Cemeteries, cess pits, sewers and shipwrecks

At the Paignton Heritage Society this was the title  of the talk by Peter Marsden, an archaeologist, who spent most of his career at the Museum of London, and later on the Mary Rose project. He talked wistfully, and with great interest about the days spent exploring ancient cess pits, and burial grounds. Archaeology involved making sense of a previous generations rubbish. In an operation to save a shipwreck at Blackfriars, Spike Milligan and staff and actors from the Mermaid Theatre helped with the digging. He had illustrations of fabulous finds among the rubbish, and a few arifacts, including the above lump in the photograph. Whatever is it?  This would defeat anyone if it was in a mystery object quiz.
The answer is it is fossilised poo, from an Ichthyosaur, a large extinct marine reptile. It was found in a London cess pit. Amazing how they know these things.

https://en.m.wikipedia.org/wiki/Ichthyosaur