Mesopithecus delsoni BONIS , BOUVRAIN, GERAADS et KOUFOS, 1990

Mesopithecus delsoni BONIS, BOUVRAIN, GERAADS et KOUFOS, 1990

Text-fig. 3

L o c a l i t y. Thermopigi.

M a t e r i a l. Humerus left SIT 986, tibia right SIT 980, calcaneus left SIT 1100.

D e s c r i p t i o n. The humerus retains a curved diaphysis, with proximal and distal metaphyses ( Text-fig. 3a, Tab. 1). It shows a high degree of modification or deformation due to neotectonic events, which is quite common – it has been observed in several bones of large mammal remains from Thermopigi. Epiphyses are missing, likely due to the young juvenile (neonate?) age of the individual. The diaphysis shows a curvature in the middle and torsion of a relatively large cercopithecid. The maximum height of the chord from the axis of the bone is 15.22 mm. The distal portion is slender, whereas the proximal portion, particularly the collum area, is robust.

The tibia retains the distal end, with a small part of diaphysis ( Text-fig. 3b, Tab. 1). The epiphysis was not fused, and a suture is visible around the entire perimeter of the distal end. The part of the shaft appears relatively robust, is posteriorly rounded and anteriorly flat. The anterior surface is partly eroded. The malleolus tibiae is well developed with a sub-triangular facies articularis malleoli. The facies articularis distalis tibiae is sub-square but narrow, whereas the facies articularis malleoli fibulae is semicircular and wide. The cross-section of diaphysis at 40 mm from distal end is relatively rounded.

The calcaneus is complete, and well preserved ( Text-fig. 3c, Tab. 1). It is robust, with a slightly concave squarishshaped tuber below it, and laterally, a well-marked fossa. The lateral side of the calcaneal body is eroded, and the posteromedial and disto-medial edges of the sustentaculum tali are broken. From the lateral and posterior view, the sustentaculum tali curves anteriorly and medially respectively in relation to the manubrium. The articular surface for the cuboideum is broad and hollow. Detailed descriptions of the calcaneus can be found in Youlatos et al. (in press).

D i s c u s s i o n. The morphology of the humeral shaft implies a rather pronounced deltopectoral ridge, although it is difficult to judge, as it is rather incomplete. The shaft appears relatively robust and seems to be mediolaterally compressed. This morphology is reminiscent of more terrestrial Macaca LACÉPÈDE, 1799 species. All these characters suggest quadrupedal adaptations related to terrestrial activities ( Harrison 1989).

The tibial morphology is similar to most terrestrial cercopithecids (e.g., Papio DESMAREST, 1820 ), but different from the more medially expanded shafts of more arboreal colobines ( DeSilva et al. 2010). The medial malleolus is moderately developed, triangular, and standing at an almost vertical angle to the trochlear facet. The overall morphology is similar to that of most terrestrial cercopithecids like Papio and differs substantially from the medio-laterally robust and protruding malleolus of hominoids ( DeSilva et al. 2010). The malleolus bears a slightly concave articular surface, ending distally at a pronounced ball-shaped area, a feature typical to cercopithecids, but absent in hominoids ( Harrison 1989). Although the antero-distal part of this end is broken, the bulbous malleolar end is reminiscent of that of more terrestrial cercopithecines (e.g., Papio , Theropithecus I. GEOFFROY, 1843 ). This condition is different from that observed in the more arboreal colobines and from that in Paradolichopithecus NECRASOV et al., 1961 from Vatera ( Sondaar et al. 2006). In distal view, the antero-posterior axis of the malleolus is set parasagitally, a condition observed in more semi-terrestrial and terrestrial cercopithecines (e.g., Macaca , Theropithecus ), and is different from the medial tilt of most arboreal colobines (e.g., Colobus ILLIGER, 1811 ) ( Youlatos 1994). The postero-distal part of the epiphysis and the malleolus are slightly damaged, and the groove that accommodates the tendons of the posterior tibialis and the long digital flexor is not visible ( Ford 1986). In this character, SIT-980 is different from Mesopithecus pentelicus , where the groove is deep ( Ingicco 2008), but is similar to most terrestrial cercopithecids. On the distal surface, the trochlear facet has a trapezoid shape, is smoothly concave, and mediolaterally narrow. The latter morphology is typical of cercopithecids ( Harrison 1989), and more particularly that of more terrestrial forms ( Laird et al. 2018). The facet bears an underdeveloped, relatively concave median antero-posterior ridge. The ridge is slightly perpendicular to the medio-lateral plane of the bone, is bordered by shallow medial and lateral depressions for articulation with the trochlear surface of the talus, and ends anteriorly to a V-shaped tubercle. The protuberance of the tubercle is reminiscent of that in Macaca and Theropithecus , and differs from that of more arboreal colobines ( Youlatos 1994, Laird et al. 2018). The posterior border of the trochlear facet is smooth and relatively low. The anterior border appears sharper and relatively higher, and bears the anterior tubercle. On the lateral side of the bone and towards the anterior part of the trochlear facet lies the relatively shallow crescent of the fibular facet. This morphology is similar to that of cercopithecids. Overall, these characters of the distal tibia suggest terrestrial quadrupedal activities.

