Very large unidentified elongate and rounded fossilized bones of uncertain origin recovered from different Late Triassic localities across Europe have been puzzling the paleontological community since the second half of the 19th century. Different hypotheses have been proposed regarding the nature of these fossils: giant amphibian bones, dinosaurian long bone shafts, and giant ichthyosaurian jaw bone segments. A new study by paleontologists from the Rheinische Friedrich-Wilhelms Universität Bonn and the Natural History Museum of Los Angeles County’s Dinosaur Institute rules out dinosaurs as possible sources of the mysterious large bones, thus rejecting the dinosaur hypothesis and instead supporting the giant ichthyosaur hypothesis.
The Triassic ‘dinosaur bones’ are from the lower jaw of a gigantic ichthyosaur. Image credit: Marcello Perillo / University of Bonn.
In 1850, the British naturalist Samuel Stutchbury reported a mysterious find in a scientific journal: a large, cylindrical bone fragment had been discovered at Aust Cliff, a fossil deposit near Bristol, the United Kingdom.
Similar bone fragments have since been found in various different places around Europe, including Bonenburg in North Rhine-Westphalia and in the Provence region of France.
More than 200 million years ago, these areas were submerged beneath a huge ocean that covered vast swathes of Western and Central Europe.
Fossil remains from the animal world of that time — including marine and coastal dwellers — have been preserved in the sediment.
There is still some debate to this day about the animal group to which these large, fossilized bones belonged.
Stutchbury assumed in his examination of the first finds that they came from a labyrinthodontia, an extinct crocodile-like land creature.
However, this hypothesis was questioned by other researchers, who believed instead that the fossils came from long-necked dinosaurs (sauropods), stegosaurs or a still completely unknown group of dinosaurs.
“Already by the beginning of the 20th century, some other researchers had theorized that the fossils could possibly be from a gigantic ichthyosaur,” said Marcello Perillo, a paleontologist at the Rheinische Friedrich-Wilhelms Universität Bonn.
In the study, Perillo and Professor Martin Sander examined the microstructure of the fossilized bone tissue.
“Bones of similar species generally have a similar structure. Osteohistology — the analysis of bone tissue — can thus be used to draw conclusions about the animal group from which the find originates,” Perillo said.
“We compared specimens from South West England, France and Bonenburg. They all displayed a very specific combination of properties. This discovery indicated that they might come from the same animal group.”
The researchers then used a special microscope to prove that the bone wall had a very unusual structure.
It contained long strands of mineralized collagen, a protein fiber, which were interwoven in a characteristic way that had not yet been found in other bones.
Interestingly, fossils from large ichthyosaurs from Canada also have a very similar bone wall structure.
“However, this structure is not found in fossil samples from other animal groups that I have studied,” Perillo said.
“Therefore, it seems highly probable that the fragments in question also belong to an ichthyosaur and that the findings refute the claim that the bones come from a land-living dinosaur.”
“It is likely that the fossils come from the lower jaw of a sea creature.”
“By comparing the size of the fragments with the jaws of other species in this animal group, it is possible to deduce the length of the animals: they could possibly have reached a length of 25 to 30 m, as proponents of the ichthyosaur theory had originally speculated in an earlier study.”
“However, this number is only an estimate and far from certain — until, that is, we find more complete fossil remains. Nevertheless, they were certainly exceptionally large.”
“The first ichthyosaur lived in the ancient oceans in the Early Triassic epoch around 250 million years ago.”
“Species as big as whales existed early on but the largest creatures only appeared around 215 million years ago.”
“Almost all species of ichthyosaurs then died out at the end of the Triassic period more than 200 million years ago.”
“The unusual structure of their bone walls — which is similar to carbon fiber reinforced materials — probably kept the bone very stable while allowing for fast growth.”
“These huge jaws would have been exposed to strong shearing forces even when the animal was eating normally.”
“It is possible that these animals also used their snouts to ram into their prey, similar to the orcas of today. However, this is still pure speculation at this time.”
The team’s results were published online in the journal PeerJ.
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M. Perillo & P.M. Sander. 2024. The dinosaurs that weren’t: osteohistology supports giant ichthyosaur affinity of enigmatic large bone segments from the European Rhaetian. PeerJ 12: e17060; doi: 10.7717/peerj.17060