Eocene Of Asia Research Articles

New remains of Siamochoerus banmarkensis Ducrocq, Chaimanee, Suteethorn & Jaeger, 1998 (Artiodactyla, Suidae) from the late Eocene of Thailand

We report here new craniodental remains from the late Eocene Krabi coal mine in Thailand that can be attributed to the suoid Siamochoerus banmarkensis Ducrocq, Chaimanee, Suteethorn & Jaeger, 1998. This material that includes the complete lower dentition and isolated upper molars provides new information on the dental morphology of this species and makes S. banmarkensis, together with Egatochoerus jaegeri Ducrocq, 1994 from Krabi, the second best documented early suoid in the Eocene of Asia. A few dental features that can be observed on these new remains suggest that S. banmarkensis might be more closely related to Suidae Gray, 1821, but it also illustrates the difficulty to attribute a precise taxonomic position to Eocene taxa.

Unraveling weak and short South Asian wet season in the Early Eocene warmth

AbstractThe timing and causative factors underlying the Asian summer monsoon initiation remain contentious as recent proxy data and modeling studies suggested the existence of a wet-dry monsoon-like climate from the Cretaceous period. Leveraging an ensemble of deep-time climate simulations focused on the early Eocene (DeepMIP-Eocene), we show that the early Eocene Asian wet season was notably weaker and shorter than present-day in the absence of an elevated heat source like the Tibetan Plateau. The deficient upper-tropospheric meridional temperature gradient was insufficient to propel the seasonal northward migration of the rainband over South Asia. The weaker cross-equatorial moisture flow encountered obstruction of Gangdese mountain along the southern edge of Asia, leading to significant South Asian aridity. The enhanced greenhouse effect was inadequate to augment the seasonal circulation and rainfall variability to current levels. The altered wet and dry seasonality across South Asia during the early Eocene does not meet the criteria to be classified as a monsoon, suggesting that South Asian monsoon initiation is likely contingent upon a specific level of Tibetan Plateau upliftment.

A distinctive Eocene Asian monsoon and modern biodiversity resulted from the rise of eastern Tibet

The uplift of eastern Tibet, Asian monsoon development and the evolution of globally significant Asian biodiversity are all linked, but in obscure ways. Sedimentology, geochronology, clumped isotope thermometry, and fossil leaf-derived numerical climate data from the Relu Basin, eastern Tibet, show at ∼50–45 Ma the basin was a hot (mean annual air temperature, MAAT, ∼27 °C) dry desert at a low-elevation of 0.6 ± 0.6 km. Rapid basin rise to 2.0 ± 0.9 km at 45–42 Ma and to 2.9 ± 0.9 km at 42–40 Ma, with MAATs of ∼20 and ∼16 °C, respectively, accompanied seasonally varying increased annual precipitation to > 1500 mm. From ∼39 to 34 Ma, the basin attained 3.5 ± 1.0 km, near its present-day elevation (∼3.7 km), and MAAT cooled to ∼6 °C. Numerically-modelled Asian monsoon strength increased significantly when this Eocene uplift of eastern Tibet was incorporated. The simulation/proxy congruence points to a distinctive Eocene Asian monsoon, quite unlike that seen today, in that it featured bimodal precipitation and a winter-wet regime, and this enhanced biodiversity modernisation across eastern Asia. The Paleogene biodiversity of Asia evolved under a continually modifying monsoon influence, with the modern Asian monsoon system being unique to the present and a product of a long gradual development in the context of an ever-changing Earth system.

A proto-monsoonal climate in the late Eocene of Southeast Asia: Evidence from a sedimentary record in central Myanmar

