Late Oligocene Research Articles

Lithospheric evolution and uplift of the Tibetan Plateau during continental convergence: evidence from Early Oligocene pseudoleucite phonolites from southern Qiangtang, central Tibet

Abstract The Cenozoic collision of India and Eurasia clearly built the high-elevation Tibetan Plateau, but how this collision was accommodated, and the Tibetan Plateau uplifted, remains an area of study. The widespread occurrence of Cenozoic potassic-ultrapotassic lavas provides a valuable opportunity to constrain the relationship between surface deformation and underlying geodynamic processes. In this study, we report sanidine Ar–Ar dating, whole-rock geochemical and clinopyroxene Sr–Nd isotope analyses on pseudoleucite phonolites from the Yulinshan area of south Qiangtang (central Tibet) to determine their petrogenesis and regional tectonic significance. Sanidine Ar–Ar dating yields an Early Oligocene age of ca. 30 Ma, in agreement with previous studies. A high modal abundance of pseudoleucite in the samples indicates their ultrapotassic and silica-undersaturated composition. All of the rocks have typical arc-like geochemical signatures and enriched whole rock and clinopyroxene Sr–Nd isotope signatures. Based on thermodynamic phase equilibria modeling and our detailed study of the textural and in-situ compositional information of clinopyroxene, the intermediate to felsic potassic magmas are interpreted to have been formed through differentiation of primitive, mantle-derived, potassic magmas at shallow crustal levels. The inferred enrichment of the lithospheric mantle is probably related to subduction of the Songpan-Ganze continental lithosphere beneath Qiangtang after India-Asia collision. Voluminous Late Eocene-Early Oligocene magmatism in central Tibet may reflect partial removal of the mantle lithosphere, perhaps triggered by instabilities associated with previous lithospheric thickening. Our work provides additional constraints on the timing of mantle root loss that likely contributed to the surface uplift of central Tibet, which post-dated the cessation of upper crustal shortening and deformation.

The Cibarco anticline of the northernmost San Jacinto fold belt: Possible influence of mud diapirism on the Neogene to the recent evolution of the structure where the first oil and gas wells were drilled in Colombia

The Cibarco anticline, located in the northwestern South American convergent margin, is a long (approximately 40 km) and narrow (2.5 km wide), south-southwest–north-northeast-trending structure that separates two much wider depocenters, the Tubará-Juan de Acosta depocenter (approximately 20 km wide) to the west, and the Sabanalarga syncline (approximately 11 km wide) to the east. Detailed geologic surface mapping and subsurface seismic interpretation suggest that the formation and evolution of the Cibarco anticline have been strongly influenced by mud diapirism, which has been focused along a west-southwest–east-northeast-trending, inverted pre-Oligocene extensional fault. Seismic interpretation and outcrop analyses also confirmed that the source of mobile shales is the Upper Oligocene marlstones of the Lower Ciénaga de Oro Formation. We suggest that the main mechanism that caused the overpressures that triggered and controlled shale mobilization was disequilibrium compaction related to the sedimentary loading of impermeable seals and the shortening and inversion of pre-Oligocene extensional faults. Growth strata indicate that the Upper Oligocene mud started moving in Early Miocene times during the deposition of the thick Porquero mudstones, and it stopped migrating in Latest Miocene times. After an uplift and erosion event, mud diapirism remained inactive in the Early Miocene, and it was then reactivated in Late Pliocene times. Slight mud migration focused on the inverted fault continued throughout the Pleistocene and to the present. Today, the slight mud migration to the surface is observed in volcanoes located in the northern strand of the Cibarco anticline, although the rest of the anticline has become welded. Our new interpretations, carried out more than a century after the first hydrocarbon exploration activities began in Colombia, will hopefully allow us to increase our understanding of shale tectonics, petroleum systems, and plays in this portion of the convergent margin.

