Are interphylum spiralian relationships resolvable?

Area of Science:

• * Zoology
• * Evolutionary Biology
• * Phylogenetics

Background:

• * The major animal clade Spiralia includes diverse phyla, but their interrelationships lack consensus.
• * Molecular phylogenetics has clarified spiralian phyla but not their evolutionary tree.
• * Taxonomic instability suggests rapid evolutionary radiation.

Purpose of the Study:

• * To investigate the evolutionary relationships among the five largest spiralian clades.
• * To explore the hypothesis of rapid evolutionary radiation in Spiralia.
• * To understand the causes of taxonomic instability in spiralian phylogeny.

Main Methods:

• * Utilized two large phylogenomic datasets.
• * Applied site-bootstrapping and taxon-jackknifing analyses.
• * Analyzed 105 possible rooted trees and 15 unrooted trees.

Main Results:

• * Interphylum branches within Spiralia were found to be very short.
• * A long-branch artefact was identified, potentially favoring rooting on Platyhelminthes.
• * A preferred tree topology was observed in most analyses, but statistical support was not significant.

Conclusions:

• * Spiralian phyla likely emerged in rapid succession, indicating a difficult-to-resolve radiation.
• * The inferred deep history of Spiralia has significant implications for interpreting Cambrian fossils.
• * This rapid radiation impacts our understanding of the evolution of key traits like biomineralisation, segmentation, and larvae.