The Burma Terrane has yielded some of the earliest pieces of evidence for monsoonal rainfall in the Bay of Bengal. However, Burmese ecosystems and their potential monsoonal imprint remain poorly studied. This study focuses on the late Eocene Yaw Formation (23° N) in central Myanmar, which was located near the equator (c. 5° N) during the Eocene. We quantitatively assessed the past vegetation, climate, and depositional environments with sporomorph diagrams, bioclimatic analysis, and sequence biostratigraphy. We calculated the palynological diversity and drew inferences with rarefaction analysis by comparing with four other middle to late Eocene tropical palynofloras. Palynological results highlight a high floristic diversity for the palynoflora throughout the section formed by six pollen zones characterized by different vegetation. They indicate that lowland evergreen forests and swamps dominated in the Eocene Burmese deltaic plains while terra firma areas were occupied by seasonal evergreen, seasonally dry, and deciduous forests. This vegetation pattern is typical to what is found around the Bay of Bengal today and supports a monsoon-like climate at the time of the Yaw Formation. Bioclimatic analysis further suggests that in the late Eocene, the Yaw Formation was more seasonal, drier, and cooler compared to modern-day climate at similar near-equatorial latitude. More seasonal and drier conditions can be explained by a well-marked seasonal migration of the Intertropical Convergence Zone (ITCZ), driver of proto-monsoonal rainfall. Cooler temperatures in the late Eocene of central Myanmar may be due to the lack of adequate modern analogues for the Eocene monsoonal climate, while those found at other three Eocene Asian paleobotanical sites (India and South China) may be caused by the effect of canopy evapotranspirational cooling. Our data suggest that paleoenvironmental change including two transgressive–regressive depositional sequences is controlled by global sea level change, which may be driven by climate change and tectonics. The high diversity of the Yaw Formation palynoflora, despite well-marked seasonality, is explained by its crossroads location for plant dispersals between India and Asia.

The origin of Asian monsoons: a modelling perspective

Abstract. The Cenozoic inception and development of the Asian monsoons remain unclear and have generated much debate, as several hypotheses regarding circulation patterns at work in Asia during the Eocene have been proposed in the few last decades. These include (a) the existence of modern-like monsoons since the early Eocene; (b) that of a weak South Asian monsoon (SAM) and little to no East Asian monsoon (EAM); or (c) a prevalence of the Intertropical Convergence Zone (ITCZ) migrations, also referred to as Indonesian–Australian monsoon (I-AM). As SAM and EAM are supposed to have been triggered or enhanced primarily by Asian palaeogeographic changes, their possible inception in the very dynamic Eocene palaeogeographic context remains an open question, both in the modelling and field-based communities. We investigate here Eocene Asian climate conditions using the IPSL-CM5A2 (Sepulchre et al., 2019) earth system model and revised palaeogeographies. Our Eocene climate simulation yields atmospheric circulation patterns in Asia substantially different from modern conditions. A large high-pressure area is simulated over the Tethys ocean, which generates intense low tropospheric winds blowing southward along the western flank of the proto-Himalayan–Tibetan plateau (HTP) system. This low-level wind system blocks, to latitudes lower than 10∘ N, the migration of humid and warm air masses coming from the Indian Ocean. This strongly contrasts with the modern SAM, during which equatorial air masses reach a latitude of 20–25∘ N over India and southeastern China. Another specific feature of our Eocene simulation is the widespread subsidence taking place over northern India in the midtroposphere (around 5000 m), preventing deep convective updraught that would transport water vapour up to the condensation level. Both processes lead to the onset of a broad arid region located over northern India and over the HTP. More humid regions of high seasonality in precipitation encircle this arid area, due to the prevalence of the Intertropical Convergence Zone (ITCZ) migrations (or Indonesian–Australian monsoon, I-AM) rather than monsoons. Although the existence of this central arid region may partly result from the specifics of our simulation (model dependence and palaeogeographic uncertainties) and has yet to be confirmed by proxy records, most of the observational evidence for Eocene monsoons are located in the highly seasonal transition zone between the arid area and the more humid surroundings. We thus suggest that a zonal arid climate prevailed over Asia before the initiation of monsoons that most likely occurred following Eocene palaeogeographic changes. Our results also show that precipitation seasonality should be used with caution to infer the presence of a monsoonal circulation and that the collection of new data in this arid area is of paramount importance to allow the debate to move forward.

Emergence of hystricognathous rodents: Palaeogene fossil record, phylogeny, dental evolution and historical biogeography

