Of all the dinosaur groups, the sauropods (long-necked giants like Brontosaurus and Maryland’s own Astrodon) were among the most successful and long-lasting. They first appeared in the Late Triassic, 210 million years ago, and persisted all the way to the end of the Age of Dinosaurs, 66 million years ago. Many sauropod species came and went over time, but their basic body plan remained remarkably consistent. This is surprising given the massive environmental changes that occurred during their existence. The climate changed, the plant communities on which sauropods fed were replaced many times over, and new plant-eating competitors like ceratopsians and duck-bills evolved. And yet, sauropods thrived through it all.
In North America, sauropods were at their most diverse during the Late Jurassic, about 150 million years ago. At this time, around twenty sauropod species co-existed in the western United States. Most of these species fell into one of two major sub-groups: the diplodocids and the macronarians. Diplodocids like Brontosaurus and Diplodocus were relatively gracile, with very long necks and tails. Their mouths were packed with dozens of narrow, peg-like teeth that would self-replace every few weeks. In contrast, macronarians like Camarasaurus and Brachiosaurus were stocky and high-shouldered. They had a smaller number of teeth, which were broad and spoon-shaped. Traditionally, paleontologists have assumed diplodocids and macronarians could co-exist because they fed on different kinds of plants. Diplodocids’ lower stance and more delicate teeth were ideal for eating soft vegetation that grew close to the ground, like ferns and cycads. Meanwhile, macronarians held their necks up high, and used their heavy-duty teeth to munch on woody branches and cones at the tops of trees.
Nevertheless, something strange happened at the start of the Cretaceous Period that disrupts this just-so story. First, the diplodocids went extinct, leaving the macronarians as the only sauropod group on Earth. One might think that changes in plant communities meant there was no longer a niche for narrow-toothed grazers, or that the diplodocids were muscled out in the competition for low-growing vegetation by the newly evolved duck-billed dinosaurs. However, no sooner were the diplodocids gone that new macronarian species, including Astrodon, evolved with narrow, diplodocid-like teeth. Narrow-toothed macronarians completely replaced their broad-toothed ancestors, but the rest of the robust, high-shouldered macronarian body plan remained largely unchanged.
So what happened? We don’t really know. Nobody has been able to point to a specific change in plant life during the Cretaceous that would have driven diplodocids to extinction. And even if there was such a link, it wouldn’t explain why macronarian dentition would evolve to be more like the allegedly obsolete diplodocids. Flowering plants (angiosperms) arose in the Cretaceous, nearly replacing the conifers that Jurassic sauropods fed upon. But flowering plants did not become a significant part of the North American flora until after narrow-toothed macronarians were well-established. Narrow teeth with a fast replacement rate might be an adaptation for eating abrasive plants, but again there is no evidence that angiosperms or any other Cretaceous plants were harder on teeth than Jurassic greenery.
What we’re left with is a sauropod-sized mystery. Dinosaur Park and other Early Cretaceous fossil sites may yet hold clues about how and why sauropods changed their teeth. But like any lingering questions about the prehistoric world, the answers won’t come until we find more fossils!
Button, D.J., Rayfield, E.J., and Barrett, P.M. (2014). Cranial biomechanics underpins high sauropod diversity in resource-poor environments. Proceedings of the Royal Society B 281: 1795.
Chure, D., Britt, B.B., Whitlock, J.A., and Wilson, J.A. (2010). First complete sauropod dinosaur skull from the Cretaceous of the Americas and the evolution of sauropod dentition. Naturwissenschaften 97: 379-391.
D’Emic, M.D., Whitlock, J.A., Smith, K.M., Fisher, D.C., and Wilson, J.A. (2013). Evolution of High Tooth Replacement Rates in Sauropod Dinosaurs. PLoS One 8: 7: e69235.