I think what Lithoid-Man is saying is that the fossil pictured on the AiG site is of the genus Mesolimulus. I'm a complete know-nothing on this but if you put
Solnhofen and
horseshoe into Google you will find lots of references to
Mesolimulus fossils - specifically
Mesolimulus walchi.
There are a few (and it is very few) references to
Limulus walchi fossils from Solnhofen. It would take somebody who knows their fossils to say what the one pictured on AiG is. Actually given how few references to Limulus walchi there are I wonder if they are misidentifications.
Edit: I see from the phylogeny that Belfry posted that there is no such thing as
Limulus walchi so I guess they are indeed misidentifications or mistakes and should (I guess?) be
Mesolimulus walchi.
Edited by MangyTiger, : Add update about non-existence of Limulus walchi.
Oops! Wrong Planet