Krabi Basin Research Articles

New fossil remains from Bang Mark locality, Krabi Basin, southern Thailand

ABSTRACT We describe several mammal taxa from the poorly known late Eocene locality of Bang Mark, Krabi Basin in southern Thailand. Most of them were unknown in that locality that now includes 19 distinct taxa. The new material corresponds to dental remains that can be attributed to a carnivoran, a dichobunid, ruminants, anthracotheres and perissodactyls. These remains provide information on the affinities of several genera that were uncertain so far. Archaeotragulus might be more closely related to Siamotragulus from the Miocene of Southeast Asia, Pakistan and East Africa, and the tragulid status of Krabitherium is supported by the morphology of its p4. We also describe the smallest known anthracothere, Geniokeryx nanus, sp. nov., and a new species of eomoropid chalicothere, Eomoropus meridiorientalis, sp. nov., that represents the first record of that family in the Krabi fauna. This work also supports the contemporaneity of Bang Mark with Wai Lek and Bang Pu Dam.

A new Late Eocene primate from the Krabi Basin (Thailand) and the diversity of Palaeogene anthropoids in southeast Asia

According to the most recent discoveries from the Middle Eocene of Myanmar and China, anthropoid primates originated in Asia rather than in Africa, as was previously considered. But the Asian Palaeogene anthropoid community remains poorly known and inadequately sampled, being represented only from China, Myanmar, Pakistan and Thailand. Asian Eocene anthropoids can be divided into two distinct groups, the stem group eosimiiforms and the possible crown group amphipithecids, but the phylogenetic relationships between these two groups are not well understood. Therefore, it is critical to understand their evolutionary history and relationships by finding additional fossil taxa. Here, we describe a new small-sized fossil anthropoid primate from the Late Eocene Krabi locality in Thailand, Krabia minuta, which shares several derived characters with the amphipithecids. It displays several unique dental characters, such as extreme bunodonty and reduced trigon surface area, that have never been observed in other Eocene Asian anthropoids. These features indicate that morphological adaptations were more diversified among amphipithecids than was previously expected, and raises the problem of the phylogenetic relations between the crown anthropoids and their stem group eosimiiforms, on one side, and the modern anthropoids, on the other side.

New remains ofEgatochoerus jaegeri(Mammalia, Suoidea) from the late Eocene of Peninsular Thailand

Abstract: We report here additional remains referred toEgatochoerus jaegerifrom the late Eocene locality of Ban Mark in the Krabi basin, Thailand. The new material described, comprising upper and lower cheek teeth, deciduous premolars and partial cranial remains, makesE. jaegerithe best‐documented Eocene representative of Old World Suoidea at present. Detailed study and comparison of their cheek teeth structure reveal a homogeneity of the molar crest and groove patterns of Old World and New World Palaeogene suoids.

Coal facies in a Cenozoic paralic lignite bed, Krabi Basin, southern Thailand: Changing peat-forming conditions related to relative sea-level controlled watertable variations

Abstract The Cenozoic Krabi Basin in the southern part of peninsular Thailand contains about 112 million tons proven coal reserves. At present, coal is only produced from the Bang Mark mine located in the southern part of the basin, where the main lignite bed is 7–20 m thick. The lignite bed occurs in an overall paralic succession. The present paper investigates the depositional conditions of an approximately 8 m thick lignite bed (main seam) in the Bang Mark mine using organic petrography, including maceral ratios, and geochemistry. The results are further interpreted in a sequence stratigraphic context. The lignite is of low rank and is completely dominated by huminite indicating generally oxygen-deficient conditions in the precursor mire. Very low inertinite contents suggest rare occurrences of wildfires. The lower part of the lignite bed represents a topogenous fresh water peat mire with open water areas that in few cases may have experienced influx of saline water. The peat mire was subjected to periodic inundations and deposition of siliciclastics. Tissue preservation was relatively poor. The upper part of the lignite bed represents a slightly domed fresh water ombrogenous peat mire with a stable watertable and a balance between peat accumulation and accommodation space creation that favoured preservation of plant tissues. In general, the mire vegetation changed from less woody in the topogenous mire to more arborescent in the ombrogenous mire, where plants with suberinised wood cell walls also were more frequent. Decompacted, the lignite bed corresponds to a minimum ~ 11 m thick peat deposit that records from ~ 22,000 to 55,000 years of peat accumulation. Watertable rise in the peat mire was controlled overall by relative sea-level rise. In a sequence stratigraphic context, the lignite bed overlies a terrestrialisation surface (TeS; sensu Diessel, 2007) and the lowermost part records peat formation during a falling watertable and a decreasing accommodation/peat accumulation ratio (terrestrialisation). An accommodation reversal surface (ARS; sensu Diessel, 2007) indicates a change to paludification style of peat formation characterised by rising watertable and a high accommodation/peat accumulation ratio. Another ARS marks a gradual change to a situation with a balanced accommodation/peat accumulation ratio. The overall watertable rise throughout peat formation, but at a gradually slower rate from base to top, suggests that the lignite bed could be located in the late transgressive systems tract (TST).

