Anatomy of Umkomasia (Corystospermales) from the Triassic of Antarctica

Abstract

The permineralized, corystosperm, cupulate, ovule-bearing organ Umkomasia resinosa is described from the early Middle Triassic of Antarctica. This is the first description of anatomically preserved Umkomasia, which consists of a determinate cupulate branch with helically arranged, recurved, pedicellate megasporophylls, each of which bears one or two abaxially attached unitegmic ovules. Cupules are ovoid, bilobed with elongate ventral and dorsal openings or unlobed with a single ventral opening, and have a two-zoned parenchymatous cortex and abundant secretory cavities. Ovules are small, orthotropous, and possess a thin integument that contains numerous secretory cavities. The ovules are broadly attached at the base, with a bifid integumentary apex that extends past the cupule lobes. The cupulate branch displays stem-like anatomy, producing paired traces into each cupule stalk. These structurally preserved ovulate organs can be related to other corystosperm organs from Antarctica, particularly the pollen-organ Pteruchus fremouwensis. Both anatomical and morphological features support interpretation of corystosperm reproductive structures as branching systems rather than as compound sporophylls. As a result of an increased understanding of the organization of Umkomasia, it appears doubtful that any direct relationship exists between the corystosperm and angiosperm lineages.

Perspective: The Origin of Flowering Plants and Their Reproductive Biology?a Tale of Two Phylogenies

“…The cupules were curved downward relative to the axis, implying that the ovules were abaxial. In a study of petrified material, Klavins et al (2002) confirmed this interpretation based on orientation of the xylem and phloem in the vascular bundles of the cupule: the ovules were on the phloem side of the bundles, which was presumably abaxial. Therefore I have rescored corystosperms as having paddlelike megasporophylls (1), rather than uncertain (0/1) in Doyle (1996), and abaxial ovules (1), rather than (1/2).…”

“…Because there is much confusion about this point, it is important to realize this comparison is based on the presumed adaxial position of the nucellus and inner integument (together considered equivalent to the original seed plant ovule) relative to the outer integument, not on the adaxial position of the bitegmic ovule on the carpel. Klavins et al (2002) took the contrast between the abaxial position of ovules in the corystosperm cupule and the adaxial position of ovules in the angiosperm carpel as evidence against a relationship between the two groups, but if the angiosperm bitegmic ovule corresponds not to a gymnosperm ovule but rather to a cupule containing an ovule, this is not the relevant comparison.…”

“…An exciting recent discovery (Axsmith et al 2000, Klavins et al 2002 was that cupules of corystosperms from the Triassic of Antarctica had a similar orientation. Some authors had interpreted the branched structures bearing these cupules as compound sporophylls (Harris 1951, p. 38;Doyle and Donoghue 1986), but compression specimens with cupules arranged in terminal pseudowhorls (Axsmith et al 2000) indicated that they were branches and the cupules were simple sporophylls, as in peltasperms, as argued by Thomas (1933) and Nixon et al (1994) and accepted as one of two possibilities by Doyle (1996).…”

“…Although the pollen chamber of Gnetum is somewhat intermediate, Martens (1971) considered it more like that of Welwitschia than Ephedra. In Doyle 1996 I scored corystosperms as unknown, but according to Klavins et al (2002) they had a nucellar beak, so I have rescored them as (0), as in Rothwell and Serbet (1994). Reymanówna (1974) contrasted the small pollen chamber of Caytonia with the large chamber of Callistophyton (Callospermarion).…”

“…Rothwell and Serbet (1994) scored corystosperms as having a lagenostome, but in Doyle (1996) I considered the character unknown. However, Klavins et al (2002) showed a nucellar beak in petrified material, so I have rescored the group as (0). 98 (D96 57 modified).…”