Millerocaulis richmondii sp. nov., an osmundaceous fern from Mesozoic strata near Little Swanport, Tasmania, Australia

Abstract

A permineralised rhizome from Mesozoic strata south of Little Swanport, Tasmania, represents a new species of Millerocaulis (M. richmondii).Its stem is 6 x 7 mm across and is surrounded by adhering leaf bases with each having a stipular expansion typical of the Osmundaceae. The xylem of the ectophloic siphonostele comprising this stem is dissected by leaf gaps and consists of 14 xylem strands in its cylinder.Twenty leaf traces occur in a transverse section of its cortices. The xylem of its leaf traces and petiolar vascular strands is generally curved adaxially with a single protoxylem cluster being median on the trace. This proroxylem cluster bifurcates into two protoxylem groups immediately after leaving the stem and upon entering the petiole. The sclerotic ring of the petiole base is uniform in width and cell-wall thickness. A mass of sclerenchyma present in the adaxial concavity of the petiolar vascular strand expands at higher levels of the petiole until it fills the concavity of the strand and becomes immediately adjacent to the sclerotic ring. A large, round cellular mass of sclerenchyma occurs in the stipular expansions midway between their sclerotic rings and their tips. Millerocaulis richmondii is an additional species in the family Osmundaceae which was very abundant in Tasmania during mid-Mesozoic time.