A new tiny fossil penguin from the Late Oligocene of New Zealand and the morphofunctional transition of the penguin wing

ABSTRACT The Late Oligocene is a period of high penguin diversity, following major changes in the marine environment at the Eocene/Oligocene boundary and prior to the emergence of crown penguins in the Miocene. Historically, a large morphological gap existed between the most crownward Platydyptes among the Oligocene penguins from New Zealand and the Early Miocene stem penguins such as Palaeospheniscus from South America. Here we describe a new species that contributes to filling this gap. Pakudyptes hakataramea gen. et sp. nov. is the earliest tiny penguin, overlapping in size with the little penguin Eudyptula minor. Its distinctive combination of a well-developed proximal end of the humerus and an archaic elbow joint provides clues to the evolution of penguin wings. Phylogenetic analysis indicates that penguin wings evolved rapidly from the Late Oligocene to the Early Miocene, together with the acquisition of morphofunctional and hydrodynamical characteristics that enable the excellent swimming ability of modern penguins. As an indicator of aquatic adaptation, bone microanatomy shows a comparable structure to that of Eudyptula. The appearance of the smallest body size and the evolution of modern wings may have led to the ecological diversity of modern penguins, which confirms the importance of Zealandia in penguin evolution.

The first recognition of the enigmatic fossil shark genus Megalolamna (Lamniformes, Otodontidae) from the lower Miocene of Europe and M. serotinus (Probst, 1879) as the newly designated type species for the genus

Megalolamna is an elusive extinct shark genus (Lamniformes: Otodontidae) previously known from the upper Oligocene–Miocene of the USA, Colombia, Ecuador, Peru, and Japan. Here, we document the first occurrence of Megalolamna from Miocene marine deposits in Europe, specifically from Austria, France, Germany, and Italy. Although the taxon is regarded as a monospecific genus, this study reveals that the species name serotinus has the priority over any previously used names for the taxon and thus redescribes it as Megalolamna serotinus (Probst), comb. nov. This study is also significant because it demonstrates the existence of Megalolamna in the Mediterranean and Paratethys seas during the Miocene. Megalolamna inhabited the tropical–mid-latitudinal zones with spotty but wide geographic distribution.

Mitogenomic Architecture of Atlantic Emperor Lethrinus atlanticus (Actinopterygii: Spariformes): Insights into the Lineage Diversification in Atlantic Ocean.

The evolutionary history of emperors, particularly in the Atlantic and Indo-West Pacific Oceans, remains largely unmapped. This study explores the maternal lineage evolution of Lethrinids by examining the complete mitogenome of Lethrinus atlanticus, which is endemic to the Eastern Atlantic Ocean. Utilizing advanced next-generation sequencing, we found that the mitogenome spans 16,789 base pairs and encompasses 37 genes, including 13 protein-coding genes (PCGs), two ribosomal RNAs, 22 transfer RNAs, and an AT-rich control region (CR). Our analysis indicates a preference for AT base pairs in the L. atlanticus mitogenome (53.10%). Most PCGs begin with the ATG codon, except for COI, which starts with GTG. Relative synonymous codon usage reveals high frequencies for alanine, leucine, proline, serine, and threonine. The ratio of nonsynonymous to synonymous substitutions suggests strong negative selection across all PCGs in Lethrinus species. Most transfer RNAs exhibit typical cloverleaf structures, with the exception of tRNA-serine (GCT), which lacks a dihydrouracil stem. Comparative analysis of conserved sequence blocks across the CRs of three Lethrinus species shows notable differences in length and nucleotide composition. Phylogenetic analysis using concatenated PCGs clearly distinguishes all Lethrinus species, including L. atlanticus, and sheds light on the evolutionary relationships among Spariformes species. The estimated divergence time of approximately 20.67 million years between L. atlanticus and its Indo-West Pacific relatives provides insights into their historical separation and colonization during the late Oligocene. The distribution of Lethrinids may be influenced by ocean currents and ecological factors, potentially leading to their speciation across the Eastern Atlantic and Indo-West Pacific. This study enhances our understanding of the genetic diversity and phylogenetic relationships within Lethrinus species. Further exploration of other emperor fish mitogenomes and comprehensive genomic data could provide vital insights into their genetic makeup, evolutionary history, and environmental adaptability in marine ecosystems globally.

