Antarctic Glossopterids: Clarifying and Comparing Reproductive Structures

Abstract

Glossopterid reproductive structures, both impressions and permineralizations, have been known from Antarctica for decades, but little detailed work has been done on them to date. Impression fossils from the Central Transantarctic Mountains reveal that at least four genera of reproductive organs are present on the continent. Plumsteadia is a genus of multiovulate megasporophylls found on all Gondwanan continents and distinguished by tightly compacted ovules surrounded by a wing which contains a row of ovules at its base. Numerous specimens of Plumsteadia ovata Kyle add additional information on anastomosing venation in the sporophyll and a fluted margin on the wing to the species description. A new species of Rigbya, an ovulate organ, is characterized by the laminar appearance of the cupules/scales and a crenate margin at the apex of the cupules. Eretmonia singulia sp. nov., a microsporangiate structure, is distinguished by the terminal attachment of a single sporangium on a dichotomizing stalk rather than a whorl of sporangia as found in other species. Finally, Arberiella inflectada sp. nov., a cluster of pollen sacs, includes microsporangia with a distinctive, recurved base. These impression genera are found across Gondwana, suggesting that Antarctica, with its central location in the supercontinent, was a bridge for the distribution of glossopterid genera from one Gondwanan continent to another. The relative scarcity of information garnered from these impression specimens presents a conservative estimate of the diversity of the glossopterid clade, but information from Antarctica adds considerably to our knowledge of this group. Permineralized reproductive structures have been known from Antarctica for the past four decades. No formal descriptions have been available for this material, however, leaving a gap in our knowledge of the glossopterid clade. Lakkosia kerasata gen. et sp. nov. is a multiovulate, megasporangiate structure found in silicified peat from the Late Permian Skaar Ridge locality in the Central Transantarctic Mountains. Ovules are borne in depressions on the adaxial surface of the megasporophyll and enclosed in thin strips of tissue that arises from the sporophyll. Transfusion tissue with scalariform wall thickenings is present in the sporophyll and may have acted as a storage or conducting tissue. In longitudinal sections of the ovules, the sclerotesta forms two appressed, hemispherical masses of lignified parenchyma cells which create the micropyle. Tissue of the sarcotesta overarches these parenchymatous masses, creating a chamber above the micropyle. Lakkosia is compared to previously described permineralized material from Antarctica, permineralized reproductive structures from the Bowen Basin of Australia, and impression material found in Antarctica. Arberiella is an aggregation of pollen sacs associated with all the microsporangiate structures attributed to the Permian Glossopteridales. Microsporophylls are characterized by scale leaves (smaller leaves with similar morphology as Glossopteris leaves) that bear clusters of sporangia (Arberiella) at the end of pairs of pedicels. Late Permian permineralized specimens of Arberiella from the Central Transantarctic Mountains in Antarctica reveal that the pollen sac walls consist of two types of cells. Rectangular parenchyma cells form a tissue one to two layers thick and constitute the majority of the sac wall. Lignified elongated cells are dispersed randomly among the parenchyma cells and probably correspond to striations on the surface of the pollen sac. This differentiation of cells in the pollen sac has not been described in any Arberiella species. No pollen sac is preserved in its entirety, but they appear to be bean shaped with a bulbous apex. Up to 12 pollen sacs were identified by the presence of aggregations of bisaccate pollen grains surrounded by remnants of pollen sac walls. Arberiella schopfii is most similar to the impression species A. vulgaris, but with the limited amount of information available from impression material, classifying the permineralized Antarctic specimens as A. vulgaris is premature. The simplicity of the pollen sacs does not suggest a specialized form of pollination, but rather that the glossopterids were wind pollinated. The morphology of these sporophylls is similar to that of pollen cones of other gymnosperms and most especially the Cycadales. The organization of glossopterid microsporophylls supports a gymnospermous affinity for the Glossopteridales. A comparison of these new Antarctic taxa to species found on other Gondwana continents is provided to illustrate the diversity and geographic range of glossopterid reproductive structures.