The paleobiology of Amphipithecidae, South Asian late Eocene primates

Abstract

Analysis of the teeth, orbital, and gnathic regions of the skull, and fragmentary postcranial bones provides evidence for reconstructing a behavioral profile of Amphipithecidae: Pondaungia, Amphipithecus, Myanmarpithecus (late middle Eocene, Myanmar) and Siamopithecus (late Eocene, Thailand).

At 5–8 kg, Pondaungia, Amphipithecus, and Siamopithecus are perhaps the largest known Eocene primates. The dental and mandibular anatomy suggest that larger-bodied amphipithecids were hard-object feeders. The shape of the mandibular corpus and stiffened symphysis suggest an ability to resist large internal loads during chewing and to recruit significant amounts of muscle force from both the chewing and non-chewing sides of the jaw so as to increase bite force during mastication. The large spatulate upper central incisor of Pondaungia and projecting robust canines of all the larger amphipithecids suggest that incisal food preparation was important. The molars of Siamopithecus, Amphipithecus, and Pondaungia have weak shearing crests. This, and the thick molar enamel found in Pondaungia, suggests a diet of seeds and other hard objects low in fiber. In contrast, Myanmarpithecus was smaller, about 1–2 kg; its cheek teeth suggest a frugivorous diet and do not imply seed eating.

Postcranial bones (humerus, ulna, and calcaneus) of a single large amphipithecid individual from Myanmar suggest an arboreal quadrupedal locomotor style like that of howler monkeys or lorises. The humeral head is rounded, proximally oriented, and the tuberosities are low indicating an extremely mobile glenohumeral joint. The great thickness of the midshaft cortical bone of the humerus implies enhanced ability to resist bending and torsion, as seen among slow moving primate quadrupeds. The elbow joint exhibits articular features for enhanced stability in habitually flexed positions, features also commonly found in slow moving arboreal quadrupeds. The short distal load arm of the calcaneus is consistent with, but not exclusive to, slow arboreal quadrupedalism, and suggests no reliance on habitual leaping. A recently recovered talus of an amphipithecid suggests a possibly more active arboreal quadrupedalism.

Introduction

Large primate taxa Pondaungia, Amphipithecus, and Siamopithecus from the later middle and late Eocene of Myanmar and Thailand form a monophyletic clade called Amphipithecidae (Chaimanee et al., 1997; Ducrocq, 1998, Ducrocq, 1999b). Kay et al. (2004)allocate the smaller Myanmar taxon Myanmarpithecus to this clade. The fossil evidence for these taxa consists mainly of teeth and jaws. One large-bodied amphipithecid specimen from the Pondaung Formation of Myanmar, assignable to either Amphipithecus or Pondaungia, is known from parts of the humerus, ulna, and calcaneus (Ciochon et al., 2001; Gunnell et al., 2002). Another specimen associated with a maxilla of Amphipithecus, preserves a part of the frontal bone (Gunnell et al., 2002; Shigehara et al., 2002; Shigehara and Takai, 2004; Takai et al., 2003). Myanmarpithecus is represented by teeth and jaws (Takai et al., 2001) while Siamopithecus is represented by teeth, jaws and undescribed facial fragments.

A full historical account of the debate about the phyletic position of amphipithecids is provided by Ciochon and Gunnell (2002b)and Takai and Shigehara (2004). First to be described were Pondaungia (Pilgrim, 1927) and Amphipithecus (Colbert, 1937), each known from just a few poorly preserved jaws and cheek teeth. In 1994, two papers (Ciochon and Holroyd, 1994; Godinot, 1994) recognized that Amphipithecus and Pondaungia are closely related taxa within a group now called Amphipithecidae (see Holroyd et al., 2002; Shigehara et al., 2002). Recovery of less fragmentary material of Amphipithecus and Pondaungia (Jaeger et al., 1998a,b; Chaimaneeet al., 2000; Ciochon et al., 2001; Gunnell et al., 2002; Shigehara et al., 2002), the addition of Myanmarpithecus (Takai et al., 2001) and Siamopithecus to the Amphipithecidae (Ducrocq et al., 1995b; Chaimanee et al., 1997; Ducrocq, 1998, Ducrocq, 1999b), as well as better documentation of the orbital anatomy and postcranium has fueled the debate about whether the group has adapoid or anthropoid affinities.

A cladistic analysis of dental, cranial, and postcranial anatomy by Kay et al. (2004)re-examined the phylogenetic puzzle. Kay et al. (2004)reported that it is slightly more parsimonious to root amphipithecids within anthropoids than it is to root them within adapoids. This result is driven largely by similarity in the dental and gnathic regions. In spite of this, they argued that a linkage with anthropoids is questionable because of dissimilarities between the amphipithecid humerus and calcaneus and those of late Eocene and early Oligocene African anthropoids (oligopithecids, parapithecids, and propliopithecids). At the very least, this postcranial evidence seems to indicate that the amphipithecids are outside the clade of the late Eocene-Oligocene African anthropoids. A recently described amphipithecid talus from Myanmar (not included in Kay et al.’s phylogenetic analysis) possesses a number of anthropoid characters strengthening support for the anthropoid status of amphipithecids (Marivaux et al., 2003). On the other hand, claims for the anthropoid status of amphipithecids appear especially to be weakened by the evidence about postorbital closure—a key adaptive innovation assumed to have evolved only once at the base of the Tarsius+anthropoid clade (Cartmill, 1980; Simons and Rasmussen, 1989; but see Beard and MacPhee, 1994; Ross, 1994). The frontal bone indicates that Amphipithecus did not possess postorbital closure (Shigehara et al., 2002; Shigehara and Takai, 2004). Therefore, if amphipithecids are anthropoids, then postorbital closure must have evolved independently twice—or it was lost in amphipithecids. We consider either scenario unlikely. Moreover, the dental evidence suggesting an anthropoid relationship is unconvincingbecause dental similarities lie more with the later, more specialized, anthropoids from the early Oligocene-taxa like Propliopithecus and Aegyptopithecus—whereas amphipithecids are less like the smaller more primitive African anthropoids of similar geologic age (late Eocene) such as oligopithecids or early parapithecids.

Thus, interpreting the phyletic position of amphipithecids depends on the weight one wishes to place on one or another part of the known anatomy. All of us agree that recovery of crucial parts of the basicranium and skeleton could resolve this question.

Whatever the true phylogenetic position ofamphipithecids, the available anatomical evidence tells a very interesting story about their paleobiology. In this paper, we summarize this evidence as a first step toward reconstructing their behavioral profile and the community structure of the late Eocene primates.