Two species of Allonema: Problematic sclerobionts that survived the end-Ordovician extinction in Baltica

Two species of the encrusting calcitic sclerobiont Allonema have been identified for the first time in the Ordovician. Allonema moniliforme and A. botellus were originally known only from the Silurian but are here recorded from the Katian (Upper Ordovician) of Estonia. A. moniliforme reappears in the Rhuddanian–Aeronian of Estonia, and A. botellus emerges again in the Sheinwoodian of Gotland. Both species are unknown in the Hirnantian (latest Ordovician), and both survived the end-Ordovician extinction. Both species lived in similar, calm, muddy environments during their Ordovician and Silurian appearances. During the Silurian, Allonema was a much more common and widespread encruster than in the Ordovician, occurring on a broad range of biogenic hard substrates. This difference may reflect ecological and evolutionary shifts following the Late Ordovician mass extinction, which reorganized marine ecosystems and opened new niches. A. botellus maintained a stable morphology across the Ordovician–Silurian boundary, A. moniliforme underwent a narrowing of its morphological variability, possibly reflecting evolutionary canalization or shifts in ecological pressures over time. © 2026 Nanjing Institute of Geology and Palaeontology. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.