A meeting in the cave: Taphonomy and ecology of scutelluid trilobites in the Devonian Hamar Laghdad elevation, eastern Anti-Atlas, Morocco

Densely packed accumulations of scutelluid trilobite sclerites, found in subseafloor vent fissures and cavities within the Devonian Hamar Laghdad elevation (Anti-Atlas, Morocco), were investigated to understand their taphonomic, ecological, and behavioural significance. The assemblages are monospecific and composed of disarticulated exuviae, with a predominance of pygidia and no complete trilobite carapaces. Two Pragian-age lumachelles, deposited in subseafloor cavities within bioclastic packstones, are dominated by Platyscutellum massai , while five Emsian-age lumachelles occurring in fissures within the Kess-Kess mounds contain Cavetia furcifera . This study provides key insights into the moulting behaviour, life habits, and palaeoecological preferences of scutelluid trilobites during the Early Devonian. Morphological features indicates that both trilobite species were adapted to open marine environments, ruling out a permanent cave-dwelling lifestyle. The assemblages are interpreted as the result of mass moulting behaviour within subseafloor fissures and cavities. Size-frequency data suggest synchronised, size-segregated moulting events. Although a reproductive function for these aggregations remains speculative, parallels with modern arthropods suggest that mass moulting may have conferred both protective and reproductive advantages. These findings reinforce the interpretation that the mass gathering of trilobites, particularly during ecdysis, may have been an adaptive strategy in response to ecological pressures in Early Devonian marine environments. The preferred concave-up orientation of sclerites suggests passive gravitational settling within fissures and cavities, rather than transport by horizontal currents, while the breakage of smaller sclerites points to reworking by episodic hydrothermal fluid discharge. • Early Devonian trilobite accumulations of disarticulated exuviae discovered in vent fissures. • No evidence of adaptations for a permanently cave-dwelling lifestyle. • Size-frequency analyses revealed synchronised, size-segregated moulting behaviour. • Sclerite deposition influenced by gravity-driven settling and hydrothermal fluid discharge.