Development of arcuate faults and controls on hydrocarbon accumulation in the Tiantai slope, Xihu Depression, East China Sea basin

The strike-slip movement of NW-trending transfer zone in the Tiantai slope of the Xihu Depression governed the development of the Cenozoic arcuate faults. These faults significantly influenced trap formation, sand-bodies distribution, and hydrocarbon migration. Based on seismic, drilling data and fault activity analysis methods, this paper describes the origin and evolution of the arcuate faults and their impact on hydrocarbon accumulation. These arcuate faults exhibit a change from NE to NW strike orientation in plan view, and are distributed in the hanging wall of the NW-trending basement faults. The main arcuate faults penetrate the bottom of the Cenozoic strata and extend upward through the top of the lower member of the Huagang Formation, markedly affecting the depositional dynamics of the Pinghu Formation. The segments of the arcuate faults began to grow during the Middle Eocene, and experienced hard linkages in the Late Eocene. The activity of these arcuate faults diminished significantly and gradually stopped during the Middle and Late Oligocene. The segmentation and linkage growth processes of the arcuate faults controlled the formation and evolution of traps. Additionally, the activities of these faults influenced the distribution of sandbodies, with the segmentation points of the arcuate faults serving as important conduits for sediment input. Moreover, the arcuate faults served as effective pathways for vertical hydrocarbon migration. The area with the arcuate faults present favorable conditions for hydrocarbon accumulation in the Tiantai slope of the Xihu Depression.

Occurrence characteristics of authigenic pyrite in the deep-sea environment and its paleoceanographic implications based on core sediments from IODP Expedition 342 Site U1406

Although authigenic pyrite (FeS2) in marine sediment is an important proxy for oxygen conditions and microbial activities at its formation site, very little is known about its morphological characteristics in the deep-sea (> 3500 m) environment and their paleoceanographic implications. Here, we report the occurrence characteristics of authigenic pyrite including texture, size distribution, and relative content in North Atlantic deep-sea sediments from Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406. The results of electron microscopy show octahedral pyrite microcrystals of 1.5–4.0 μm, the spherical texture of greigite (Fe3S4), and large pyrite framboids (average, 25 μm) relative to those of the continental margin. These features may demonstrate an unchanged supersaturation level of the pore water for octahedral pyrite after its initial formation, replacement of the precursor greigite by pyrite, and an expanded pyrite framboid formation zone with a low sulfate reduction rate in deep-sea sediments, respectively. Although the size distribution of pyrite framboids measured in this study falls into the range of pyrites formed in the sulfate–methane transition zone, based on onboard geochemical data and previous research, these pyrites most likely formed through organoclastic sulfate reduction during diagenesis. We assessed the implications of these findings for paleoceanographic changes, as the formation of diagenetic pyrite is controlled by the supply rate and reactivity of organic matter, which are closely related to ocean productivity and circulation. A comparison between the profiles of relative pyrite content and benthic foraminiferal oxygen isotope (δ18Obf) values indicated that these factors were anti-phased during the late Oligocene. Based on our results, we conclude that the formation of diagenetic pyrite was favored during interglacial periods due to increases in the amount and reactivity of organic matter driven by more energetic formation of proto-North Atlantic Deep Water. Our findings provide new insights into the formation of authigenic pyrite in deep-sea environments and its potential as a paleoceanographic proxy.

Geochemical modeling of diagenetic reactions between albitization of K-feldspar and plagioclase feldspar in sandstone reservoirs under the influence of CO2 partial pressure

