Pteridophytic features in some Lower Carboniferous seed megaspores

Abstract

Some Lower Carboniferous seed megaspores have triradiate sutures–unequivocal evidence of their being arranged in a tetrahedral configuration. This type of tetrad arrangement is unknown in the ovules of modern gymnospermS. Some of the fossil tetrads consist of one large functional megaspore and three smaller abortive spores attached at the apex of the large one. All four spores have an exinous covering. Spores arranged in tetrahedral tetrads, and hence with triradiate sutures, are characteristic of many modern and fossil free‐sporing pteridophyteS. In possessing this feature the fossil seed megaspores are more similar to the spores of these pteridophytes than to the megaspores of modern gymnosperms.

A Tetrahedral Megaspore Arrangement in a Seed Fern Ovule of Pennsylvanian Age

“…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.…”

A reappraisal of the Dinantian floras at Oxroad Bay, East Lothian, Scotland. 1. Floristics and the development of whole-plant concepts

“…Pettitt's (1969, pis 2C, 3C) ovule 'type B' (c. 2-7 mm x c. 1-9 mm) corresponds to our type 3; he too believed that it represented E. oxroadense. Pettitt (1969) demonstrated the attachment of three abortive megaspores at the apex of the functional megaspore, together forming a tetrahedral tetrad. Pettitt's (1969, pis 2A-B, 3A, C) ovule 'type A' (megaspore 3-0 mm x 1-2 mm) is most likely to represent the permineralised ovule-species Stamnostoma sp.…”

The Inevitable Seed

“…2 summariees six early stages in seed evolution which may be recognised in Palaeozoic fo88ils, showing different stages in the reduction of megaspore number to a single tetrad (Bothrodendrortrobur), abortion of two members of the tetrad to give only two sizeahle (functional ?) megaspores (Bensonite1), or one (Archaeosper11U1), and the subsequent reduction in size of the abortive spores (Cardiocarpur, see Pettitt, 1969), and eventually to a transient linear tetrad of which the abortive members never receive an exine (e.g. Cycar, and probahly various Palaeozoic seeds).…”

“…Pettitt andBeek, 1968 andGillespie et coU., 1981). The megasporangia show a considerable range in spore number per sporangium, from 20 (i.e.…”