Phylogenomic resolution of sea spider diversification through integration of multiple data classes

Despite significant advances in invertebrate phylogenomics over the past
decade, the higher-level phylogeny of Pycnogonida (sea spiders) remains
elusive. Due to the inaccessibility of some small-bodied lineages, few
phylogenetic studies have sampled all sea spider families. Previous
efforts based on a handful of genes have yielded unstable tree topologies.
Here, we inferred the relationships of 89 sea spider species using
targeted capture of the mitochondrial genome, 56 conserved exons, 101
ultraconserved elements, and three nuclear ribosomal genes. We inferred
molecular divergence times by integrating morphological data for fossil
species to calibrate 15 nodes in the arthropod tree of life. This
integration of data classes resolved the basal topology of sea spiders
with high support. The enigmatic family Austrodecidae was resolved as the
sister group to the remaining Pycnogonida and the small-bodied family
Rhynchothoracidae as the sister group of the robust-bodied family
Pycnogonidae. Molecular divergence time estimation recovered a basal
divergence of crown group sea spiders in the Ordovician. Comparison of
diversification dynamics with other marine invertebrate taxa that
originated in the Paleozoic suggests that sea spiders and some crustacean
groups exhibit resilience to mass extinction episodes, relative to mollusk
and echinoderm lineages.