AbstractAlthough phylogenetic trees imply Asia as the ancestral homeland of the Hystricognathi clade (Rodentia: Ctenohystrica), curiously the oldest known fossil occurrences of hystricognathous rodents are not from Asia, but from Africa and South America, where they appear suddenly in the fossil record of both landmasses by the Late Middle Eocene. Here we performed cladistic and Bayesian (standard and tip-dating analyses) assessments of the dental evidence documenting early ctenohystricans, including several Asian ‘ctenodactyloids’, virtually all Palaeogene Asian and African hystricognaths known thus far and two representatives of the earliest known South American hystricognaths. Our results provide a phylogenetic context of early hystricognaths (with implications on systematics) and suggest that some Eocene Asian ‘ctenodactyloids’ could be considered as stem hystricognaths and pre-hystricognaths, although they were not recognized as such originally. However, this view does not fill the gap of the Eocene Asian hystricognath record, as the proposed results imply many ghost lineages extending back to the Middle Eocene for several Asian and African taxa. They also imply a complex early historical biogeography of the group, involving multiple dispersal events from Asia to Africa (and possibly from Africa back to Asia) and then to South America sometime during the Middle Eocene. Based on these phylogenetic considerations, we discuss the emergence of hystricognathous rodents from a morpho-anatomical perspective by analysing the differentiation of their masticatory apparatus and chewing movements, notably through the evolution of their dental patterns.

Biostratigraphy and Diversity of Paleogene Perissodactyls from the Erlian Basin of Inner Mongolia, China

Extant perissodactyls (horses, rhinos, and tapirs) comprise a small portion of living mammals, but fossil perissodactyls were more diverse and commonly dominated Paleogene faunas. Unfortunately, the taxonomy and distribution of some Chinese Paleogene perissodactyls remain controversial and unclear, hampering the correlation of Asian paleofaunas with paleofaunas from other continents. Here we clarify the temporal and spatial distribution of Paleogene perissodactyl species from the Erlian Basin based on published specimens, archives, and our recent fieldwork. The strata of the Erlian Basin range nearly continuously from the late Paleocene to the early Oligocene, and almost all Eocene Asian Land Mammal Ages (ALMA) are based on corresponding faunas from the Erlian Basin. We revise the most complete section of deposits at Erden Obo (= Urtyn Obo) that range in age from the late Paleocene to the early Oligocene in the Erlian Basin, and correlate it with other type formations/faunas in the basin based mainly on the perissodactyl biostratigraphy and lithostratigraphy. Furthermore, we discuss perissodactyl faunal components and their diversity from the early Eocene to the early Oligocene in the Erlian Basin, as well as the correlation between middle Eocene ALMAs and North American Land Mammal Ages based on perissodactyl fossils. The general decrease in perissodactyl diversity from the middle Eocene to the late Eocene can probably be attributed to a global climatic cooling trend and related environmental changes. The diversity of perissodactyls declined distinctly during the Eocene-Oligocene Transition, when global average temperatures dropped considerably.

The divergence and dispersal of early perissodactyls as evidenced by early Eocene equids from Asia

The earliest perissodactyls are represented by some basal equoid fossils from Euramerica near the Paleocene/Eocene boundary. Unequivocal early equoids have yet to be reported from the early Eocene of Asia, although other groups of early perissodactyls were indeed present in Asia. Here we report the earliest Eocene Asian equid, Erihippus tingae gen. et sp. nov., based on partial specimens initially assigned to the ceratomorph Orientolophus hengdongensis, from the Hengyang Basin of Hunan Province, China. The specimens previously assigned to ‘Propachynolophus’ hengyangensis from the same Lingcha fauna are split and now reassigned as an ancylopod Protomoropus? hengyangensis and a brontothere Danjiangia lambdodon sp. nov. The nearly simultaneous appearance of equids, ceratomorphs, ancylopods, and brontotheres in the Hengyang Basin suggests that the four main groups of perissodactyls diverged as early as, or no later than, the beginning of the Eocene (about 56 Ma), and displayed different dispersal scenarios during the early Eocene.

Global cooling and enhanced Eocene Asian mid-latitude interior aridity

Tibetan Plateau uplift has been suggested as the main driving force for mid-latitude Asian inland aridity (AIA) and for deposition of thick aeolian sequences in northern China since the Miocene. However, the relationship between earlier AIA and Tibetan Plateau mountain building is uncertain because of a lack of corresponding thick aeolian sequences with accurate age constraints. We here present results for a continuous aeolian sequence that spans the interval from >51 to 39 Ma from the eastern Xorkol Basin, Altun Shan, northeastern Tibetan Plateau. The basal age of the studied sequence postdates initial uplift of the Tibetan Plateau by several million years. Our results indicate that the local palaeoclimate was teleconnected strongly to the overall global cooling pattern, so that local enhanced aridification recorded by the studied aeolian sequence is dominantly a response to global climatic forcing rather than plateau uplift.

What enhanced the aridity in Eocene Asian inland: Global cooling or early Tibetan Plateau uplift?

