The paleobiology of Amphipithecidae, South Asian late Eocene primates
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.
Section snippets
Geological setting and age
Amphipithecids have been found in Myanmar and Thailand. In Myanmar, the specimens come from several levels of the Pondaung Formation, a package of continental sediments that consists of variegated mudstones and sandstones deposited bymeandering rivers and streams at low elevation adjacent to the northern shore of the Tethys Sea (Aung, 1999).The Tethys in later middle and late Eocene times was a discontinuous strait dividing most of present-day Afro-Arabia to the south from Asia and Europe to
The species
Amphipithecids are known from four or perhaps five species (Table 1): Siamopithecuseocaenus (Thailand), Amphipithecus mogaungensis (Myanmar) and Myanmarpithecus yarshensis (Myanmar), and Pondaungia (Myanmar). The latter seems to be represented by two species, P. cotteri and P. savagei (Takai and Shigehara, 2004), although these could also represent a single sexually dimorphic species. One argument for the former interpretation is that the ratio of the size of the lower canine (or its root
Body size
Determination of body mass for amphipithecids is based on their molar dimensions as presented in Table 2, Table 3.
Based on the relationship among extant primates between lower first or second molar size and body mass, the larger specimens of Pondaungia (P. savagei) had a mass of between 5.3 and 9.3 kg (depending on which taxonomic sample of extant primates is used as the model). An m2 assigned to P. cotteri indicates a weight of 5.2 kg for this species. (Here and elsewhere in the text, upper
Summary and conclusions
Amphipithecidae—Pondaungia, Amphipithecus, and Myanmarpithecus (late middle Eocene, Myanmar) and Siamopithecus (late Eocene, Thailand)—is a clade of middle and late Eocene primates. Opinions are divided as to whether amphipithecids are specialized adapoids or anthropoids.
The upper and lower teeth, mandibular structure, as well as humeral and calcaneal fragments, provide detailed evidence of the behavioral profile of amphipithecids. At 5–8 kg, Pondaungia, Amphipithecus, and Siamopithecus were as
Acknowledgements
This work began when the first author spent four months in Japan as a guest of Prof. N. Shigehara and Dr. M. Takai of the Primate Research Institute of Kyoto University. Also, it is a contribution to the first author's continuing study of world wide climate change towards the end of the Eocene, supported by the US National Science Foundation. We thank Professor Elwyn Simons and Mr. P. Chatrath for access to osteological specimens at the Duke University Primate Center. We thank authorities of
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