Endocranial cast and morphology of the olfactory bulb of Amphipithecus mogaungensis (latest middle Eocene of Myanmar)

Early anthropoid primates: New data and new questions

Although the evolutionary history of anthropoid primates (monkeys, apes, and humans) appears relatively well-documented, there is limited data available regarding their origins and early evolution. We review and discuss here the earliest records of anthropoid primates from Asia, Africa, and South America. New fossils provide strong support for the Asian origin of anthropoid primates. However, the earliest recorded anthropoids from Africa and South America are still subject to debate, and the early evolution and dispersal of platyrhines to South America remain unclear. Because of the rarity and incomplete nature of many stem anthropoid taxa, establishing the phylogenetic relationships among the earliest anthropoids remains challenging. Nonetheless, by examining evidence from anthropoids and other mammalian groups, we demonstrate that several dispersal events occurred between South Asia and Afro-Arabia during the middle Eocene to the early Oligocene. It is possible that a microplate situated in the middle of the Neotethys Ocean significantly reduced the distance of overseas dispersal.

Amphipithecine primates are stem anthropoids: cranial and postcranial evidence

Since their discovery in 1927, the phylogenetic status of the Myanmar amphipithecines has been highly debated. These fossil primates are recognized either as anthropoids or as adapiform strepsirrhines. This uncertainty was largely the consequence of a limited fossil record consisting mostly of jaw fragments but lacking the critical cranial elements that might resolve this debate. We report here cranial remains associated with an ulna from a single individual pertaining to the amphipithecine Ganlea megacanina. In addition to anthropoid-like dentognathic characters, Ganlea displays several ulna and skull features that testify to its anthropoid affinities (e.g. short subvertically oriented lacrimal duct, lacrimal foramen and bone inside the orbit, maxillary contribution to the lower orbital rim, fused metopic suture). By contrast to crown anthropoids, however, Ganlea lacks postorbital closure, confirming that postorbital closure appeared later than many anthropoid dentognathic characters and evolved convergently in extant tarsiers and anthropoids. Thus, amphipithecines must now be recognized as stem anthropoids offering a unique window on the early evolution of cranial and skeletal features in anthropoids, and reinforcing the hypothesis of an origin and early diversification of anthropoids in Asia.

The face of Siamopithecus: new geometric-morphometric evidence for its anthropoid status

Amphipithecids assume a key position in early primate evolution in Asia. Here we report on new maxillofacial and associated mandibular remains of Siamopithecus eocaenus, an amphipithecid primate from the Late Eocene of Krabi (Thailand) that currently represents the most complete specimen belonging to this group. We used synchrotron microtomography and techniques of virtual reconstruction to recover the three-dimensional morphology of the specimen. Geometric-morphometric analysis of the reconstructed specimen within a comparative sample of recent and fossil primates clearly associates Siamopithecus with the anthropoids. Like modern anthropoids, Siamopithecus displays a relatively short face and highly convergent and frontated orbits, the lower rim of which lies well above the alveolar plane. The cooccurrence of spatially correlated anthropoid features and classical anthropoid dental characters in one individual represents a strong argument to support the anthropoid status of Siamopithecus. It is, thus, highly unlikely that amphipithecids are specialized adapiforms exhibiting complete convergence with anthropoids.

A new primate from the Eocene Pondaung Formation of Myanmar and the monophyly of Burmese amphipithecids

The family Amphipithecidae is one of the two fossil primate taxa from Asia that appear to be early members of the anthropoid clade. Ganlea megacanina, gen. et sp. nov., is a new amphipithecid from the late middle Eocene Pondaung Formation of central Myanmar. The holotype of Ganlea is distinctive in having a relatively enormous lower canine showing heavy apical wear, indicating an important functional role of the lower canine in food preparation and ingestion. A phylogenetic analysis of amphipithecid relationships suggests that Ganlea is the sister taxon of Myanmarpithecus, a relatively small-bodied taxon that has often, but not always, been included in Amphipithecidae. Pondaungia is the sister taxon of the Ganlea + Myanmarpithecus clade. All three Pondaung amphipithecid genera are monophyletic with respect to Siamopithecus, which is the most basal amphipithecid currently known. The inclusion of Myanmarpithecus in Amphipithecidae diminishes the likelihood that amphipithecids are specially related to adapiform primates. Extremely heavy apical wear has been documented on the lower canines of all three genera of Burmese amphipithecids. This distinctive wear pattern suggests that Burmese amphipithecids were an endemic radiation of hard object feeders that may have been ecological analogues of living New World pitheciin monkeys.

Anatomy of the bony pelvis of a relatively large-bodied strepsirrhine primate from the late middle Eocene Pondaung Formation (central Myanmar)

Recent survey of the fossiliferous variegated mudstones of the PK1 locality (Sabapondaung) in the late middle Eocene Pondaung Formation (central Myanmar) has led to the recovery of a partial right innominate of a relatively large-bodied primate. Given its size and provenance, this bone probably belongs to the same individual represented by the NMMP 20 primate partial skeleton described previously from the same locality. The new fossil, which preserves the region around the acetabulum and the adjacent part of the ilium, clearly exhibits strepsirrhine rather than anthropoid affinities. This addition to our knowledge of the NMMP 20 partial skeleton allows us to reassess the different locomotor interpretations that have been proposed for this specimen. Aspects of pelvic morphology suggest that the NMMP 20 partial skeleton documents a primate that probably engaged in active arboreal quadrupedalism similar to that practiced by medium-sized Malagasy lemurids rather than lorislike slow moving and climbing. Given the conflicting phylogenetic signals provided by NMMP 39 (a talus showing anthropoid affinities) and NMMP 20 (a partial skeleton bearing adapiform affinities), it appears that two higher-level taxonomic groups of relatively large-bodied primates are documented in the Pondaung Formation. The recent discovery of two taxa of sivaladapid adapiforms from the Pondaung Formation indicates that the assumption that the NMMP 20 partial skeleton belongs to an amphipithecid can no longer be sustained. Instead, this specimen apparently documents a third large-bodied sivaladapid species in the Pondaung Formation.

