The three-dimensionally preserved Haliestes dasos from the Silurian (Wenlock) Lagerstätte is the most complete fossil sea spider and the oldest unambiguous pycnogonid known from the fossil record. The discovery of two new specimens to add to the holotype reveals new features including proximal annulations of the appendages and segmentation of the trunk end, critical details for comparison with pycnogonids from the Devonian (Emsian) Hunsrück Slate and for the interpretation of the evolutionary significance of Palaeozoic genera. There is some evidence of sexual dimorphism. H. dasos was nektobenthic and its morphology indicates an unusual mode of feeding compared to living pycnogonids. The new morphological features of H. dasos are closely similar to those in Palaeoisopus problematicus from the Hunsrück Slate and it clearly belongs, together with that species, in stem Pycnogonida and not the crown group.

Specimens of Haliestes dasos were serially-ground and photographed at 20 μm intervals using a digital micropscope camera. The images were aligned, edited, and used to generate 3D virtual fossils with SPIERS software-suite (http://spiers-software.org). Data provided are (a) registered image-sets from serial-grinding,  one set per 'part', and (b) VAXML/STL format models of specimens generated using SPIERS and manual editing (segmentation) and mark-up of the data into separate anatomical elements.

Models are in VAXML/STL format. These are ideally viewed with SPIERSview (open source and available at http://spiers-software.org), but the mesh-models comprise a series of STL format files which can be combined and viewed in any mesh-viewing software. Note that meshes are correctly positioned within a single coordinate-space to facilitate the latter mode.