The reconstructed cranium of Pierolapithecus and the evolution of the great ape face

Abstract

Pierolapithecus catalaunicus (~12 million years ago, northeastern Spain) is key to understanding the mosaic nature of hominid (great ape and human) evolution. Notably, its skeleton indicates that an orthograde (upright) body plan preceded suspensory adaptations in hominid evolution. However, there is ongoing debate about this species, partly because the sole known cranium, preserving a nearly complete face, suffers from taphonomic damage. We 1) carried out a micro computerized tomography (CT) based virtual reconstruction of the Pierolapithecus cranium, 2) assessed its morphological affinities using a series of two-dimensional (2D) and three-dimensional (3D) morphometric analyses, and 3) modeled the evolution of key aspects of ape face form. The reconstruction clarifies many aspects of the facial morphology of Pierolapithecus . Our results indicate that it is most similar to great apes (fossil and extant) in overall face shape and size and is morphologically distinct from other Middle Miocene apes. Crown great apes can be distinguished from other taxa in several facial metrics (e.g., low midfacial prognathism, relatively tall faces) and only some of these features are found in Pierolapithecus , which is most consistent with a stem (basal) hominid position. The inferred morphology at all ancestral nodes within the hominoid (ape and human) tree is closer to great apes than to hylobatids (gibbons and siamangs), which are convergent with other smaller anthropoids. Our analyses support a hominid ancestor that was distinct from all extant and fossil hominids in overall facial shape and shared many features with Pierolapithecus . This reconstructed ancestral morphotype represents a testable hypothesis that can be reevaluated as new fossils are discovered.

The Miocene primate Pliobates is a pliopithecoid

“…The cranial morphology of Pliobates is also ambiguous because some similarities with hylobatids (anteriorly situated orbits, broad interorbital distance, short face, low zygomatic roots) are also displayed by the pliopithecoid Epipliopithecus 27 , stem hominoids such as nyanzapithecids 2 , and small-bodied catarrhines from East Africa such as the dendropithecid Micropithecus 28 . It is thus likely that the hylobatid-like features of Pliobates are either symplesiomorphic for crown catarrhines and/or homoplastic among pliopithecoids, hylobatids, and some stem hominoids, maybe being related to small body size 29 . However, while the inclusion of cranial characters does not alter the topology recovered on the basis of teeth, the postcranial dataset has a major influence on the most parsimonious topology, suggesting that, due to rampant homoplasy, it is introducing more ‘noise’ than phylogenetic signal in the case of Pliobates .…”