period or incomplete fossils to reconstruct such events[5,6] Such evidence indicated that the last common ancestor of cercopithecoids and hominoids had a small olfactory bulb and enhanced visual system, reflecting a change from reliance on olfactory to visual reproductive signalling[7,8,9]. Using highresolution computed tomography (CT) we digitally extracted and reconstructed the endocast of KNM-MB 29100 to assess its bearing on the evolutionary relationship between brain size and complexity in the cercopithecoid lineage, and catarrhines in general
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