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12/10/15 Séminaire Européen: Robert Spicer (Open University, UK)

(à ENS amphi L, du 12/10/2015 14:00 au 12/10/2015 15:00)
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Environmental Constraints on Terrestrial Vertebrate Behaviour and Reproduction in the High Arctic of the Late Cretaceous

Robert A. Spicer1, Alexei B. Herman2 and Teresa E.V. Spicer3

1 Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Milton Keynes, MK7 6AA, UK. 2 Geological Institute, Russian Academy of Sciences, 119017 Moscow, Russia. 3 State Key Laboratory of Systematic and Evolutionary Botany, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China.

The polar climate and light regime were key constraints on ecosystem functioning and dynamics throughout the Late Cretaceous high Arctic (palaeolatitudes> 70°N) where a variety of invertebrates, insects, the flightless bird Hesperornis, placental and marsupial mammals, turtles, and numerous large and small dinosaurs are known to have lived.  Dinosaurs are preserved as skeletal remains, footprints and even skin impressions. Spatial reconstructions of deltaic floodplain vegetation, growth rings and plant-based proxy reconstructions of the Late Cretaceous Arctic temperature and moisture regimes, provide detail of the light regime, length of the growing season, summer maximum and winter minimum temperatures and evapotranspirational capacity, all of which determined seasonal primary productivity. Moreover such constraints have important implications for dinosaur feeding, nesting behaviour and the capacity to live year round in these near-polar environments.

The most northerly and coldest terrestrial environment in which there are rich vertebrate remains is that represented by the early Maastrichtian parts of the Prince Creek Formation in Northern Alaska (palaeolatitide ~ 82-85°N). Here the forests were dominated by a single deciduous conifer, Parataxodium wigginsii Arnold et Lowther similar in form to the extant genus Metasequoia. Trees were of limited height and small (~ 20 cm) girth with narrow, often false, growth rings. The forest canopy was semi-open over a groundcover of ferns and Equisetites. Ponds supported abundant floating aquatic angiosperms belonging to Quereuxia. Pollen evidence also suggests the presence of herbaceous angiosperms. Abundant fossil charcoal is indicative of frequent forest fires and likely points towards a partially open landscape consisting of a patchwork of burned areas with recovery vegetation dominated by ferns, Equisetites and herbaceous angiosperms between surviving areas of conifer forest. River margins were likely dominated by weedy flowering plants admixed with ferns and Equisetites.

At these latitudes winter darkness lasted for ~120 days and the spring and autumn twilight period for ~ 15 days. Palaeoclimate reconstructions and modeling indicate a more or less permanently cloudy and mist-shrouded environment with a mean annual temperature (MAT) of 6-7°C, a warm month mean temperature (WMMT) of 14.5 ± 3.1 °C and a cold month mean temperature (CMMT) of -2 ± 3.9 °C. During the summer growth rings in wood suggest the temperature frequently fell below + 10°C. In the winter temperatures as low as -10°C may have occasionally lasted for several contiguous 24 hour periods. Spring bud break would have been around late February to early March at the earliest and leaf fall in early October at the latest, limiting the time when fresh food was available in any quantity to not more than 6 months.

The diversity of body sizes and skeletal structures imply a range of strategies for coping with limited good quality food, cold and dark during the winters. These include both hibernation and migration. However, year-round residency in these harsh conditions requires reproduction. For small mammals and dinosaurs burrowing and enclosed nest building was clearly an option for overwintering, egg incubation and rearing young, but for larger animals this was not possible. To date no dinosaur egg remains have been found in Northern Alaska but they are known to occur some 6° latitude further south in the Early Maastrichtian Kakanaut Formation of Northeastern Russia. Here the forests were more diverse, the winter darkness shorter (45 days), and the temperature regime warmer (MAT 10°C, WMMT 19°C, CMMT +3°C). The growing season (temperatures > 10°C) was ~6.3 months and fresh food available in quantity for slightly longer that in Northern Alaska. Taken together these two localities and their reconstructed summer temperature regimes constrain the thermal regime of nest environments and appear to suggest sophisticated nest management strategies to assist the necessary rapid incubation and hatching before the onset of winter.

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