Skates are egg-layers. A female encloses each embryo in a tough keratinous capsule with a horn at every corner, anchored to the seabed by fibres and sticky threads. Development is slow, commonly four to five months and longer in cold water, and the young emerge as miniature adults with no larval stage.
Empty capsules float free and strand along the tideline, which is why records collected by the public dominate what is known about their distribution. The Shark Trust's Great Eggcase Hunt has logged hundreds of thousands of British finds, and the commonest of them is not a skate at all but the small-spotted catshark. Among skate cases the thornback's leads: roughly the size of a palm, rectangular, with a fibrous coat and short horns.
The cases identify to species, which is what makes the strandings useful: a beach record indicates an egg-laying population offshore. What it cannot show is abundance, since drift, wind and coastline shape determine where cases pile up as much as ray numbers do.