Some observations on burrows of thalassinidean Crustacea in chalk hardgrounds

Upper Cretaceous Chalk hardgrounds (horizons of early-diagenetic cementation beneath intraformational erosion surfaces) characteristically contain extensive, branching systems of burrows. The morphology of these burrows is described and compared with that of the ichnogenus Thalassinoides Ehrenberg. They were constructed before the sea-floor was cemented, and are attributed to sediment-feeding crustaceans similar to the living Callianassa. The burrows remained occupied after the sea-floor sediment was cemented. This may be explained either by a change in feeding-habits of the burrower, or by the occupation of the burrow by another crustacean which fed by plankton-straining or predation. The existence of burrows comparable to those in hard-grounds, throughout much of the White Chalk, is evinced by (i) the appearance of burrows at sedimentary junctions where a change of colour takes place; (2) the common occurrence of burrow-shaped flint nodules; and (3) the development of ‘nodular chalk’.