The Rhizomorph Apex of Paurodendron; Implications for Homologies Among the Rooting Organs of Lycopsida

Abstract

A rhizomorph of Paurodendron with an intact apex recently has been discovered in Upper Pennsylvanian sediments of Ohio, and this provides the anatomical evidence necessary to interpret structure, ontogeny and homologies among lycophyte rooting organs. The basal meristem of Paurodendron is radial and lenticular, and produces an apical plug of parenchymatous tissue similar to a root cap. The plug is surrounded by a furrow associated with radially aligned cells that demonstrate a developmental correspondence to the furrow(s) of Isoetes. Based on external structural similarities at the rhizomorph apices of Paurodendron, Stigmaria, and young Nathorstiana, and on the anatomical similarities of Paurodendron to Isoetes, Stigmaria, Chaloneria, and Lepidocarpon embryos, all are interpreted as having a rooting organ that represents a modified shoot system that is fundamentally unlike the primary root system of seed plants. Likewise, the rootlets of rhizomorphic lycophytes are interpreted as leaves modified for rooting, and that have the equivalent of exogenous origin. As such, they are fundamentally unlike the adventitious roots of rhizomatous lycophytes like Lycopodium and Selaginella.

The Rhizomorphic Lycopsids: A Case-Study in Paleobotanical Classification

“…Stewart and Rothwell (1993) chose to assign both Oxroadia and Paumd&ndmn to the Isoetales, despite their fundamentally radially symmetrical rhizomorphs (cf. Rothwell and Erwin 1985;Long 1986;Bateman 1992a).…”

“…Although the term 'rhtzomorph' is used to describe certain fungal structures, this is far less likely to confound interpretation than use of the same term (rhizophore) for non-homologous structures in sister-groups vjithin the Lycopsida. Third, the term rhizomorph is already available in the literature as a descriptor of the rooting structure of stigmarian lycopsids (Bierhorst, 1971;Rothwell and Erwin 1985;Pigg 1992). Use of this established, accurate descriptor seems preferable to coining yet another term.…”

“…Other characters diagnostic of the clade that reflect shoot organization and vegetative anatomy and are not enumerated here (see ). The phylogenetic and ontogenetic significance of this suite of characters has been widely discussed elsewhere (Eggert 1961(Eggert ,1972Paolillo 1963;Karrfalt 1984;Rothwell and Erwin 1985;Rothwell and Pryoc 1991;Bateman 1994).…”

“…and appears to have evolved by changes in developmental timing (Bateman and DiMichele 1991). Paurodendronfraipontii Fry (Phillips and Leisman 1966;Schlanker and Leisrnan 1969;Rothwell and Erwin 1985) was a sprawling, centrally rooted pseudoherb, smaller than but similar in vegetative grovvfth habit to the only Lower Carboniferous species in the analysis, Oxroadia gracihs Alvin (Alvin 1965;Long 1986). Chahneria cormosa Pigg and Rothwell (Pigg and Rothwell 1983a, b) was columnar and unbranched, with an estimated height of 1-2 m.…”

“…la). Despite the lack of an analytical context, there is strong circumstantial evidence ttiat several lycopsid genera with bilaterally symmetrical rhizomorphs form a distinct clade, which diverged from (or alternatively was ancestral to) the radially symmetrical lineage during the Devonian (e.g., Jennings et al 1983;Rothwell and Erwin 1985;Pigg 1992). The isoetacean clade is well represented throughout the fossil record by genera that on average became smaller-bodied and less branched with time, though probably not gradually nor in the unidirectional, linear reduction series envisaged by Magdefrau (1956).…”