Geological evidence shows that the Asian inland environment experienced enhanced aridity from the Early to the Late Eocene. This enhanced Eocene aridity in the Asian inland was related to combined impacts from global cooling, topographic uplift and land–sea reorganization. However, the primary cause of the enhanced Eocene aridity in this region is still under debate and varies between global cooling and early Tibetan Plateau uplift. To distinguish between the importance of these factors, we evaluate the climatic impacts of these factors from a modeling point of view. Consistent with geological evidence, our simulations support the observed enhanced Eocene aridity in the Asian inland. Both global cooling (induced by atmospheric CO2 decrease) and topographic uplift contributed to intensified Asian inland aridity, while land–sea redistribution did not. The uplift of the central Tibetan Plateau during the early stage of the India–Asia collision is emphasized more to be responsible for the long-term Asian inland aridification during the Eocene, playing at least an equally important role as the global cooling induced by decrease in atmospheric CO2.

First record of a diacodexeid artiodactyl in the middle Eocene Pondaung Formation (Myanmar)

A fragmentary maxilla referred to a new diacodexeid artiodactyl, Magwetherium burmense, n. gen. and sp., is described from Sabapondaung locality in the middle Eocene Pondaung Formation in Myanmar. Comparisons with other basal artiodactyls known in the Eocene of Asia, Europe, and North America reveal that this unexpected new taxon displays an unusual combination of derived and primitive features. Magwetherium further displays more morphological affinities with the diacodexeid Jiangsudon from the middle Eocene Shanghuang fissure fillings in eastern China than with any other Holarctic dichobunoids. This study also suggests that fossil remains from Pondaung recently described as indeterminate artiodactyls can likely be referred to dichobunids. The occurrence of a diacodexeid artiodactyl in Southeast Asia extends the fossil record of the family in this part of the continent, it represents the youngest representative of the family, and it further documents the adaptive radiation of diacodexeids in Asia during the Eocene.

New records of archaic ungulates from the Lower Eocene of Sanshui Basin, Guangdong, China

Phenacolophidae is a group of little known archaic ungulates from the Late Paleocene to Middle Eocene of Asia. Its phylogenetic relationships with other altungulates have remained uncertain, partly because most phenacolophids are represented by poorly preserved material. Here we report a new phenacolophid, Sanshuilophus zhaoi gen. et sp. nov., from the Lower Eocene Huayong Formation of the Sanshui Basin, Guangdong, China. Although still fragmentary, the new specimens show that the new taxon is characterised by relatively large body size (except for Zaisanolophus), sub-molariform premolars, relatively higher bilophodont molars that lack the mesostyle, and tooth enamel microstructure with true prisms and typical Hunter-Schreger bands (HSB). With the new specimens and a review of the published phenacolophid material, we are able to provide an alternative identification for the tooth loci for the type specimen of Phenacolophus and further present an emended diagnosis for Phenacolophidae. The tooth morphology and enamel microstructure provide new evidence to support the notion that phenacolophids differ from species of Embrithopoda in having low-crown teeth, considerably slanting lophids, distinct paralophids and lacking the arsinoitheriid radial enamel. Phenacolophidae should not be included in Tethytheria but probably represent a stem group for altungulates, if not for all archaic ungulates.

New Ctenodactyloid Rodents from the Erlian Basin, Nei Mongol, China, and the Phylogenetic Relationships of Eocene Asian Ctenodactyloids

ABSTRACT During the last decade, numerous ctenodactyloid rodent fossils have been systematically collected from at least six horizons of the strata that are distributed from the upper part of the Nomogen Formation to the lower part of the Irdin Manha Formation in the Huheboerhe-Nuhetingboerhe area of the Erlian Basin, Nei Mongol (Inner Mongolia). The ages of these fossiliferous horizons range from the Earliest Eocene to the Middle Eocene. These fossils represent the best-known ctenodactyloid assemblages with a high species diversity and reliable stratigraphic and chronological constraints from one locality in central Asia. The fossils show a relatively continuous record of ctenodactyloids and the earliest radiation of rodents in central Asia beginning from the earliest Eocene. These data are important for biostratigraphic correlation of the Paleogene in central Asia and for understanding the taxonomy of Asian ctenodactyloids and the earliest diversity and evolution of rodents. Among the new fossils, we re...

