We found a smaller Flat-spined Brittle Star (Ophiopteris papillosa) last year, but the size of this individual was notable. The central disc alone was ~3 cm (~1 inch) across!
Here's a close-up of the disc:
And a view of the flattened spines at the tip of one arm:
Amazingly, the largest specimens of this species have central discs up to 4.5 cm across!
Many thanks to Joe and Sam for sharing this impressive brittle star.
For a look at the smaller individual we photographed last year, along with a short video clip, see the post from 1 March 2017.