Fossils believed to date back more than 300 million years have been destroyed by arson attacks at an historic steelworks site near Wrexham.
Hundreds of samples had been stored at the derelict Brymbo Steelworks site after being collected from Brymbo Fossil Forest.
Up 150 fossils are still in tact but the rest are badly damaged.
The forest pre-dates dinosaurs by 70 million years.
Colin Davies, from the Brymbo Heritage Group, which is looking to create a heritage and visitor centre at the site, said: "We would love to know what did happen and who is responsible for it.
"There were two metal containers that were set alight - there must have been accelerant used and resulted in a complete write off of the containers.
"One of the containers had literally hundreds of fossils and we have managed to salvage about 100-150 but the rest are completely and utterly useless."
Mr Davies said many of the fossils were wrapped securely for future exhibitions and others had already been moved.
He said the incident had been a set-back for the group's regeneration plans but it would not put them off.
"Our plan and vision is to get the site, the building, the ironworks and blast furnace to make them into a major visitor centre."
North Wales Fire Service confirmed it was called to the site twice at the weekend to deal with fires which were started deliberately.
The forest fossils were discovered in 2006 during excavations at the steelworks site, and have been dated to the Carboniferous period.