Early Oligocene Research Articles

Dental eruption and adult dentition of the enigmatic ptolemaiid Qarunavus meyeri from the Oligocene of the Fayum Depression (Egypt) revealed by micro-computed tomography clarifies its phylogenetic position

Abstract The Palaeogene is a very important time period for mammalian evolution because it documents the first occurrence of many groups. One such group is the enigmatic Ptolemaiida, best known from the Early Oligocene of the Fayum Depression in Egypt, where it is represented by three genera, Ptolemaia, Qarunavus, and Cleopatrodon, including five species. Among these, Qarunavus meyeri is especially rare, as it is only known from two partial juvenile mandibles. Herein, the affinities of this taxon are re-investigated by applying micro-computed tomography (μCT), thus enabling the study and comparison of its unerupted permanent dentition with that of other ptolemaiidans. The μCT analysis confirms that Q. meyeri is a valid species, distinct from all other ptolemaiidans. Both the comparison and a phylogenetic analysis suggest a closer relationship of Qarunavus to the genus Cleopatrodon than to Ptolemaia; thereby, placing Qarunavus in the family Ptolemaiidae. Furthermore, the μCT analysis enabled the assessment of the dental morphology of Q. meyeri revealing an unusual combination of traits, such as a very small single incisor next to a large canine. The eruption sequence of the permanent dentition in Q. meyeri implies a late dental eruption for the species, which would support the hypothesis of Ptolemaiida belonging to Afrotheria.

Monnina (Polygalaceae), a New World monophyletic genus full of contrasts

Abstract Endemic to the Neotropics, Monnina is the second largest genus of Polygalaceae, yet little is known about its phylogenetic history, biogeography, and morphological character evolution. To address these knowledge gaps, we conducted Bayesian and maximum likelihood (ML) analyses of nuclear ITS and plastid trnL–F regions to test the monophyly of Monnina s.l. We used this phylogenetic framework to (i) infer divergence time estimates of lineages within the genus and reconstruct their historical biogeography; (ii) reconstruct the evolution of morphological characters of putative ecological and evolutionary importance in Monnina; and (iii) test for correlations between our phylogenetic hypothesis and environmental data. Our results reveal that Monnina is monophyletic with an indehiscent, 1–2-seeded fruit as a synapomorphy for the genus. We identify six clades within Monnina based on our combined phylogenetic results: Clades A, B, and D are primarily distributed in southern and eastern South America, Clades C and E are primarily Central Andean, and Clade F is chiefly distributed in the Northern Andes and Central America. The ancestor of the Monnina stem lineage dispersed from Australia/Africa to South America during the late Eocene to early Oligocene. The divergences of major lineages within the genus began in the early Miocene. We inferred the most recent common ancestor of Monnina to be an herbaceous plant with one-seeded samaroid fruits. The origins of fleshy fruits and shrubby habits are phylogenetically correlated within Monnina, and their concerted convergent evolution may have promoted increased net diversification rates in the two most species-rich subclades of the genus.

First report of the genus Eggysodon from Asia

ABSTRACT We described a new pair of incomplete mandibles from the Qingshuiying locality, Northwest China, and referred it to Eggysodon, E. lingwuensis sp. nov. The new species has a typical mosaic assemblage of features: dp1 (or p1) is present at adult stage, but the symphysis is posteriorly elongated and the diastema distance between the canine and dp1 (or p1) is further shortened. In terms of the evolutionary stage, E. lingwuensis sp. nov. is more advanced than E. osborni, but comparable to that of E. gaudryi. Together with other mammals from the Qingshuiying locality, we suggested that the strata-produced new material was deposited during the Early Oligocene. On the other hand, the continental fauna exchange during the Palaeogene between Europe and Asia had very scarce records. Eggysodon has four species from the Oligocene of Europe. This record definitely confirmed that there is a migration route between Central Europe and East Asia during the Early Oligocene.