Diagenetic albitization has been observed in sedimentary basins around the world. This process significantly changes the original composition of sandstones and the chemistry of the formation waters under the influence of partial pressure of CO2. The transformation of detrital feldspars into albite is considered a crucial diagenetic process in the Gulf Coast and North Sea reservoirs. Earlier studies suggest that plagioclase albitization typically happens before that of K-feldspar. In the Gulf Coast's Frio Sandstone, located in the Upper Oligocene at depths between 900 and 2400 m, detrital plagioclase is often dissolved and replaced by albite, while K-feldspar mostly dissolves without much substitution. Similarly, in the North Sea reservoirs, especially in the upper section of the Upper Triassic Lunde Formation at depths beyond 2900 m, plagioclase tends to undergo albitization, whereas K-feldspar remains largely unaffected or experiences minimal transformation. This research focuses on analyzing the differences in the albitization patterns of detrital and K-feldspar plagioclase through the KINDISP and Geochemist's Workbench (GWB) geochemical modeling tools, aiming to compare them. These diagenetic processes are crucial for reservoir geology, as they influence the concentration of silica in water, which, in turn, affects quartz cementation. This study aims to explore the variations in the albitization behavior of detrital and K-feldspar plagioclase using the KINDISP and Geochemist's Workbench (GWB) geochemical models and conduct a comparative analysis between them. Understanding these diagenetic reactions becomes relevant for reservoir geology analysis, as such phenomena control the aqueous silica concentration to some extent, which is consequently reflected in the quartz cementation. The dissolution of plagioclase and K-feldspar releases silica into the pore fluids. As the concentration of silica in the fluid increases, it leads to the precipitation of quartz as overgrowths on detrital quartz grains, a process known as quartz cementation. This was observed particularly in simulations involving temperature increases up to 150 °C, where the equilibrium between albite and anorthite was closely linked to the stability of quartz (Ben et al., 1993). The removal of feldspar through albitization reduces porosity and permeability but contributes silica to the system, which promotes quartz cementation. This, in turn, decreases the reservoir quality by filling pore spaces with secondary quartz, reducing the rock's ability to store and transmit fluids. Thus, the study highlights the importance of these diagenetic processes in reservoir evaluation, as the balance between feldspar dissolution and quartz cementation ultimately controls reservoir properties.

Early Eocene Metasequoia and Taxodium (Cupressaceae) from Lingbao, Henan Province and their implication for a perhumid climate in central China

In this study, fossil leaves of Metasequoia Hu and Cheng, along with leaves and cones of Taxodium Richard, from the lower Eocene of the Xiangcheng Group in Lingbao, Henan Province, central China, were examined. These fossils have been taxonomically assigned to Metasequoia sp. (cf. M. glyptostroboides Hu and Cheng) and Taxodium dubium (Sternberg) Heer. The closest modern relatives are Metasequoia glyptostroboides and Taxodium distichum (Linnaeus) Richard, which typically thrive in humid environments, indicating that similar conditions may have prevailed in early Eocene. Through the Species Distribution Model (SDM) experiment, it was suggested that T. distichum is significantly influenced by precipitation levels, highlighting the importance of moisture in their historical distribution. Moreover, the analysis of the survival environments of native Metasequoia and Taxodium indicates that the annual precipitation in Lingbao during the early Eocene exceeded 1000 mm, which is roughly double the current annual precipitation in the area. This precipitation level is comparable to the present-day subtropical climate zone. During the late Eocene and early Oligocene, the seasonal variation in precipitation was intensified due to the strengthening of the East Asian monsoon, which constrained the seed germination and seedling survival. This may have contributed to the decline in the populations of Metasequoia and Taxodium in East Asia during that period.

Evolutionary palaeoecology of European rhinocerotids across the Oligocene-Miocene transition.

The Oligocene-Miocene transition witnessed great environmental and faunal changes, spanning from late Oligocene to early Miocene (MP28-MN3). Its drivers and consequences on mammals are, however, poorly understood. Rhinocerotoids are among the most affected taxa, reflected by great taxonomical and morphological changes. However, potential associated changes in ecology have not been explored. Here, we investigated the palaeoecology of 10 rhinocerotid species coming from 15 localities across Western Europe and ranging from MP28 to MN3. We explored evolutionary trends for diet, physiology and habitat via dental wear, hypoplasia, body mass and stable isotopy. All rhinocerotids studied were C3 feeders, whether browsing or mixed-feeding, but clear dietary differences were observed at some localities and between Oligocene and Miocene rhinocerotids. The prevalence of hypoplasia was low (less than 10%) to moderate (less than 20%), but there were great differences by loci, species and localities. Body mass covaried with hypoplasia prevalence, suggesting that larger species might be more susceptible to stresses and environmental changes. We reconstructed similar warm conditions at all localities except Gaimersheim, but found greater variations in precipitation. Indeed, a clear shift in δ13C values was noticed at the end of the Oligocene, consistent with climatic and vegetation changes reported at that time.