A new primitive alligatorine from the Eocene of Thailand: relevance of Asiatic members to the radiation of the group

Despite the report of fragmentary remains from China, the fossil record of primitive Alligatorinae is unbalanced with most of its members reported from North America and Europe. Here, we describe cranial, mandibular, and some postcranial elements of a new alligatorine taxon from the Late Eocene of Krabi Basin, southern Thailand. Krabisuchus siamogallicus gen. et sp. nov. was a small animal probably not surpassing 2 m in total length. Despite deformation, the remains show a dorsally elevated cranium similar to Arambourgia gaudryi and Osteolaemus tetraspis. The new alligatorine is characterized by a very short snout and a blunt dentition in the posterior region. Krabisuchus siamogallicusis the first fossil alligatorine from Asia to be incorporated into cladistic analysis. Results indicate a close relationship of the newtaxon to Procaimanoidea, Arambourgia gaudryi, and Allognathosuchus polyodon, all of them being sister groups to the Alligator clade. Alligatorine were widespread as early as the Late Eocene across the northern hemisphere. Scenarios of alligatorine dispersal during the Palaeogene are discussed in comparison with the European and North American alligatorine fossil record. Basal alligatorines were probably land-dwelling animals contrary to modern-day crocodylians. Such a mode of life may have allowed this group to colonize vast territories, especially during periods of global warm climates. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158, 608–628.

First Middle Miocene Rodents from the Mae Moh Basin (Thailand): Biochronological and Paleoenvironmental Implications

We report here the first discovery of microvertebrates from the Mae Moh coal mine, Lamphang Province, northern Thailand. This discovery sheds new light on the age and paleoenvironments of the Miocene basins of northern Thailand. Previous investigations of paleomagnetic stratigraphy in the Mah Moh Basin demonstrated that the fossiliferous Q and K coal seams there date between 13.12–13.3 Ma. The microvertebrate fauna from the Q and K coal seams includes Tarsius sp., erinaceid insectivores and rodents, the last being represented by the monospecific genera Prokanisamys and Neocometes. These microvertebrate fossils are extremely similar to elements of the Mae Long fauna from the Li Basin, which had been estimated to date between 16–18 Ma. Accordingly, the Mae Long fauna in the Li Basin appears to be contemporaneous with the fauna from the Q and K coal seams in the Mae Moh Basin, suggesting that previous estimates of the age of the Mae Long fauna were 3–5 Ma too old. The discovery of numerous fragments of the primitive deer Stephanocemas cf. rucha and the pig Conohyus thailandicus in the Mae Moh Basin confirms the correlation suggested by the microvertebrates. As a result, there is no longer any reason to advocate a west-east chronological gradient for the opening of the intermontane basins of northern Thailand. Alternatively, dating of other Tertiary basins in Thailand suggests a south-north gradient. The Krabi Basin, located in peninsular Thailand, is of latest Eocene age, the Nong Ya Plong Basin in central Thailand dates to the late Oligocene, and basins in northern Thailand are middle Miocene in age. Analysis of mammalian faunas and pollen indicate that the middle Miocene was a period of dramatic climatic fluctuations in northern Thailand. The Q and K coal seams in the Mae Moh Basin show a temperate-tropical vegetation, whereas the uppermost coal layers testify to warmer tropical vegetation. Based on magnetostratigraphic data, these climatic fluctuations were synchronous with those observed in Antarctic deep waters. Climatic fluctuations during the middle Miocene cannot be used for purposes of correlation, because they appear to have occurred in an iterative fashion.

