A peculiar reworking of Ophiomorpha shafts in the Miocene Nangang Formation, Taiwan

Abstract

In the Miocene sandstones of the Nankang Formation in north-eastern Taiwan, different varieties of the trace fossil Ophiomorpha are abundant. In certain beds, a peculiar reworking of vertical Ophiomorpha shafts was observed. This reworking consists of an inner, lined tube positioned in the centre of the shaft. The shaft is lined by thin walls with small knobs and is distinctly different from the shafts of the Ophiomorpha nodosa mazes found in the same beds which have thick walls and large knobs. Because both outer and the inner tube walls were constructed by small sub-pellets, a diagenetic origin can be ruled out. The presence of sub-pellets further indicates that the inner tubes were also constructed by crustaceans and not commensal organisms such as worms or fish. The abundance of these vertical shafts suggests that they were constructed for a specific purpose, and the similarities in sub-pellets indicate that they likely were constructed by different generations of the same crustacean species. Because brooding chambers were not observed and are rare among extant marine crustaceans, we suggest that the vertical shafts were constructed to encourage juvenile shrimp to resettle in the parental burrows after they had completed their pelagic larval stages.