Fossil Assemblages Research Articles

The lower Cambrian Cranbrook Lagerstätte of British Columbia

Discovered over a century ago, the lower Cambrian (Series 2, Stage 4) Cranbrook Lagerstätte of southeastern British Columbia's Eager Formation is one of the oldest Burgess Shale-type deposits in North America. This Konservat-Lagerstätte is rich in olenelloid trilobites, but also yields a very low-diversity soft-bodied fossil assemblage including Tuzoia and Anomalocaris , and a low-diversity ichnofauna. Its scientific study, however, remains limited. A 2015 field-based investigation by the Royal Ontario Museum has revealed new information about the site's biota, depositional environment and taphonomic conditions. Not only is the Cranbrook Lagerstätte significant for early Cambrian biostratigraphy and comparisons with other Burgess Shale-type deposits, it also reveals some of the little-known diversity of life in a distal outer shelf environment during the Cambrian period. Supplementary material: Three supplementary figures (historical map, Tuzoia outlines, Anomalocaris specimens), supplementary material and methods, and a supplementary data file (generic presence/absence matrix) are available at https://doi.org/10.6084/m9.figshare.c.6850810

Provenance and tectonic settings of the lower Permian siliciclastic rocks in central Inner Mongolia: Implications for the evolution of the Paleo-Asian Ocean

The fine siliciclastic rocks of the Early Permian Shoushangou Formation in West Ujimqin Banner, central Inner Mongolia are a hot spot for Late Paleozoic oil and gas exploration and hold key implications for the tectonic evolution during the closure of the Paleo-Asian Ocean (PAO). Therefore, the sedimentary characteristics of the Shoushangou Formation in West Ujimqin Banner, along with the geochemical and Sm–Nd isotopic characteristics of the clastic rock samples in Well MXD1, were investigated to privde additional constraints on its provenance and tectonic background.The mudstones contain large proportions of quartz, feldspars and igneous debris, and the CIAcorr (0.52–0.66), the A–CN–K plot and the lack of Ce anomalies in REE show a weakly weathered provenance and the chemically unaltered parent rocks, suggestive of the proximal deposition and rapid burial. Slip-roll beddings, gravity flow, Bouma sequences and the (trace) fossil assemblages in the profile indicate that the Shoushangou Formation is mainly a shallow- (semi-)abyssal submarine fan and the lithological information indicates the presence of microfaces including main and lesser water supply channels, interchannels, lobes, natural levees. Furthermore, a narrow continental shelf above the submarine fan and the concentrated zircon age population of the Shoushangou formation suggest that the tectonic background is most likely a retroarc basin.The Al2O3/TiO2 ratio (19–24, average 22), the multivariable principal element source identification diagram and the plots of Th/Sc vs. Zr/Sc and Co/Th vs. La/Sc indicate that the source is mainly felsic, with a minor proportion of intermediate rocks. Trace elemental characteristics (e.g., Th, Zr, Sc, Co, Nd, and REE) deomonstrate that the tectonic setting of the Shoushangou Formation is confined by continental arcs.The whole rock Sm–Nd isotopic spectra of the Carboniferous and Permian strata indicate that the Baolidao arc is the main source of central Inner Mongolia. Additionally, the comparison of the Nd model ages and the zircon U–Pb ages of the Baolidao arc and other entities in the Central Asian Orogenic Belt (CAOB) is in support of a prolonged subduction of the PAO since ∼510 Ma, suggesting that the Baolidao arc was developed from the southern continental arc of the original South Mongolia microcontinent. Accordingly, it is suggested that the Shoushangou Formation was deposited in a retro-arc basin which was strongly influenced by the steep subduction of the PAO and the results collectively suggest that the PAO terminated no earlier than the middle to late Permian.

