Reinvestigation of the Enigmatic Carboniferous Sphenophyte Strobilus Cheirostrobus Scott and Implications of In Situ Retusotriletes Spores

Premise of research. Cheirostrobus pettycurensis Scott is an early Carboniferous sphenophyte known only from its anatomically preserved strobili, which bear highly dissected sporophylls comprising a lower vegetative part and an upper fertile part bearing inwardly facing sporangia. Since Scott's pioneering work over 100 years ago, the genus has not been reinvestigated such that it remains poorly characterized and its systematic position enigmatic. We reinvestigate Cheirostrobus, providing new insights into its structure, anatomy, and in situ spores and thereby addressing long-standing questions concerning its considerable systematic and evolutionary importance. Methodology. Historically prepared slides of Cheirostrobus from museum collections were examined using transmitted light microscopy to facilitate a detailed description using modern terminology, allowing for more effective comparisons and interpretations. Pivotal results. The strobilus axis comprises a stellate protostele with 9-13 protoxylem poles showing exarch maturation. The protoxylem has circular and metaxylem scalariform pitting. The main vascular bundles depart from the stele in superposed whorls and divide vertically close to the cortex, from which the lower (abaxial) daughter bundle supplies two or three bracts and the upper (adaxial) daughter bundle supplies two or three sporangiophores, with each bract subtending one sporangiophore. Individual sporangiophores have a foliar apex and typically bear four inwardly facing, longitudinally elongated sporangia disposed in two vertical rows. In situ spores are homosporous and trilete, have a prominent margo with leavigate exine, and range from 48 to 77 mu m in diameter. Spores conform to the sporae dispersae Retusotriletes incohatus Sullivan 1964. Conclusions. Cheirostrobus has an organization similar to members of the Sphenophyllales and the Calamitales; it is interpreted as a member of the Sphenophyllales that is on the evolutionary stem lineage leading to the stratigraphically younger Calamitaceae. Retusotriletes-type spores are reported in situ for the first time within sphenophytes, having previously been found within rhyniophytes, zosterophylls, trimerophytes, and algae, further emphasizing the polyphletic nature of the genus and convergent spore evolution across multiple lineages. Retusotriletes incohatus in the dispersed record ranges from the late Famennian (Devonian) to the late Visean (Carboniferous) and is common in basal Carboniferous deposits, suggesting that Cheirostrobus (or closely related plants producing the same kind of spores) had a longer stratigraphic range than was previously recognized and an earliest Carboniferous acme.