Boring a mobile domicile: an alternative to the conchicolous life habit

Conspicuous borings, belonging to the new ichnotaxon Trypanites mobilis isp. n., recorded in the bulbous spines of the echinoid Tylocidaris and in the spherical calcareous sponge Porosphaera globularis, are common in Late Cretaceous (Cenomanian) to Early Paleocene (Danian) strata of Central and NW Europe. It is suggested that the borings were produced post mortem by sipunculan worms to create a lightweight and mobile domicile, offering effective shelter and protection. This strategy, which is hitherto not known from any other animal group, represents an alternative to the conchicolous life habit. Both habits evolved almost simultaneously in sipunculans at the end of the Early Cretaceous, suggesting escalation as a response to increased predation. Moreover, it bears a number of advantages such as superior substrate availability, the avoidance of replacing the substrate during ontogeny, as well as comparatively lightweight domiciles. On the other hand, the high degree of dependence on only very few suitable specific host taxa is a major drawback of this strategy. It ultimately led to the disappearance of this mode of life in favour of the still widespread conchicolous habit at the end of the Danian when the hosts vanished together with the chalk-sea ecosystem.