I love living in dinosaur country. Not only are there plenty of bones to discover, but Mesozoic mysteries abound. And one of the most persistent has to do with the 150 million year old environments that are preserved here and there in eastern Utah. Those Late Jurassic sites are brimming with the bones of huge sauropods – long-necked, hefty dinosaurs such as Apatosaurus, Camarasaurus, and Diplodocus, just to name a few. In fact, there are so many species and individuals that paleontologists are left with a tough question: how did so many giants coexist?
In general form, one sauropod might look more or less like another. Tiny head, long neck, column-like legs, long tail, and so on. But the variety of dinosaurs that tromped around Jurassic Utah – and other parts of the Morrison Formation, where they were entombed in other states – were actually quite different. While Diplodocus had a long, low, narrow skull tipped in a square muzzle bearing peg-like teeth, its neighbour Camarasaurus had a tall, blunt skull with a rounded muzzle with spoon-shaped teeth.
By skull shape alone, we can hypothesize that Diplodocus and Camarasaurus probably preferred different items on the Jurassic salad bar. But how did they feed, and how did both dinosaurs find enough tasty vegetation in environments where sharp swings between the wet and dry seasons limited the food supply?
To unpack this puzzle, University of Bristol’s David Button and Emily Rayfield teamed up with Natural History Museum, London paleontologist Paul Barrett to investigate the biomechanics of Camarasaurus and Diplodocus skulls. More specifically, the paleontologists used virtual models to see how the skulls of these dinosaurs responded to the stresses and strains of feeding.
As you might expect from skull shape, the two sauropods turned out to have different ways of munching on plants. Camarasaurus had more muscle mass in its jaws, Button and colleagues found, and had a stronger bite force than Diplodocus. This hints that Camarasaurus may have relied on tougher fare like conifer branches. And even though the skull of Diplodocus was still relatively resistant to the stresses involved in biting, the paleontologists suspect that the dinosaur was limited to a softer diet of plants like horsetails and ferns.
But these dinosaurs only represent two extremes found in the Morrison Formation. At some sites, as many as five different sauropod species have been found. While the researchers didn’t directly investigate the skull biomechanics of these other Jurassic giants, they compared their skull shapes and proportions to Camarasaurus and Diplodocus. These dinosaurs – such as Apatosaurus and Giraffatitan – had skull shapes that were distinct, and so hint that the sauropods had differentiated diets. Some, such as Diplodocus and Apatosaurus, were probably specialists that plucked up certain soft foods, while Camarasaurus and Giraffatitan were generalists that could supplement their coarse diet with some lush greens. Along with body size that shielded them from drought and allowed them to walk long distances to food, the variations in skull shape and diet allowed these dinosaurs to make a living in places where succulent greens could sometimes be hard to come by.
It’s important to remember, though, that these dinosaurs did not chew. They could wrench branches off trees and crop ferns, but they didn’t have anything like molars. Sauropods swallowed their meals whole. This, along with strange stones sometimes found with their bones, led some paleontologists to propose that sauropods swallowed stones on purpose to grind their meals in their stomachs.
There’s a snag, though. The problem is that very few sauropods have ever been found with these gastroliths – “stomach stones” – in place. In a review of the evidence published around the same time as the biomechanics study, Museum für Naturkunde paleontologist Oliver Wings concluded that only a handful of sauropod individuals of different species had swallowed stones, and in the Morrison Formation the only solid case is a skeleton of Diplodocus.
There’s no good evidence that sauropods regularly ingested rocks to help crush their food. Instead, paleontologists expect, their guts retained vegetation for longer amounts of time for biochemical breakdown. Of course, this would all be easier if we had a living Camarasaurus or Apatosaurus to observe, but paleontologists are still able to tell a surprising amount about dinosaur biological basics through their fossilized remains.
Button, D., Rayfield, E., Barrett, P. 2014. Cranial biomechanics underpins high sauropod diversity in resource-poor environments. Proceedings of the Royal Society B. doi: 10.1098/rspb.2014.2114
Wings, O. 2014. The rarity of gastroliths in sauropod dinosaurs – a case study in the Late Jurassic Morrison Formation, western USA. Fossil Record. 18: 1-16. doi: 10.5194/fr-18-1-2015