Hearing abilities of a late-surviving archaeocete (Cetacea: Kekenodontidae), and implications for the evolution of sound in Neoceti.

Kekenodontids are the only known archaeocetes (stem cetaceans) from the late Oligocene. They possess a unique combination of morphological features seen in both more primitive Eocene basilosaurid archaeocetes and more derived Neoceti (mysticetes and odontocetes). However, much remains unknown about the clade, including its acoustic biology. Based on its phylogenetic position crownward to basilosaurids as the latest-diverging archaeocete, we hypothesize that kekenodontids would be specialized for hearing low-frequency sounds. Here, we provide the first report on the cochlear anatomy of a kekenodontid using the holotype of Kekenodon onamata from New Zealand. We compare the cochlear morphology of K. onamata to a sample of extinct and extant cetaceans and quantify shape differences using three-dimensional geometric morphometrics. The analyses show that K. onamata was indeed adapted to hear low frequencies and suggests low-frequency hearing may be a characteristic of raptorial macrophagous fossil cetaceans in contrast to infrasonic bulk filter-feeding mysticetes and ultrasonic echolocating odontocetes.

Palynostratigraphy, palynofacies and reservoir characteristics of Upper Oligocene - Lower Miocene sediments in the southwestern Cuu Long Basin, Vietnam

Palynostratigraphy, palynofacies and reservoir characteristics of Upper Oligocene - Lower Miocene sediments in the southwestern Cuu Long Basin, Vietnam

Petrophysical and seismo-stratigraphic analysis of the tertiary formations of the South onshore of the Rio Del Rey Basin (Cameroon margin, Gulf of Guinea) from well logs and seismic data

The present study used well log and seismic data to determine the main lithology types, characterize and evaluate the reservoirs properties and discuss about the hydrocarbon quality and productivity in the onshore south of the Rio Del Rey Basin (RDRB). The main results show seven lithologies inside the Isongo, Diongo and Nguti including clays, sandstones and clayey sandstones, the most dominant, and limestones, dolomites, sandstone limestones and clayey limestones, the less dominant. Seventeen reservoir zones with fifteen gas-bearing and two oil-bearing were identified in the Miocene sands of the Isongo, Diongo and Nguti Formations and very few in the Upper Oligocene sands of Akata Formation. The average values of petrophysical parameters obtained are Vshale (13.26–31.3%), NTG (64.48–87.07%), ∅eff (18.54–26.60%), ∅avg (23.27–33.59%), Sw (4.84–11.10%), Shc (88.90–95.16%), and K (1258.96–2187.26 mD). It shows that the identified reservoirs are of the good quality and can be commercially viable for the gas. Knowing that the RDRB and the neighbouring ones (Niger Delta, Rio Muni and Gabon basins which are OPEC members) have good petrophysical parameters, findings from this investigation show that the key difference between RDRB and others lay on the low continuity and vertical extension of these reservoirs, as well as the lower sedimentary thickness (6–7 km), the tectono-stratigraphic architecture and the production essentially focused on the Miocene horizons.

Evidence for a floristically diverse rainforest on the Falkland archipelago in the remote South Atlantic during the mid- to late Cenozoic

Abstract We report the discovery of an ancient forest bed near Stanley, on the Falkland Islands, the second such ancient deposit identified on the South Atlantic island archipelago that is today marked by the absence of native tree species. Fossil pollen, spores and wood fragments preserved in this buried deposit at Tussac House show that the source vegetation was characterized by a floristically diverse rainforest dominated by Nothofagus-Podocarpaceae communities, similar to cool temperate Nothofagus forests/woodlands and Magellanic evergreen Nothofagus rainforests. The age limit of the deposit is inferred from the stratigraphic distribution of fossil pollen species transported by wind, birds or ocean currents from southern Patagonia, as well as similar vegetation types observed across the broader region. The deposit is suggested to be between Late Oligocene and Early Miocene, making it slightly older than the previously analysed Neogene West Point Island forest bed (200 km west of Tussac House). The combined evidence adds to our current knowledge of the role of climate change and transoceanic dispersal of plant propagules in shaping high-latitude ecosystems in the Southern Hemisphere during the late Palaeogene and Neogene.

