Evolving Earth最新文献

Humans have been considered ecologically unspecialized, and our evolution a compromise path through a maze of conflicting influences. Generalist ecological roles contrast with ecological polarities such as competitor, breeder or tolerator. Ecological polarities can be approximated in fossil mammals by relative size of canines for competitors, incisors for breeders, and molars for tolerators. Considered in this way, early Miocene apes of Kenya were generalists, but show a greater range of ecological polarity than modern apes, including ecological polarities found in living apes, macaques, and vervets. As many as six primates in a single Miocene aleosol show diverse ecological polarities, implying competitive exclusion. Middle Miocene monkeys and apes were more molarized and marginally more tolerant than early Miocene primates, so more like humans in that respect. This adaptive shift of 15 ​Ma was at a time of climatic aridity and open vegetation indicated by associated paleosols. A 20 ​m.yr record of Kenyan paleoprecipitation from paleosols indicates that 15 ​Ma was unusually dry after exceptionally wet paleoclimate of 16 ​Ma. This new Kenyan paleosol record of paleoclimate is from the same localities as the fossil apes, and uninfluenced by whole ocean mixing, salinity and ice volume effects which compromise similar marine isotopic proxies of global change. Molarization of apes at 15 ​Ma ago was only one of a series of adaptations selected by at least 9 dry alternating with wet episodes which shifted forest to shrubland ecotones over the past 20 ​m.yr. Our evolutionary lineage ran a gauntlet of Neogene climatic and vegetation changes in Africa by adopting a generalist ecological role.

A fossil without provenance data is problematic because it cannot be placed into meaningful paleoecological and paleobiogeographic contexts. This problem is particularly acute when the fossil is suspected or known to have originated from a formation within which a major interval of biotic and/or paleoenvironmental change has been recognized that could change the paleobiological implications of the taxon in question. Two reptile fossil specimens (a testudinate and a crocodylomorph) preserved within ex situ mudstone cobbles of the Lower Jurassic (Pliensbachian-Toarcian) Nishinakayama Formation in Yamaguchi, Japan exemplify this problem. Both specimens are preserved alongside associated ammonoid fossils that biostratigraphically constrain them to the Toarcian interval of the Nishinakayama Formation, but it is unclear whether their stratigraphic provenance is below, within, or above the interval of the formation that preserves the chemostratigraphic markers of the Toarcian Ocean Anoxic Event (T-OAE, ∼182.5 ​Ma), which were first established at the nearby locality Sakuraguchidani. Herein, we used isotope ratio mass spectrometry and portable energy-dispersive X-ray fluorescence to investigate the geochemistry of sedimentary matrix sampled from each fossil and a new measured stratigraphic section of the Nishinakayama Formation close to where the fossil specimens were found. We interpret a ∼2‰ positive shift in δ¹³C_org at the base of the section as the recovery of the negative carbon isotope excursion associated with the T-OAE, providing additional evidence of the event from a new locality within the Nishinakayama Formation. Linear discriminant analysis (LDA) of the total geochemical dataset was then used to explore the multivariate separateness of binned intervals of the composite section and predict the provenance of each fossil. The results suggest with 93.33% confidence that both fossils were derived from strata above the T-OAE interval. This predictive method can be applied to any fossil collected ex situ with preserved rock matrix and for which the general provenance is known or suspected.

The early Paleocene (Danian) period included three major events: the Cretaceous-Paleogene (K-Pg) extinction at 66.0 ​Ma, the Dan-C2 hyperthermal at 65.2 ​Ma, and the latest Danian hyperthermal at ∼62.2 ​Ma. In this paper, we investigate benthic foraminiferal diversity patterns, morphotypes, and oxygen conditions along with the carbonates and magnetic susceptibility records at IODP Site U1457 (Laxmi Basin, Northern Indian Ocean) to understand the effects of these Danian events on the marine community in the Indian Ocean. Findings suggest that foraminifera persisted across these major events. Species belonging to Bolivina, Glandulina, Hoeglundina, Parrelloides and Quadrimorphina genus were dominant above the K-Pg boundary whereas Bolivina, Bulimina, Cassidulina, Cornuspira, Gyroidinoides, Melonis, Oolina, Pullenia, Reussoolina and Rutherfordoides dominated across subsequent hyperthermal events. We calculated the average oxygen content at 0.16 ​ml/L in accordance with oxyphilic species abundance, which shows that the Laxmi Basin comprised, mostly, a suboxic to dysoxic environment. The benthic foraminiferal diversity patterns, primary anomalies of calcium carbonates, and magnetic susceptibility, integrated with previously-published global datasets of carbon and oxygen isotopes, help to define the major geologic events at the study site and show how biotas responded to global change during the early Paleocene greenhouse mode.

Exposures of lamproitic volcanic rocks have been used to constrain the age of the evaporitic basin-fill in the Fortuna Basin of southern Spain. K/Ar dates initially suggested a Messinian age assignment for these deposits; however, subsequent ⁴⁰Ar/³⁹Ar dates indicated a Tortonian age, implying the early onset of evaporite formation in the eastern Betic Cordillera, in an event termed the “Tortonian Salinity Crisis”. In this paper, we critically review the published chronology of the key igneous outcrops and provide new ⁴⁰Ar/³⁹Ar ages, which show that the Cabezos Negros lamproites lie close to the Tortonian-Messinian boundary (c. 7.25 ​Ma). We also provide new paleomagnetic measurements from each of the three lamproitic outcrops (Cabezos Negros, Derramadores, and El Tale), which demonstrate normal magnetic polarity coincident with episodes of volcanic and subvolcanic activity. The presence of peperites and pillow structures indicate that volcanism at Cabezos Negros was coeval with deposition of the lower part of Rambla Salada Gypsum Member. Therefore, the normal polarity identified in this unit should be equivalent to the normal polarity on the Cabezos Negros lamproites. These results indicate that the initial marine restriction and evaporitic deposition of Fortuna Basin occurred more than a million years before the Messinian Salinity Crisis.

The Miaolingian and Furongian epochs of the Cambrian period have been identified as a time of limited metazoan reef development. The aim of this paper is to improve understanding of the biological and geochemical conditions that affected reefs during this interval, and to propose a hypothesis for understanding why metazoan reef development was inhibited. To address these issues, a global dataset of fossil occurrences (N ​= ​25,307) spanning Cambrian Stage 4 to the early Ordovician (Tremadocian) was extracted from the Paleobiology Database, Paleoreef Database, and a review of the primary literature. Findings show that the proportion of reefs constructed by metazoans fell from 40% in the Wuliuan age to 0% in the Drumian age, with reefs being overwhelmingly dominated by microbial ecosystems through the remainder of the Cambrian. The proportion of skeletal material constructed from carbonate fell from 85% in the Wuliuan age to 63% in the Drumian age across all the fossil occurrence data, before recovering. These findings suggest that environmental conditions may have not been favorable to carbonate organisms, but this does not fully explain the prolonged reduction of metazoans within reefs throughout this interval. A hypothesis proposed here is that Miaolingian to Furongian metazoan reef abundances were low because of two factors: (1) shallow water anoxia – and other factors such as elevated temperatures and ocean acidification – caused the extinction of metazoan reef builders in the late-early Cambrian and (2) deep water anoxia and marine regression, resulted in a loss of habitat. These inhibiting conditions were not necessarily concurrent but are inferred to have collectively suppressed the growth of metazoan reefs until the Early Ordovician when more shelf space for new reef development occurred. This hypothesis provides a first step in exploration of these conditions during the middle and late Cambrian and for reef development in general.