Inferred nutrient forcing on the late middle Eocene to early Oligocene (~40–31 Ma) evolution of the coccolithophore Reticulofenestra (order Isochrysidales)

AbstractThe first size reduction (FSR) in the Reticulofenestra-Gephyrocapsa-Emiliania (RGE) lineage (order Isochrysidales), which occurred in the early Oligocene (~32 Ma), is of great significance for understanding the Lilliput effect that has affected coccolithophore communities from the late Eocene to this day. We conducted a morphologic analysis on the coccoliths of Reticulofenestra species that lived during the late middle Eocene to early Oligocene (~40–31 Ma), using marine sediments from the South Atlantic Ocean. Our data show increasing size and decreasing abundance of the large species during the late Eocene, leading to their disappearance at the FSR, and a concurrent decrease in the size variability of the small- to medium-sized coccoliths whose central opening diameter had become very reduced. Although the cosmopolitan late Paleogene through Neogene size decrease in coccolithophores has been linked to the concomitant long-term decline in global pCO2, we suggest here that the FSR was the result of environmental destabilization caused by the expansion of eutrophic environments following the late Eocene establishment of overturning circulation associated with ice buildup on Antarctica. This study also leads us to propose a hypothetical model that links coccolith morphology of species of the RGE lineage and trophic resources in the upper ocean: the small- to medium-sized, r-selected coccolithophores with smaller coccolith central openings live in nutrient-rich waters where they rely mostly on photosynthesis and little on mixotrophy, whereas the larger, K-selected species with larger coccolith central openings live in oligotrophic waters where they are more dependent on mixotrophy.

Variability of syn-rift geometry in Pearl River Mouth Basin, China: implications for faulting patterns in two-phase rift basins

Recent studies of two-phase non-coaxial analogue experiments and natural rifts suggest that the pre-existing faults that form in the first-phase rift could strongly influence the fault development during a subsequent phase of extension. However, related models from natural examples are still lacking. Here we compare the fault geometry and evolution with different pre-existing fault arrays in two adjacent areas (Xijiang and Lufeng sags) from the Pearl River Mouth Basin in the South China Sea. This basin has experienced two-phase rifting including the middle Eocene rift phase 1 (NW–SE extension) and a late Eocene–early Oligocene rift phase 2 (N–S extension). The Xijiang Sag developed an approximately NE-striking listric fault system, while the Lufeng Sag formed complex structure is composed of six major faults with various orientations. We demonstrate that the first-phase fault network in the Xijiang Sag is a colinear listric fault system, while that in the Lufeng Sag comprises two sets of (non-colinear) faults. In the Xijiang Sag, the second-phase fault network consists of the reactivated first-phase faults, newly formed faults abutting against or cross-cutting the pre-existing reactivated faults or occurring between the pre-existing faults. In contrast, in the Lufeng Sag, the second-phase faults include partially reactivated first-phase non-colinear faults and new non-colinear faults. The influence of first-phase faults on second-phase faults is manifested in several distinctive ways. Two sets of major non-colinear faults led to the development of more complicated oblique faults in the Lufeng Sag. The development of the second-phase faults is the result of a combination of geometry and heterogeneity of pre-existing fabrics, variation of extension direction and local perturbations set up by the geometry of pre-existing faults.

Eocene to Oligocene cooling and ice growth based on the geochemistry of interglacial mudstones from the East Antarctic continental shelf

AbstractThe Eocene-Oligocene Transition at c. 34 million years ago (Ma) marked the global change from greenhouse to icehouse and the establishment of the East Antarctic Ice Sheet (EAIS). How the ice-sheet behaviour changed during interglacials across this climate transition is poorly understood. We analysed major, trace and rare earth elemental data of late Eocene interglacial mudstone from Prydz Bay at Ocean Drilling Program Site 1166 and early Oligocene interglacial mudstone from Integrated Ocean Drilling Program Site U1360 on the Wilkes Land continental shelf. Both sites have comparable glaciomarine depositional settings. Lithofacies and provenance at Site 1166 in Prydz Bay are indicative of a late Eocene glacial retreat in the Lambert Graben. Palaeoclimate proxies, including the Chemical Index of Alteration, mean annual temperature and mean annual precipitation, show a dominant warm and humid palaeoclimate for the late Eocene interglacial. In contrast, at Site U1360, in the early Oligocene, the provenance and interglacial weathering regime remained relatively stable with conditions of physical weathering. These results confirm that the EAIS substantially retreated periodically during late Eocene interglacials and that subglacial basins probably remained partially glaciated during interglacials in the earliest Oligocene.