Revisions, new taxa, and venation transformations of the sawfly family Blasticotomidae sensu lato (Hymenoptera: Tenthredinoidea) highlight the evolution of the basal Hymenoptera

Abstract Wing venation pattern is particularly important in the taxonomy and classification of insects, especially for fossil material. There are recognized transformation series that apparently often represent a mainstream in the evolutionary trend of the wing venation of Hymenoptera. One notable instance is the gradual reduction of the subcosta veins from multiple branches to their total absence. Herein, we place four new genera and six new species in the family Blasticotomidae (=Xyelotomidae) of Hymenoptera. They are Xyelocerus abruptus sp. nov., Liberitoma tenella gen et sp. nov., Liberitoma compta sp. nov., Liberitoma incompleta sp. nov., Aduantoma insolita gen. et sp. nov., and Apertoma gen. nov. from the Middle Jurassic of China, and Enspeletoma oligocaenica gen. et sp. nov. from the Upper Oligocene of Germany. The newly discovered species of Liberitoma from China possessed a supernumerary hind branch of vein Sc in its forewing, which has apparently never been recorded before in other hymenopterans, and the unique venation pattern of Aduantoma suggests the possibility of two additional steps of vein Sc transformation before its complete loss. Based on the new findings, Blasticotomidae are proposed to be divided into four subfamilies: Blasticotominae Thomson 1871, Dahurotominae subfam. nov., Pseudoxyelocerinae subfam. nov., and Undatominae subfam. nov.

Settings Trace elements and garnet formation in a distal skarn zone: a case study of the Rudnik deposit, Central Serbia

The Rudnik Pb-Zn deposit is hosted in skarns and hornfels formed in the late Oligocene by contact metamorphism of limestones, sandstones and shales. Garnets, together with epidote, represent the main non-metallic minerals in the Rudnik skarn. In the distal skarn zone, the garnets are rare and their occurrence is related to the flow path of hydrothermal fluids. To constrain the hydrothermal and physicochemical conditions, in situ elemental SEM-WDS and LA-ICP-MS analyses, and fluid inclusion microthermometric measurements, were made. The Rudnik garnets from the distal skarn zone are predominantly of andradite-grossular composition (Adr39.3–88.9Grs2.9–53.9Alm0.5–10.0), with a small amount of spessartine. Generally, the Fe-rich garnets show a positive Eu anomaly with LREE enrichment and a HREE flat pattern, with homogenization temperatures and salinities of fluid inclusions ranging from 373 to 392°C and from 14.25 to 15.27% NaCl equivalent, respectively. The trace elements and microthermometric properties indicate that the garnets formed at moderately high temperatures, mildly acidic pH levels and increased oxygen fugacity

Revised timing of rapid exhumation in the West Qinling: Implications for geodynamics of Oligocene-Miocene Tibetan plateau outward expansion

Two contrasting age models for initial mountain building in the northeastern (NE) Tibetan Plateau (Paleocene-early Eocene versus late Oligocene-early Miocene) have led to the debate on how the deformed continental lithosphere absorbs plate convergence in general. The initial compressional deformation in the West Qinling (WQL) of the NE Tibetan Plateau figures prominently in this ongoing debate. Here, apatite (U-Th)/He (AHe) thermochronology combined with geomorphological analysis are used to refine the onset of compressional deformation in the WQL. New AHe ages from two vertical transects and an updated reconstruction of an obliquely-tilted erosion surface document the accelerated exhumation in the northern WQL at 23-22 Ma, interpreted as the onset of north-vergent thrusting. The AHe results, together with sedimentary records in the intermontane and foreland basins, suggest that the entire WQL began experiencing compressional deformation in the late Oligocene-early Miocene. When integrated with previous studies, our findings show that the northern plateau boundary has not remained stationary since the collision, but has instead experienced ∼750 km of outward expansion during the late Oligocene to middle Miocene. This phase of rapid plateau growth is coeval with the ∼30–50 % reduction of the India-Eurasia convergence rate, which suggests that the increased gravitational potential energy of orogenic belts played a key role in plate motion changes.

