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in Perillo & Sander, 2024. DOI: 10.7717/peerj.17060 |
Abstract
Very large unidentified elongate and rounded fossil bone segments of uncertain origin recovered from different Rhaetian (Late Triassic) fossil 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: (1) giant amphibian bones, (2) dinosaurian or other archosaurian long bone shafts, and (3) giant ichthyosaurian jaw bone segments. We call the latter proposal the ‘Giant Ichthyosaur Hypothesis’ and test it using bone histology. In presumable ichthyosaur specimens from SW England (Lilstock), France (Autun), and indeterminate cortical fragments from Germany (Bonenburg), we found a combination of shared histological features in the periosteal cortex: an unusual woven-parallel complex of strictly longitudinal primary osteons set in a novel woven-fibered matrix type with intrinsic coarse collagen fibers (IFM), and a distinctive pattern of Haversian substitution in which secondary osteons often form within primary ones. The splenial and surangular of the holotype of the giant ichthyosaur Shastasaurus sikanniensis from Canada were sampled for comparison. The results of the sampling indicate a common osteohistology with the European specimens. A broad histological comparison is provided to reject alternative taxonomic affinities aside from ichthyosaurs of the very large bone segment. Most importantly, we highlight the occurrence of shared peculiar osteogenic processes in Late Triassic giant ichthyosaurs, reflecting special ossification strategies enabling fast growth and achievement of giant size and/or related to biomechanical properties akin to ossified tendons.
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| A reconstruction of a gigantic ichthyosaur - floating dead on the surface of the ocean. Remains of ichthyosaurs have been found in ocean sediment in various places around Europe. Image: Marcello Perillo/University of Bonn |
Conclusions:
Paleohistology can be a powerful tool for determining the taxonomic affinity of fragmentary bone specimens, as has been demonstrated in dinosaur studies previously (e.g., Garilli et al., 2009; Hurum et al., 2006). However, paleohistology can also be used to show that dinosaur-sized fragmentary bones do not belong to dinosaurs at all. Our study does just that, ruling out Sauropodomorpha and Stegosauria as possible sources of the mysterious large bone segments and fragments found in the European Rhaetian, thus rejecting the Dinosaur Hypothesis and instead supporting the Giant Ichthyosaur Hypothesis laid out by Lomax et al. (2018).
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Marcello Perillo and P Martin 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
Do some mysterious bones belong to gigantic ichthyosaurs?
A study sheds light on a mystery that has puzzled paleontologists for 150 years