Late Cenomanian Plenus event in the Western Interior Seaway

The Late Cenomanian Plenus Cold Event is one of the most enigmatic paleoclimate episodes in Earth history with potential to inform understanding of global climate system variability under greenhouse warming conditions, as well as internal feedback pathways that modulate such variability. Following an interpreted massive addition of volcanic CO2 to the atmosphere and warming that led to a major ocean anoxic event (OAE2), there was a brief interval of cooling recorded in oxygen isotopes and biogeographic data. Here we present evidence that cooling was absent or muted within the Western Interior Seaway (WIS). Clumped isotope data from the basin suggest persistent extreme warmth during the Late Cenomanian, macroinvertebrate fossil assemblages do not record a decrease in temperature, and changes in other paleoceanographic proxies do not correlate temporally with Plenus interval signals from other locales. Using select proxy data to guide construction of GCM model simulations, we explore possible hypotheses to explain these observations. Our results suggest that the paleogeographic configuration of the basin and its gateways to adjoining oceans, which evolved in association with changing pCO2 and sea level, influenced winter sea ice formation at the northern aperture of the seaway, water mass circulation, salinity, temperature, and water column stratification. We propose that northward advection of warm Tethyan water muted expression of Plenus cooling in the seaway. Understanding the unique character of the Western Interior paleoceanographic record provides critical input for the development of robust models of ancient Earth System dynamics and should aid predictions of future climate system dynamics.

The late Eocene rise of SE Tibet formed an Asian ‘Mediterranean’ climate

Southeastern (SE) Tibet forms the transition zone between the high interior Tibetan Plateau and the lowlands of southwest China. So understanding the elevation history of SE Tibet, a biodiversity hotspot, enlightens our understanding of the interactions between tectonics, monsoon dynamics and biodiversity. Here we reconstruct the uplift history of the Markam Basin, SE Tibet, during the middle−late Eocene based on U − Pb dating, plant fossil assemblages, and stable and clumped isotope analyses. Our results suggest that the floor of the Markam Basin was at an elevation of 2.6 ± 0.9 km between 42 Ma and 39 Ma, where the mean annual air temperature (MAAT) was 13.2 ± 2.4 °C. The basin then rose rapidly to 3.8 (+0.6/−0.8) km before 36 Ma. Integrated with existing paleoelevation data, we propose that the high plateau boundary (∼3.0 km) of SE Tibet formed during the late Eocene. Numerical climate modeling with realistic paleo-landscapes shows that with the rise of SE Tibet, a Mediterranean-like climate developed in the region characterized by bi-modal precipitation with two wet seasons in boreal spring and autumn. The high topographic relief of SE Tibet, coupled with this distinctive Mediterranean-like climate system, helped develop the high biodiversity of the Hengduan Mountains.

Monsoon influence on plant diversity in northern Indochina: Evidence from the late Miocene Yen Bai flora, northern Vietnam

Indochina, as a global biodiversity hotspot, offers the potential for understanding the evolution of biodiversity. However, the historical narrative of plant diversity in this region remains enigmatic due to limited fossil records. Here, we report a newly discovered megafossil flora from the late Miocene of the Yen Bai Basin, northern Vietnam. This megafossil flora suggests that the late Miocene vegetation in northern Vietnam comprised mixed tropical evergreen and deciduous broadleaved forest components mainly characterized by Fabaceae, Fagaceae, and Lauraceae bearing strong resemblance to the modern vegetation in northern Vietnam. Paleoclimate reconstruction for this plant fossil assemblage using the Coexistence Approach indicates a mean annual temperature (MAT) of 18.5–23.0 °C and a mean annual precipitation (MAP) of 1183.1–2078.5 mm. Similarly, employing the Climate-Leaf Analysis Multivariate Program yields an MAT of 21.3 ± 2.3 °C and a growing season precipitation (GSP) of 1328.6 ± 606.0 mm. The length of the growing season was about eleven months. Overall these results indicate a modern-like warm and humid tropical monsoon climate in northern Vietnam during the late Miocene. Climatic comparison of the Yen Bai with other fossil floras in northern Vietnam and southern China reveals the relative stability of temperature seasonality since the middle Eocene but obvious long-term variation in precipitation seasonality, particularly in respect of precipitation during the three consecutive driest months (X3.DRY). This suggests the Asian monsoon in northern Vietnam underwent important changes from the middle Eocene to the late Miocene, and intensified significantly during the middle and late Miocene. Our results show the modernization of plant diversity in northern Indochina had its origin in the Paleogene and further developed in the late Miocene, and was linked to the evolution of the Asian monsoon mainly in terms of changes in dry season precipitation.

