STEREOMIC MICROSTRUCTURES IN CLYPEASTEROIDA: A CASE STUDY OF PLEISTOCENE SAND DOLLARS FROM WESTERN TAIWAN

Stereomic microstructures known as stereom are the fundamental building blocks of echinoderm ossicles. Stereom consists of three-dimensional mesh of trabeculae filled with interconnecting pores. Trabeculae are composed of a high-magnesium calcite which behaves as a single crystal. At least nine types of stereom, including rectilinear, microperforate, laminar, galleried, fascicular, labyrinthic, retiform, perforate and imperforate stereom, have been reported from both living and fossil echinoids. Stereom studies are crucial to understand the phylogeny, growth rate and soft tissue type of Echinoidea.

The basic body plans of sea urchins or echinoids can be categorized into two groups. The regular urchins bear a nearly perfect pentaradial symmetry, and the irregular urchins have a bilateral symmetry. Among irregular urchins, sand dollars are members of Clypeasteroida that inhabits in the relatively shallow sandy nearshore areas. The goal of this study is to study the stereom preservation of a fossil clypeasteroid Scaphechinus mirabilis recovered from the Pleistocene strata in the 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 Miaoli Toukoshan Formation in Taiwan.

To understand better the stereom and ontogeny 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 studied also and 65 well-preserved individuals, ranging from 0.7 cm to 6 cm in size, were selected for making thin sections. Among the 73 thin sections, 55 sections were prepared and cut through the bilateral symmetry, 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 galleried and labyrinthic stereom. The average pore size is 14.6 micron and the average minimum trabeculae thickness is 12.2 micron for the exterior surface of a typical petaloid plate.