Selaginella

From Palaeos

Selaginella is currently a cosmopolitan genus of about 700 species of small superficially fern-like plants, though numerous additional fossil taxa are also known (see below).

Descriptions

Parent taxa: Lycopodiopsida, Selaginellales.

Age: Carboniferous to Recent (see below).

Description: (from PlantNet and Thomas, 1997) Terrestrial or rarely epiphytic, annual or perennial; rhizome usually long-creeping, subterranean or running over the surface of the ground; aerial parts erect or creeping. Leaves simple and scale-like, ligulate, spirally arranged and all alike (isophyllous) or in 4 rows with the dorsal leaves reduced in size and appressed to the stem (anisophyllous - larger climbing species may only be anisophyllous on their branches). Modified axillary leaves may be present at branching points. Leaves usually single-veined (some taxa with three veins). Heterosporous; cones (strobili) terminal on branches or branchlets. Sporophylls may be all alike and in tetragonous cones, or dimorphous and in bilateral cones.

Comments: Selaginella, as might be expected of a genus containing some 700 species, is very variable. Most species of Selaginella possess rhizophores (aerial roots) except for a small number of species such as S. selaginoides, and molecular phylogenetic analysis supports recognising the rhizophore-bearing species as a single clade (Korall & Kenrick, 2004).

Isophyllous fossil specimens resembling modern Selaginella have been found from as early as the Lower Carboniferous, with anisophyllous forms appearing by the Late Carboniferous. However, many authors have separated these fossils from modern Selaginella into the "genus" Selaginellites. Thomas (1992, 1997) demonstrated that the taxonomic distinction between fossil and recent taxa was an artificial one (indeed, many species assigned to Selaginellites can be identified to certain living subgroups of Selaginella), and Thomas (1997) recommended that most fossil taxa be assigned to the recent genus, reserving Selaginellites for fossils of anisophyllous and solely vegetative specimens.

References

Korall, P., & P. Kenrick. 2004. The phylogenetic history of Selaginellaceae based on DNA sequences from the plastid and nucleus: extreme substitution rates and rate heterogeneity. Molecular Phylogenetics and Evolution 31 (3): 851-864.

Thomas, B. A. 1992. Paleozoic herbaceous lycopsids and the beginnings of extant Lycopodium sens. lat. and Selaginella sens. lat. Annals of the Missouri Botanical Garden 79 (3): 623-631.

Thomas, B. A. 1997. Upper Carboniferous herbaceous lycopsids. Review of Palaeobotany and Palynology 95: 129-153.

Credits

CKT080214