This is a fantastic Jurassic aged horseshoe crab known as  Mesolimulus walchi. These species are very closely related to modern day horseshoe crabs as can be apparent from their visual appearance. Fossils of this specimen are rare due to their thin shells and scarcity as a species. This particular fossil comes from the famous Solnhofen lithographic limestone Lagerstätte of Germany. They are known for finding world class fossils with excellent preservation and detail. This speciminen is no exception.
 
Fossil plate measures 12" wide by 7.5" tall with the horseshoe being roughly 5" long.