Tiny inhabitants of a large Middle Devonian reef of northern Gondwana: Sclerobionts of the coral-stromatoporoid Aferdou el Mrakib buildup, eastern Anti-Atlas, Morocco

Abstract

For the first time, a large number of macrofossils (rugose and tabulate corals, chaetetid sponges and brachiopods) collected from the deposits surrounding a large Middle Devonian (Givetian) reef at Aferdou el Mrakib (Mader Basin, eastern Anti-Atlas, Morocco) were inspected with respect to their sclerobiont assemblages. Although the preservation state of the hosts did not enable us to retrieve precise information on the abundance of the sclerobionts, their general diversity was deciphered and compared to a non-reefal assemblage from the Mader Basin and other reefal assemblages from elsewhere. The sclerobiont inhabitants of the Aferdou el Mrakib reef are diverse, represented by foraminifers, sponges (including stromatoporoids), microconchids, cornulitids, anticalyptraeids, crinoids, brachiopods, rugose and tabulate corals, bryozoans, hederelloids and enigmatic ascodictyids, as well as bioerosion traces after sponges (Clionolithes) and worm organisms (Trypanites borings). Evidently, the diversity of the epibionts is biased toward the robust skeletons, and delicate taxa, such as e.g., ascodictyids and foraminifers, were in most cases taphonomically erased. Additionally, the presence of malformations and bioclaustrations in some of the hosts point to their syn-vivo colonization by now unpreserved mineralized and non-mineralized sclerobionts. Although the sclerobiont assemblages are quite similar on the off-reef rugose corals and reef-front brachiopods, and less diverse on tabulates and chaetetids from the reef-core, when combined together, the whole assemblage shares some similarities with a coeval, non-reef community from the nearby locality of Madène el Mrakib. This pattern may indicate that the sclerobiont species pool was similar to some extent across the Givetian Mader Basin. Circular Oichnus borings, present on some of the brachiopod shells, additionally point to drilling predation and/or parasitism activities in the Aferdou el Mrakib reef habitats.