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

A Dryopithecine Talus From Abocador de Can Mata (Vallès-Penedès Basin, NE Iberian Peninsula): Morphometric Affinities and Evolutionary Implications for Hominoid Locomotion

OBJECTIVES

The functional interpretation of postcranial remains of Middle Miocene great apes from Europe (dryopithecines) suggests a combination of quadrupedalism and orthograde behaviors without modern analogs. We provide further insights based on an isolated dryopithecine talus (IPS85037) from the Middle Miocene (11.7 Ma) Abocador de Can Mata locality ACM/C8-B* (Vallès-Penedès Basin, NE Iberian Peninsula), which represents the most complete one known to date.

MATERIAL AND METHODS

We compare the specimen with an extant anthropoid sample (n = 68) and the stem hominoid Ekembo heseloni (KMN RU 2036, ~18 Ma, Kenya) using 3D geometric morphometrics. For the two fossil tali, we assess their phenetic affinities using a between-group principal components analysis (bgPCA), estimate body mass based on centroid size, and make locomotor inferences using a partial least-squares regression (PLSR) between talar shape and locomotor repertoire.

RESULTS

Its large inferred body mass (~38 kg) and the possession of several modern hominoid-like features (albeit combined with more plesiomorphic traits) support the attribution of IPS85037 to a male dryopithecine. The bgPCA indicates that IPS85037 falls close to the extant hominoid variation and is less cercopithecoid-like than that of Ekembo, whose inferred locomotor repertoire is vastly dominated by quadrupedalism (81%). In contrast, the locomotor repertoire inferred from IPS85037 combines important quadrupedal (32%) and vertical climbing/clambering (50%) components with only moderate suspension (10%).

DISCUSSION

Our results align with previous inferences derived from other postcranial elements of Middle Miocene dryopithecines and, given their classification as crown hominoids, support the hypothesis that certain suspensory adaptations shared by extant hylobatids and hominids likely evolved independently.

Reanalysis of Samburupithecus reveals similarities to nyanzapithecines

Samburupithecus kiptalami is an ape found in Late Miocene deposits (ca. 9.5 Ma) of northern Kenya. Initial assessments of the holotype specimen (KNM-SH 8531), a female-gorilla-sized maxillary fragment preserving the postcanine tooth row, noted similarities to gorillas or to African apes more broadly. More recently, primitive features of the maxilla and dentition have been used to propose a stem hominoid position for Samburupithecus. In particular, Samburupithecus shares some dental features with orepithecids (nyanzapithecines and Oreopithecus). To evaluate these competing hypotheses, and investigate possible affinities to oreopithecids, we reanalyzed the dentition of Samburupithecus quantitatively and assessed qualitative dental and maxillary features shared by oreopithecids and Samburupithecus. Based on the results of our analyses, we suggest that Samburupithecus is a late-occurring African oreopithecid, which we regard as a long-lived family of stem hominoids. The inclusion of Samburupithecus within Oreopithecidae highlights that stem hominoids and oreopithecids, in particular, spanned a large range of body sizes, similar to the range of size variation seen among all extant apes. Finally, the presence of oreopithecids in Africa on either side of a notable gap in the Late Miocene African fossil record of apes (from ∼13 to 10 Ma) demonstrates that the rarity of fossil African apes (i.e., nonhominin hominines) during this period is likely due to sampling biases rather than a recent immigration back into Africa from Eurasia.

Additional analyses of stem catarrhine and hominoid dental morphology support Kapi ramnagarensis as a stem hylobatid

Fossil gibbons are exceedingly rare, with much of the hylobatid fossil record and, consequently, hylobatid evolutionary history remaining unknown. Kapi ramnagarensis was described as a stem hylobatid on the basis of an isolated lower right M3 from ∼13.0-12.5 Ma deposits surrounding Ramnagar (J&K), India. This interpretation was recently challenged, with alternative hypotheses suggesting that it is instead a stem catarrhine or a strangely derived pliopithecoid that has converged on hylobatid morphology. A series of morphological features were said to distinguish Kapi from fossil and extant hylobatids; notably, however, none of these features were examined or compared using quantitative analyses. Here, we further examine the dental morphology of Kapi, providing quantitative analyses to critically evaluate the hypothesis that Kapi represents a stem catarrhine or pliopithecoid rather than a stem hylobatid. Results demonstrate that none of the claimed differences between Kapi and hylobatids hold up under closer scrutiny, and multivariate discriminant analyses taking size and shape into account strongly support Kapi as a hylobatid with high posterior probabilities. Although only represented by a single lower molar, Kapi remains the most compelling candidate for the earliest known hylobatid in the fossil record and thus likely documents the simultaneous arrival of lesser and great apes to Asia during the Middle Miocene.

First evolutionary insights into the human otolithic system

The human otolithic system (utricle and saccule), housed within the bony vestibule of the inner ear, establishes our sense of balance in conjunction with the semicircular canals. Yet, while the morphological evolution of the semicircular canals is actively explored, comparative morphological analyses of the otolithic system are lacking. This is regrettable because functional links with head orientation suggest the otolithic system could be used to track postural change throughout human evolution and across primates more broadly. In this context, we present the first analysis of the evolution of the human otolithic system within an anthropoid primate setting. Using the vestibule as a morphological proxy for the utricle and saccule, we compare humans to 13 other extant anthropoid species, and use phylogenetically-informed methods to find correlations with body size, endocranial flexion, and head-neck posture. Our results, obtained through micro-CT of 136 inner ears, reveal two major evolutionary transitions in hominoids, leading to distinctive vestibular morphology in humans, characterized by otolithic morphology resembling squirrel monkeys (possibly due to reversal), with a pronounced supraovalic fossa. Finally, we find a positional signal embedded in the anthropoid bony vestibule, providing the foundation to further explore the evolution of human head-neck posture using inner ear morphology.

The Miocene primate Pliobates is a pliopithecoid

The systematic status of the small-bodied catarrhine primate Pliobates cataloniae, from the Miocene (11.6 Ma) of Spain, is controversial because it displays a mosaic of primitive and derived features compared with extant hominoids (apes and humans). Cladistic analyses have recovered Pliobates as either a stem hominoid or as a pliopithecoid stem catarrhine (i.e., preceding the cercopithecoid-hominoid divergence). Here, we describe additional dental remains of P. cataloniae from another locality that display unambiguous synapomorphies of crouzeliid pliopithecoids. Our cladistic analyses support a close phylogenetic link with poorly-known small crouzeliids from Europe based on (cranio)dental characters but recover pliopithecoids as stem hominoids when postcranial characters are included. We conclude that Pliobates is a derived stem catarrhine that shows postcranial convergences with modern apes in the elbow and wrist joints-thus clarifying pliopithecoid evolution and illustrating the plausibility of independent acquisition of postcranial similarities between hylobatids and hominids.