UC San Diego

Holasteroida represents one of the most morphologically distinctive and ecologically specialized deep-sea clades of echinoids, yet their evolutionary relationships have remained poorly resolved due to limited taxon sampling, morphological convergence, and the scarcity of molecular data. Previous studies relied heavily on morphology and fossil traits, which often yield incongruent results and are easily affected by homoplasy. Here, I present the first complete mitogenome-based phylogenetic reconstruction of Holasteroida, including seven newly assembled mitogenomes representing five families: Corystusidae, Urechinidae, Plexechinidae, Pourtalesiidae, and Ceratophysidae. Maximum-likelihood analyses of both nucleotide and amino acid datasets strongly support the monophyly of all sampled families but show inconsistency and uncertainty in the placement of Plexechinidae. Inter-family relationships are more stable and congruent with published morphological hypotheses in amino acid-based trees, which are less affected by DNA noise. Corystusidae consistently emerges as the most distantly related lineage, and the recovered sister-group relationship between Plexechinidae and Pourtalesiidae matches predictions from morphological studies, particularly the apical-system reductions and plate rearrangements identified by (Mooi and David 1996) as reliable synapomorphies. Ceratophysidae, a newly established family by (Minin et al. 2024), forms a strong monophyly with Pourtalesiidae. In contrast, characters such as fascioles, historically used to define broader clades like the former “Urechinina,” are likely homoplastic and may explain discrepancies between morphology-based and nucleotide-based topologies. Overall, this study establishes the first mitogenomic framework for Holasteroida and highlights the importance of integrating molecular and morphological evidence to reconstruct their evolutionary history.