Six vertebrae, which are all fused. Viewed laterally, the sacrum is slightly arched, with continuous neural blade formed by the neural spines. A platform projects from the bases of the neural spines and links with the diapophyses. There is a shallow sulcus along the ventral surface of the fused sacral vertebrae and the first two vertebrae have pleurocoels, set in deep excavations.

Little bit of history, as this entry’s a bit short. Ornithodesmus was originally described as a bird, was then re-described as a pterosaur, was then discovered to be a dinosaur, was classified as a troodontid (although the material it was compared to eventually turned out to be misidentified itself), then a dromaeosaur. Basically, no-one really knows what it is, although most palaeontologists agree it is a dromaeosaur. Even if it is, it is not the same as the large velociraptorine(s) also known from the Isle of Wight.

Ornithodesmus – Wikipedia, the free encyclopedia

NAISH, D., HUTT, S. and MARTILL, D. M. 2001. Saurichian (sic) dinosaurs 2: theropods. In MARTILL, D. M. and NAISH, D (eds). Dinosaurs of the Isle of Wight. The Palaeontological Association. Field Guide to Fossils 10, 242-309.

NAISH, D. 2011. Theropod dinosaurs. In Batten, D. J. (ed.) English Wealden Fossils. The Palaeontological Association (London), pp. 526-559.