Turtles from the late Eocene – early Oligocene of the Krabi Basin (Thailand)

Abstract Two new species of geoemydid turtles from the late Eocene – early Oligocene of Krabi Basin in southern peninsula of Thailand are described on the basis of shells and other postcranial material: Mauremys thanhinensis n. sp. and Hardella siamensis n. sp. These early representatives of testudinoid turtles from South-East Asia shed light on the early evolution of extant geoemydids.

New primates from the late Eocene of Thailand: a contribution to primate diversity in the Paleogene of Asia

We report the discovery of two new primates from the late Eocene Krabi Basin, southern Thailand. One isolated upper molar displays morphological features (protocone united with hypocone by the prehypocrista, postprotocrista extended distobuccally) suggesting possible phylogenetic relationships with Amphipithecidae, while an isolated lower molar is tentatively referred to as a new tarsiiform, mainly on the basis of its paraconid and entoconid morphology. Although very scarce, these remains expand the record of Paleogene primates in Southeast Asia, and testify to their successful radiation in that area.

New strepsirrhine primate from the late Eocene of Peninsular Thailand (Krabi Basin)

In this paper, we describe the newly discovered lower jaw of a primate from the late Eocene Krabi coal mine (Bang Mark pit) of Peninsular Thailand. We performed microtomographic examinations at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) to analyze different morphological aspects of the jaw and teeth. Although partially preserved, this fossil mandible reveals a set of distinctive dental traits (e.g., double-rooted P(2) and molarized P(4)) that allow us to describe a new stepsirrhine adapiform: Muangthanhinius siami, new genus and species. This taxon is somewhat atypical among Paleogene adapiforms, and more specialized than the sivaladapid adapiforms (hoanghoniines) that existed in Asia in the same epoch. In fact, Muangthanhinius shows a degree of dental specialization approximating that of some modern strepsirrhine lemuriforms, although it lacks the highly specialized anterior dentition characterizing this living primate group (canine + incisors forming a very procumbent toothcomb). In contrast, Muangthanhinius exhibits a large canine deeply anchored within the dentary that probably protruded high above the toothrow. Finally, despite the development of a molarized P(4) as in Miocene sivaladapid sivaladapines, Muangthanhinius differs in molar morphology from this group, and the position of this new taxon within the Adapiformes remains indeterminate. Clarification of its phylogenetic position will require more morphological evidence than is currently available.

Early rhinocerotids (Mammalia: Perissodactyla) from South Asia and a review of the Holarctic Paleogene rhinocerotid record

The earliest rhinocerotids from South Asia are identified on the basis of few dental remains originating from the Late Eocene of Thailand (Wai Lek mine, Krabi Basin) and the Early Oligocene of Pakistan (Paali nala C2, Bugti Hills). Once synthesized, the Holarctic Paleogene rhinocerotid record points out a westward diachronism of rhinocerotid First Appearance Data, from North America to Europe via Asia, throughout mid-Cenozoic times. The faunal similarity among mammal localities from the Late Eocene and Early Oligocene of peninsular Thailand, southern China, and Pakistan suggests the existence of a single South Asian paleoprovince during this interval and the persistence of a tropical–subtropical climate. Substantial faunal changes recorded in eastern Balochistan reveal a significant climatic deterioration from the middle part of the Oligocene. Neither provinciality nor endemism is noticeable for rhinocerotoid taxa recognized in the Oligocene of the Indian subcontinent: neither the Himalayas nor the Tibet Plateau was a paleogeographic barrier for large mammals during this interval.

