Monoplacophoran Igorella-type pore-channel structures from the Lower Cambrian in China

Monoplacophoran Igorella-type pore-channel structures from the Lower Cambrian Meishucunian Stage in China have been examined. A well-preserved shell shows that the pores are round or elliptical in transverse profile, elevated on the shell surface, and almost equally spaced and arranged along growth lines on the whole dorsal and lateral sides of the shell while the ventral side is smooth. The distribution of these Igorella-type pore-channel structures is different not only from the Maikhanellaʹs concentric distribution but also from the Auriculaspiraʹs spiral distribution. The pore is 10–20 μm in diameter, generally 15 μm, and the elevated shell wall is 0.1 μm thick. The pore peristome is slightly contracted and covered by a ring of granular crystals. These pores are apparently different in shape from the mastoids of Maikhanella and Auriculaspira. The pore channel arrangement is similar to that of recent molluskan bivalved shells, but the pore channels of the monoplacophoran Igorella-type shells are obviously bigger than those of the latter. It is speculated that the porosity of the shell may be useful for exchanging between the soft body and the surrounding water and relieving pressure on the thin shell from water. This study on pore-channel structures provides new evidence for understanding the skeletonization and ecological functions of early mollusks.