Taxonomic status of purported primate frontal bones from the Eocene Pondaung Formation of Myanmar

Two isolated cranial fragments from the late middle Eocene Pondaung Formation of central Myanmar have previously been interpreted as frontal bones of the amphipithecid primate Amphipithecus mogaungensis. Aside from a few maxillary fragments, these specimens provide the only potential source of information currently available regarding the cranial anatomy of Amphipithecidae. Were this taxonomic attribution correct, these specimens would indicate that amphipithecids retained numerous primitive skull features, including the absence of a postorbital septum, the retention of a voluminous olfactory chamber, and strong separation between the forebrain and the orbital fossa. However, several anatomical details observable on these specimens are incompatible with their attribution to any primate and strongly suggest that they cannot be ascribed to Mammalia. Particularly problematic in this regard are the extreme thickness of the dermal bone, the odd structure of the alleged "frontal trigon," and the mediolateral orientation and uniquely robust construction of the descending process of the frontal bone (which partially segregates the orbital and temporal fossae). Because these isolated elements can no longer be attributed to Amphipithecus, the anatomical, phylogenetic, and behavioral inferences regarding amphipithecid paleobiology that have been drawn from these specimens can no longer be sustained.

Distribution of olfactory epithelium in the primate nasal cavity: are microsmia and macrosmia valid morphological concepts?

The terms "microsmatic" and "macrosmatic" are used to compare species with greater versus lesser olfactory capabilities, such as carnivores compared to certain primates. These categories have been morphologically defined based on the size of olfactory bulb and surface area of olfactory epithelium in the nasal fossa. The present study examines assumptions regarding the morphological relationship of bony elements to the olfactory mucosa, the utility of olfactory epithelial surface area as a comparative measurement, and the utility of the microsmatic concept. We examined the distribution of olfactory neuroepithelium (OE) across the anteroposterior length of the nasal fossa (from the first completely enclosed cross-section of the nasal fossa to the choanae) in the microsmatic marmoset (Callithrix jacchus) compared to four species of nocturnal strepsirrhines (Otolemur crassicaudatus, O. garnetti, Microcebus murinus, and Cheirogaleus medius). Adults of all species were examined and infant C. jacchus, O. crassicaudatus, M. murinus, and C. medius were also examined. All specimens were serially sectioned in the coronal plane and prepared for light microscopic study. Distribution of OE across all the turbinals, nasal septal surfaces, and accessory spaces of the nasal chamber was recorded for each specimen. The right nasal fossae of one adult C. jacchus and one neonatal M. murinus were also three-dimensionally reconstructed using Scion Image software to reveal OE distribution. Findings showed OE to be distributed relatively more anteriorly in adult C. jacchus compared to strepsirrhines. It was also distributed more anteriorly along the nasal septal walls and recesses in neonates than adults. Our findings also showed that OE surface area was not a reliable proxy for receptor neuron numbers due to differing OE thickness among species. Such results indicate that nasal cavity morphology must be carefully reconsidered regarding traditional functional roles (olfaction versus air conditioning) assigned to various nasal cavity structures. At present, the microsmatic concept itself lacks a basis in nasal chamber morphology, since OE may have varying patterns of distribution among different primates.

Microsmatic primates: reconsidering how and when size matters

The terms "microsmatic" and "macrosmatic" refer to species with lesser or greater levels, respectively, of olfactory function. Historically, primates are considered microsmats (olfactory sense reduced) with a concomitant increased emphasis on vision. The olfactory bulbs (forebrain centers that receive peripheral olfactory input) are proportionately smaller in primates compared to most other mammals. Similarly, the regions of the nasal cavity that are covered with olfactory epithelium (containing receptor cells) have proportionately less surface area in primates than other mammals. Thus, the generalization that primates are microsmatic is most frequently stated in terms of the proportional rather than absolute size of olfactory structures. Yet the importance of scaling to body size is unclear in regard to the chemical senses such as the olfactory or vomeronasal systems-do chemosensory structures such as olfactory bulbs and olfactory epithelium exhibit the same neural relationship to body mass that is seen for neural tissues that supply innervation to musculature or the skin? Previous studies examining neuronal density, volume, and/or surface area of the olfactory epithelium illustrate that different conclusions may be supported based on the parameter used. Plots of olfactory bulb volume versus body mass that generated for large-scale taxonomic studies or growth studies benefit from body mass (or total brain volume) with a comparative perspective. However, our examination of proportional versus absolute measurements implies that in comparisons within taxa, body size adjustments needlessly distort the data. As a final consideration, another embryonic derivative of the nasal placode, the vomeronasal organ, may warrant consideration regarding a definition of microsomia versus macrosomia.

The paleobiology of Amphipithecidae, South Asian late Eocene primates

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 large-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 forces 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.