Primate Morphotype Locomotor Mode

The realization that scandentians, as best represented by Ptilocercus, are probably the closest sister group of primates, has implications for scenarios of primate origins. The present author has always been reluctant to accept the leaping part of the grasp-leaping theory (Dagosto, 1988, 1990; Szalay and Dagosto, 1980, 1988; Szalay and Delson, 1979), for a variety of reasons: (1) the fact that leaping is a behavior that strongly shapes morphology and would subsequently leave strong anatomical signals (e.g., elongated tarsals in Otolemur crassicaudatus); (2) grasp-leaping would presumably require powerful grasping hands, which, as believed, were not present in the primate ancestral morphotype, or a rapid shift to vertical supports and vertical clinging and leaping (leaping on horizontal supports seems mechanically problematic with regard to landing with grasping hands); and (3) grasp-leaping would imply the coordinated reversal of a lot of characters in the early simiiforms, something considered very unlikely during the basal phase of an adaptive radiation. This locomotor reversal was considered likely because it was presumed to be linked to a shift from grasp-leaping toward a more quadrupedal above-branch locomotion, probably also linked with an increase in size (Dagosto, 1990; Gebo, 1986). Taking Ptilocercus as a primitive reference would diminish the support for this view because some of the primitive primate characters (e.g., elongated astragalus with a relatively high body) presumed to reflect grasp-leaping are present in Ptilocercus, while frequent leaping is not; however, it would be important to better document the locomotion of Ptilocercus (expert climber, hopping on the ground, Nowak, 1999; see Le Gros Clark, 1926; Stafford and Thorington, 1998; Sargis, 2001). More importantly, the description of the small tarsals from the Shanghuang fissures provides arguments in favor of the author's view (contra Gebo et al., 2001). One astragalar character in particular, the reduced medial facet, is considered by Gebo et al. (2001) to be primitive for mammals but reversed from a derived prosimian state in eosimiids. This is incredibly unlikely! Now that the character has been documented in

Middle Eocene primates of 50-125 g, it is much more probably primitive for primates, and also for simiiforms. A reappraisal of the tarsal characters of the Shanghuang primates is beyond the scope of this chapter. However, a few other characters need to be mentioned. The more salient sustentaculum of the "protoanthropoid—new taxon" projects medially to a greater degree than in the other taxa, rendering this calcaneum wider than the others. This character is said to be especially "platyrrhine-like" by Gebo et al. (2001). However, it is also Ptilocercus-like, and evidently primitive. The short distal part of the calcaneum of the "new protoanthropoids" is also Ptilocercus-like and probably primitive. The very circular shape of the calcaneocuboid joint in eosimiids is Ptilocercus-like, and thus could be primitive, whereas the removed wedge on the plantar side is derived and appears as one of the rare derived anthropoidlike features of these tarsals. The small Shanghuang tarsals are exciting because they document some new primitive tarsal characters that must be included in the primate morphotype (because primitive characters are not indicative of phylogenetic affinity, their link with simiiforms remains weak). Their comparison with Ptilocercus should enlighten our understanding of early primate locomotor evolution and phylogeny. It is the conviction of this author that leaping and climbing specializations are more accentuated in prosimians, which are derived for the related characters (and many parallel acquisitions of these characters are to be expected during the early radiation of primates), whereas the more quadrupedal eosimiids and "new protoanthropoids" are more similar to Ptilocercus, and more likely approach the primitive primate morphotype. Simiiforms are likely primitive for many tarsal characters; however, tarsals from early simiiforms (= "telanthropoids") will be critical to test this hypothesis (the posterior astragalar shelf present in Ptilocercus and eosimiids needs further study; its absence in simiiforms might be derived). Let us simply add that the simiiform ovoid entocuneiform facet for the first metatarsal is more likely to be derived from a more primitive (? ptilocercine-like) morphology than reverted from the prosimian sellar joint (contra Szalay and Dagosto, 1988). Entocuneiforms might be too small to be found in eosimiids; however, first metatarsals when found should test these ideas.

In sum, the primitive reference offered by Ptilocercus and the eosimiid tarsals lead to further question the leaping component of the grasp-leaping theory and to favor a hypothesis of rapid grasp-quadrupedalism for the primitive primate morphotype locomotor mode. This would be associated with insect manual catching. This hypothesis avoids multiple postcranial character reversals in the origin of simiiforms and considers leaping features as apo-morphies developing in prosimian groups. This sketchy view should be expanded in much more detailed scenarios (see Dagosto, this volume).

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