Tasselia ordamensis: A biogenic structure of probable deposit-feeding and gardening maldanid polychaetes

The internal structure and growth direction of well-preserved Tasselia ordamensis specimens from the Upper Cretaceous–Cenozoic of Antarctica and Tierra del Fuego are not compatible with its current interpretation as a domichnion/equilibrichnium trace fossil. Accordingly, its functional morphology and the likely ethology of the producer are revised. Tasselia is a protrusive structure composed of a lined inner tube surrounded by vertically stacked sediment disks and ending in a basal chamber. Each disk has two domains of active burrow fills. The micritic, outer burrow fill domain consists of radial petaloid elements containing abundant radiolaria, calcispherules, diatoms and plant debris. This domain was formed by excavation due to deposit feeding; thereby non-ingested particles remained in the chamber and ingested particles passed through the gut to the sediment surface. Microfossil-rich material was collected at the sediment surface, placed in the feeding chamber, and rearranged within the petaloid elements probably after pulsed delivery of organic matter. The wineglass-shaped inner burrow fill domain is enriched in coarser particles and its petaloid elements crosscut the outer burrow fill domain. This domain probably functioned as “culturing compartment” for bacteria and its secondary reworking implies some form of ‘gardening’ and feeding on these bacterial populations at depth. The concentric striae on the organic-lined inner tube and the vertically stacked series of sediment-disks suggest maldanid polychaetes as the producers of T. ordamensis.