Lower Devonian crinoid pluricolumnals from Morocco are severely infested by sclerobionts: Evidence for a parasitic outbreak on the shelf of northern Gondwana

Since early Palaeozoic time, stalked crinoids provided a unique substrate for attachment of various epi- and endobiontic organisms (sclerobionts), which therefore had access to suspended food particles and did not compete with benthic filter-feeding organisms on the sea floor below them. However, many epibiontic colonizers were partly embedded by the crinoid stereom, so the syn vivo (during life of the host) biotic interactions may be uniquely preserved in the fossil record. Numerous crinoid-sclerobiont associations and their symbiotic relationships are reported in literature; however, the previous studies were mainly devoted to specific epibiontic colonizers. Here, we present a comprehensive study of the whole assemblage of colonizers on Lower Devonian (Pragian) crinoid stems from the famous locality at Hamar Laghdad, Morocco, providing a uniquely broad picture of crinoid-sclerobiont biocenosis and symbiosis. We show that more than half of the Schyschcatocrinus pluricolumnals were colonized syn vivo. The most abundant symbionts are: Tremichnus pits-producing epibionts (found in 43% of host crinoids), different bioclaustrations and stems possessing galls/swellings. In addition, some crinoids, tabulate and rugose corals are embedded by the hosts’ stereom. In comparison to crinoid-sclerobiont associations from other sites, those from Hamar Laghdad show the most severe, so far recorded, syn vivo infestation by parasitic Tremichnus-producing epibionts. The picture emerging from our data and previous observations from another site (Tala n’Taleb, south-western Morocco) shows that Early Devonian crinoid biocoenoses of the southern Rheic Ocean realm contained abundant ectoparasitic epibionts, which are interpreted to have easily infected the hosts forming such dense populations. Additionally, a single Ferestromatopora stromatoporoid found encrusting a pluricolumnal is the oldest record of this genus, also demonstrating a wider geographic range than previously known during the Early Devonian, at a time when stromatoporoids were globally impoverished.