The 55 fossils were loaned to Doncaster Museum in the mid-1960s, and although now the normal practice is to loan items for a year, they had been there ever since.
But after expert restoration work by palaeontologist Nigel Larkin, paid for by the Esmée Fairbairn Foundation, they are now on display at Scarborough’s Rotunda Museum.
They include a block of ammonites which features at least two different species and is about 200 million years old, collected from Robin Hood’s Bay, and several very rare fish, thought to be 180 million years old, which were found in the Whitby area.
The fish, believed to be leptolepis and lepidotus, are extinct and their fossils are no longer easily collected.
The fossils have been restored as part of the Catalogued, Interpreted, Researched, Conserved, Accessible (CIRCA) project.
The geologist and head of public programmes at Scarborough Museums Trust, Will Watts, said: “We’re delighted to welcome these specimens back after an extended vacation.
“They’re in superb condition thanks to the CIRCA project, and several – including a leg bone believed to be from a middle Jurassic period dinosaur – will go on immediate display as part of the Scarborough’s Lost Dinosaurs exhibition at the Rotunda Museum.”
Dean Lomax, the assistant curator of palaeontology at Doncaster Museum (CIRCA project) and a visiting scientist at Manchester University, added: “The specimens were brought to Doncaster by John Lidster and Elphinstone Forrest Gilmour, who were past curators at the old Woodend Museum in Scarborough.
“The Doncaster Museum and Art Gallery opened in 1964, and we believe these remains formed part of a new display on the geology and fossils of Doncaster and the wider region – we know there was a section dedicated to the Jurassic Coast of Yorkshire around Scarborough.”
The aim of the CIRCA project is to research, revitalise, and care for museum fossil collection.