Cj. Phipps et al., OSMUNDA (OSMUNDACEAE) FROM THE TRIASSIC OF ANTARCTICA - AN EXAMPLE OFEVOLUTIONARY STASIS, American journal of botany, 85(6), 1998, pp. 888-895
Compressed specimens of the fern Osmunda are described from the Triass
ic of the Allan Hills, Antarctica. The specimens consist of a once pin
nate, deeply pinnatifid fertile frond as well as several sterile speci
mens. Six pinnae are present on the partial fertile rachis, with two s
terile pinnae above four fertile pinnae. Both sterile and fertile spec
imens are virtually identical to the modern species Osmunda claytonian
a. Entire fronds are fragmentary; the longest is 21 cm in length. Ster
ile pinnae are alternate and deeply pinnatifid, with slightly toothed
pinnules and dichotomous venation. Fertile pinnae are 1-1.3 cm long, o
nce pinnate, and lack vegetative lamina. Sporangia are clustered, each
300-375 mu m in diameter, and possess a transverse annulus 6-8 cells
long; dehiscence is by a vertical slit. Fronds arise from a rhizome 4
cm long by 1 cm wide; two croziers are present on the rhizome. Two fro
nd segments up to 6 cm long and three deeply pinnatifid pinnae are pre
sent on the uppermost part of one rachis. Pinnules are similar to 4 mm
long and 2-3 mm wide. The presence of this Osmunda species in the Tri
assic demonstrates stasis of frond morphology, both fertile and vegeta
tive, for the genus.