Identification and current palaeobiological understanding of "Keratosa"-type non-spicular demosponge fossils in carbonates: with a new example from the lowermost Triassic, Armenia

Fossilised fibrous skeletons of non-spicular demosponges have been reported in carbonates throughout the Phanerozoic and recently from rocks dated back to 890 Ma ago. These records expanded our understanding of metazoans in multiple aspects, including their early evolution, the cryptic ecology in fossil reefs, and recovery after mass extinction events. However, debates and confusion persist on the differentiation of these fossils from other biogenic or abiotic structures. Here we propose six identification criteria based on established taphonomical models and biological characteristics of these sponges ("Keratosa" sensu Minchin, 1900). Besides, sponge fossils of this kind from the lowermost Triassic of Chanakhchi (Armenia) are described with a 3-D reconstruction to exemplify the application of these criteria in the recognition of such organisms. Subsequently, the state-of-the-art understanding of the taxonomy and evolution of these fossil sponges, a previously poorly addressed topic, is summarized. The morphology of the Triassic Chanakhchi fossils indicates an affinity with Verongimorphs, a group that may have evolved by the Cambrian Age 3. Other than that, more efforts are still required to explore the taxonomic diversity and evolutionary details in the accumulating data of these fossil non-spicular demosponges.