Endobiotic Rugosan Symbionts in Stromatoporoids from the Sheinwoodian (Silurian) of Baltica

Abstract

A paleoecological study of stromatoporoid endobionts was carried out to discern the relationships between symbiotic rugosans and their stromatoporoid hosts. The earliest endobiotic rugosan symbiont Palaeophyllum sp. in Baltica has only been found in the stromatoporoid Ecclimadictyon astrolaxum from Saaremaa, Estonia. The rugosans are vertically oriented inside the stromatoporoid skeleton. Numerous rugosans have their corallites open at the upper, external surface of stromatoporoids, but many are completely embedded within the stromatoporoids. Stromatoporoid hosts were presumably beneficial for rugosans as elevated substrates on a sea floor that offered a higher tier for feeding. Relative substrate stability in the hydrodynamically active shallow waters may have also been beneficial for the rugosans.

Earliest known rugosan-stromatoporoid symbiosis from the Llandovery of Estonia (Baltica)

“…A symbiotic rugosan has been recently described from a stromatoporoid of Pridoli of Saaremaa, Estonia (Vinn and Wilson, 2012), but in this association only a couple of rugosans occur in a relatively large stromatoporoid. In contrast, the late Sheinwoodian stromatoporoidrugosan association from Saaremaa (Vinn and Mõtus, 2014a) is generally similar to the association described here. The rugosans in this described association have a larger size than their Sheinwoodian equivalents, but the occupied area of the feeding surface of the host is similar.…”

“…Similar rugosan-stromatoporoid associations have been described from the late Sheinwoodian of Estonia (Vinn and Mõtus, 2014a). A symbiotic rugosan has been recently described from a stromatoporoid of Pridoli of Saaremaa, Estonia (Vinn and Wilson, 2012), but in this association only a couple of rugosans occur in a relatively large stromatoporoid.…”

“…Endobiotic rugosans often had symbiotic relationships with stromatoporoids, and such interactions are especially common in the Silurian of Baltica (Kershaw, 1987;Vinn and Wilson, 2012;Vinn and Mõtus, 2014a). In addition to rugosans, the tabulate coral Syringopora is also a common endobiotic symbiont in Silurian stromatoporoids of Baltica (Kershaw, 1987;Vinn et al, 2014).…”

Differentiating Parasitism and Other Interactions in Fossilized Colonial Organisms

“…Solitary (Figure 2(b)) and occasionally ramose colonial rugose corals can also be found embedded within stromatoporoids in Silurian and Devonian deposits (Kershaw, 1987;Stel and Stoep, 1982;Soto and Méndez-Bedia, 1985;Nestor et al, 2010;Vinn and Mõtus, 2014a). Individual stromatoporoids sometimes host both rugose corals and caunopores (Figure 2(a)), but it is unclear whether the rugose coral symbionts were parasites of the host stromatoporoids.…”

“…Individual stromatoporoids sometimes host both rugose corals and caunopores (Figure 2(a)), but it is unclear whether the rugose coral symbionts were parasites of the host stromatoporoids. Vinn and Mõtus (2014a) postulated benefits to the corals in terms of being able to feed at a higher tier in the water column and having a more stable substrate than usual, while the stromatoporoid hosts may have incurred advantages in being protected against predation by the nematocysts of the corals. However, the latter would have been balanced by loss of some of their feeding surface area.…”