A TETRAHEDRAL MEGASPORE ARRANGEMENT IN A SEED FERN OVULE OF PENNSYLVANIAN AGE

A tetrahedral arrangement of one functional and three aborted megaspores has been found in the fossil seed fern ovule Conostoma anglo-germanicum. This is the first conclusive report of a Pennsylvanian age ovule with such a primitive megaspore arrangement, suggesting that this feature was more widespread and persistent among Paleozoic pteridosperms than previously assumed. THE pteridosperms or seed ferns included a diverse group of Paleozoic and Mesozoic gymnospermous plants which bore ovules and pollen organs on fern-like foliage. Although the pteridosperms became extinct during the Mesozoic Era they are generally considered to have played an important evolutionary role as ancestors to the cycads, cycadeoids, and possibly even the angiosperms (Takhtajan, 1969; Stebbins, 1974). During the Pennsylvanian the pteridosperms were represented by members of three families: the Lyginopteridaceae, Medullosaceae, and Callistophytaceae (Stidd and Hall, 1970). In the past ten years much has been deduced concerning the ontogeny and reproductive biology of members of the Callistophytaceae including ovule and megagametophyte development (Rothwell, 1971b), microsporogenesis and microgametophyte development (Hall and Stidd, 1971; Millay and Eggert, 1974), pollination mechanism (Stidd and Hall, 1970; Rothwell, 1977) and fertilization (Rothwell, 1972). In general, this evidence suggests that callistophytacean ovules and pollen were developmentally and functionally similar to those of extant gymnosperms such as cycads and Ginkgo. OBsERvATIoNs-There is now, however, evidence that at least some Pennsylvanian seed ferns had ovules which clearly differed from those of modern gymnosperms in regards to the basic megaspore configuration. This evidence comes from a petrified pteridosperm ovule found in a coal ball collected from strata of the Desmoinesian Series near Oskaloosa, Iowa. The ovule is assignable to Conostoma anglo-germanicum as characterized by Oliver and Salisbury 1 Received for publication 17 July 1978; revision accepted 21 February 1979. Dr. Joseph Wood, University of Missouri, is gratefully acknowledged for providing lab facilities to the junior author. (1911) and Rothwell (197 la) and was probably borne by a member of the Lyginopteridaceae (Arnold 1947; p. 219). A cellulose acetate peel preparation of a transverse section through the micropylar end of the petrified ovule (Fig. 1) shows the central column, circular pollen chamber, and the surrounding sclerotic layer of the integument with its four prominent ribs. Serial preparations made just beneath the pollen chamber show the presence of a group of three distorted spores (Fig. 2) which presumably represent aborted megaspores. Lying proximal to these spores is a single large megaspore bearing a triradiate mark (Fig. 3). The presence of such a trilete mark provides verification of the apparent tetrahedral arrangement of the megaspore tetrad. DISCUSSION-The occurrence of a tetrahedral arrangement of megaspores in gymnosperm ovules is considered to be a primitive feature that has previously been observed only in fossil ovules of Devonian and Mississippian age (Pettitt and Beck, 1967; Pettitt, 1969). Such a tetrahedral configuration is clearly different from the linear tetrad arrangement produced in modern seed plants. In most extant gymnosperms and angiosperms the four megaspores are arranged in a vertical row with the three spores closest to the micropyle aborting and breaking down (Foster and Gifford, 1974). The oldest documented occurrence of this linear type of tetrad is that in fossil ovules of cycadeoids from the Cretaceous Period (Crepet and Delevoryas, 1972). The discovery of the tetrahedral tetrad in Conostoma anglo-germanicum is significant in that it represents the first Pennsylvanian age ovule in which the tetrad arrangement has been conclusively demonstrated. Although Hoskins and Cross (1946) previously reported tetradedral arrangements of megaspores in the Pennsylvanian medullosan ovule Pachytesta vera, the evidence