Coralliths of tabulate corals from the Devonian of the Holy Cross Mountains (Poland)

Tabulates, an extinct Palaeozoic group of corals developed diverse colony morphologies during the Silurian to Devonian peak of reef development. Coralliths, or circumrotatory colonies, are passively motile coral colonies constantly overturned by wave action or currents. Such overturning allows tissue growth on all sides of the colony. They are among the most rarely reported growth forms. Recent corallith-forming scleractinian corals mostly inhabit the shallowest reef environments, but coralliths can also develop at greater depths in areas of low topographic relief, unconsolidated substratum, low coral cover and high water movement. Here, we report on Devonian (Givetian Favosites goldfussi and Frasnian Alveolites? tenuissimus) coralla from the Holy Cross Mountains, Poland. Our analysis suggests these colonies are coralliths, although less mature than usually reported. These corals, unlike previously described growth forms of this kind, lived in relatively deep environments: probably the upper mesophotic (Givetian, Miłoszów), or deep reef fore-slope (Frasnian, Jaźwica and Kowala). Microfacies analysis of the Jaźwica site suggests unconsolidated substratum and high hydrodynamic energy. We conclude that these corals lived in deeper environments where bottom currents caused their autorotation. A good modern analogue for such a corallith-forming environment is the Wistari Channel (Southern Great Barrier Reef), where bottom tidal currents at nearly 30 m of depth are strong enough to overturn colonies of Stylocoeniella cf. guentheri reaching 15 cm in diameter. Our discovery shows that the spectrum of coral growth forms during the Devonian peak of reef development was broader than previously assumed, and that tabulate corals displayed numerous adaptive strategies to various environments.