A relatively warm and humid Oligocene climate in the northeastern Tibetan Plateau based on a high-resolution clay mineralogical and geochemical record

The Oligocene was the earliest cool period in the Cenozoic, and thus its terrestrial climatic evolution is of significant interest but remains poorly constrained for Inner Asia. In this study, we obtained a high-resolution paleoclimate record based on clay minerals and silicate chemical weathering proxies of the clay fraction (<2 μm) in sedimentary rocks from the Xining Basin on the northeastern Tibetan Plateau. We compared these data with those for surrounding basins to reconstruct the Oligocene climatic evolution of the northeastern part of the plateau. Illite, chlorite, smectite (I/S; including irregular mixed-layer illite–smectite), and kaolinite are the four main clay minerals in the Oligocene strata. Multiple proxies indicated three characteristic climatic periods. During the late Eocene to early Oligocene, a weak silicate weathering intensity is indicative of an arid climate in the northeastern Tibetan Plateau. A stronger silicate weathering intensity and enrichment in smectite (I/S) and kaolinite indicate more warm and humid climate conditions during 32–25 Ma. During the Oligocene, before the high topography of the northern and northeastern parts of the Tibetan Plateau had developed, the relatively warm and humid climate conditions since 32 Ma were likely caused by enhanced transport of water vapor in response to relatively warm temperatures. The decrease in the degree of silicate weathering from 25 to 23 Ma was possibly caused by regional tectonism and global climatic changes. These relatively warm and humid conditions would explain the occurrence of large Oligocene mammals in the northeastern Tibetan Plateau.

Systematics of the late Eocene ‘oreonetine’ oreodonts (Merycoidodontidae: Artiodactyla)

ABSTRACT The small, primitive ‘oreonetine’ oreodonts from the late Eocene (Chadronian) of North America have long been a systematic mess with gross oversplitting, failure to recognise post-mortem distortion of specimens and many errors in their descriptions. We describe some new fossils and review the group for the first time since 1956. Of the six genera and nine species recognised by Schultz & Falkenbach (1956), only three genera (Oreonetes, Limnenetes, and Bathygenys) and six species are now considered valid. The ‘Oreonetinae’ is a paraphyletic wastebasket group because some members are closer to more derived oreodonts than others, and Limnenetes is a primitive leptauchenine, not a merycoidodontine. These small, rare late Eocene oreodonts found only in Wyoming, Montana, and Texas are the precursors to the enormous radiation of oreodonts in the early Oligocene.

Controls on sedimentation in a deep‐water foredeep: Central Pindos foreland basin, western Greece

AbstractThis study provides a sedimentological, stratigraphic and palaeocurrent investigation of Upper Eocene to Lower Oligocene deep‐sea fan deposits found in the central part of the Pindos foreland basin in western Greece. According to facies analysis, the examined succession at Amfilochia area has 13 sedimentary facies and 10 facies and sub‐facies associations. Depositional elements include abyssal plain pelagics, outer fan, inner fan, and slope deposits. Outer fan sediments are classified as lobe‐axis, lobe‐off‐axis, lobe‐fringe, and distant lobe‐fringe deposits, while inner fan sediments are classified as channel‐fill, crevasse‐splay, internal and external levee deposits. The stratigraphic study shows an upward shift from abyssal plain pelagics to outer, inner fan, and finally slope deposits, implying submarine fan system progradation and progressive infilling of a deep‐water sediment depocentre. The sediments were deposited in the foredeep of the Pindos foreland system and correspond to the system's underfilled stage, when sedimentation was unable to exceed the accommodation provided by lithospheric flexure. They point to deposition near the onset of the Pindos orogen, after the closure of the Pindos basin because of the collision of the Apulian with the Pelagonian microplate during the Cretaceous‐Palaeogene period. Palaeocurrent data from sole marks show bipolar directions associated with two distinct spreading sub‐marine fan deposits. As a result, the study region was split into Upper (major SE‐direction flow) and Lower (major NW‐direction flow) parts, indicating that axial flows were predominant during sediment deposition. However, as the deposition of the elements continued, the progradation of both systems constrained the space accommodation because of the increased basin sediment supply and forced an increase in the degree of basin confinement that changed the compensational to aggradational stacking pattern. The goals of this research are to develop an updated facies model for these deep‐sea fans as well as a robust correlation framework for the various stratigraphic units in the central Pindos foreland basin. This research also connects the stratigraphic development of deep‐sea fan deposits to the evolutionary phases of the Pindos foreland system, providing fresh insights into the palaeogeographic circumstances in the Pindos foreland basin.

