Bj. Axsmith et al., New perspectives on the Mesozoic seed fern order Corystospermales based onattached organs from the triassic of Antarctica, AM J BOTANY, 87(6), 2000, pp. 757-768
A new Triassic corystosperm is described from the Shackleton Glacier region
of Antarctica. The compression fossils include cupulate organs (Umkomasia
uniramia) and leaves (Dicroidium odontopteroides) attached to short shoot-b
earing branches. The cupulate organs occur in groups near the apices of the
short shoots, and each consists of a single axis with a pair of bracts and
a subapical whorl of five to eight ovoid cupules. This unique architecture
indicates that the cupules are individual megasporophylls rather than leaf
lets of a compound megasporophyll. A branch bearing an attached D. odontopt
eroides leaf provides the first unequivocal evidence that Umkomasia cupulat
e organs and Dicroidium leaves were produced by the same plants. Although t
his had previously been assumed based on organ associations, the new specim
ens are important in demonstrating that a single species of corystosperm pr
oduced the unique cupulate organs described here and the geographically and
stratigraphically widespread and common D. odontopteroides leaf. Therefore
, biostratigraphic, paleoecological, and phylogenetic studies that treat Di
croidium leaf morphospecies as proxies for biological species of entire pla
nts should be reconsidered. Phylogenetic analysis suggests that the corysto
sperm cupule is an unlikely homologue for the angiosperm carpel or outer in
tegument.