First record of Arachnostega burrows in trilobite remains from the lower Silurian of Estonia

Arachnostega gastrochaenae Bertling forms straight to slightly curved burrow systems beneath trilobite cephalia, consistently located under the glabella. In Estonia, its occurrence in trilobites ranges from the Rhuddanian to the Sheinwoodian, with no records from the upper Silurian. The ichnotaxon is restricted to deeper carbonate platform and slope settings characterized by lowenergy, fine-grained carbonate-rich clay muds, likely deposited below the storm wave base within the lower photic zone. Arachnostega distribution in trilobite remains suggests a shift from shallower Ordovician to deeper Silurian environments in the Baltic Basin, possibly driven by increased substrate instability. Smaller Silurian trilobite remains provided less stable and less suitable microhabitats, promoting offshore migration of Arachnostega trace makers to more stable settings.