As part of the 'History of the World' project, BBC Leicester are looking for the remarkable stories behind objects.
John Foster from Leicester was literally stopped in his tracks when he happened across his special item.
The fossil was caught up in the blades of his mower as he worked on the grounds of Leicester General Hospital on 25 July 1994.
The piece of limestone rock contains fossil shells and an ammonite may date back to over 200 million years.
John was mowing an area near the main gates of the hospital when he hit something hard.
On getting out to check the mower blades, he discovered the unusual piece of rock nestled amongst the grass.
He pocked the item, and immediately after his shift made his way down to New Walk Museum in Leicester to consult the experts.
A sea-based predator
Ammonites became extinct, along with the dinosaurs, about 65 million years ago.
In these fossil ammonites you can clearly see the individual chambers
They belonged to a group of sea-living predators called cephalopods, with includes the living octopus, squid, cuttlefish and nautilus.
The ammonite's coiled shell was hollow and divided into chambers filled with gas which gave the creature buoyancy.
These chambers can be seen in John's fossil, as the shell has been broken open.
Fossil ammonites are common in the bedrock of much of eastern Leicestershire, formed from the mud that rested at the bottom of the sea which covered most of Britain 190-210 millions years ago.
However New Walk believed that John's specimen was not local rock, and may have been transported to the area with the southern movement of glaciers in the last Ice Age, around 10,000 years ago.
Although a remarkable object, John's fossil cannot be added to the 'History of the World' main site as it is not man-made.
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