They looked, as one researcher said, like "rubbish," eight seemingly inconsequential Cretaceous Period dinosaur fossils that sat in a London museum's collection for more than a century after being found in Canada's Alberta province.
But a fresh examination revealed the 75-million-year-old fossils to be quite remarkable, boasting microscopic structures that appear to be red blood cells containing nuclei as well as rope-like collagen fibers, scientists said on Tuesday.
It is extraordinarily rare for such ancient soft-tissue structures to be preserved as fossils, and some of the few similar discoveries in the past have been greeted with doubt.
It is even more surprising considering these fragmentary fossils, from a number of different dinosaurs, are not especially well preserved like the earlier ones that harbored soft-tissue remains, the researchers said.
They conducted a series of examinations using sophisticated microscopes and sliced samples using a focused ion beam to check the internal structures.
"We have attempted to apply the correct amount of skepticism, but, yes, I think it's fair to say that neither of us can think of anything else that these structures might be," said paleontologist Susannah Maidment, referring to her study co-leader at University College London, biomedical physical scientist Sergio Bertazzo.
An ungual claw from an indeterminate theropod where the erythrocyte-like structures were found, is pictured in this undated handout photo provided by Laurent Mekul, June 9, 2015
Apparent red blood cells were found in a claw that may be from the forelimb of meat-eater Gorgosaurus, which reached 30 feet (nine meters) long.
Other fossils from the collection at London's Natural History Museum appeared to preserve remains of collagen, the main structural protein in various types of tissues including bone and skin, and fragments of the protein's constituent amino acids.
But do not expect the findings to lead to the creation of live dinosaurs via cloning as in the "Jurassic Park" movies, including the new "Jurassic World" film.
"Although we have found dense internal structures that we have interpreted as nuclei in our cells, and the cells we found appear to preserve original components of blood, there is no evidence of any organelles or DNA within the nuclei," Maidment said.
"But even if one was to find some fragments of DNA, we would not be able to reconstruct a dinosaur 'Jurassic Park-style' because we would need the complete genome to figure out where the holes in the DNA are," Maidment said.
The findings, however, may buttress earlier similar discoveries including one by paleontologist Mary Schweitzer involving Tyrannosaurus rex.
"Her work has been extremely controversial, but our research, which uses different techniques, strongly vindicates her findings," Maidment said. "The difference between her work and ours is that our fossils are, frankly, really crappy, whereas hers were very well-preserved."
This suggests such remains may be more common than previously thought, Maidment said.
The structures they found looked like red blood cells from birds, which evolved from small feathered dinosaurs, and contained an apparent nucleus. "Bird red blood cells have nuclei and, therefore, a dinosaur red blood cell should have a nucleus," Bertazzo added.
Tests showed that these had striking similarities to blood cells from an emu, a large Australian flightless bird.
The research appears in the journal Nature Communications.