The utility of alpine cave fossil assemblages for zoological census: an example from northern Utah, United States

Abstract Assemblages of mammal skeletal remains provide a powerful tool for censusing wildlife populations to establish zoological baselines required for evaluating biogeographic trends over varying timescales. Caves provide an ideal depositional setting to preserve these skeletal remains despite potential time averaging and taphonomic filtering. We describe a Holocene paleontological assemblage from Boomerang Cave in the Bear River Range of Cache County, northern Utah, United States, at an elevation of 2,231 m, and at the boundary between the Great Basin and Rocky Mountain biogeographic provinces. We analyzed 1,228 surface-collected specimens from six areas within the cave, and identified a minimum of 22 nonoverlapping mammalian taxa, comprising all size classes present in the region. Compared to museum records for mammals from the Bear River Range and individuals trapped or observed in the vicinity of the cave, specimen-based rarefaction demonstrates that our assemblage captures most of the mammalian diversity expected in the area. This is particularly apparent for carnivorans and soricids, which are particularly well-represented in the Boomerang Cave assemblage, with the former clade represented by at least nine taxa. This high level of diversity can be attributed to the relatively random nature of natural trap cave deposition, reducing accumulation biases due to size or diet. We also record the first occurrence of Merriam’s Shrew (Sorex merriami) from the Bear River Range. Our analysis does not indicate any mammalian changes between late Holocene and present-day communities, but these data establish a new zoological baseline for an alpine community at the interface between two key biogeographic provinces in western North America. Our work highlights the value of collecting skeletal remains from cave assemblages as a convenient and fast method for censusing terrestrial mammalian communities.

A new model for Early Paleozoic ichnostratigraphy based on trace fossil assemblages from Brazil

Trace fossils are not generally utilized as biostratigraphic indicators due to their long stratigraphic ranges. Despite the use of intricate behavioral traces in the absence of other indicators, existing models like the Precambrian–Cambrian boundary and Cruziana stratigraphy encounter limitations due to crucial data gaps and regional constraints. To surmount these challenges, in this paper, we critically assess established models and present a new framework for Early Paleozoic strata, drawing on trace fossils from the intracratonic basins of Brazil. Our ichnostratigraphic model is calibrated using ichnological data from the Parnaíba, Paraná, and Amazonas basins, including new data. The analysis focuses on trace fossils in strata that are independently dated using chitinozoan, miospore, and acritarch biozonation. Key ichnotaxa, such as Arthrophycus and Cruziana, are identified as prominent indicators of the Llandovery Stage in Brazil. Occurrences of Heimdallia and Musculopodus in the Tianguá Formation also may be used to suggest a Llandovery interval. Notably, Bifungites, found widely across Brazilian basins, emerges as a potential ichnomarker for the Early to mid-Paleozoic interval, with a global presence throughout Cambrian to Mississippian deposits. While current ichnostratigraphic models lack robust calibration with chronostratigraphic or biostratigraphic data, our new proposed model integrates key ichnotaxa, including Bifungites, Climactichnites, Heimdallia, Oldhamia, and Musculopodus, surpassing those pre-existing zonations based on Cruziana and arthrophycids. These ichnotaxa exhibit unique features and narrow temporal ranges, meeting essential biostratigraphic criteria. Although their spatial distribution is somewhat limited, our new model, which is continually evolving with new data, holds promise for enhancing global stratigraphic correlations, particularly where independent age information is available.

