Studies of Paleozoic Seed Ferns: Additional Studies of Microspermopteris aphyllum Baxter

Abstract

The genus Microspermopteris is characterized as a small lyginopterid pteridosperm with a pentarch protostele, irregular cortical ridges, and multicellular trichomes. We describe the frond architecture and variability among axillary branches previously unknown for the genus. Fronds are small and delicate, exhibiting three orders of branching. Clasping V-shaped petioles produce primary pinnae alternately to suboppositely from adaxial projections. Primary pinnae in turn produce secondaries that bear two-, three-, or four-lobed ultimate laminar pinnules. Pinnules reconstructed from serial sections are morphologically similar to Sphenopteris-like compressions; however, the overall frond is reduced in size and complexity. While some axillary branches produce secondary xylem, others are composed entirely of primary tissues. Some axillary branches produce scalelike leaves in a tight helix; distally, immature buds are surrounded by flattened bud scales. Although Microspermopteris shows similarities to Heterangium in stelar and cortical anatomy, the two taxa are distinct. Microspermopteris is interpreted as a delicate, scrambling vine, liana, or shrublike plant in contrast to the more robust lyginopterids Lyginopteris, Schopfiastrum, and Heterangium.