Skip to main content
Dryad

Are interphylum spiralian relationships resolvable?

Serra Silva, Ana1; Telford, Maximilian1

Published Apr 16, 2026 on Dryad. https://doi.org/10.5061/dryad.280gb5n3j

Data files

Apr 16, 2026 version files 5.41 GB

Click names to download individual files

Abstract

While the membership of the major animal clade of Spiralia has been relatively stable since the introduction of molecular sequence data, the relationships between the constituent phyla are less clear. Focusing on the five largest phyla (Annelida, Brachiopoda, Mollusca, Nemertea, and Platyhelminthes), we find previous analyses have supported all 15 possible unrooted trees that could relate them, suggesting a hard-to-resolve node. We have used two recent phylogenomic data sets to explore this remarkable example of taxonomic instability. Using a combination of taxon-jackknifing and bootstrapping on empirical and simulated data, we explored the support for all 105 rooted and 15 unrooted 5-phylum topologies under site-homogeneous and site-heterogeneous substitution models. Rooted analyses suggested that the preference for rooting Spiralia on Platyhelminthes is due to artefactual long-branch attraction between this clade and Gnathifera. Most unrooted analyses showed a marginal and non-significant preference for the same 5-phylum topology. In all unrooted trees, the branches relating to spiralian phyla were shorter, on average, than the short (and contested) Deuterostome branch. Our results suggest that spiralian phyla likely emerged in rapid succession, in a difficult to resolve adaptive radiation. Resolving these very short branches will require large data sets to overcome stochastic errors, as well as efforts to address systematic errors arising from both branch-length and site-compositional heterogeneity. The lack of clarity has implications for our understanding of the clade’s history, the interpretation of Cambrian fossils, and for clarifying the evolutionary history of traits, including biomineralization, segmentation, and larvae.