KLAVINS, SHARON D.*, EDITH L. TAYLOR, and THOMAS N. TAYLOR. Department of Ecology and Evolutionary Biology and Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, KS 66045. - Anatomy of the ovulate cupules of Umkomasia (Corystospermales) from the Triassic of Antarctica.
Umkomasia is a cupulate ovule-bearing organ belonging to the
Corystospermales, a small group of pteridosperms known from the
Triassic. Since their initial description, these reproductive
structures have been known only from compressions. Although many
species of Umkomasia have been described, important details of
morphology have remained ambiguous due to the lack of anatomical
information. Conflicting interpretations of morphology, as well as
missing information on key characters, have resulted in poor
resolution of the position of the corystosperms in seed plant
phylogeny. Anatomically preserved Umkomasia has recently been
identified for the first time in permineralized peat from the early
Middle Triassic of Antarctica. These specimens provide the opportunity
to correlate the morphological details of Umkomasia with
anatomical information. Umkomasia is a determinate cupulate
branch with helically arranged, recurved, pedicellate cupules, each of
which bears one to two abaxially-attached unitegmic ovules. Cupules
are ovoid and either bilobed with elongate ventral and dorsal
apertures or unlobed with an elongate ventral aperture. The cupule
cortex is two-zoned and includes sclerified cells and abundant
secretory cavities. Ovules are small, orthotropous, broadly attached
basally, and also possess secretory cavities in the thin integument.
The bifid apex of the integument extends past the cupule lobes. The
cupulate branch is ovoid and displays stem-like anatomy, producing
paired traces into each cupule stalk. We speculate that the cupules
can be related to other corystosperm organs from Antarctica,
particularly the pollen-organ Pteruchus fremouwensis, based on
similar secretory cavities. These specimens further support the
interpretation of corystosperm reproductive structures as branching
systems. In addition to aiding in the assessment of homologies among
the Mesozoic pteridosperms, these new specimens may provide critical
information on the placement of the corystosperms in seed plant
evolution.
Key words: anatomy, Antarctica, Corystospermales, pteridosperms, Triassic, Umkomasia