Asterothecaceae

Introduction

The carboniferous was often described as the “Age of Ferns”, however it is now known that some of these fern fossils actually belonged to early gymnosperms, because they were found in association with seeds. One can however not be certain about sterile fronds, and these have been placed in a number of form genera. One of these form genera is Pecopteris, with a large number of described species known. Some of these were certainly gymnosperms, while others were certainly Marattioid ferns, because they bore sori of thick-walled sporangia. These fronds, sometimes as large as 6 m long, were many times pinnate, and the pinnules were attached along their entire base, each with a single midrib. The lateral veins were branched dichotomously once or twice or remaining simple, a venation type that is very similar to most extant Marattiales.

Asterotheca is the name given to pecopterid fronds bearing sessile sori made up of four or five sporangia fused at the base into a synangium, but with the distal part free. The sori were commonly arranged in two series along the pinna, each associated with a veinlet in the lamina. Example: Asterotheca candolleana.

Scolecopteris was similar to Asterotheca, except that the sorus was elevated on a short pedicel, or receptacle. Example: Scolecopteris incisifolia.

Fronds of the types above are often found in association with stout trunks, having a superficial resemblance to modern tree-ferns. These had a remarkably complex stelar anatomy and are generally referred to as Psaronius. Some of these were as much as 75 cm across, but most of this width was occupied by a thick root mantle. The stele of most Psaronius fossils (and in fact also most modern Marattiales) is a polycyclic dictyostele which, in the more complex types, contains as many as eleven interconnecting coaxial cylinders. Each cylinder is dissected into a number of mesarch meristeles completely surrounded by phloem, and the leaf traces at any particular level arose from the outermost system, while the inner systems were concerned with the origin of leaf traces at higher levels. The earliest examples had a simpler internal anatomy. Psaronius renaultii for instance had an endarch solenostele.

References

Christenhusz, M. J. M. 2007. Evolutionary History and Taxnomy of Neotropical Marattioid Ferns: Studies of an ancient lineage of plants. Annales Universitatis Turkuensis ser. AII, tom. 216, pp. 1-134.

Sporne, K. R. 1962. The morphology of pteridophytes, the structure of ferns and allied plants, pp. 127-135. Hutchison & Co. London.