The calcaneal features indicate a typical cercopithecid calcaneus, morphologically different from extant and fossil hominoids ( Rose 1986, Strasser 1988). The proximal calcaneo-astragalar facet is relatively short, squared off, tightly curved, and tilts obliquely and faces more dorsally, whereas fossil and extant hominoids possess longer, gently curved, and acutely tilted facets ( Langdon 1986, Rose 1986, Strasser 1988, Gebo 1989). Additionally, there is a distinct pressure facet for the fibula-calcaneal ligament on the lateral side of the proximal calcaneo-astragalar facet, a feature absent in hominoids ( Strasser 1988). More distally, the middle and distal calcaneo-astragalar facets on the sustentaculum tali are discontinuous and acutely oriented relative to each other. This condition is different from the confluent facets of fossil and extant hominoids ( Rose 1986, Strasser 1988). The middle facet is almost flat, with a disto-medial orientation, whereas the distal one is also flat and adjacent to the calcaneocuboid facet ( Ford 1988). At the distal end, the calcaneocuboid joint is dorso-plantarly low and relatively flat, with a very shallow medial pit, but not rounded and deeply concave, as in most hominoids ( Rose 1986). These features support the attribution of SIT-1100 to cercopithecids. The overall size of the calcaneus appears larger than that of M. pentelicus from Pikermi, and is more like that of large colobines, such as Semnopithecus DESMAREST, 1822 and Pygathrix LINNAEUS, 1771 . However, as calcaneal features that distinguish between the two subfamilies demonstrate considerable variation, related to both phylogenetic and ecological constraints, it is difficult to classify this calcaneus at a subfamilial level. The calcaneal body is relatively robust and high, and is similar to that of extant colobines, and to a lesser extent to Mesopithecus . It is generally straight, a condition more reminiscent of terrestrial cercopithecines ( Pina et al. 2011). On the dorsal surface of the calcaneal body lies the proximal calcaneo-astragalar facet. In SIT-1100, the facet is quite low, relatively narrow (like in Erythrocebus SCHREBER, 1775 and Semnopithecus ), comparatively short, and tilts disto-medially, similar to that of Papio and Erythrocebus . It is generally reminiscent of the morphologyofsemi-terrestrialandterrestrialcercopithecines. Finally, on the dorso-lateral side of the facet, the pressure facet is smooth and short, but relatively wide. This condition is more similar to that of more terrestrial cercopithecids. The proximal region of the calcaneus is relatively high and curves medially, and its relative size seems intermediate between Papio , Erythrocebus , and Semnopithecus and the more arboreal colobines (e.g., Colobus ) and cercopithecines (e.g., Cercopithecus LINNAEUS, 1758 ). On the lateral side near the proximal end of the calcaneus, SIT-1100 possesses a marked relatively deep fossa that accommodates the calcaneofibular ligament. This condition is similar to that of Mesopithecus and the terrestrial cercopithecines, and probably indicates a strong ligament that counteracts lateral tensions and transversely stabilizes the foot. At the proximal end of the calcaneus, the tuber calcanei, where mm. triceps surae insert, is very wide and relatively low. It ends dorsally on a rounded surface and proximally forms an extended, lightly concave surface. This morphology is similar to that of Mesopithecus from Pikermi and that of more terrestrial (e.g., Erythrocebus ) and semi-terrestrial (e.g., Chlorocecus J. E. GRAY, 1870, Cercocebus É. GEOFFROY, 1812 ) cercopithecines. Compared to Mesopithecus pentelicus from Kryopigi, the SIT-1100 calcaneus has a more squarish tuber than that of KRY, which is distinctly smaller in size and has an oval-shaped tuber, in proximal view. In SIT-1100, the distal region of the calcaneus appears relatively long and morphologically reminiscent of the calcanei of semi-terrestrial Macaca . On the distal and medial part of the calcaneus, the sustentaculum tali, which accommodates the middle and distal astragalo-calcaneal facets, is relatively short and narrow. The middle facet faces medio-plantarly and is relatively flat and relatively steeply inclined. The distal facet is also steeply set and is relatively flat. The overall morphology is similar to that of most terrestrial and semi-terrestrial cercopithecines (e.g., Macaca ). The surface between the facets lacks the prominent fossa observed in some calcanei of Mesopithecus , indicating a weak astragalar neck ligament ( Lewis 1989, Youlatos 1990). On the plantar side of the sustentaculum, the wide and very shallow groove that runs on the latero-plantar side of the calcaneal body suggests a less developed hallucal flexor. On the distal end of the calcaneus, the calcaneocuboid facet of SIT-1100 is quite flat and relatively rounded, with a very shallow medial pit. This morphology is similar to that of Mesopithecus and terrestrial cercopithecines (e.g., Papio , Theropithecus I). The facet appears slightly symmetrical, similar to Papio and Semnopithecus , when compared to the highly asymmetrical facets of more arboreal colobines (e.g., Pygathrix ). Moreover, on the plantar side of the calcaneocuboid facet, there is a quite well-developed plantar tubercle, similar to that observed in more terrestrial cercopithecines, such as Papio and Theropithecus . A well-developed tubercle indicates a strong calcaneocuboid ligament that stabilizes the joint ( Langdon 1986). Overall, these characters most likely suggest quadrupedal adaptations on more stable, terrestrial substrates.

In Greece, Mesopithecus delsoni was established for the first time in Ravin des Zouaves 5, Axios Valley, Macedonia (type locality), of Late Miocene age and biozone MN 11 ( Bonis et al. 1990). In addition, probable occurrences M. cf. delsoni are reported at Perivolaki (MN 12; Koufos 2006b) and Ravin-X in Axios Valley (MN 11; Koufos 2022d). In the revision, Koufos (2022d) referred to Mesopithecus sp. at Nikiti 2 in Chalkidiki (MN 11), and in Perivolaki and Vathylakkos (MN 12). On the other hand, Mesopithecus pentelicus is established at Pikermi in biozone MN 12, as well as at Chomateres (Attica), Dytiko 1, 3 (Axios Valley) and Kryopigi (Chalkidiki). Therefore, based on the presence of this early form of Mesopithecus and the rest of the associated fauna, the stratigraphic age of the Thermopigi locality can be estimated between late MN 11 and pre-Pikermian MN 12 biozones.