THE CORYSTOSPERM POLLEN ORGAN PTERUCHUS FROM THE TRIASSIC OF ANTARCTICA

The permineralized corystosperm pollen organ Pteruchus is described fr om the early Middle Triassic of Antarctica. Pteruchus fremouwensis con sists of an axis bearing numerous, helically arranged microsporophylls , each of which terminates in a distal flattened head. The axis is 1-2 mm in diameter and eustelic. Spherical to elliptical secretory caviti es are present in the ground tissue of the axis, microsporophyll, and pollen sac wall. The basal stalk of the microsporophyll is vascularize d by a C-shaped strand that gives rise to a midvein and numerous later al veins in the distal head. At least 38 pollen sacs are borne on the abaxial surface of the microsporophyll head. These are arranged in pai rs on either side of lateral veins. Each pollen sac is sessile, elonga ted, and uniloculate. The pollen sac wall is several cell layers thick early in ontogeny, but reduced to a single layer in thickness when ma ture. Dehiscence is longitudinal along the inner surface. Pollen is mo nosulcate and bisaccate, and of the Alisporites-type. The Triassic spe cimens are the first structurally preserved pollen organs of the Pteru chus-type and can be related to the associated corystosperm stem and l eaf genera based on the presence of unique secretory cavities. The mor phology of Pteruchus and the relationship of this pollen organ with ot her Mesozoic and Paleozoic pollen organs is discussed.