New research by Dr. Jordan Mallon, Canadian Museum of Nature, Dr. Donald Henderson, Royal Tyrrell Museum of Palaeontology, Dr. Colleen McDonough and Dr. W.J. Loughry, Valdosta State University, explains the upside-down preservation of ankylosaurs in North America.
Ankylosaurs were armoured, herbivorous dinosaurs. By the Late Cretaceous, ankylosaurs had diversified into two families, Ankylosauridae and Nodosauridae. They lived across the globe and were an important part of Late Cretaceous ecosystems as low-browsing bulk feeders.
Palaeontological work during the early 20th century revealed numerous ankylosaurs preserved upside-down. C.M. Sternberg was an American-Canadian fossil collector who worked for the Geological Survey of Canada. After a 70-year career collecting fossils in western Canada, Sternberg noted that almost all ankylosaur specimens he collected were preserved upside-down. Sternberg’s observations have been widely accepted by the palaeontological community but never verified.
This new research surveyed ankylosaur specimens from Cretaceous Period rocks in Alberta to verify Sternberg’s claims and tested several hypotheses to explain why ankylosaurs are so commonly preserved upside-down.
To verify Sternberg’s claim, researchers analysed all known ankylosaur material from Cretaceous Period rocks in Alberta to determine what position they were discovered in. Using field notes, field photographs, conferring with original collectors and considering indicators of exposed bone, researchers determined the orientation of 36 ankylosaurs. Of the 36 specimens, 26 (70.3%) were preserved upside-down. This indicates strong evidence in support of Sternberg’s observations.
Researchers then tested four commonly proposed hypotheses to determine why ankylosaurs commonly preserve upside-down. Only the “bloat-and-float” model stood up to scientific examination. This model suggests that when ankylosaurs washed into an adjacent river or stream, they would flip upside-down due to the configuration of the centres of mass and buoyancy, and the cross-sectional shape of the body. Carcass bloating due to decomposition would create further instability, making the body easily tipped over. The carcass would either sink to the bottom of the channel or sea floor, or be deposited in an overbank and become preserved in sediment.
To test this hypothesis, Dr. Henderson generated three-dimensional, digital models of an ankylosaur (Euplocephalus tutus) and a nodosaur (Sauropelta edwardsi) using special software. The models were built to include the body masses and the centres of mass and buoyancy of the animals.
Dr. Henderson’s models show that the bloating of an ankylosaur carcass in water would distend the belly. This would cause the floating carcass to become unstable and easily overturned with the slightest bump. The preservation of upside-down ankylosaurs in fluvial (of, or, found in a river) or marine environments also supports the bloat-and-float method. Only ankylosaurs from fluvial and marine deposits in North America are preserved on their backs. Ankylosaurs from inland deposits in Asia are consistently preserved right-side up.
This research confirms Sternberg’s observations and explains the century old puzzle of why ankylosaurs are commonly preserved upside-down in North America.