Do you have any other examples of slate with fossils ? I
Slate fossils are pretty rare it seems after your question got me trawling across the net. Large ones are even rarer... They do exist however: http://www.fossilmuseum.net/Fossil_Sites/Bundenbach/Bundenbach.htm
I wonder whether those aren't fossils but rather inclusions or xenoliths in the slate (i.e. another peice of rock trapped in the strata as the slate was formed)?