Stereomic microstructure of Clypeasteroida in thin section based on new material from Pleistocene strata in Taiwan

The goal is to document the stereom preservation of a fossil clypeasteroid Scaphechinus mirabilis recovered from the Pleistocene strata in western Taiwan. The adult size of S. mirabilis ranges from 4 to 7 cm. Although the living species can be found along the coast in Japan, Korea, and China, only fossil materials have been reported from the Toukoshan Formation in Miaoli, Taiwan. To understand better the fossil stereom preservation of S. mirabilis, a total of 971 specimens deposited at the Department of Geosciences, National Taiwan University (NTUG) were examined. An additional 572 specimens were also studied and 65 well-preserved fossil specimens and one modern sample were selected for making thin sections. Among the 86 thin sections, 68 sections were prepared and cut parallel to the bilateral symmetry plane of the echinoid, and the other 18 thin sections were cut through different regions of the test, including apical system, petaloid, and interambulacral areas. Types of stereom and the associated tissues can be determined by calculating the pore size and minimum thickness of trabeculae. Results show that plate boundaries, growth lines, and stereom are clear and identifiable under a polarized microscope. Dominant types of stereom include labyrinthic, rectilinear and galleried stereom.