A New Species of Millerocaulis (Osmundaceae) from the Middle Jurassic of China and Its Implication for Evolution of Osmunda

Abstract

A new species, Millerocaulis preosmunda, is established based on specimens collected from the Lanqi Formation (Middle Jurassic) near Changgao, Liaoning Province, China, and provides evidence for tracing the evolution of living Osmunda subg. Osmunda. The specimens represent two stems surrounded by a mantle of stipular petiole bases and adventitious roots. The stems, 10–13 mm in diameter, consist of siphonostele and a two‐layered cortex containing 32–35 leaf traces. When departing from the stele, the leaf traces are a flattened C shape, with an endarch protoxylem that bifurcates at the base of the petiole. Sclerenchyma rings are heterogeneous and change from an abaxial band at the base of stipular petiole bases to two lateral strands at the top. A heterogeneous sclerenchyma mass is present in each stipular wing at the midpoint of the petiole base. Millerocaulis preosmunda shows similarity in the arrangement of sclerenchyma of the petiole base to Osmunda subg. Osmunda but not to other Osmunda subg. Plenasium and Osmundastrum or to genera Todea and Leptopteris in extant Osmundaceae. The similar structures in the petiole base indicate that the new species could be a close relative or direct precursor of Osmunda subg. Osmunda.