Lire le passé forestier dans un bourbier : histoire écologique et dynamique contemporaine d’une population marginale nordique d’érable rouge (Acer rubrum)

ABSTRACT Populations at the edge of a species range are vulnerable to climate change. Studying their ecological history helps understanding resilience processes acting at the limit of the species’ tolerance envelope. In the mixedwood forest ecotone, several thermophilic species, such as white pine and red maple, reach the northern limit of their geographical range. Here we rely on macrofossil analysis of charcoal particles and plant macroremains of a forest hollow to reconstruct the local history of thermophilic species in a marginal red maple stand isolated within the mixedwood forest ecotone. Taxonomic identification and radiocarbon dating of charred particles and wood remains extracted along the peat accumulation profile indicated dominance of white pine during the Holocene Climate Optimum (6200 to 4300 cal yr BP). The fossil assemblage suggests the transition of a black spruce - feathermoss (>9500 to 6200 cal yr BP) to a balsam fir forest that developed after the decline of white pine (since 4300 cal yr BP). Red maple is not detected in the fossil assemblage suggesting a late arrival and/or expansion on the site, likely favored by anthropogenic activities and the climate warming ongoing since the end of the Little Ice Age.

Empirical record, geochronology and theoretical determinates of Mesozoic climate in the Junggar Basin, NW China, in relation to other basins in NE China

Abstract Mesozoic continental basins of northern China, including the Junggar Basin, provide some of the most spectacular and important fossil assemblages in the world, but their climatic and environmental contexts have been shrouded in uncertainty. Here we examine the main factors that determine those contexts: palaeolatitude; the effects of changing atmospheric gases on the radiative balance; and orbitally paced variations in insolation. Empirical evidence of these factors is accumulating rapidly and promises to upend many long-standing paradigms. We focus primarily on the Junggar Basin in Xinjiang, NW China, with the renowned Shishugou Biota, and the basins in Liaoning, Hebei and Inner Mongolia with their famous Jehol and Yanliao biotas. Accurate geochronology is necessary to disentangle these various factors, and we review the Late Triassic to Early Cretaceous U–Pb ages for these areas and supply a new laser ablation inductively coupled plasma mass spectrometry age for the otherwise un-dated Sangonghe Formation of Early Jurassic age. We review climate-sensitive facies patterns in North China and show that the climatic context changed synchronously in northwestern and northeastern China consistent with a previously proposed huge Late Jurassic–earliest Cretaceous true polar wander event, with all the major plates of East Asia docked with Siberia and moving together since at least the Triassic when the North China basins were at Arctic latitudes. We conclude that this true polar wander shift was responsible for the coal beds and ice-rafted debris being produced at high latitudes and the red beds and aeolian strata being deposited at low latitudes within the same basin. The climatic and taphonomic context in which the famous Shishugou, Yanliao and Jehol biotas were preserved was thus a function of true polar wander, as opposed to local tectonics or climate change.

Shelly coprolites record durophagous predation in the Late Ordovician Bohdalec Formation (Katian; Prague Basin, Czech Republic)

Trace fossils can illustrate important palaeobiological interactions within a fossil assemblage that body fossils do not record. A group of trace fossils that showcase feeding ecology, and evidence of predation, are coprolites. Shelly coprolites are useful for documenting records of durophagous predators or scavengers within a substrate. To expand the record of these traces from the lower Paleozoic, here we present 12 shelly coprolites from the Late Ordovician (Katian) Bohdalec Formation of the Czech Republic. These coprolites contain abundant Onnia superba (Bancroft, 1929) fragments with marked breakages across exoskeletal sections. Rarer evidence for gastropods, bivalves, crinoid debris, and another indeterminate shelly material are also observed within the coprolites. While the producer cannot be irrevocably determined, possible options are explored. We propose that larger, co-occurring trilobites and predatory cephalopods likely made the majority of coprolites. Furthermore, large unbiomineralised arthropods, such as phyllocarids and eurypterids are highlighted as possible producers. Continued examination of these trace fossils will highlight when and where similar interactions between trophic levels had occurred.