A new and primitive species of Protophiomys (Rodentia, Hystricognathi) from the late middle Eocene of Djebel el Kébar, Central Tunisia

Based on fossil discoveries and phylogenetic studies, an Eocene Asian origin for hystricognathous rodents and anthropoid primates has gained strong support in recent years. The two groups then invaded both Africa and South America, which promoted their evolutionary success. However, the fossil record has so far failed to constrain the nature and precise timing of these pivotal dispersal events. In Africa, given the apparent absence of hystricognaths and anthropoids in early to early middle Eocene localities, it is suggested that these mammal groups dispersed from Asia to Africa sometime during the middle Eocene. In this paper, we report the discovery of several isolated teeth of a rodent from a new vertebrate locality situated in central Tunisia (Djebel el Kebar, KEB-1), dating from the late middle Eocene (Bartonian, ~39.5 Myr). These fossils document a diminutive new species of Protophiomys (P. tunisiensis nov. sp.), a basal genus of hystricognathous rodents which is well known from several North African mammalian-bearing localities of the end of the Eocene. The teeth of P. tunisiensis display a suite of anatomical details comparable with those observed in the other species of the genus, but with a lesser degree of development. Such an apparent primitive evolutionary stage is corroborated by the greater antiquity of this Tunisian species. P. tunisiensis nov. sp. is so far the most ancient representative of hystricognaths in Africa. However, it can be expected that hystricognaths were already present on that landmass given the new data on early caviomorphs recently reported from South America (at ~41 Myr). The arrival of hystricognaths in Africa from South Asia certainly predates the depositional period of the Kebar sediments, but perhaps not by much time.

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.

Early Eocene Asian climate dominated by desert and steppe with limited monsoons

Paleoenvironmental reconstructions illustrated that Asia was dominated by a zonal climate pattern in the Eocene epoch, with an almost latitudinal arid/semiarid band extending from the eastern coast of China to Central Asia. However, the mechanism behind the formation of the broad zonal arid/semiarid region and the possible occurrence of a monsoonal system remains unclear for that period. Here, using a reconstructed land-sea distribution and topography of 50 Ma, we employed a global climate model to simulate the Early Eocene climate in China. A set of experiments with consideration to different topography, orbital parameters, vegetation, sea surface temperature and atmospheric CO 2 concentration demonstrated that a large extent of arid/semi-arid region with annual precipitation less than 800 mm was distributed latitudinally between 20°N and 40°N paleolatitude in China. This arid region was controlled by desert (BWh) and steppe (BSh) climates (according to the Köppen climate classification systems). On the north of the zonal desert/steppe region, a Mediterranean-like climate occurred in Central Asia. These simulations agree well with the geological reconstructions of a zonal desert/steppe region and indicate that this pattern was primarily caused by subtropical high pressures associated with strong descending air. However, it is likely that the aridity in the zonal desert/steppe was reduced occasionally by intensification of monsoon circulations over East Asia. The change in orbital parameters, causing enhanced insolation in summer, was the most likely factor that could enlarge the land-sea thermal contrast and further intensify the monsoon circulations in the Eocene epoch, though its effect was reversible and impermanent. In addition, simulations show that East Asian monsoon did not play a leading role in Chinese Paleogene climate although it may have affected Eastern China occasionally.

The earliest lagomorph (Lagomorpha, Mammalia) from the basal Eocene of Mongolia

The earliest representatives of the order Lagomor-pha are known from the Eocene of Asia and NorthAmerica. Early Eocene lagomorphs have only beenrecorded in Asia. In the Middle Eocene, they were moreabundant and diverse in this continent. This agrees withtheir origin in Asia. To date, the following three taxadated terminal Early Eocene have been described:

The Middle Eocene North American Myomorph Rodent Elymys, Her Asian Sister Aksyiromys, and Other Eocene Myomorphs

Abstract The early Bridgerian (early middle Eocene) myomorph rodent Elymys complexus, from the Sheep Pass Formation near Ely, Nevada, is more variable in its dental morphology than originally described. Its similarity to Aksyiromys and other middle Eocene myomorphs from China and Central Asia suggests close relationships, which requires intercontinental interchange of these small rodents during the middle Eocene. At present, the biogeographic scenario for the origin and early evolution of Myomorpha that best fits the known record of occurrences is: evolution of Myomorpha from Sciuravidae in North America at least as early as the beginning of the middle Eocene, emigration of an Elymys-like myomorph from North America to Asia in the early middle Eocene, diversification of dipodoids and muroids in the later Eocene of Asia, emigration of Eucricetodon-like cricetids from Asia to North America (eumyine cricetids) in the latest Eocene, followed later by dipodids in the Miocene.

