The Middle Miocene ape Pierolapithecus catalaunicus exhibits extant great ape-like morphometric affinities on its patella: inferences on knee function and evolution

Abstract

The mosaic nature of the Miocene ape postcranium hinders the reconstruction of the positional behavior and locomotion of these taxa based on isolated elements only. The fossil great ape Pierolapithecus catalaunicus (IPS 21350 skeleton; 11.9 Ma) exhibits a relatively wide and shallow thorax with moderate hand length and phalangeal curvature, dorsally-oriented metacarpophalangeal joints, and loss of ulnocarpal articulation. This evidence reveals enhanced orthograde postures without modern ape-like below-branch suspensory adaptations. Therefore, it has been proposed that natural selection enhanced vertical climbing (and not suspension per se) in Pierolapithecus catalaunicus. Although limb long bones are not available for this species, its patella (IPS 21350.37) can potentially provide insights into its knee function and thus on the complexity of its total morphological pattern. Here we provide a detailed description and morphometric analyses of IPS 21350.37, which are based on four external dimensions intended to capture the overall patellar shape. Our results reveal that the patella of Pierolapithecus is similar to that of extant great apes: proximodistally short, mediolaterally broad and anteroposteriorly thin. Previous biomechanical studies of the anthropoid knee based on the same measurements proposed that the modern great ape patella reflects a mobile knee joint while the long, narrow and thick patella of platyrrhine and especially cercopithecoid monkeys would increase the quadriceps moment arm in knee extension during walking, galloping, climbing and leaping. The patella of Pierolapithecus differs not only from that of monkeys and hylobatids, but also from that of basal hominoids (e.g., Proconsul and Nacholapithecus), which display slightly thinner patellae than extant great apes (the previously-inferred plesiomorphic hominoid condition). If patellar shape in Pierolapithecus is related to modern great ape-like knee function, our results suggest that increased knee mobility might have originally evolved in relation to enhanced climbing capabilities in great apes (such as specialized vertical climbing).

Figure 1. Patella of Pierolapithecus catalaunicus.

IPS 21350.37 is shown in anterior (A), posterior (B), lateral (C), medial (D), proximal (E) and distal (F) views.

Figure 2. Boxplots representing patellar size (GM) and Mosimann shape variables.

A, patellar size (GM); B–F, shape variables standardized by GM based on the four original variables. Vertical lines represent the median, boxes the interquartile range (between 25th and the 75th percentiles), whiskers the extreme values, circles the outliers and asterisks the extreme outliers. Colors indicate the major taxonomic groups: blue, cercopithecoids; red, platyrrhines; orange, hylobatids; green, great apes; purple, humans. Pierolapithecus is highlighted by a star and the grey vertical line in every boxplot is meant to facilitate the visual comparison with the remaining taxa.

Figure 3. Patellar 3D virtual models of selected extant anthropoid genera and Pierolapithecus.

Anterior (left), posterior (middle), and side (right) views of: A, Cercopithecus (right, reversed); B, Colobus (right, reversed); C, Papio (right, reversed); D, Ateles (right, reversed); E, Cebus (left); F, Hylobates (left); G, Symphalangus (right, reversed); H, Gorilla (left); I, Pan (right, reversed); J, Pongo (right, reversed); K, Homo (left); and L, Pierolapithecus (IPS 21350.37, left). Patellae were scanned by a superficial laser scan (provided by P. Ibáñez). Then 3D models were created and scaled to the same proximodistal size for a better visualization. Scale bars equal 1 cm.

Figure 4. Allometric bivariate plots.

A, mediolateral breadth (ML) vs. body mass (BM); B, ML vs. patellar size (GM). The OLS and PGLS allometric regression equations are reported in Table 1; black line denotes female means of non-humans primates OLS regression (see text for further explanation). Because of the isometric relationship between ML and BM, the former can be used as a surrogate of BM (see text).

Figure 5. Between Groups Principal Components Analysis (BgPCA) performed on extant taxa and individual fossil patellae.

The first two axes explain up to 91.6% of the total variance (BgPC1, 61.4%; BgPC2, 30.3%). Major taxonomic groups are indicated by colors as follows: blue, cercopithecoids; red, platyrrhines; orange, hylobatids; green, great apes; purple, humans. Pierolapithecus is highlighted by a star. See Material and methods and Table 1 for more details.