Contents
Abstract- INTRODUCTION
- VALIDITY OF PTERIDOPHYTES
- CLASSIFICATION OF THE PTERIDOPHYTES
- EVOLUTIONARY SIGNIFICANCE OF SPHENOPSIDE
- CLASSIFICATION
- THE PTEROPSIDA
- EVOLUTION OF SPHENOPSIDE AND PTEROPSIDA
- Conclusion
- References
Description
A mini research was done on the topic "EVOL.SIGNIFICANCE OF PTROPSIDA AND SPHENOPSIDE." It utilizes "whorled appendages," an unambiguous feature uniting the two major groups of sphenopsids, Equisetales (sensu Good) and Sphenophyllales, and two other characters, with states partially redefined by us, which we regard as significant to the problem. These characters are: 1) a morphological series involving dissection of a ribbed protostele and loss of central xylem ultimately resulting in the equisetalean type of stele, and 2) a proposed series suggesting homology between equisetalean and sphenophyllalean "sporangiophores" and the sporangial complexes of some cladoxylopsids. The distribution of known taxa among defined character states forces us to conclude that the three characters are mutually incompatible, that is, no character supports the same estimate of phylogeny as any other. However, if we consider one character to be "true," then only a few parallelisms or reversals are necessary to explain the data. Allowing each character in turn to be "true" generates a total of six partially resolved phylogenetic hypotheses, only three of which are consistent with stratigraphic evidence. Each of our hypotheses is unique, but the set as a whole shares some features that do not occur in the hypothesis of Skog and Banks. Reasons for this, we believe, are: 1) increased understanding of members of Iridopteridales, and 2) rejection by us of the idea that similar features of cladoxylopsids are necessarily the result of parallelism. However, we are unable to find a testable argument that supports a greater likelihood of "truth" in thus character compared to the others. Therefore, we do not believe that relationships suggested by it are more likely to be true.
