The Silurian mesophotic coral ecosystems: 430 million years of photosymbiosis

Abstract

The Silurian Period witnessed one of the most profound intervals of reef development in the history of the Earth, formed in large part by tabulate and rugose corals and stromatoporoid assemblages. One of the best-known examples of Silurian reefs (bioherms) is those exposed on the Baltic island of Gotland (Sweden). The stratigraphic sequence below these bioherms is represented by the Visby Beds (Lower Wenlock, ca. 430 Ma) that record in their lower sections (Lower Visby Beds) environments near the lower limit of the euphotic zone. Here, we describe platy tabulate coral assemblages from the uppermost Lower Visby Beds that represent a mesophotic coral ecosystem (MCE) potentially spread over 40 km. This MCE is dominated by platy tabulate corals, with accessory branching tabulates and solitary and phaceloid rugose corals. Algae are absent, likely as a result of taphonomic bias. It is possible that other MCEs of similar age are recorded on other Silurian tropical shelves: Anticosti Island (Laurentia, present Arctic Canada) and Hiiumaa (Baltica, present Estonia), in particular, may possess candidate early Silurian MCE assemblages. At a minimum, however, tabulate corals acquired photosymbionts during the early Silurian (Wenlock), which resulted in the development of both MCEs and shallow-water reef systems and contributed to the great expansion of the mid-Palaeozoic reef complexes.

Unusual shallow water Devonian coral community from Queensland and its recent analogues from the inshore Great Barrier Reef

“…In contrast, previously reported middle Palaeozoic coral communities with a large contribution from foliaceous tabulates likely represent mesophotic settings, deeper than the Fanning River community (Zapalski et al 2017b;Zapalski and Berkowski 2019). These mesophotic communities differ from the Fanning River community by the near absence of massive colonies, higher diversity of co-occurring fauna (abundant brachiopods and crinoids), and presence of chaetetid sponges.…”

“…Such morpho-functional convergences between Palaeozoic rugose and tabulate and Recent scleractinian corals have enabled identification of mesophotic coral ecosystems (MCEs) from the Silurian (ca. 430 Ma) of Sweden (Zapalski and Berkowski 2019) and the Devonian (ca. 390 Ma) of Poland (Zapalski et al 2017b).…”

Patterns of sclerobiont colonization on the rugose coral Schlotheimophyllum patellatum (Schlotheim, 1820) from the Silurian of Gotland, Sweden

“…At the base of the Upper Visby Beds, a distinct positive δ 13 C excursion was also noted by Munnecke et al (2003). Like the Lower Visby Beds, the Upper Visby Beds consist of limestone‐marl alternations, but the bedding is less regular than in the Lower Visby Beds (Calner et al 2004; Adomat et al 2016; Zapalski & Berkowski 2019). The deposits also contain abundant fossils (rugose and tabulate corals, stromatoporoids, brachiopods, trilobites, bryozoans) and numerous reef mounds formed by tabulate corals and stromatoporoids (Adomat et al 2016; Berkowski & Zapalski 2018; Zapalski & Berkowski 2019).…”

“…Like the Lower Visby Beds, the Upper Visby Beds consist of limestone‐marl alternations, but the bedding is less regular than in the Lower Visby Beds (Calner et al 2004; Adomat et al 2016; Zapalski & Berkowski 2019). The deposits also contain abundant fossils (rugose and tabulate corals, stromatoporoids, brachiopods, trilobites, bryozoans) and numerous reef mounds formed by tabulate corals and stromatoporoids (Adomat et al 2016; Berkowski & Zapalski 2018; Zapalski & Berkowski 2019). Unlike the Lower Visby Beds, which are considered to have been deposited in a relatively deep, distal shelf environment, below storm wave base and most likely in the aphotic zone (Samtleben et al 1996; Adomat et al 2016; Zapalski & Berkowski 2019), the deposits of the Upper Visby Beds are thought to represent shallower settings, probably within the photic zone, between storm wave base and fair weather wave base in a proximal shelf environment as evidenced by the presence of erosional surfaces, ripple marks and algae (Samtleben et al 1996; Adomat et al 2016).…”

Mapping Ecological Units in Mesophotic Coral Ecosystems of San Andrés Island (Southwestern Caribbean)

“…The Mesophotic Coral Ecosystems (MCEs) are those usually found from 30 to over 150 m depth in tropical and subtropical waters, where light-dependent corals and associated communities are thriving with significant reduction in light penetration [1,2]. This kind of marine habitat has been around since the Silurian period, as suggested by recent studies on fossil reefs [3]. MCEs have received increased attention in the last decades due to the introduction of technical diving in scientific research allowing safe diving over 40 m deep, and the increasing availability of remotely operated vehicles-ROVs [4].…”