Paleontology, Stratigraphy and Biogeography of the Late Precambrian and Phanerozoic Formations of Russia

Abstract —This special issue is dedicated to discussing the possibility of the stage division of the Vendian system, specifically, a composite section of the Siberian Platform is proposed as a stratigraphic standard. For the Ordovician system of the Siberian Platform and Northeastern Russia, certain changes in stratigraphic charts are proposed, nomenclative issues and different variants of correlations of regional horizons with the General Stratigraphic Scale (GSS) of Russia are considered. A geochemical analysis of Bajocian and Bathonian belemnite rostra of Arctic Siberia was carried out and for the first time a comprehensive C, O, and Sr isotope characterization of the sections was obtained, making it possible to propose a comparison of the studied sections with the European standard and to develop paleogeographic models. Comprehensive studies of the Upper Jurassic and Lower Cretaceous series of the Anabar Bay deposits (Laptev Sea) led to an improved biostratigraphic division for this reference section and to the identification of the levels for interregional and global correlation. The magnetostratigraphic division of the Jurassic–Cretaceous boundary interval and its comparison with the Tethyan regions are discussed. Taking into account biostratigraphic data, the δ13Corg variation curve can also serve as a correlation tool. The clinoform structure of Upper Jurassic strata was revealed in the northwest of the Anabar–Khatanga depression, based on the interpretation of seismic profiles calibrated against studied sections. The formation of this complex allows us to consider this stage as the initiation of the Verkhoyan orogeny in this folded area. Analysis of variations in the thickness of sediments allows us to deduce that intense tectonic-sedimentary processes took place during the Late Jurassic epoch, along with subsidence of the bed of the sedimentary basin. Analysis of a specific assemblage of lacustrine fossils from the reference section of the upper part of the Lower Cretaceous series of Transbaikalia and of published data showed that this fossil assemblage was widespread in East Asia. Biofacies analysis made it possible to reconstruct the paleoenvironments of this time.

First Eocene–Miocene anuran fossils from Peruvian Amazonia: insights into neotropical frog evolution and diversity

AbstractAnurans are one of the most diverse vertebrate groups, particularly in Amazonia, where species richness exceeds that of anywhere else. Amazonian frogs belong to three main lineages (Hyloidea, Microhylidae and Pipidae), each of which diversified during the Cenozoic. However, due to the virtual absence of an anuran fossil record in that area, the evolutionary history of modern lineages has so far remained accessible only via molecular data. In recent decades, a series of field campaigns in Peruvian Amazonia led to the discovery of an unparalleled set of anuran bone fragments, scattered across different sites spanning the Eocene–Miocene time interval. Here, we describe these first Palaeogene and early Neogene anurans from Peru with a focus on humeral and ilial morphology, identifying five humeral and five ilial morphotypes. Humeral morphotypes suggest the presence of different lineages of Brachycephaloidea in Peruvian fossil assemblages, whereas ilial morphotypes suggest the presence of Leptodactylidae, although leptodactylid‐like ilia also occur in some extant brachycephaloids. Pipids were also identified based on both humeral and ilial fragments. This study fills a major temporal and geographical gap in the evolutionary history of South American anurans, while further uncovering a lack of knowledge in the skeletal morphology of extant anuran families, as well as their inter‐ and intra‐species variability.

A new method for examining the co-occurrence network of fossil assemblages

Currently, studies of ancient faunal community networks have been based mostly on uniformitarian and functional morphological evidence. As an important source of data, taphonomic evidence offers the opportunity to provide a broader scope for understanding palaeoecology. However, palaeoecological research methods based on taphonomic evidence are relatively rare, especially for body fossils in lacustrine sediments. Such fossil communities are not only affected by complex transportation and selective destruction in the sedimentation process, they also are strongly affected by time averaging. Historically, it has been believed that it is difficult to study lacustrine entombed fauna by a small-scale quadrat survey. Herein, we developed a software, the TaphonomeAnalyst, to study the associational network of lacustrine entombed fauna, or taphocoenosis. TaphonomeAnalyst allows researchers to easily perform exploratory analyses on common abundance profiles from taphocoenosis data. The dataset for these investigations resulted from fieldwork of the latest Middle Jurassic Jiulongshan Formation near Daohugou Village, in Ningcheng County of Inner Mongolia, China, spotlighting the core assemblage of the Yanliao Fauna. Our data included 27,000 fossil specimens of animals from this deposit, the Yanliao Fauna, whose analyses reveal sedimentary environments, taphonomic conditions, and co-occurrence networks of this highly studied assemblage, providing empirically robust and statistically significant evidence for multiple Yanliao habitats.

