IN SITU GYMNOSPERM POLLEN FROM THE MIDDLE JURASSIC OF YORKSHIRE

Abstract

In this paper the morphology of pollen grains yielded by male Gymnosperm fructifications from the Jurassic flora of Yorkshire is studied and discussed. Several new male fructifications were found and described: Hastystrobus gen. nov. was erected for male cones yielding the Eucommiidites type of pollen grains. This genus is monotypic and the type species Hastystrobus muirii yielded pollen grains that agree with Eucommiidites troedssonii. Hastystrobus muirii very probably has Cycadalean affinities, because the whole abaxial surface of the microsporophylls is covered with sporangia. For the first time the male fructification of Ginkgo huttoni (Heer) Sternberg is described. It resembles in general the male fructification of the recent Ginkgo biloba L., and the pollen grains agree with those of Ginkgo biloba. Male cones associated with Brachyphyllum crucis Kendall have been found and described. They yielded pollen grains that after short maceration were identifiable as Circulina, while after prolonged maceration they could be assigned to Classopollis multistriatus Burger. Brachyphyllum crucis is provisionally assigned to the Hirmerella-group on the basis of its male cone and pollen grains. The cones were compared with other male cones containing Classopollis pollen, which were also attributed to the Hirmerella-group. It is suggested that all members of the Hirmerella-group have an epidermis with a special type of stoma. Masculostrobus harrisii sp. nov. is described. This male cone resembles closely the male cone of Elatides williamsoni (Brgt) Sew., but its pollen grains are of the Inaperturopollenites-type, instead of the Perinopollenites-type. The new species Pityanthus scalbiensis yielding bisaccate pollen grains of a rather primitive type is described; there is almost no saccus infrastructure present. The Czekanowskia-group (including Leptostrobus and Solenites) in all probability does not belong to the Ginkgoales but to the Conifers. An interesting find was that the pollen grains of the Araucariaceous Brachyphyllum mamillare Brgt are of two types: In type 1 the nexine and sexine are attached to each other, while in type 2 the nexine is loose from the sexine and is somewhat shrunk. In the latter type also pollen grains were found that are somewhat trilobate. A similar dimorphism of pollen grains was also found in the recent Araucaria araucana (Molina) K. Koch. The pollen grains from the various male fructifications were compared with those from other male fructifications related to them. If the cones were assigned to recent taxa, the pollen grains were compared with recent pollen grains from those taxa (Cycadales, Ginkgoales and Coniferospermae). The pollen grains were also compared with dispersed pollen grains, mainly from the Jurassic.