New remains of primitive ruminants from Thailand: evidence of the early evolution of the Ruminantia in Asia

A new tragulid, Archaeotragulus krabiensis, gen. n. et sp. n., is described from the late Eocene Krabi Basin (south Thailand). It represents the oldest occurrence of the family which was previously unknown prior to the Miocene. Archaeotragulus displays a mixture of primitive and derived characters, together with the M structure on the trigonid, which appears to be the main dental autapomorphy of the family. We also report the occurrence at Krabi of a new Lophiomerycid, Krabimeryx primitivus, gen. n. et sp. n., which displays affinities with Chinese representatives of the family, particularly Lophiomeryx. The familial status of Iberomeryx is discussed and a set of characters is proposed to define both Tragulidae and Lophiomerycidae. Results of phylogenetic analysis show that tragulids are monophyletic and appear nested within the lophiomerycids. The occurrence of Tragulidae and Lophiomerycidae in the upper Eocene of south-east Asia enhances the hypothesis that ruminants originated in Asia, but it also challenges the taxonomic status of traguloids within the Ruminantia.

Eocene Krabi basin (southern Thailand): Paleontology and magnetostratigraphy

We present the magnetostratigraphy of the late Eocene Krabi section from the Krabi basin, Thailand. The investigated section is about 105 m thick and consists of red and yellow siltstones, sandstones, gray claystones, lignites, and limestones. We collected 86 samples at 43 different stratigraphic levels from the 105-m-thick section. The samples yielded a paleomagnetic direction, and paleomagnetic analyses give favorable demagnetization behaviors; two magnetic components can be isolated. Rock magnetic experiments show that the natural remanent magnetization generally results from the presence of titanomagnetite. The first component has roughly the direction of the present-day field. The second component displays only reversed polarity, with a mean direction: declination = 192.34, inclination = −15.5, N = 71, α 95 = 4.7, k = 14 (after bedding correction). Mammalian biostratigraphy indicates a late Eocene age. Correlation to the Geomagnetic Polarity Time Scale relies on biostratigraphic data previously proposed for the fauna found in the same section. These correlations have been achieved by comparing variations in the sedimentation rate derived from the alternative correlation. The proposed correlation puts the Krabi section in chron C12r or chron C13r. This suggests sedimentation rates of 4.9 and 9.4 cm/k.y., respectively. This correlation suggests that the Krabi section is older than 31 Ma and younger than 34 Ma.

A new baluchimyine rodent from the Late Eocene of the Krabi Basin (Thailand): palaeobiogeographic and biochronologic implications

The latest fieldworks in the Krabi Basin from Thailand have provided accurate stratigraphic, magnetostratigraphic and faunal settings. A complete left maxilla of a new baluchimyine rodent, Baluchimys krabiense nov. sp., described herein, has been found in the Bang Mark pit from the Krabi mine for which a Late Eocene age is attributed. The discovery of such an Eocene baluchimyine in South Asia casts serious doubts on the previous Early Miocene age assignment for Pakistani baluchimyines. Moreover, the comparison with a Late Eocene taxa from North Africa allows us to suggest close phylogenetic relationships, implying that this group had a widespread African–Asian distribution during the Palaeogene. In that context, the endemism assumption related to the Indian Subcontinent isolation throughout the Eocene until the Early Miocene is refuted here.

Eocene nimravid carnivorans from Thailand

ABSTRACT Dental remains assigned to nimravid carnivores have been discovered in southern Thailand. These specimens come from the upper Eocene Krabi Basin that has already yielded numerous vertebrate taxa important for the knowledge of mammal evolution. The fossils described here are among the oldest remains belonging to the Nimravidae, and they are attributed to Nimravus cf. intermedius and Hoplophoneus sp. The occurrence of nimravid carnivores in Southeast Asia implies exchanges between Asia and North America during the Late Eocene, and it supports a larger geographical distribution and an origin of the family older than previously known.

Paleontology and magnetostratigraphy of the Eocene Krabi basin (Southern Thailand)