Rearranging and completing the puzzle: Phylogenomic analysis of bark beetles Dendroctonus reveals new hypotheses about genus diversification

Studies carried out on bark beetles within Dendroctonus have been extensive and revealed diverse information in different areas of their natural history, taxonomy, evolution, and interactions, among others. Despite these efforts, phylogenetic hypotheses have remained obscured mainly due to limited information analyzed (taxonomic, gene sampling, or both) in studies focused on obtaining evolutionary hypotheses for this genus. With the aim of filling these gaps in the evolutionary history for Dendroctonus, we analyzed ∼1800 loci mapped to a reference genome obtained for 20 of the 21 species recognized to date, minimizing the impact of missing information and improving the assumption of orthology in a phylogenomic framework. We obtained congruent phylogenetic topologies from two phylogenomic inference strategies: loci concatenation (ML framework) and a multispecies coalescent model (MSC) through the analysis of site pattern frequencies (SNPs). Dendroctonus is composed of two major clades (A and B), each containing five and four subclades, respectively. According to our divergence dating analysis, the MRCA for Dendroctonus dates back to the early Eocene, while the MRCA for each major clade diverged in the mid-Eocene. Interestingly, most of the speciation events of extant species occurred during the Miocene, which could be correlated with the diversification of pine trees (Pinus). The MRCA for Dendroctonus inhabited large regions of North America, with all ancestors and descendants of clade A having diversified within this region. The Mexican Transition Zone is important in the diversification processes for the majority of clade A species. For clade B, we identified two important colonization events to the Old World from America: the first in the early Oligocene from the Arctic to Asia (via Beringia), and the second during the Miocene from the Arctic-Western-Alleghany region to Europe and Siberia (also via Beringia). Our genomic analyses also supported the existence of hidden structured lineages within the frontalis complex, and also that D. beckeri represent a lineage independent from D. valens, as previously suggested. The information presented here updates the knowledge concerning the diversification of a genus with remarkable ecological and economic importance.

Late Paleocene to early Oligocene fire ecology of the south Mongolian highland

Changes in fire ecology during warm and cold periods in the geological past are important because of their effects on terrestrial ecosystems and the global carbon cycle. We examined the charcoal concentrations of the Erden Obo section in Inner Mongolia to reconstruct the evolution of wildfire and their relationship to the regional vegetation from the Late Paleocene through Early Oligocene. Our data show that fire frequency were relatively high from the end of the Paleocene to the beginning of the Eocene, in accord with other paleofire records worldwide. However, low fire frequency occurred during the Early Eocene Climate Optimum (EECO), coincident with the change in the regional vegetation from shrubland to forest due to the strengthening of the regional rainfall, and we suggest that the humid climate may have been responsible for this decrease. High frequency fire occurred after the Middle Eocene, near-synchronously with the transition of the regional vegetation from forest to steppe. The high-frequency fire was most likely triggered by regional drought during the aridification process after the Middle Eocene. We propose that these temporal changes in the fire ecology were consistent within the northern temperate zone from the Late Paleocene through Early Oligocene, and we suggest that studies of global wildfires need to be evaluated within the context of paleovegetation zones and ecosystem evolution.

Paleogeographic approach for the oil and gas potential assessment of the Khadum Formation (Lower Oligocene) in Pre-Caucasus basin

Authors used the paleogeographic approach to identify the different facies generation potential, mark out the areas with the highest organic matter and its types, analyse the reservoir quality for oil and gas potential assessment of the Pre-Caucasus basin Khadum formation. The greatest prospects in the Western and Central parts of the basin characterized by terrestrial and partly mixed organic matter types and are associated with the facies of the Sarpinsk-Armavir channel where discharge currents from the Volga-Don basin occurred. The same organic matter types can be related to clastic fans drawn from the north. In the Eastern part the main prospects are associated with unconventional black shale reservoirs with the predominantly aquatic organic matter, isolated on the top and base by the thick series of seal rock.