Disentangling environmental drivers of subarctic dinocyst assemblage compositional change during the Holocene

Abstract. Analysis of compositional changes in fossil organic-walled dinoflagellate cyst (dinocysts) assemblages is a widely used tool for the quantitative reconstruction of past environmental parameters. This approach assumes that the assemblage composition is significantly related to the reconstructed environmental parameter, which requires an independent correlation between the assemblage and the variable, meaning that the variable explains a dimension of the assemblage variance that is not also explained by other parameters. Theoretically, dinocyst assemblages can be used to reconstruct multiple environmental variables. However, there is evidence that primary and subordinate drivers for assemblage compositions regionally differ, and it remains unclear whether a significant relationship to specific parameters in the present ocean always implies that this relationship is significant in fossil assemblages, questioning if past changes in these multiple parameters can be reconstructed simultaneously from fossil assemblages. Here, we analysed a local subset of the Northern Hemisphere dinocyst calibration dataset (n=1968), including samples from the Baffin Bay area (n=421), and evaluated the benefits of a local versus a more regional or global calibration for the environmental reconstruction of Baffin Bay oceanography during the Holocene. We determined the dimensionality of the dinocyst ecological response and identified environmental drivers in the Baffin Bay area for the modern dataset. We analysed four existing Holocene records along a north–south transect in the area and evaluated the statistical significance of downcore reconstructions by applying the local and global datasets with different techniques: the modern analogue technique (MAT), the weighted average partial least square (WA-PLS) and maximum likelihood (ML). We evaluated reconstructions tested as significant in light of the existing state of knowledge about West Greenland's Holocene palaeoceanography. Our analyses imply that present-day and Holocene dinocyst assemblages in the Baffin Bay area are primarily driven by salinity changes; other parameters were less important in driving assemblage compositions, and their contribution differed among the studied records. We show that the objectively occurring and temporally coherent shifts in dinocyst assemblages in the Holocene of Baffin Bay can be robustly interpreted only by transfer functions that are locally calibrated. Transfer functions based on the broad Northern Hemisphere calibration yielded many insignificant environmental reconstructions. At the same time, we show that even in the local calibration, not all parameters that appear to significantly affect dinocyst assemblages in the calibration dataset can be meaningfully reconstructed in the fossil record. A thorough evaluation of the calibration dataset and the significance of downcore applications is necessary to reveal the region-specific information contained in dinocyst assemblage composition.

Pedogenic processes and climatic conditions from Cretaceous (Albian) tropical paleosols of the Itapecuru Formation, Parnaíba Basin, northeast Brazil

Paleosols are an under-utilized tool for paleoenvironmental and paleoclimatic reconstruction in Brazilian intracontinental basins. The Itapecuru Formation, from the Lower Cretaceous of the Parnaíba Basin, consists of a 600-m-thick succession of interbedded very fine-to-fine-grained sandstone, mudstone, and paleosols containing rich fossil assemblages. This paper provides a detailed description of the macro and microscopic pedogenic features of a hydromorphic paleosol profile whithin the Itapecuru Formation. Analyses of clay mineralogy and whole-rock geochemistry in the hydromorphic paleosols at the Prata locality are used to infer pedogenic processes, paleoclimate, paleohumidity and paleovegetation. Estimates of paleoprecipitation and paleotemperature using climofunctions, chemical index of alteration without potassium, and paleosol weathering index proxies (PWIs) show values ranging from 810 to 1042 mm/yr (SE = ±78.1 mm/yr) and 10.9 to 11.8 °C (SE = ±0.23 °C), respectively. Climofunction values (MAP and MAT), paleohumidity, Kӧppen aridity index, and pedogenic clay mineral content suggest a humid paleoclimate during soil-forming and weathering processes. Based on these climate data and previously published paleogeographic and palynological studies, the Cretaceous (Albian) Prata pedotype was formed within the equatorial humid belt with vegetation of wet tropical forests.

