March 29, 2012
Tenontosaurus is a difficult dinosaur to describe. This beaked herbivore—a distant, roughly 110-million-year-old cousin of the more famous Iguanodon—didn’t have any spectacular spikes, horns, plates, or claws. In short, Tenontosaurus was a vanilla dinosaur, and is probably most famous for being the prey of the “terrible claw” Deinonychus. But there is something very important about the unassuming plant-eater: Paleontologists have collected a lot of them. There are at least 30 complete or partial Tenontosaurus skeletons in museums across the country, including everything from very young dinosaurs to adults. With such a sample size, paleontologists can compare skeletons to dig into the dinosaur’s biology, and University of California at Berkeley paleontologist Sarah Werning has done just that. In a paper just published in PLoS One, Werning details how Tenontosaurus grew up.
The secret to Tenontosaurus growth is in the bones themselves. The microscopic structure of dinosaur bone contains clues to how rapidly the dinosaurs grew and what was happening to them at the time of death. For this study, Werning created slides from sections of Tenontosaurus long bones—the humerus, ulna, femur, tibia and fibula—to tease out the history of each animal and the larger pattern of how the dinosaur changed with age.
During early life, Tenontosaurus grew quickly. “Throughout early ontogeny and into subadulthood,” Werning writes, “Tenontosaurus tilletti is characterized by bone tissues associated with fast growth.” But the dinosaur didn’t maintain this quick pace during its entire life. Sometime in its adolescence, perhaps around the time Tenontosaurus began reproducing, the dinosaur’s growth rate slowed. (Working with colleague Andrew Lee, Werning previously found that Tenontosaurus and other dinosaurs started having sex before they reached full size.) The dinosaur kept growing, but at a much slower rate, until it eventually reached skeletal maturity and its growth all but ceased.
This kind of growth pattern wasn’t unique to Tenontosaurus. Similar and closely related dinosaurs, such as Rhabdodon and Zalmoxes, appear to have grown quickly in their youth before slowing down sometime in their subadult lives. But not all ornithopod dinosaurs grew this way.
Tenontosaurus, Rhabdodon, Zalmoxes and similar dinosaurs were all on branches near the base of a major dinosaur group called the Iguanodontia. This group also contains Iguanodon itself and the full swath of hadrosaurs (think Edmontosaurus and Parasaurolophus). And, as Werning points out, hadrosaurs and the closer kin of Iguanodon grew extremely rapidly. These dinosaurs grew faster than Tenontosaurus and sustained the high growth rates until their skeletons were fully developed—there was no extended period of slow growth as the dinosaurs approached skeletal maturity.
This different pattern might explain why dinosaurs like Edmontosaurus were so much bigger than their archaic cousins. A really big, mature Edmontosaurus could reach more than 40 feet in length, but Tenontosaurus topped out at around 25 feet. Perhaps the rapid, sustained growth rate of the hadrosaurs and their close kin allowed them to attain huge sizes, while the more variable growth rates of Tenontosaurus constrained the dinosaur’s size to the middle range.
As paleontologists study other dinosaurs, perhaps the details of how iguanodontian growth rates shifted will become clearer. And Werning has set an excellent precedent for other researchers delving into dinosaur histology. Not only is her paper open-access, but Werning also uploaded multiple high-resolution images of the Tenontosaurus bone slides to the website MorphoBank. Other scientists can readily download the images and investigate the slides for themselves. I hope the Tenontosaurus images are just the start of what will become on online library of dinosaur histology—a resource that will undoubtedly help researchers further investigate the biology of these amazing animals.
Werning, S. (2012). The Ontogenetic Osteohistology of Tenontosaurus tilletti PLoS ONE, 7 (3) DOI: 10.1371/journal.pone.0033539
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