Excavations undertaken since several years in the Eocene sites of the Krabi basin (southern Thailand) has yielded six genera of reptiles and 31 different species of mammals, among which are several forms new to Southeast Asia. Nearly all vertebrate remains have been collected from lignite levels, and their study indicates a rather closed area submitted to a tropical climate. In addition to considerably increase our knowledge ofthe early Tertiary terrestrial vertebrate faunas of southern Asia, these Thai remains allow to propose an accurate age for this mammal fauna. They also can be related to known late Eocene and/or early Oligocene forms from Europe, North Mrica, Asia, or even North America. The study of the evolutionary stages ofthe different taxa of the Krabi fauna, and comparisons with other well dated mammalian faunas from Eurasia and North Mrica, led us to propose a late Eocene age for the mammal fauna (about ±37 to ±34 Ma). A magnetostratigraphic study has been carried out on the late Eocene fluviatilell acustrine Krabi deposit (southern Thailand). 87 samples in 43 different stratigraphic levels were collecte d from a thickness of 105 m of the section. These paleomagnetic samples were analysed, and they yielded a paleomagnetic direction. Rock magnetic experiments show that the NRM generally results from the presence of magnetite. The Krabi section that has been sampled displays only reverse polarity. The mean direction for reversed polarity samples was D = 192.34, I = -15.5 (N = 71, <X95 = 4.7, k = 14). The correlation to the Geomagnetic Polarity Time Scale (GPTS) relies on the biostratigraphic data previously proposed for the fauna found in the same section. These correlations have been achieved by comparing variations in the sedimentary rate derived from the alternative correlation. The proposed correlation puts Krabi section in the chron C13r (34.1 Ma) or C15r (35.14 Ma). This suggests sedimentation rates of 9.4 and 26 cm/ky respectively, but the second sedimentation rate is too high for this type of sediments (lignite). Hence, the most probable correlation for the Krabi section is with chron C13r (33.54-34.65 Ma).

Dental anomalies in Upper Eocene Anthracotheriidae: a possible case of inbreeding

Three fragmentary maxillae and an isolated upper molar from the Upper Eocene locality of Wai Lek (Krabi Basin, South Thailand) belonging to the small anthracotheriid Siamotherium krabiense display strongly abnormal morphologies that are particularly unusual among the dental anomalies recorded in both extant and fossil mammals. Variation and atavism can be ruled out, and the other hypothetical origins of these anomalies, particularly the occurrence of odontomes and inbreeding, are discussed.Dental anomalies, Anthracotheriidae, Asia, atavism, variation, inbreeding. Stephane Ducrocq and Jean-Jacques Jaeger, Laboratoire de Paleontologie, Institut des Sciences de l'Evolution, Case Courrier 064, Universite Montpellier II, Place Eugene Bataillon, F-34095 Montpellier Cedex 5, France (SD's current address: Staatliches Museum fur Naturkunde, Rosenstein 1, D-70191 Stuttgart, Germany); Yaowalak Chaimanee and Varavudh Suteethorn, Department of Mineral Resources, Geological Survey, Rama VI Road, Bangkok 10400, Thailand; 14th November, 1994; revised 27th July. 1995.

Mammalian faunas and the ages of the continental Tertiary fossiliferous localities from Thailand

The paleontological study of the mammalian fossils of Tertiary fossiliferous localities from Thailand, has led to the description of numerous new taxa, the modification of several extant paleobiogeographical patterns and identification of accurate dates for the Tertiary continental basins. The Krabi Basin yielded 27 distinct mammal species, most of them representing new forms, and an Upper Eocene age is now well established for this southern locality. The use of the Krabi assemblage as a reference fauna also led to the reassessment of the ages of several other Asian localities and suggests that the Thai assemblage lived in a forest under a tropical climate. All localities from northern Thailand occur in a time span ranging from 16 to 14 Ma. The use of the cenogram method, allowed the paleoenvironment of the Neogene Thai localities to be compared with contemporaneous faunal communities from the Pakistan Siwaliks and China. The middle Miocene environment in South Asia seems to have been quite open, with a likely monsoonal climate. Finally, the ages of different fossiliferous localities from Thailand are correlated with the geodynamic events following the India-Asia collision.

New primate from the Palaeogene of Thailand, and the biogeographical origin of anthropoids

Abstract Recent field work in the Eocene Krabi Basin (south Thailand) has yielded a fragment of a primate lower jaw containing the second and third molar teeth attributed to a new genus and species, Wailekia orientale. The specimen described here is morphologically very close to Oliopithecus savagei from the Fayum (Egypt), and the presence of Wailekia in the Upper Eocene of Thailand suggests that South East Asia may be an important centre of origin for anthropoids.