Plant Traces Resembling Skolithos

Abstract

The ichnogenus Skolithos Haldeman 1840 is a simple tubular trace fossil that was initially described as a “fucoid” or seaweed impression, i.e., a fossilized marine plant. Today the tracemaker is commonly considered to have been a vermiform marine invertebrate, although this interpretation is not acceptable to all. It is the name giver to the archetypal, Seilacherian Skolithos ichnofacies, which is a widely acknowledged indicator of relatively high energy, shallow water, nearshore to marginal marine environments. Here we record and describe analogous cylindrical structures, unquestionably reflecting plant tracemakers. These have been recognized in Quaternary consolidated “coffee” sandrock and Recent coastal dune, marsh or swamp, and salt meadow settings from northern New Zealand and Sapelo Island, USA.

We conclude that Skolithos is not an unequivocal indicator of shallow marine settings. Our observations confirm the opinions of a number of previous workers who have expressed the need for caution when using this ichnotaxon as a shallow-water, higher energy palaeoenvironmental indicator, especially when supporting evidence is otherwise lacking. In particular we note that in high intertidal and non-aquatic settings, Skolithos may be a plant trace fossil rather than serve as evidence for invertebrate activities. At the present time there is pressing need for a thorough systematic revision of the Skolithos ichnotaxon.

Keywords:

• Skolithos
• Quaternary and Recent
• dune settings
• rushes
• Spartina
• limonitic preservation
• plant trace-makers
• rhizoliths

ACKNOWLEDGEMENTS

KAC and MRG had funding support through grants (9349/3602468 and /3421326) from The University of Auckland Research Committee. They are also indebted to Andrea Alfaro and Neville Hudson for field and logistics assistance in the far north and to Louise Cotterall for art work. Jon Garbisch, University of Georgia Marine Institute, provided AJM with lodgings on Sapelo and Ruth Schowalter assisted with field observations. Despite having made enquiries, it is unfortunate that we have had to cite Haldeman's original text without being able to sight it. We acknowledge Ichnos reviewers G. Retallack and B. White for their perceptive and demanding comments—when we started down the phytoturbation track we were somewhat ignorant of what lay underneath. The observations made and conclusions drawn are the authors’ alone.