Khoratpithecus et la radiation des hominoïdes en Asie du Sud-Est au Miocène

La découverte d'hominoïdes fossiles en Thaïlande et au Myanmar a révélé de nouveaux épisodes de l'histoire évolutive du groupe. Ces fossiles ont été exhumés sur trois sites distincts, qui ont été caractérisés géologiquement et précisément datés : la mine de Chiang Muan, au Nord-Ouest de la Thaïlande, le plateau de Khorat, au Nord-Est de la Thaïlande, et la formation de l' Irrawaddy, dans le bassin central birman. L'âge de ces sites est compris entre 8,8 et 13 Ma. La description des fossiles a permis leur attribution au genre Khoratpithecus, au sein duquel trois espèces ont été reconnues. Morphologiquement, Khoratpithecus présente des affinités claires pour le genre Pongo et, dans une moindre mesure, pour le genre Sivapithecus des Siwaliks. La morphologie des racines dentaires des hominoïdes a été étudiée grâce aux techniques d'imagerie numérique de la morphologie interne. Il est apparu que cette morphologie présentait une forte valeur taxonomique, et qu'elle était utilisable dans les analyses phylogénétiques. Une analyse morphométrique des couronnes dentaires, ainsi qu'une reconstitution virtuelle de fossiles déformés ou brisés, ont également été effectuées grâce aux techniques de microtomographie X. L'intégration des nouveaux fossiles et des nouvelles données dans les analyses phylogénétiques a confirmé la position de Khoratpithectus en groupe frère des orang-outangs. En revanche, le genre Lufengpithectus, classiquement attribué au clade Pongo, s'avère plus proche des Dryopithectus européens. Cette phylogénie renforce l'idée d une migration d' Ouest en Est, au sud de l Himalaya, pour le clade Pongo, et une migration depuis l'Europe par le Nord de l'Himalaya pour le genre LufengpithectusThe recent discovery of fossil hominoids in Thailand and Myanmar unveiled new episodes of the evolutionary history of this group. Those fossils were unearthed in three distinct sites: Chiang Muan in north-western Thailand, Khorat plateau in north-eastern Thailand and the Irrawaddy formation in Central Myanmar. These sites have been geologically characterized and precisely dated between 8.8 and 13 Ma. The descriptions of those fossils allowed an attribution to Khoratpithecus. Within this genus, three species have been recognized. Morphologically, Khoratpithecus display clear affinities to extant Pongo and, to a lesser extend, Sivapithecus from Siwaliks. The morphology of dental roots within hominoids has been studied using inner anatomy imaging techniques. This morphology displays high taxonomical value, and can be used in phylogenetical studies. Morphometrical analyses of dental crowns, along with virtual reconstruction of shattered or distorted fossils, have been performed using X-ray microtomography. Phylogenetical analyses integrating new specimens and new data confirmed Khoratpithecus as a sister-group of orang-utans. On the other hand, the genus Lufengpithecus, despite classically attributed to Pongo clade, appears closer to Dryopithecus than to Pongo. Such a phylogeny strengthens the hypothesis of a West-East migration through the South of Himalaya for Pongo clade, and a migration from Europe through the North of Himalaya fot Lufengpithecus.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

First hominoid from the Late Miocene of the Irrawaddy Formation (Myanmar).

For over a century, a Neogene fossil mammal fauna has been known in the Irrawaddy Formation in central Myanmar. Unfortunately, the lack of accurately located fossiliferous sites and the absence of hominoid fossils have impeded paleontological studies. Here we describe the first hominoid found in Myanmar together with a Hipparion (s.l.) associated mammal fauna from Irrawaddy Formation deposits dated between 10.4 and 8.8 Ma by biochronology and magnetostratigraphy. This hominoid documents a new species of Khoratpithecus, increasing thereby the Miocene diversity of southern Asian hominoids. The composition of the associated fauna as well as stable isotope data on Hipparion (s.l.) indicate that it inhabited an evergreen forest in a C3-plant environment. Our results enlighten that late Miocene hominoids were more regionally diversified than other large mammals, pointing towards regionally-bounded evolution of the representatives of this group in Southeast Asia. The Irrawaddy Formation, with its extensive outcrops and long temporal range, has a great potential for improving our knowledge of hominoid evolution in Asia

MFI 89, upper molar of <i>Khoratpithecus</i>.

<p>A: right M² MFI89 in occlusal view. B: interpretive drawing of MFI89. C–D: interpretive drawings of <i>Khoratpithecus chiangmuanensis</i> left M² TF 6176 (mirror image) and right M² TF 6169. The upper arrows point the valley between the paracone and the protocone which is deviated in MFI89. Scale bar: 1 cm. C–D are not to scale.</p

Principal component analysis plots of the elliptic Fourier coefficients of the symphyseal outlines of <i>Khoratpithecus</i> MFI-K171 and TF 6223 and extant hominoids.

<p>Only the outlying individuals are represented for the extant hominoids. For each component, extreme outlines and expressed variance are given.</p

Comparison of the corpora measurements and indices of MFI-K171, TF 6223, and <i>Ouranopithecus macedoniensis</i>.

a<p>depth/squareroot(M₁ length*M₁ breadth).</p>b<p>thickness/squareroot(M₁ length*M₁ breadth).</p>c<p>thickness/depth*100.</p><p>Corpus depths are means of buccal and lingual measurements. Corpus measurements and indices of <i>O. macedoniensis</i> are given with the total range on the first line and the intrasexual ranges (♀/♂) on the second line.</p

Magnetic polarity stratigraphy of the hominoid section.

<p>A: lithology and stratigraphic position of the sampled levels. B: latitude of the virtual geomagnetic pole vs. stratigraphic position. C: polarity column (black bar: normal polarity zone) and potential correlations with the GPTS of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017065#pone.0017065-Gradstein1" target="_blank">[35]</a>.</p

δ¹³C and δ¹⁸O values obtained from six <i>Hipparion (s.l.)</i> specimens from the hominoid section.

<p>δ¹³C and δ¹⁸O values obtained from six <i>Hipparion (s.l.)</i> specimens from the hominoid section.</p

Location of the <i>Khoratpithecus</i>-bearing localities of Thailand and Myanmar.

<p>a: map of Southeast Asia. b: general map of Myanmar. c: local map of the region of Magway displaying the Irrawaddy Formation outcrops <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017065#pone.0017065-Bender1" target="_blank">[6]</a> in light grey.</p

Paleomagnetic analyses of the samples from the hominoid section.

<p>A–C: Orthogonal vector diagrams (closed/open symbols correspond to the horizontal/vertical component), stereoplots (crosses: upper hemisphere, circles: lower hemisphere), intensity and step plots for representative samples after thermal and alternating field demagnetization. D: Equal-area stereographic projections of site mean ChRM directions. Open triangle: site mean direction (Declination = 7.3°; Inclination = 23.9°; α₉₅ = 7; k = 23; n = 20), Ellipse: 95% confidence ellipse for mean direction. Solid star: direction derived from the 10-Ma apparent polar wander path. E: Isothermal remanent magnetization (IRM) acquisition curves (normalized values) for two representative samples. F–G: Stepwise thermal demagnetization of the differential IRM components (X, Y, and Z). 99B022A: medium grained sandstone. 99B039A: grey claystone.</p