New evidence for late Eocene-early Oligocene uplift of Aves Ridge and paleogeography of GAARlandia

The GAARlandia hypothesis has produced vigorous debate among biologists regarding whether now-submerged landforms that existed in the Caribbean region during the late Paleogene might have acted as a barrier for marine organisms and as a bridge for terrestrial biotas migrating from South America into the Greater Antilles. This concept derived from the hypothesized emergence history of the Aves Ridge. In the quarter century since GAARlandia was first proposed, new paleontological, geological and geophysical information has greatly extended the database available. Here we reaffirm that GAARlandia was a positive topographic feature from middle Eocene, and was exposed above sea level between late Eocene and early Oligocene when it facilitated biotic colonization of the northern Greater Antilles and their satellite islands, whether as a series of closely spaced islands or as a continuous peninsula projecting from northeastern South America along the crown of the rise.

Eocene to present evolution of the Pearl River, China: A quantitative provenance interpretation on large detrital zircon U–Pb geochronological data

AbstractDrainage evolution of the Pearl River, one of the major rivers in the eastern margin of the Asian continent, has important implications for the tectonomorphic evolution of the northern margin of the South China Sea and the southeastern Tibetan Plateau. Previous reconstructions using different methods suggested discrepant timings for the formation of river, ranging from Early Oligocene to Middle Miocene. Here, we address this heavily debated topic using quantitative unmixing modelling of detrital zircon data from the Pearl River Mouth Basin in the northern margin of the South China Sea. In this study, we develop a novel approach for estimating the relative contributions of detrital zircon sources to their shared sink, and propose to use the correlation coefficients among zircon contribution models to evaluate the trade‐off among sources with similar age spectra, to avoid the potential overinterpretation of individual contributions. Our new method is applied to new (997) and published detrital zircon U–Pb data from offshore boreholes and modern Pearl River samples to quantitatively interpret and reconstruct the sediment provenance of the Pearl River Mouth Basin and the development of the Pearl River. Our findings reveal that the provenance change of the Pearl River Mouth Basin can be divided into three main stages. Eocene sediments were mainly sourced in the intra‐basinal highlands and the eastern coastal tributaries, indicating a local drainage system. Early Oligocene provenances extended westward, as shown by the increase in sediment contribution from the central and western parts of the Pearl River (28% in total). Since the Late Oligocene, the eastern, central and western parts of the modern Pearl River have contributed equal amounts of zircons to the Pearl River Mouth Basin, indicating the establishment and long‐term stability of the modern‐like drainage system, as highlighted by our new data acquired from the borehole Miocene strata. The Late Oligocene westward expansion of the Pearl River is consistent with the timing of the coeval breakup and spreading of the South China Sea and the intensified Asian monsoon precipitation, highlighting the importance of base level fall and climate in controlling the drainage evolution.

Multi-locus phylogeny of the catfish genus Ictalurus Rafinesque, 1820 (Actinopterygii, Siluriformes) and its systematic and evolutionary implications

BackgroundIctalurus is one of the most representative groups of North American freshwater fishes. Although this group has a well-studied fossil record and has been the subject of several morphological and molecular phylogenetic studies, incomplete taxonomic sampling and insufficient taxonomic studies have produced a rather complex classification, along with intricate patterns of evolutionary history in the genus that are considered unresolved and remain under debate.ResultsBased on four loci and the most comprehensive taxonomic sampling analyzed to date, including currently recognized species, previously synonymized species, undescribed taxa, and poorly studied populations, this study produced a resolved phylogenetic framework that provided plausible species delimitation and an evolutionary time framework for the genus Ictalurus.ConclusionsOur phylogenetic hypothesis revealed that Ictalurus comprises at least 13 evolutionary units, partially corroborating the current classification and identifying populations that emerge as putative undescribed taxa. The divergence times of the species indicate that the diversification of Ictalurus dates to the early Oligocene, confirming its status as one of the oldest genera within the family Ictaluridae.

