BORED BRYOZOANS FROM THE ORDOVICIAN OF ESTONIA: A BIOLOGICAL REINTERPRETATION OF THE ICHNOGENUS SANCTUM ERICKSON AND BOUCHARD, 2003

Trepostome bryozoans have often provided domiciles or refuges for a variety of organisms, and their borings have been variously attributed to Palaeosabella or Trypanites. In 2003 Erickson and Bouchard described the new ichnogenus and species Sanctum laurentiensis from the Ordovician of North America that they attributed to the activity of crustaceans. 150 hemispherical and ramose trepostome bryozoans collected from fifteen localities in the Middle to Upper Ordovician Darriwilian to Caradoc Stages of Estonia contain similar traces. Of the 68 ramose colonies 71% were bored with over two tubes per colony; in longitudinal section 10% of colony area was bored, with tubes 2.1 mm in diameter and on average 7.1 mm long. Hemispherical colonies were more susceptible (89%) to being bored than ramose forms; in cross-section each contains over 3 borings that mined 8% of the cross-sectional area, with tubes being the same diameter as in ramose colonies. Borings on surfaces of hemispherical colonies are more common on the upper surface (83% with 11 openings per bored colony) than on the flat base that usually sat on the sediment surface (74% with 6 openings per bored colony). In 18 colonies, borings contain clear infills, which we interpret as being ‘ghosts' of the original trace-producing organism. These had a circular cross-section 1.2 mm in diameter and were up to 3.9 mm in length. Its burrows are nearly 1 mm in diameter wider than its body, but were probably excavated by feathery lateral appendages now not preserved. Margins of burrows comprise shattered autozooecial chamber walls that suggest the organism produced borrows by mechanical destruction rather than by dissolution. Rare regrowth surfaces suggest the bryozoans and trace organism lived commensally. It is unlikely that the producer targeted the bryozoan colonies as a nutrient source as no grazing traces were found along surfaces of colonies where bryozoan zooids were concentrated. The producer of Sanctum laurentiensis was probably an annelid filter-feeder rather than a group of arthropods as originally postulated. The boring shapes originally described by Erickson and Bouchard are more a function of bryozoan colony internal morphology than the borer's morphology. The borer used the bryozoan colony as a domicile to elevate it above the sediment surface into clearer water.