The earliest Petroxestes borings were excavated in large trepostome bryozoans in the Sandbian (earliest Late Ordovician) of Estonia. The Estonian specimens are morphologically similar to the type material from the later Katian of North America. Petroxestes pera is rare in the Sandbian of Estonia and occurs only in biogenic hard substrates. Petroxestes borings occur in muddy environments that were preferred by macroborers in the Hirnantian and early Silurian of North America. It is possible that muddy environments supported higher bioerosion intensities and higher diversity of bioerosional traces in the shallow epicontinental seas of the Late Ordovician. The discovery of Petroxestes in the Sandbian of Estonia supports the idea that there was an earliest Late Ordovician peak in the diversification of borings in Baltica. It is possible that there was a migration of bioerosional trace makers from Baltica to Laurentia in the Late Ordovician.
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Vol. 34 • No. 10