Abstract Detail

Horsetails Through Form, Space, and Time

Stein Jr, William E [1], Berry, Christopher [2].

Horsetails: enigmatic Devonian source.

Horsetails (Equisetales & Sphenophyllales) are commonly encountered fossil plants from the Carboniferous Period worldwide, with the modern genus Equisetum the only extant representative. Although molecular evidence may suggest a common origin of Equisetales with the ferns, little light has yet been thrown on the important morphological or anatomical changes required for the origin(s) of horsetails from Devonian precursors. We briefly survey our current understanding of Devonian groups (Cladoxylopsida, Hyeniales, Iridopteridales) often considered to be “preferns” or “protoarticulates”. In particular, our understanding of Pseudosporochnales (within Cladoxylopsida) has improved most dramatically in recent years resulting in reconstructions now far more familiar to us as plants, but very different from the rhizomatous fern or horsetail precursors they were once thought to be. We now know that at least some of these plants were towering trees with main trunk bearing ephemeral, regularly abscised and highly ramified branch systems serving the same function as fronds in modern tree ferns or palms. Base of the trunk also shows remarkable specializations for extended growth and new root insertion required by lengthy development associated with large size. It also appears increasingly likely that there is little evidence supporting Hyeniales as a distinct group based primarily on procumbent habit. In addition, Iridopteridales, although distinct with wholed organotaxis and erect sporangia, may well turn out to be variant “fronds” within a more general pseudosporochnalean body plan. With increased clarity of overall form, one naturally turns to reappraisal of synapomorphies currently employed to diagnose high-level groupings (Euphyllophytes, Monilophytes, Radiatopses, Lignophytes) intended to incorporate both fossil and extant taxa. In most if not all instances, features cited retrospectively as phylogenetically important in recognized monophyletic groups, were already implicit, although variably expressed, earlier in time. This points to the critical importance of considering developmental processes underpinning observed form, that is, the necessity of treating this information as homology in the capacity to produce form in differing local developmental contexts rather than homology in overall form per se. We’ll look at what’s known concerning lateral branch deployment and phyllotaxis perhaps leading to the whorled condition in horsetails. We’ll also consider underlying developmental homology between “radiate protoxylem” and “permanent protoxylem” groups. Both comparisons suggest a continuum developmental outcomes rather than discrete states. Much remains to be done using this approach.

1 - State University of New York, Department of Biological Sciences, Binghamton, NY, 13902-6000, USA
2 - Cardiff University, School of Earth & Ocean Sciences, Cardiff, CF10 3AT, UK

Keywords:
Pseudosporochnales
horsetails.

Presentation Type: Symposium Presentation
Session: SY03, Horsetails through form, space and time
Location: Oglethrope Auditorium/Savannah International Trade and Convention Center
Date: Monday, August 1st, 2016
Time: 2:00 PM
Number: SY03003
Abstract ID:331
Candidate for Awards:None