Prehistoric mud-mound. Illustration of the marine invertebrates found on a prehistoric structure known as a mud-mound. This is a non-skeletal structure, a form of carbonate platform, similar to today's coral reefs but formed by different processes and in different conditions. The fossil records indicates that mud-mounds were more common in prehistoric times. The number labels indicate: 1. mud-mound (external and cross-section); 2. sea fan; 3. vase-shaped bryozoan; 4. crinoids; 5. hollow cavities with bryozoan roofs; 6. cemented bryozoan; 7. sediment; 8. debris sediment infill; 9, graduated sediments; 10. sponge. A scale bar (0.5 metres) is at lower right.