Monday, 31 August 2015

Glenrosa carentonensis: A new species of Conifer from the Early Cretaceous of Charente-Maritime, France.


Conifers of the genus Glenrosa were first recorded from the Early Cretaceous Glen Rosa Formation of Texas in 1984 (although specimens of the plant had been collected in the area since the 1890s), and subsequently recorded from widely distributed Early Cretaceous from North America, Europe and Asia, suggesting that for a while it was a widespread and dominant plant in the Northern Hemisphere. Glenrosa is distinguished by having small fleshy triangular leaves held close to the stems, with stomata (breathing pores) grouped together in deep stomatal crypts (indentations on the leaves). Such crypts are known only from a single other species of Conifer, Sedites rabenhorstii, which also lived in the Cretaceous, making interpretation of its ecological significance difficult, however similar structures are found in some modern Angiosperms (flowering plants), the majority of which live in arid climates, suggesting that the pores may help to regulate water loss in dry conditions.

In a paper published in the journal PLoS One on 19 August 2015, Jean-David Moreau of Géosciences Rennes at Université Rennes and the European Synchrotron Radiation Facility, Didier Néraudeau, also of of Géosciences Rennes at Université Rennes, Paul Tafforeau also of the European Synchrotron Radiation Facility and Éric Dépré of the Groupement d’Etude et de Contrôle des Variétés et Semences, describe a new species of Glenrosa from material included in flint nodules collected from Early Cretaceous deposits at Font-de-Benon Quarry, between the villages of Archingeay and Les Nouillers in the Charente-Maritime Department of western France.

The new species is named Glenrosa carentonensis, meaning ‘from Charente’ (Carentonia is the Latin name for the River Charente). The species is described from a number of specimens of fragmentary material up to 50 mm in length preserved in flint nodules. Since it is essentially impossible to extract plant remains from hard flint, the specimens were examined using propagation phase-contrast X-ray synchrotron microtomography at the European Synchrotron Radiation Facility.

Plant fossils embedded inside flint nodules. (A) Broken flint nodule with diverse conifer inclusions, SIL_ARC_2. (B) Leafy twigs of Glenrosa carentonensis. (C) Leafy twigs of Glenrosa carentonensis. (D) Leafy axis of Glenrosa carentonensis bearing distally a microsporangiate cone on its distal tip. (E) Detail of the same cone. Moreau et al. (2015).

The leaves of Glenrosa carentonensis are arranged helically around the stems; they are flesh and somewhat claw-shaped, with a waxy cuticle and numerous stomatal crypts, each containing 7-12 stomata. Several male cones were found in the sample, the first time these have been recorded for any species of Glenrosa. These are 2.8–5.0 mm long and 2.3–4.1 mm wide and bear up to 40 microsporophylls regularly distributed along the cone axis. Each microsporophyll bears 6-7 pollen sacs in clusters up to 0.5 mm wide. The pollen could be determined to be 17.3–20.3 μm long and 10–16 μm wide, but no details could be discerned.

Propagation phase-contrast X-ray synchrotron microtomograph, virtual sections of Glenrosa carentonensis reconstructed using a single distance phase retrieval process. (A) Longitudinal section of leafy axis. (B) Transversal section of leafy axis. (C) Longitudinal section of twig and cone. Moreau et al. (2015).

The stomatal crypts, fleshy leaves and thick waxy cuticles of Glenrosa carentonensis all appearto be adaptations to an arid climate, while the inclusion of the material in flint nodules implies a coastal environment (all flint deposits are thought to have originated in shallow-marine environments). Moreau et al. suggest the plants were a major component of a dry coastal Conifer forest, with occasional storm events and possibly haline influences (salt laden winds).

Propagation phase-contrast X-ray synchrotron microtomograph 3D renderings of Glenrosa carentonensis. (A) Helically arranged leafy axis. (B–C) Leafy axes bearing a microsporangiate male cone. Moreau et al. (2015).

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