Last of the Famennian Phacopids: A New Omegops from Central Armenia and Its Biostratigraphic Significance

Trilobites were abundant and diverse marine arthropods throughout the Devonian, occupying habitats from shallow nearshore to deep marine environments. Their diversity, however, declined sharply during the Late Devonian due to environmental crises such as major sea-level fluctuations and widespread anoxia. The uppermost Famennian marks their reappearance after their near-complete disappearance at the end of the Frasnian. Although the biodiversity decline and subsequent rebound represent notable events, the Famennian trilobite record remains limited, particularly in shallow-water settings. In this context, the Upper Devonian successions of Armenia, deposited on the northern margin of Gondwana, are of particular interest, as they consist of fossil-rich shallow-water deposits․ Among trilobites, the latest Famennian phacopids are documented from the Armash locality (Ararat Province) and are represented by the genus Omegops, which includes a newly described species, Omegops armeniensis. This species represents the last known phacopids in the region prior to the Devonian–Carboniferous boundary. Omegops armeniensis is distinguished by six pairs of pygidial pleural ribs, distinctive lateral pits, and a higher number of lenses per eye file compared to closely related species. These features expand our understanding of morphological diversity within Omegops, including variation in postocular pads and pygidial structures, possibly reflecting ecological adaptations to shallow, nearshore environments. The species occurs in bioclastic limestones alongside abundant brachiopods and corals, consistent with a tropical–subtropical palaeolatitude. This study further highlights the biostratigraphic and paleobiogeographic significance of Omegops and provides new insights into the evolutionary patterns, ecological preferences, and extinction dynamics of Late Devonian phacopids.