Bioclaustrations in Upper Ordovician bryozoans from northern Estonia

Two new bioclaustrations, Anoigmaichnus zapalskii sp. nov. and A. bretti sp. nov., are described in Mesotrypa expressa colonies of Katian age. Both species were borings in the initial phase of growth and bioclaustrations in the later phase of growth. In addition, a new semiconical bioclaustration is described from Mesotrypa expressa. New data from the Late Ordovician of Baltica indicate that there may have been a peak in the diversity of bryozoan symbionts in the Ordovician. Late Ordovician trepostomes were especially prone for forming bioclaustrations around their soft bodied symbionts. There is great increase in diversity of bioclaustrations in the Late Ordovician which coincides with peaks in diversity of many Ordovician invertebrates. Diversity of bioclaustration ichnospecies is highest in colonial animals and bryozoans were the favoured hosts in the Ordovician.