The mobile domicile boring Trypanites mobilis revisited – new observations and implications for ecosystem recovery following the Cretaceous – Palaeogene mass extinction

Cylindrical borings assigned to ichnospecies Trypanites mobilis Neumann, Wisshak & Bromley, 2008, are common in bulbous echinoid spines (Tylocidaris baltica and Ty. vexilifera) and globular calcareous sponges (Porosphaera globularis) from the Maastrichtian chalk and Danian limestone of eastern Denmark. Trypanites mobilis was originally described as being restricted to these substrates but is here reported also from crinoid fragments and asteroid ossicles. A revised diagnosis of Tr. mobilis is proposed to accommodate the broader spectrum of host bioclasts and the fact that some bioclasts, notably Porosphaera, are fully penetrated. Trypanites mobilis is interpreted as a mobile shelter for sipunculan worms and the first appearance of this trace fossil around the Early/Late Cretaceous transition is inferred to reflect intensified predation pressure, and a general shortage of hard substrates on the muddy chalk sea floor. Trypanites mobilis is very common in the Maastrichtian of Denmark. It then nearly disappears in the lower Danian, re-occurs in the middle Danian, at first with smaller sizes and in low numbers, before becoming numerous and large-sized again around the middle/upper Danian transition. The near absence of Tr. mobilis in the lower Danian and its low abundance in the middle Danian is suggested to reflect that the ‘substrate makers’ were affected by ecological instability following the Cretaceous–Palaeogene mass extinction. Trypanites mobilis disappeared after the mid/late Danian transition, i.e. before extinction of its Danian substrate-maker Ty. vexilifera.