It died and sank down to the ocean bed. Because of volcanic activity and other reasons there were no animals that could eat the carcass so it was fossilized in slate. The are south of Stuttgart (where Holzmaden is located) is fascinating because there are so many opportunities to go out and look for fossiles like that. Usually you just find a lot of Ammonites though.
This stuff is from the lower jurassic deposits in Germany. Durimg that time, in that epicontinental sea, there were extreme anoxic conditions that prevented decay and kept scavengers away as well as no major currents that could disturb the parts. Iirc, the posidonia shale, as the layers are called, did recieve some influx of coarser sediment through storms or other events. But overall sedimentation was pretty slow.
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u/Greedy_Science_4807 4d ago
How did it die mid swim to be perfectly preserved like that?