Taphonomic analysis of microfossil bonebeds from Western Gondwana: a case study from the late Permian Corumbataí Formation (Paraná Basin, Brazil)

ABSTRACT Storm deposits are common in geological records and encompass a variety of fossil assemblages affected by taphonomic processes that can biased fossil preservation. Here, we describe a microfossil bonebed to demonstrate the role played by certain filters in the final composition of storm-generated fossil accumulations. These layers are recurrent in the Paraná Basin and possibly in other localities of Gondwana, outcropping over 1000 km in the Permian units of the basin. Thirty samples were systematically collected from the state of São Paulo and their components were analysed. These samples consisted of clay matrix, coprolites, scales, and teeth of lower actinopterygian (Palaeonisciformes) and Xenacanthiformes fish. The taphonomic signatures demonstrated that the layer formed as a lag deposit in a distal aquatic environment. Two major taphonomic filters were recognised. The first filter selected bioclasts according to size and density, whereas the second filter preserved only the original phosphatic elements. The third and minor filter were related to breakage, which occured during reworking and diagenesis, respectively. All of these processes contributed to the generation of an assemblage with limited composition and species richness. Thus, distal tempestites might not be useful for estimating palaeodiversity, but may still offer some valuable information about intraspecies variation.

Fossil vertebrates from the Cerro Fortaleza Formation (Upper Cretaceous), Santa Cruz Province, Argentina

The Cerro Fortaleza Formation is a stratigraphic unit that crops out near the Cerro Fortaleza hill, northwestern Santa Cruz province, Argentina. This unit has been well-known since the end of the XIX century, and because of the finding of abundant dinosaur bones, it was named as the “Capas con dinosaurios” (=Dinosaur Beds). Intensive work since the end of the 1990's resulted in the finding of diverse dinosaur species and other vertebrates. The fossil assemblage recovered includes lamniform sharks, lepisosteiforms, the dipnoan Atlantoceratodus iheringi, chelid turtles, peirosaurid crocodyliforms, mosasaur teeth, and diverse dinosaur remains. Dinosaurs include a partial tail of a small-sized titanosauriform, and another, identified as Dreadnoughtus, shows derived procoelous caudals. Isolated sauropod teeth indicate the presence of a derived lithostrotian and the enigmatic Clasmodosaurus spatula Ameghino, 1899. Among theropods, abelisaurid and megaraptorids are reported. A partial ulna, manual claw and metatarsal belonging to the holotype of the enigmatic theropod Austrocheirus isasii are also described. The morphology of the new elements does not comfortably fit with abelisaurid or megaraptorid theropods, and the affinities of Austrocheirus among theropods still remain elusive. The fossil vertebrate content of the Cerro Fortaleza Formation indicates close similarities with the faunistic assemblage previously reported from the Mata Amarilla Formation (Cenomanian-Turonian), and strong differences are present with the overlying Chorrillo Formation (Maastrichtian). This may indicate that some kind of change among terrestrial vertebrates occurred at the Campanian/Maastrichtian boundary in the Austral Patagonia Basin.

