Gymnospermes s.l. du Jurassique supérieur du massif Manamana, Sud-Ouest de Madagascar: une nouvelle paléoflore gondwanienne

Resume

Des plantes fossiles du Jurassique supérieur rapportées aux Gymnospermes s.l. provenant du massif Manamana, au sud-ouest de Madagascar, ont été étudiées. Elles sont attribuées aux Ptéridospermales et aux Coniférales. Une nouvelle espèce, Brachyphyllum madagascariense nov. sp. est décrite, avec l’aide des caractères épidermiques observés au microscope électronique à balayage (MEB). Il s’agit de la première description d’une telle paléoflore dans le Jurassique supérieur de Madagascar. Des comparaisons de composition systématique sont faites avec d’autres paléoflores de même âge du Gondwana.

Abstract

Fossil plants from Upper Jurassic sediments belonging to the Gymnosperms s.l. have been studied in the Manamana massif from the southwestern part of Madagascar. The levels are Upper Oxfordian in age on the basis of the palaeontological content, especially with ammonites. The majority of fossil plants are devoided of organic matter but in some cases collodion peels have been made to present epidermal characters. The cuticle of Brachyphyllum madagascariense nov. sp. has been observed with scanning electron microscope (SEM). This new flora is constituted by Pteridospermales and Coniferales. Fragmentary fronds of Pteridospermales have been collected of Pachypteris sp. A and Pachypteris sp. B. Some epidermal characters are described. Coniferales are represented by leafy stems. The diagnosis of Brachyphyllum madagascariense nov. sp. is given with morphological and epidermal characters. Others conifers have been determined: Brachyphyllum sp. A, Brachyphyllum sp. B, Elatocladus sp., Cupressinocladus sp., Cyparissidium sp.. Araucarites sp. remains are probably isolated fertile scales of a female cone. Complete cones of Conites sp. are also present.

Comparisons of the systematic composition are made with other Gondwana floras of the same age from Antarctica, South America, Madagascar, India, South Africa, New Zealand and Australia. This flora shows xerophytic adaptations. With the Filicales described by Appert (1973a, b) from the same localities, it is now the most diverse flora from the Upper Jurassic of Madagascar, despite the lack of Cycadales, Bennettitales and Angiosperms.