Reducing taxonomic noise in problematic fossils: revision of the incertae sedis genus Allonema based on shape analysis

Organisms of unknown biological affinity are widespread in Paleozoic ecosystems and play key ecological roles. Recognition of these roles relies on consistent identification and sound taxonomy, yet many microproblematica lack complex morphological features that could be the basis for qualitative taxonomic diagnoses. Allonema is an incertae sedis calcareous sclerobiont widespread in Paleozoic carbonates. At least seven species of Allonema have been proposed, but their diagnoses based on size ranges overlap. In this work, dimensions of 590 individuals from the Silurian of Estonia and Sweden have been measured. Regression analysis of the dimensions revealed two groups, overlapping in size ranges, but differing significantly in proportions. These groups could be matched with previously described A. botellus and A. moniliforme, but both showed a wider range of sizes than previously reported. Another previously described species, A. minimum, fell within the range of dimensions of A. moniliforme and is proposed to be its junior synonym. Differences in proportions revealed in the study do not seem to be dependent on sedimentary environment or local growth conditions such as competition for space, and thus may represent consistent interspecific variability. Although the affinity of Allonema remains unknown, this quantitative approach to its taxonomy reduces unnecessary species in the genus. The success of this approach may encourage its application to other microproblematica populating the fossil record.