Oligocene crabs (Decapoda: Brachyura) from the Asmari Formation in Yasuj area (SW Iran)

New findings of crustacean decapod brachyurans from the Rupelian period (lower Oligocene) in Iran are presented in this study. In particular, Lophoranina sp. and Palaeocarpilius cf. P. rugifer Stoliczka, 1871, from the Rupelian strata were found in two previously unexplored localities, Abshar and Vezg near Yasuj in the southwestern region of Iran. The discovery contributes to fill the gap in the record of brachyuran decapod crustaceans in Iran during the Eocene and Miocene periods. The presence of both genera in the Oligocene of Iran suggests a certain degree of faunal similarity among brachyurans on both sides of the Tethys Realm.

New heterodont odontocetes from the Oligocene Pysht Formation in Washington State, U.S.A., and a reevaluation of Simocetidae (Cetacea, Odontoceti).

Odontocetes first appeared in the fossil record by the early Oligocene, and their early evolutionary history can provide clues as to how some of their unique adaptations, such as echolocation, evolved. Here, three new specimens from the early to late Oligocene Pysht Formation are described further increasing our understanding of the richness and diversity of early odontocetes, particularly for the North Pacific. Phylogenetic analysis shows that the new specimens are part of a more inclusive, redefined Simocetidae, which now includes Simocetus rayi, Olympicetus sp. 1, Olympicetus avitus, O. thalassodon sp. nov., and a large unnamed taxon (Simocetidae gen. et sp. A), all part of a North Pacific clade that represents one of the earliest diverging groups of odontocetes. Amongst these, Olympicetus thalassodon sp. nov. represents one of the best known simocetids, offering new information on the cranial and dental morphology of early odontocetes. Furthermore, the inclusion of CCNHM 1000, here considered to represent a neonate of Olympicetus sp., as part of the Simocetidae, suggests that members of this group may not have had the capability of ultrasonic hearing, at least during their early ontogenetic stages. Based on the new specimens, the dentition of simocetids is interpreted as being plesiomorphic, with a tooth count more akin to that of basilosaurids and early toothed mysticetes, while other features of the skull and hyoid suggest various forms of prey acquisition, including raptorial or combined feeding in Olympicetus spp., and suction feeding in Simocetus. Finally, body size estimates show that small to moderately large taxa are present in Simocetidae, with the largest taxon represented by Simocetidae gen. et sp. A with an estimated body length of 3 m, which places it as the largest known simocetid, and amongst the largest Oligocene odontocetes. The new specimens described here add to a growing list of Oligocene marine tetrapods from the North Pacific, further promoting faunistic comparisons across other contemporaneous and younger assemblages, that will allow for an improved understanding of the evolution of marine faunas in the region.

Biostratigraphic Interpretation of Lutut Beds, Kerek Formation, Based on Foraminifera Fossils

Lutut Beds are the sandstone beds exposed in the northwest margin of Kendeng Basin, which contain abundant metamorphic-quartz grains, frequent recycled sedimentary quartz, and reworked bioclasts. Lutut Beds are not shown on the regional geological map and are often neglected in determining the geological history of Kendeng Basin. However, they have significant roles because it was deposited on the basin edge, which may carry important keys. This study aims to discover the larger and smaller foraminifera fossils contained in the Lutut Beds. Outcrop samples were collected in Kali Lutut and its surrounding area, including Lutut Beds and its overlying layers. Larger foraminifera was identified within 3 of 11 thin sections, mostly of Order Rotaliida, which occur as reworked bioclasts. The smaller foraminifera was barely found in most samples except for the samples from the overlying layers of Lutut Beds. The study reveals that Lutut Beds were deposited during Early to Middle Miocene, marked by the occurrence of Miogypsina sp. and Miogypsinoides sp. and also contains reworked Eocene – Early Oligocene larger foraminifera such as Nummulites sp., Discocyclina sp., and Dictyoconus sp. Besides, the overlying layer of Lutut Beds is identified to be deposited in the Middle Miocene to Pliocene based on the occurence of smaller planktonic foraminifera, Sphaeroidinella subdehiscens. The bathymetric interpretation based on smaller benthic foraminifera showed that Lutut Beds were deposited in the upper-middle bathyal zone. We also believe that the larger benthic foraminifera fossils in Lutut Beds were transported along the slope from its original life position. Accordingly, it is considered as allochthonous fossils.

