OSMUNDA (OSMUNDACEAE) FROM THE TRIASSIC OF Antarctica: an example of evolutionary stasis

Abstract

Compressed specimens of the fern Osmunda are described from the Triassic of the Allan Hills, Antarctica. The specimens consist of a once pinnate, deeply pinnatifid fertile frond as well as several sterile specimens. Six pinnae are present on the partial fertile rachis, with two sterile pinnae above four fertile pinnae. Both sterile and fertile specimens are virtually identical to the modern species Osmunda claytoniana. Entire fronds are fragmentary; the longest is 21 cm in length. Sterile pinnae are alternate and deeply pinnatifid, with slightly toothed pinnules and dichotomous venation. Fertile pinnae are 1-1.3 cm long, once pinnate, and lack vegetative lamina. Sporangia are clustered, each 300-375 um 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 frond segments up to 6 cm long and three deeply pinnatifid pinnae are present on the uppermost part of one rachis. Pinnules are ~4 mm long and 2-3 mm wide. The presence of this Osmunda species in the Triassic demonstrates stasis of frond morphology, both fertile and vegetative, for the genus.

Phylogeny of the Fern Genus Melpomene (Polypodiaceae) Inferred from Morphology and Chloroplast DNA Analysis

“…The osmundaceous ferns (of, Figure 1a) are well-supported as sister to all other leptosporangiates (bootstrap support, BS = 100%). This position is consistent with the fossil record because the oldest leptosporangiate fossils assignable to an extant lineage are members of this clade (Galtier & al., 2001;Miller, 1971;Phipps & al., 1998;Rößler & Galtier, 2002;Tidwell & Ash, 1994). Osmundaceous ferns are placed in a single family, Osmundaceae (Osm, Figure 1a; ).…”

“…Grammatopteris, Rastropteris, and other osmundaceous ferns are common from the early Permian onward (Galtier & al., 2001;Miller, 1971;Phipps & al., 1998;Rößler & Galtier, 2002;Tidwell & Ash, 1994), and representatives belonging to the sister group of the osmundaceous ferns (i.e., the clade containing all other extant leptosporangiate ferns) also have first appearances in the Permian (e.g., Szea; Yao & Taylor, 1988). Leptosporangiate ferns from the Carboniferous cannot be readily assigned to either osmundaceous ferns or their sister group, but are instead representatives of an early radiation that yielded several now-extinct lineages (Lovis, 1977;Rothwell, 1987;Stewart & Rothwell, 1993).…”

“…2 Upper Triassic fossils assignable to the stem of Osmunda (taxon 1, Figure 2, Table 5); minimum age constraint applied to subtending node (≥ 199.6 Ma). Osmunda fossils have been described from the Upper Triassic (Phipps & al., 1998), marking its divergence from the other osmundaceous fern genera. Table 5); minimum age constraint applied to subtending node (≥ 228.0 Ma).…”

Millerocaulis (Erasmus) ex Tidwell (Osmundales, Filices) from the Carrrizal Formation (Upper Triassic) from Marayes, San Juan province, Argentina

“…En el Triásico conviven formas algo primitivas como Australosmunda (Hill et al, 1989) (género éste que Tidwell (1994) sinonimiza a Millerocaulis, opinión que aquí no se comparte) con abundantes representantes de Millerocaulis de Sud América y Antártida. Tidwell & Ash (1994) hacen un breve análisis de los géne-ros conocidos en base a su anatomía y de las numerosas frondes que estarían incluidas entre las Osmundales (sensu lato) (Phipps et al, 1998), y señalaron que todo este grupo ( Orden ?) podría haberse originado en el hemisferio sur.…”

“…Estos hallazgos en el Triásico Superior de Argentina, junto con las también triásicas M. herbsti (Archangelsky & de la Sota ) Tidwell, de la Formación Laguna Colorada (provincia de Santa Cruz), M. stipabonettii Herbst (1995) de la Formación Cepeda (provincia de San Juan), los de dos especies de Paleosmunda Gould (1970) del Pérmico y Australosmunda (Hill et al, 1989) del Triásico, ambas de Australia, M. beardmorensis (Schopf, 1978) y Ashicaulis woolfei (Rothwell et al, 2002) del Triásico de Antártida, y, finalmente, de Osmunda claytoniites (Phipps et al, 1998), muestran la amplia difusión de este grupo de helechos en el Gondwana temprano. Se puede agregar aún, el hallazgo de restos preservados sin estructura celular, ilustrados por Herbst (1977) (CTES-PB 191) más materiales idénticos, pero inéditos, (en la colección paleobotánica de la Fundación Miguel Lillo, Tucumán, LIL-PB 5303 , figuras 5G y H), que pertenecen a pecíolos de Osmundales (sensu lato) del Triásico Superior de la Formación Los Rastros (provincia de San Juan).…”