Submitted to Palaios

Trace fossils assignable to the ichnogenus Imbrichnus Hallam 1970 and associated with freshwater unionacean bivalve moulds ("Hyridella" sp.) are described from Miocene Gore Lignite Measures mudstone exposed in the banks of Hedgehope Stream at Glencoe, southern South Island, New Zealand. Imbrichnus is a sediment-filled trail, parallel or sub-parallel to bedding, that has imbricate structure. At Glencoe, irregularly meandering and commonly cross-cutting ribbon-like trace fossils named Imbrichnus glencoea (new ichnospecies) are 1. 5-2 mm wide and commonly 10-12 mm deep. Found in mottled silty mudstone, the trails are filled with 0.3-1.5 mm thick imbricated and arched laminae of quartzose silt and mud inclined at 40-50deg. The uniform sides and irregular bases of the trails are lined with a thin layer of dark grey mud. Hyridella moulds associated with Imbrichnus glencoea are 18-24 mm in length, approximately one-third the size of living New Zealand forms. Modern New Zealand hyridellids make 20-30 mm wide furrows with a narrow medial groove. The partly sediment-filled grooves represent possible analogues of Imbrichnus glencoea. Imbrichnus glencoea is thought to represent a sediment backfill of the rhythmically pulsating Hyridella foot, produced as the animal advanced across the upper few centimetres of sediment. (auth)

IGNS pre-publication Jul 1993

GN

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