Species richness disparity in tropical terrestrial herbaceous floras: Evolutionary insight from Collabieae (Orchidaceae)

Species richness is spatially heterogeneous even in the hyperdiverse tropical floras. The main cause of uneven species richness among the four tropical regions are hot debated. To date, higher net diversification rates and/or longer colonization time have been usually proposed to contribute to this pattern. However, there are few studies to clarify the species richness patterns in tropical terrestrial floras. The terrestrial tribe Collabieae (Orchidaceae) unevenly distributes in the tropical regions with a diverse and endemic center in Asia. Twenty-one genera 127 species of Collabieae and 26 DNA regions were used to reconstruct the phylogeny and infer the biogeographical processes. We compared the topologies, diversification rates and niche evolutionary rates of Collabieae and regional lineages on empirical samplings and different simulated samplings fractions respectively. Our results suggested that the Collabieae originated in Asia at the earliest Oligocene, and then independently spread to Africa, Central America, and Oceania since the Miocene via long-distance dispersal. These results based on empirical data and simulated data were similar. BAMM, GeoSSE and niche analyses inferred that the Asian lineages had higher net diversification and niche evolutionary rates than those of Oceanian and African lineages on the empirical and simulated analyses. Precipitation is the most important factor for Collabieae, and the Asian lineage has experienced more stable and humid climate, which may promote the higher net diversification rate. Besides, the longer colonization time may also be associated with the Asian lineages’ diversity. These findings provided a better understanding of the regional diversity heterogeneity in tropical terrestrial herbaceous floras.

Fluid effect on zircon O and U-Pb isotopes during ultrahigh-pressure metamorphism: Insights from the Dora-Maira Massif of the Western Alps

Zircon geochemistry such as U-Pb and O isotopes have been widely used in dating and tracing complex geological processes. However, it still remains unclear how fluid action affects zircon geochemistry during metamorphic and metasomatic processes in subduction zones. Here a systematic study on zircon U-Pb dating, O isotopes and trace elements as well as whole-rock O isotopes was carried out for the coesite-bearing whiteschists, jadeite quartzites and granitic gneisses from the Dora-Maira Massif, Western Alps. Whole-rock and zircon geochemistry supports a common protolith, i.e., Permian S-type granites, for the above three types of rocks and an intense fluid metasomatism during the Alpine orogeny to form whiteschists and jadeite quartzites. Zircon cores in all samples have nearly identical δ18O values (9‰–11‰), whereas their apparent 206Pb/238U ages show a greater variability due to Pb loss during metamorphism. Zircon rims formed in the late Eocene to early Oligocene can be categorized into two types. Type-I rims occur in granitic gneisses and jadeite quartzites. They have high δ18O values consistent with zircon cores, but much lower contents of P and Y as well as lower Th/U ratios than the cores. Their growth can be attributed to internal metamorphic fluid action at the UHP metamorphic stage. Type-II rims occur in whiteschists and jadeite quartzites. They have remarkably lower δ18O values (5‰–8‰) and Th/U ratios (<0.01), compared with zircon cores and Type-I rims. Their growth can be ascribed to external fluids during the metasomatic process. Some zircon domains in whiteschists and jadeite quartzites show a positive correlation between δ18O values and apparent 206Pb/238U ages, which suggest the simultaneous impacts on U-Pb-O isotopes during external fluid metasomatism. This process can be attributed to the fluid-assisted dissolution and recrystallization of protolith zircons. Especially, coesite inclusions that would have been expected to occur only in young zircon rims formed during UHP metamorphism are also observed in the relict magmatic zircon cores, indicating that the fluid-related metasomatism at the UHP metamorphic conditions also affected these pre-existing protolith-related cores. Therefore, fluid action in subduction zones reveals significant impacts on both the U-Pb and O isotope systems of zircon, especially when external metasomatic fluids are involved. Therefore, a detailed study on zircon, including microstructure, mineral inclusion and geochemical data of different growth and recrystallization domains, is needed in order to unravel continental crustal evolution based on zircon U-Pb ages and O isotope compositions.