Response of the Patagonian floras to climatic cooling during Oligocene−Miocene transition and the expansion of Antarctic glaciation

Pulses of Antarctic ice sheet expansion and associated climatic deterioration are thought to have driven major shifts in the world's flora during the Cenozoic Era. However, the rarity of fossils from well-constrained sedimentary strata in high palaeolatitudes makes it difficult to explore to these vegetation shifts directly. In this study, we infer changing patterns of floristic richness through the Oligocene−Miocene cooling event (∼23 Ma), based on analysis of terrestrial palynological samples in the radiometrically-constrained sediments of the Río Guillermo Formation of southern Patagonia. Our fossil assemblages include plant families typical of modern Magellanic subpolar forests such as Nothofagaceae, Podocarpaceae, Araucariaceae, and Myrtaceae. Adjusted for sample completeness (or coverage), our floristic richness estimates are, on average, higher than those from modern subpolar forests yet considerably lower than any other Paleogene or Neogene palynological records published elsewhere from the continent. The transient Oligocene−Miocene cooling episode, whose effects were probably intensified in southern Patagonia due to its close proximity to the glaciated Antarctic Peninsula, may have contributed to the steep decline in floristic richness. We infer that most Gondwanan plant relicts that survived across the Oligocene−Miocene transition endured through subsequent Neogene climatic fluctuations and contribute to present-day Magellanic subpolar forests. Our study highlights how precisely-dated fossil assemblages and robust diversity methods can be used to track biodiversity shifts in response to past climate change.

Trace fossil zonation in interdune Lakes: example from the neogene of Eastern Patagonia, Argentina

The Upper Miocene-Lower Pliocene Río Negro Formation (Río Negro Province, Argentina) contains diverse and abundant continental trace fossils. The purpose of this contribution is to recognize distinctive trace fossil assemblages in interdune lakes as response to lake expansion and contraction. The interdune deposits host 23 invertebrate, vertebrate, and plant ichnotaxa that are linked to the hydrological dynamics of the lake. During lake expansion a highly diverse subaqueous trace fossil assemblage reflects an infaunal bivalve population living in a littoral zone with swampy vegetation and semiaquatic mammals. The trace fossil assemblage recorded during Lake Highstand is less diverse and mostly linked to bivalve-generated trace fossils reflecting resting, locomotion, and escape/equilibrium behaviour (comparable to the Mermia ichnofacies). The assemblage during lake contraction exhibits high ichnodiversity and is dominated by vertebrate trace fossils reflecting foraging activities of shorebirds, cursorial birds, and herding semiaquatic mammals and ground sloths frequenting water holes. Flat sandy areas with moist substrates marginal to the interdune lake displayed a moderate ichnodiversity and its distinctive components were armadillo meniscate burrows and footprints in cross-section, as well as subvertical calcareous root casts. Neogene interdune lacustrine deposits reflect a particular zonation of trace fossils in response to changing physical conditions.

Entophysalis in the Rhynie chert (Lower Devonian, Scotland): implications for cyanobacterial evolution

AbstractThe ∼407-myr-old Rhynie chert of Scotland contains exquisite body fossils of land plants, animals and microorganisms, which provide our earliest reasonably complete snapshot of a Phanerozoic terrestrial ecosystem. These fossils have been instrumental to our understanding of the ‘greening of the land’, a major transition in the history of the Earth–life system. Among the primary producers preserved in the chert are cyanobacteria, of which only a fraction have been formally described. Here, we report the occurrence of the colony-forming cyanobacterium Eoentophysalis in the Rhynie chert. To our knowledge, this represents the first bona fide record of Entophysalidaceae from any post-Cambrian fossil assemblage or any non-marine fossil assemblage of any age. The Rhynie Eoentophysalis appears remarkably similar in appearance both to modern marine and freshwater Entophysalis ssp. and to Eoentophysalis belcherensis, a shallow-marine fossil from the ∼2 Ga Belcher Group of Canada that is perhaps the oldest convincing cyanobacterium on record. Darkened cell envelopes in the Rhynie Eoentophysalis correspond well with both E. belcherensis and modern Entophysalis, whose cell envelopes often contain the photoprotective brown pigment scytonemin. The occurrence of Eoentophysalis in the Rhynie chert supports previous claims that the fossilisable traits of entophysalid cyanobacteria are evolutionarily static through geological time. These organisms may be such effective generalists that major changes in their environment – in this case, the transition to a fully non-marine habitat – have not imposed significant selection pressure on these traits.