Improved taxonomic and gene sampling advance the knowledge of deep relationships within Macrodasyida (Gastrotricha)

Advances in Macrodasyida (Gastrotricha) phylogenetics, fuelled by new species discoveries and molecular data, are reshaping taxonomic classifications. Molecular analyses suggest polyphyly in Cephalodasyidae and Macrodasyidae, yet insufficient sampling continues to obscure precise relationships. Our study seeks to enhance the resolution of Macrodasyida's internal phylogeny through expanded taxonomic and molecular sampling. We obtained 63 new sequences from 21 Macrodasyidan species, integrating them with published data. Our dataset includes representatives from nine Macrodasyida families and 21 genera, alongside two chaetonotidans. We analysed the concatenated sequences of three genes (18S, 28S rRNA, COI mtDNA) from 51 terminals using Maximum Parsimony, Maximum Likelihood and Bayesian Inference. Our findings confirm the polyphyly of Cephalodasyidae. Dolichodasys and Paradasys cluster with Redudasyidae, while Cephalodasys and Mesodasys form unrelated lineages. Cephalodasys mahoae is nested within Paradasys rather than Cephalodasys, suggesting an original misidentification. The phylogenetic placement of Pleurodasys remains uncertain. Macrodasyidae is non-monophyletic, with Urodasys forming an independent lineage. The first molecular data ever obtained for Dendrodasys hint that the family Dactylopodolidae is likely polyphyletic as well. We propose an updated classification of Macrodasyida, introducing Mesodasyidae fam. nov., Urodasyidae fam. nov. and Paraurodasys gen. nov. Furthermore, we reassign Dolichodasys and Paradasys to Redudasyidae and Cephalodasys mahoe to Paradasys.