Nutrient Shift in the Yucatan Basin during the late Early Miocene: Evidence from calcareous nannofossils and stable isotopes at ODP Leg 165 Site 998 Hole A

Paleoceanographic reconstructions in the Caribbean Sea during past climate transitions, namely the Late Oligocene to Early Miocene (up to the Miocene Climatic Optimum and Monterey Excursion) are crucial to understanding the effects of the Eastern Pacific interchange with the Atlantic Ocean. Studies of the planktonic domain in the Yucatan Basin during the Early Miocene are limited, with most Caribbean research focused on reconstructing the series of events that precede the final closure of the Isthmus of Panama in the Early Pliocene. Furthermore, paleoceanography of the Yucatan Basin is largely unrefined during this time, even though it is an important transition for ocean gateways, and development of Atlantic deepwater and Atlantic Meridional Overturning Current (AMOC), driving global ocean circulation. Here we conduct a relative percent abundance analysis of calcareous nannofossils and stable isotopes from bulk sediment of drill cores spanning the Upper Oligocene to Lower Miocene in the central Yucatan Basin, collected by the Ocean Drilling Program Leg 165 at Site 998 Hole A. Nannofossil assemblage data record a shift from a eutrophic to oligotrophic system ∼19.2–17.65 Ma. Eutrophic taxa (Reticulofenestra spp. and Cyclicargolithus floridanus) decline at ∼20.23 Ma, while oligotrophic taxa (discoasters and sphenoliths) increase as stable carbon isotope values decline. The palaeoecological turnover coincides with a possible restriction in the Central American Seaway (CAS) ∼20.7–21 Ma. Population dynamics of the Hole 998 A nannofossil assemblage track with global declines in C. floridanus relative abundance between ∼20.2 to ∼20.5 Ma, followed by a subsequent decrease in Reticulofenestra spp. These data suggest water mass stratification in the Yucatan Basin during the Early Miocene ∼19.2–17.65 Ma, likely due to restriction of the CAS.

A new Late Miocene stem Octodontoid (Rodentia: Caviomorpha: Octochinchilloi) from the Solimões Formation (Brazil): paleobiogeographic implications

ABSTRACT Caviomorphs are the neotropical hystricognathous rodents (Ctenohystrica: Hystricognathi: Caviomorpha). Octodontoidea are today the richest and most speciose clade of caviomorphs. Their fossil record also reveals a wide array of extinct species, including Acarechimys, a taxon with a widespread South American distribution and extensive temporal range. This genus comprises at least five species documented in deposits ranging from the late Oligocene to the Middle Miocene. Here, we report the discovery of a new species of Acarechimys from Upper Miocene deposits of the Envira River (PRE06 outcrop), situated in the Acre State, western Brazilian Amazonia. The new species differs from the other species of the genus in having lower molars with a strong posterior arm of the metaconid, the presence of a mesostylid (usually prominent) and a neomesolophid forwardly directed and connected to the metalophulid I on m1 but not on m2–3, and the absence of posterior arm of the protoconid. This taxon represents the youngest known species of the Acarechimys lineage to date. Its occurrence in Western Amazonia strengthens support for the hypothesis that South American rodent lineages survived longer during the Neogene at low latitudes than in the southern regions of the continent.

The Late Oligocene–Early Pleistocene paleoclimatic pattern in the northeastern Sahara, Sohag Basin, Egypt: evidence from lithofacies and pedogenic features

The Late Oligocene–Early Pleistocene paleoclimatic pattern in the northeastern Sahara, Sohag Basin, Egypt: evidence from lithofacies and pedogenic features