A NEW HORSETAIL SPECIES WITH TUBERS FROM THE LOWER CRETACEOUS JINJU FORMATION (110-106 MYA), SOUTH KOREA, AND ITS IMPLICATION ON PALEOENVIRONMENT

We present the new species Equisetum jinjuensis from the Albian Jinju Formation (110-106 million years ago (mya)), South Korea. The depositional environment of the Jinju Formation is lacustrine, and it provides a rich vertebrate ichnofossils (of dinosaurs, birds, pterosaurs, crocodiles, lizards, mammals) and body fossils of fish, insects, spiders, etc. The paleoflora represents sporing and seed plants that fringed the lake margin including a well-preserved Equisetum specimen. The E. jinjuensis type specimen is preserved as compression, with rhizome, roots, tubers, fertile shoot and a putative strobilus present. The species is nested within the subgenus Equisetum based on combined morphological and molecular characters using parsimony phylogenetic analyses. A recent study suggests that the crown group ages of the genus Equisetum and subgenus Equisetum date back to the Late Jurassic – Early Cretaceous and the Late Cretaceous, respectively. This new find pushes back the age of the subgenus Equisetum to the late Early Cretaceous.

A notable characteristic of E. jinjuensis is the presence of tubers. These structures are reported in the subgenus Equisetum but not in the subgenus Hippochaete thus far, although not all species and individuals of the species in the subgenus Equisetum have tubers. Rhizomes and tubers are considered to be the principal means of perennation, reproduction, and dissemination. Tubers of extant Equisetum are fully developed in late fall or winter when the aerial stems die down, and contain high starch concentration which is used to produce new shoots in the next growing season. The presence of tubers of E. jinjuensis would imply that strong seasonal variation existed in the Early Cretaceous Korean Peninsula.