TOMESCU, ALEXANDRU MIHAIL FLORIAN* and GAR W. ROTHWELL. Department of Environmental and Plant Biology, Ohio University, Porter Hall, Athens, OH 45701. - Exploring the cladistic relationships of sphenopsids.
Phylogenetic relationships of Equisetum are firmly established
within the euphyllophytes, but more detailed resolution of sphenopsid
phylogeny has not been achieved. Traditional paleobotanical studies
recognize five groups of sphenopsids including Hyeniales,
Pseudoborniales, Calamitales, Sphenophyllales and Equisetales, but
these hypothesized relationships have not been fully tested using
cladistic methodology. Sphenopsids plus cladoxylalean/zygopterid
"ferns" resolved as the sister group to lignophytes in an
analysis of euphyllophytes by Rothwell (1999), but Equisetum
nested among the ferns as the sister to marattialeans in the recent
analysis of living plants by Pryer et al. (2001). Relationships among
sphenopsid groups are equally uncertain. Whereas Equisetales and
Calamitales are widely regarded as closely related, their
relationships to Pseudobornia, Hyenia and
Sphenophyllum are far less certain. Indeed, Stewart (1980) has
hypothesized that Sphenophyllum may be more closely related to
lycophytes than to euphyllophytes. To test these competing hypotheses,
we have undertaken a numerical cladistic analysis of representative
rhyniophytes, zosterophylls, lycophytes, and euphyllophytes, including
all of the hypothesized groups of sphenopsids. Relationships of these
taxa are resolved using a matrix of more than 100 morphological
characters. Principal questions addressed by this analysis include the
following. Do the sphenopsids (as traditionally circumscribed) form a
clade? What are the sister group relationships of the sphenopsid clade
that includes Equisetum? And, are Sphenophyllales
euphyllophytes, or do they nest within the zosterophyll/lycophyte
clade?
Key words: cladistics, Equisetum, phylogeny, Sphenophyllales, sphenopsids