Ancient depths: Unprecedented completeness of mesophotic coral-fish ecosystem from the Devonian of Eastern Gondwana

Abstract

Modern mesophotic coral ecosystems (MCEs) are built by zooxanthellate scleractinian corals anatomically adapted (platy growth) to light harvesting at greater depths. Ecosystems of this kind date back to the Silurian (∼430 Ma) and in the Palaeozoic were built by now-extinct tabulate corals. Palaeozoic MCEs were recognized for the first time quite recently, and are still poorly known – only from isolated occurrences on the tropical shelves of palaeocontinents of Laurussia (Silurian, Middle Devonian), with a single example from Western Gondwana (Morocco, Middle Devonian). This study reveals a Lower Devonian (Emsian, ∼395 Ma) coral biostrome at Wee Jasper (New South Wales, Australia; Eastern Gondwana). The area has long been noted for its diverse, well- preserved fossil fish fauna, and the investigated beds, besides corals, are associated with many fish remains, mainly armoured fishes – placoderms. With about 70 species identified so far (placoderms, acanthodians, osteichthyans) this is the most diverse Devonian fish assemblage known to date. We identified a platy tabulate coral (a growth form adapted to low-light conditions) assemblage. Numerous anatomical features of corals (e.g., growth forms, corallite sizes) and fish (e.g., teeth adaptations for herbivory or durophagy) are directly comparable with modern taxa. Along with sedimentological data, the entire assemblage can be interpreted as a MCE. The description of the Wee Jasper MCE highlights numerous functional similarities between Palaeozoic and Recent mesophotic ecosystems, despite major taxonomical differences. Moreover, it suggests a broad distribution of MCEs on tropical shelves of Gondwana as early as the Early Devonian, evidencing proliferation of photosymbiotic, habitat-forming tabulate corals during the reef acme in the middle Palaeozoic. We propose the Wee Jasper MCE as the most complete Palaeozoic coral-fish ecosystem known to date.