Relict from the Jurassic: New family of brittle-stars from a New Caledonian seamount

The deep-seafloor in the tropical Indo-Pacific harbours a rich and diverse benthic fauna with numerous palaeo-endemics. Here we describe a new species, genus and family of brittle-star (Ophiuroidea) from a single eight-armed specimen collected from a depth between 360 and 560 m on Banc Durand, a seamount east of New Caledonia. Leveraging a robust, fossil-calibrated (273 kbp DNA) phylogeny for the Ophiuroidea, we estimate the new lineage diverged from other ophiacanthid families in the Late Triassic or Jurassic (median=187-178 my, 95% CI = 215-143 my), a period of elevated diversification for this group. We further report very similar microfossil remains from Early Jurassic (180 my) sediments of Normandy, France. The discovery of a new ancient lineage in the relatively well-known Ophiuroidea indicates the importance of ongoing taxonomic research in the deep-sea, an environment increasingly threatened by human activities.
Methods
Micro-CT scanning. Performed at the Melbourne TrACEES (Trace Analysis for Chemical, Earth and Environmental Sciences) Platform by Dr. Jay Black (University of Melbourne) using a Phoenix Nanotom m operated using xs control and Phoenix datos|x acquisition software (Waygate Technologies). The specimen was mounted by wrapping in ethanol soak gauze, secured by bubble wrap, and placed within a plastic specimen jar. An x-ray energy of 50kV and 300 mA was used for two separate scans, one at a coarser voxel resolution of 25 micrometre to capture the full body of the specimen and a higher resolution scan at 10 micrometre focusing on a region of interest encompassing the main body. Scans were run for 10 minutes in a fast scan mode collecting 1200 projections through a 360 degree rotation of the specimen. Volume reconstruction of the micro-CT data was performed using Phoenix datos|x reconstruction software (Waygate Technologies) applying an inline median filter and ROI filter during reconstruction. The data was exported as 16-bit volume files for imaging and analysis in Avizo (Thermo Fisher Scientific).

Phylogenetic analyses. Bayesian dating estimates were obtained using BEAST2 v2.4.7 using  a codon position GTR+G+F sequence evolution model, with lognormal relaxed-clock model and Yule tree prior (both using 1/X parameterization). Fossil calibrations are outlined in electronic supplementary material Table S3. Maximum likelihood estimates were were generated in RAxML v8.1.20 using  a codon position GTR+G+F sequence evolution model by running 200 non-parametric bootstraps (the –f i command) that retain branch lengths per bootstrap. A species tree was estimated using ASTRAL-II, with local posterior support values [19], from 353 (the most data rich, electronic supplementary material, table S2) separate gene trees, drawn from the full 265kb 195 taxa dataset. We used genes (rather than exons) as independent loci [20]. Unrooted gene trees were generated by RAxML (-f d command) using a single partition GTR+G+F model (due to reduced information per gene as compared to the full concatenated data). These gene datasets comprised (median) 189 taxa and 513 sites, with each of the 195 taxa having (median) 343 gene trees (330 for specimen IE.2007.6821). The ASTRAL species tree figure is presented as rooted for ease of comparison.