Was Spinosaurus a Bison-Backed Dinosaur?

Smithsonian.com-DinosaurTracking: Was Spinosaurus a Bison-Backed Dinosaur?
Spinosaurus and Ouranosaurus were among the most prominently ornamented of all dinosaurs. Both dinosaurs—a carnivore and herbivore, respectively—had elongated neural spines sticking out of many vertebrate along their backbones, which created prominent skeletal sails. In life, these structures are thought to have been covered by a thin layer of flesh, but in 1997 paleontologist Jack Bowman Bailey proposed an alternative idea. These dinosaurs were not sail-backed, Bowman hypothesized. They were hump-backed.

Superficially, the high-spined dinosaurs appeared to be analogues of two other strange prehistoric creatures. The carnivorous Dimetrodon and the herbivorous Edaphosaurus were synapsids, our own distant cousins, that lived between approximately 280 million and 265 million years ago. Both had the skeletal rigging for prominent sails on their backs and lived in a dry, arid landscape roughly similar to the kind of habitat Spinosaurus and Ouranosaurus inhabited much later. But Bailey argued that paleontologists had selected the wrong set of analogues. Bison were a better choice.

Bailey used basic anatomical comparison to set the stage for his idea. Illustrating the skeletons of Ouranosaurus, Dimetrodon and a bison side by side, Bailey noted that the back spines of the dinosaur were most similar to the thick, flattened spines near the shoulder region of the bison and were generally unlike the spindly backbone spires of Dimetrodon. (The elongated neural spines of the bison were so high, in fact, that Bailey wondered, “If bison had become extinct prior to the emergence of our own species, would they be interpreted today as sailbacked mammals?”) The resemblance led Baily to propose that the sails were sites for the attachments of powerful ligaments and large muscles.

Bison-backed dinosaurs would have been obligated to take up a different posture to handle all that extra bulk. If Spinosaurus had a thick hump, Bailey hypothesized, then it probably walked on all fours instead of balancing on two legs like other large theropods. “Thus, it seems unlikely that Spinosaurus was an agile cat-like sprinter like many short-spined theropods (e.g., Allosaurus),” he wrote, “but perhaps used the huge mass of its bear-like body to overpower young or weak prey, or perhaps to steal the kills of smaller more agile predators.” Restored by R. E. Johnson in one of the paper’s illustrations, Bailey’s vision of Spinosaurus looks like an enormous, hunch-backed crocodile.

Spinosaurus and Ouranosaurus were not the only dinosaurs Bailey thought might have humps. Bailey also viewed the elongated neural spines of dinosaurs such as the large theropod Acrocanthosaurus, the ceratopsian Protoceratops, the plate-backed Stegosaurus and others to infer the presence of large and small humps among many dinosaurs. These structures might have allowed dinosaurs to store up large amounts of energy in harsh environments, or maybe they allowed dinosaurs to maintain high, constant body temperatures (something that Bailey did not think dinosaurs were capable of without some specialized anatomical equipment, like a hump). The idea seemed plausible to some. A few months later, in a news report printed in Science, paleontologist Paul Barrett was cited as being in favor of Bailey’s notion. More recently, a 2007 National Geographic feature on “Extreme Dinosaurs” also counted Hans-Dieter Sues as supporting the idea, and a sketch by paleontologist Jason Poole showed a typical, sail-backed Spinosaurus standing next to a hump-backed one.

Beyond these notes, however, the idea that dinosaurs were bison-backed has not caught on. Spinosaurus, Ouranosaurus, and other dinosaurs Bailey cited are most often depicted with sails or other relatively thin structures, such as the fin-like projection at the hips of the recently-described predator Concavenator. There are a few reasons for this.

At the time Bailey wrote his paper, Ouranosaurus and Spinosaurus were thought to have lived in hot, dry, arid habitats where big sails would have caused them to overheat in the hot sun. A hump, in Bailey’s alternative view, would have acted as a “heat shield” in the Cretaceous environments. But paleontologists now know that these dinosaurs lived in lush, swampy environments and probably did not require protection from the desert-like environment Bailey based his ideas on. This also means that the dinosaurs would not have needed humps to store extra energy to make it through harsh dry seasons, thereby undermining the idea that Spinosaurus and Ouranosaurus were like desert lizards that store resources for tough times. (Additionally, if Spinosaurus and Ouranosaurus really did have heat-shield humps, then it is strange that other dinosaurs from the same ancient environments did not share the same adaptation.)

The dinosaurs were also relatively unique in the shape of their elongated spine rows. In terms of maximum spine height compared to the rest of the body, the dinosaurs considered in the study had sail or hump heights intermediate between those of Dimetrodon and bison, and the long spines of Spinosaurus and Ouranosaurus jutted up over a greater length of the back than in the mammals. Whereas the elongated spines of bison typically peaked between the shoulderblades and quickly became reduced in size, the highest points of the dinosaur backs were set further back along the spine and had a more gradual slope to them. This is probably because the elongated spines of bison are sites for muscle and ligament attachments that connect to the neck and head, whereas there is no indication that Ouranosaurus, Spinosaurus, or the other sail-backs needed extra support and power in the neck region. (If this were the case, and dinosaur humps contained muscles to support the head and give the neck more power, then it is odd that huge-headed dinosaurs like Tyrannosaurus did not have a similar adaptation.) Nor is there any indication that Spinosaurus had a body adapted to walking on all fours, although Ouranosaurus likely shared the ability of its hadrosauroid relatives in being able to switch between two- and four-legged locomotion.

Why Spinosaurus and Ouranosaurus had long rows of elongated spines is unknown. The structures supported fleshy banners that almost certainly played roles in display and species recognition—these dinosaurs practically carried billboards on their backs—but beyond that, it is difficult to say. Reconstructing soft tissues on extinct animals is very difficult, and doubly so when there are no solid modern analogues for the structures in question. Though Bailey pointed to the humps of mammals, for example, the elongated spines of bison, mammoths, prehistoric deer and other creatures were related to providing support for the head and strength to the neck, which was apparently not the case with Spinosaurus and Ouranosaurus. Desert lizards with fat tails don’t appear to be good analogues, either. Spinosaurus and Ouranosaurus were fundamentally different, and they remain among the most bizarre dinosaurs yet discovered.

References:
Anonymous (1998). Dino Fins More Like Humps? Science, 279 (5354), 1139-1139 DOI: 10.1126/science.279.5354.1139d

Bailey, J.B. (1997). Neural Spine Elongation in Dinosaurs: Sailbacks or Buffalo-Backs? Journal of Paleontology, 71 (6), 1124-1146