New pedal remains of Megaladapis and their functional significance

New remains of Megaladapis from the caves within the Ankarana Range of northern Madagascar and the cave site of Ankilitelo near Toliara in southwestern Madagascar add considerably to the present sample of pedal remains for this genus. Here we describe and analyze the new pedal material and discuss the function of the Megaladapis foot in terms of positional behavior and substrate use. The northern specimens belong to the M. madagascariensis/M. grandidieri group in terms of size and morphology, whereas the new southwestern fossils are assigned to M. madagascariensis. The new specimens demonstrate that the small and intermediate sized M. madagascariensis and M. grandidieri were very similar in anatomy and inferred locomotor function, findings that also support the prior suggestion that they belong to a single widespread subgenus (Megaladapis). The new fossils provide the first examples of many pedal elements and present the first opportunity to analyze the whole pedal complex from associated remains. The foot of Megaladapis is distinctive among primates in numerous features. Intrinsic proportions of the hindlimb indicate that the foot is relatively longer than that of any other primate. The first complete calcanei reveal a large and highly modified hindfoot. The calcaneus is reduced distally, indicating an emphasis on climbing over leaping or quadrupedal walking and running. Proximally, a large, medially directed calcaneal tuberosity suggests both a strong inversion component to plantarflexion and a well-developed abductor mechanism and recalls the calcaneal morphology of the larger lorisines in some respects. Talar shape is consistent with considerable tibial rotation during plantarflexion and dorsiflexion. The subtalar joint is designed to emphasize supination/pronation and medial/lateral rotation over proximodistal translation. The distal tarsals are extremely reduced in length, and they form a high transverse arch and a serial tarsus; this configuration promotes inversion/eversion at the transverse tarsal joint. The phalanges are long and moderately curved, and the hallux is very long, robust, and abducted. Pedal morphology suggests that Megaladapis (subgenus Megaladapis) was well adapted to exploit an arboreal environment. The grasping mechanism of Megaladapis is an extreme modification of the prosimian condition, emphasizing a highly inverted set, mobility in rotation, and a powerful abduction/flexion type grasp using large hallux and the lateral abductor musculature. Such a mechanism insures a secure grasp regardless of the position of the hindlimb or the substrate. These pedal design features contrast with the grasping strategy seen in highly arboreal palaeopropithecids (or “sloth lemurs”), a group that reduces and modifies the hindfoot, culminating in Palaeopropithecus, and emphasizes extrinsic digital flexors in a more hook-like mechanism. Much less is known of M. (Peloriadapis) edwardsi. The larger body size, more gorilla-like talar articular morphology, and anatomy of the proximal fifth metatarsal suggest that this species may have been more terrestrial than the smaller forms, but other aspects of pedal morphology suggest it also exploited arboreal habitats.