Evolution of locomotion in Anthropoidea: the semicircular canal evidence

Abstract

Our understanding of locomotor evolution in anthropoid primates has been limited to those taxa for which good postcranial fossil material and appropriate modern analogues are available. We report the results of an analysis of semicircular canal size variation in 16 fossil anthropoid species dating from the Late Eocene to the Late Miocene, and use these data to reconstruct evolutionary changes in locomotor adaptations in anthropoid primates over the last 35 Ma. Phylogenetically informed regression analyses of semicircular canal size reveal three important aspects of anthropoid locomotor evolution: (i) the earliest anthropoid primates engaged in relatively slow locomotor behaviours, suggesting that this was the basal anthropoid pattern; (ii) platyrrhines from the Miocene of South America were relatively agile compared with earlier anthropoids; and (iii) while the last common ancestor of cercopithecoids and hominoids likely was relatively slow like earlier stem catarrhines, the results suggest that the basal crown catarrhine may have been a relatively agile animal. The latter scenario would indicate that hominoids of the later Miocene secondarily derived their relatively slow locomotor repertoires.

Figure 1.

Three-dimensional reconstructions of bony labyrinths from several fossil specimens used in this study. Reconstructions are scaled for body mass (BM) based on the primate regression for the mean canal radius. Each view is perpendicular to the depicted canal, labelled as follows in the top row: asc, anterior semicircular canal; psc, posterior semicircular canal; lsc, lateral semicircular canal. Axes on bottom provide general anatomical orientation for each canal view: l, lateral; r, rostral; v, ventral. Note that reconstruction for Apidium phiomense is reversed for display.

Figure 2.

Relationship between lateral semicircular canal size, BM and agility in extant and extinct primates. Legend for extant taxa provided in the figure. Fossil taxa: Cb, Catopithecus browni; Ps, Proteopithecus sylviae; Ap, Apidium phiomense; Az, Aegyptopithecus zeuxis; Pg, Parapithecus grangeri; Dg, Dolichocebus gaimanensis; Hp, Homunculus patagonicus; Lc, Lagonimico conclucatus; Th, Tremacebus harringtoni; Cc, Chilecebus carrascoensis; Sa, Saadanius hijazensis; Vm, Victoriapithecus macinnesi; Ph, Proconsul heseloni; Ob, Oreopithecus bambolii; Hl, Hispanopithecus laietanus; Rb, Rudapithecus hungaricus.

Figure 3.

Cladogram of extant and extinct haplorhine taxa with agility scores mapped onto the branches using the same colour scheme as in table 2. Major clades discussed in the text are noted. Note that strepsirrhines were included in the pGLS analyses, but are not displayed here owing to space. Ancestral agility predictions at nodes and along branches are estimated based on parsimony reconstruction method using Mesquite v. 2.75. Co-author M.L.M. disagrees with the tree as depicted and prefers alternative placement of certain fossil taxa.