Unravelling an enigma: a reappraisal of the constructional program of the ichnogenus Lapispira Lange

Abstract

The corkscrew-like ichnogenus Lapispira Lange is characterized as a double, concentric, helicoidal tubular burrow, with its coiling axis almost perpendicular to bedding, produced within a fine-grained unconsolidated sediment. Almost neglected since first described in 1932 from Hettangian beds in Germany, it received renewed attention in recent decades. Here, thepreviously available evidence is reassessed and supplemented with new data, from Lower Jurassic deposits of the Neuquén Basin in Argentina. As a result, a revised diagnosis is provided, a better understanding of its complex morphology and three-dimensional structure is revealed and two alternative fabricational models are contrasted. In Lange’s model a buildingmechanism similar to the one of Gyrolithes has been assumed, whereas in Seilacher’s model a process more closely related to the genesis of Rhizocorallium was envisaged. The finding of remarkably well-preserved materials from Pliensbachian outcrops in southern Mendoza province, that show a series of protrusive spreiten between the subparallel limbs of the external and internal coils, supports the second alternative. Consequently, the lack of spreiten in previously known cases, is attributed to an unfavourable nature of the unconsolidated sediment between both limbs when burrows were made, or to a subsequent obliteration of initial spreiten by taphonomic processes (such as activity by minute bioturbators and/or differential compaction associated with dewatering). Finally, after reviewing relevant functional hypotheses, the elaborate burrow pattern of Lapispira is interpreted as a multi-purpose biogenic structure, beneficial for deposit-feeding, temporary dwelling, deterring intruders, facilitating penetration with structural stability, and minimizing risks of crowding and interference with neighbouring organisms.