CRANIAL ANATOMY AND RELATIONSHIPS OF A NEW CTENODACTYLOID (MAMMALIA, RODENTIA) FROM THE EARLY EOCENE OF HUBEI PROVINCE, CHINA

Two remarkably complete skulls and associated mandibles from the late early Eocene of the Yuhuangding Formation of Hubei Province, central China, are described as a new genus and species of rodent, Exmus mini, and referred to the early ctenodectyloid family Cocomyidae. This is a small rodent in the size range of the early Eocene Asian Cocomys. Dental characters that link Exmus with other cocomyids include: non-molariform P4; cheek teeth increasing in size posteriorly; lower molar ectolophid weak or absent; and lower molar hypoconulid distinct, enlarged on m3. Cranial characters linking Exmus and Cocomys include: skull shape and proportions, including a short rostrum and large orbit; sagittal crest absent; large optic canal, confluent between the orbits; and no petrosal shelf posterior to the caudal tympanic process of the petrosal. Differences from Cocomys are numerous and include: absence of conules on P4 and loss of the protoconule on the upper molars; tiny meta-cone and hypocone on P4; molariform p4: and auditory bulla tightly attached, rather than loose as in Cocomys. Exmus also displays a number of autapomorphies that distinguish it from all other Eocene rodents examined in this study, such as substantial orbital roof formed by lacrimal and frontal, and a suture between the parietals that is nearly completely fused. A phylogenetic analysis based on characters of the skull, mandible, and dentition supports the concept of two clades within the ingroups, one composed of Eocene paramyids, sciuravids, and theridomyids and the other composed of Bandaomys, Exmus, Cocomys, Tamquammys, Advenimus, and Yuomys as stem taxa to ctenodactylids and hystricognaths. This dichotomy, which has been suggested by others prior to the discovery of Exmus, is here considered to represent an accurate assessment of Eocene rodent evolution and makes good paleogeographic sense. However, the desirability of adding more taxa to a phylogenetic analysis of early rodents is clearly recognized.

A New Brontothere (Brontotheriidae, Perissodactyla, Mammalia) from the Eocene of the Ily Basin of Kazakstan and a Phylogeny of Asian “Horned” Brontotheres

A new genus and species of “horned” brontothere, Aktautitan hippopotamopus, from the Ily Basin of Kazakstan is described from three skulls and nearly complete postcranial material. This material occurs in fluvio-lacustrine red beds of the upper part of the Eocene (Irdinmanhan) Kyzylbulak Formation at Aktau Mountain. Trackways occurring in the overlying layers are also attributed to this new brontothere. Additionally, several misleading problems in the taxonomy of Asian horned brontotheres are addressed. We conclude that Protitan khaitshinus Yanovskaya, 1980 is a junior objective synonym of Metatitan relictus Granger and Gregory, 1943. Protitan reshetovi Yanovskaya, 1980 is removed from the genus Protitan and possibly belongs within Metatitan. Brachydiastematherium transylvanicum Böckh and Maty, 1876, the only bona fide European brontothere, known from a single partial mandible, is morphologically consistent with Metatitan Granger and Gregory, 1943. Although B. transylvanicum is known from very fragmentary material, it is possible that Metatitan is a junior synonym of Brachydiastematherium. The first cladistic phylogeny of middle and late Eocene Asian horned brontotheres was constructed with 40 characters and 17 taxa. Aktautitan, Metatitan, Brachydiastematherium, and Embolotherium form a monophyletic clade, with Aktautitan hippopotamopus as the most basal member of this clade. Within this clade, there are two monophyletic trichotomies: a Metatitan relictus, M. primus, Brachydiastematherium transylvanicum clade and a “Metatitan” progressus, Embolotherium andrewsi, E. grangeri clade. The cladogram topology suggests that the elevated frontonasal horns shared by Aktautitan and Metatitan represent the ancestral morphology of the bizarre “battering-ram” of Embolotherium. We extend the subfamily name Embolotheriinae to include these taxa. The unusually shortened distal limb segments of A. hippopotamopus resemble those of a phylogenetically disparate group of large ungulates that have convergently evolved hippolike limb proportions. We conclude that these limb proportions probably do not indicate a semiaquatic lifestyle, as had been previously surmised.