I wonder if they were specimens that were ideally fossilized, but only in a portion of a tectonic plate that was eventually pushed below the mantle and liquefied into molten lava.
Exotic skeletons from hundreds of hyper localized species, all pristinely preserved in so much detail miraculously for millions of years due, only to eventually turn into very hot rock just before ever returning to the near surface for paleontology discovery.
Time on earth makes for very lossy data archive. Ohhh, the entropy!