Rare Arachnostega traces in brachiopod and gastropod molds from the Silurian of Gotland (Sweden) and Saaremaa (Estonia): was tropical climate unfavorable for the trace makers?

rachnostega gastrochaenae burrows occur in internal molds of the brachiopod Estonirhynchia estonica in the Wenlock of Saaremaa, Estonia, and in gastropod steinkerns in the Wenlock of Gotland, Sweden. The trace-making worms either entered the shell through the slit between the closed brachiopod valves as juveniles, or they used the brachiopod foramen to enter the shell interior. The Arachnostega traces in closed brachiopod shells are hidden until shell fragments are removed, exposing the internal mold. Because they are hidden in complete, articulated shells, Arachnostega may be more common in the Silurian of Baltica than currently recognized, though markedly less common than in the Ordovician. The trace makers responsible for burrows in brachiopods and gastropods presumably persisted from the Ordovician to the Silurian. The rarity of Arachnostega burrows in the Silurian of Baltica as compared to that of early Late Ordovician supports the view that, at least during the early Paleozoic, Arachnostega trace makers preferred colder climates