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      Early Eocene warming events and the timing of terrestrial faunal exchange between India and Asia

      , , , , , ,
      Geology
      Geological Society of America

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          Astronomical pacing of late Palaeocene to early Eocene global warming events.

          At the boundary between the Palaeocene and Eocene epochs, about 55 million years ago, the Earth experienced a strong global warming event, the Palaeocene-Eocene thermal maximum. The leading hypothesis to explain the extreme greenhouse conditions prevalent during this period is the dissociation of 1,400 to 2,800 gigatonnes of methane from ocean clathrates, resulting in a large negative carbon isotope excursion and severe carbonate dissolution in marine sediments. Possible triggering mechanisms for this event include crossing a threshold temperature as the Earth warmed gradually, comet impact, explosive volcanism or ocean current reorganization and erosion at continental slopes, whereas orbital forcing has been excluded. Here we report a distinct carbonate-poor red clay layer in deep-sea cores from Walvis ridge, which we term the Elmo horizon. Using orbital tuning, we estimate deposition of the Elmo horizon at about 2 million years after the Palaeocene-Eocene thermal maximum. The Elmo horizon has similar geochemical and biotic characteristics as the Palaeocene-Eocene thermal maximum, but of smaller magnitude. It is coincident with carbon isotope depletion events in other ocean basins, suggesting that it represents a second global thermal maximum. We show that both events correspond to maxima in the approximately 405-kyr and approximately 100-kyr eccentricity cycles that post-date prolonged minima in the 2.25-Myr eccentricity cycle, implying that they are indeed astronomically paced.
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            Whales originated from aquatic artiodactyls in the Eocene epoch of India.

            Although the first ten million years of whale evolution are documented by a remarkable series of fossil skeletons, the link to the ancestor of cetaceans has been missing. It was known that whales are related to even-toed ungulates (artiodactyls), but until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth. We also show that a major dietary change occurred during the transition from artiodactyls to whales and that raoellids were aquatic waders. This indicates that aquatic life in this lineage occurred before the origin of the order Cetacea.
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              Mammalian biodiversity on Madagascar controlled by ocean currents.

              Madagascar hosts one of the world's most unusual, endemic, diverse and threatened concentrations of fauna. To explain its unique, imbalanced biological diversity, G. G. Simpson proposed the 'sweepstakes hypothesis', according to which the ancestors of Madagascar's present-day mammal stock rafted there from Africa. This is an important hypothesis in biogeography and evolutionary theory for how animals colonize new frontiers, but its validity is questioned. Studies suggest that currents were inconsistent with rafting to Madagascar and that land bridges provided the migrants' passage. Here we show that currents could have transported the animals to the island and highlight evidence inconsistent with the land-bridge hypothesis. Using palaeogeographic reconstructions and palaeo-oceanographic modelling, we find that strong surface currents flowed from northeast Mozambique and Tanzania eastward towards Madagascar during the Palaeogene period, exactly as required by the 'sweepstakes process'. Subsequently, Madagascar advanced north towards the equatorial gyre and the regional current system evolved into its modern configuration with flows westward from Madagascar to Africa. This may explain why no fully non-aquatic land mammals have colonized Madagascar since the arrival of the rodents and carnivorans during the early-Miocene epoch. One implication is that rafting may be the dominant means of overseas dispersal in the Cenozoic era when palaeocurrent directions are properly considered.
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                Author and article information

                Journal
                Geology
                Geology
                Geological Society of America
                0091-7613
                1943-2682
                December 28 2010
                December 03 2010
                : 39
                : 1
                : 15-18
                Article
                10.1130/G31585.1
                6b929205-e6a9-4e27-bdad-cc84df0eddaa
                © 2010
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