A few weeks ago, a new species of theropod dinosaur (carnivores, like T. rex, that walked on two legs) was published in the journal Nature (1). While that in and of itself is not particularly noteworthy, what is exciting is the fact that this theropod was covered in long, filamentous feathers. Yet, even the presence of feathers on theropods is becoming commonplace. But this dinosaur, Yutyrannus, is huge AND covered in feathers, and its closest relatives were all small. Yutyrannus may have looked a lot like Big Bird. Reading the article made me realize just how different vertebrate paleontology is now than when I first decided my career path at the tender age of ten.
When I was a kid, I loved dinosaurs (of course) and, apparently, I never grew out of it. The dinosaurs of my childhood were large, lumbering, slow, awkward, and ectothermic (ectotherms are animals that acquire body heat from their environment, like lizards and snakes that bask in the sun). In my Mesozoic world (about 250 to 65 million years ago, when dinosaurs lived), organisms like the long-necked Apatosaurus, dragged their equally-long tails on the ground behind them. The bodies of dinosaurs were rotund, with impossibly thick hides of dry, scaly skin. Indeed, even the regal Tyrannosaurus rex was slothful and tail-dragging, a mindless killing machine searching out any prey it could find.
Now, I'm not that old, but our current image of dinosaurs is drastically different. In just a few decades, our understanding of the paleobiology of dinosaurs has shifted dramatically. In fact, some of paleontology's most important illustrations, the murals of Charles R. Knight, are now scientifically inaccurate. (This does not, however, detract from their historical and aesthetic value.) In the late 1960s and early 1970s, some paleontologists began to draw correlations between the upright posture of dinosaurs (their limbs extend beneath their bodies, as in a cat) and endothermy (animals that produce their own body heat like mammals and birds) (2). All groups of living endotherms have upright posture, therefore, upright posture may indicate endothermy. The sheer size of many dinosaurs led some to hypothesize that they tended to stay warm because of a low ratio of surface area to volume (think of how fast a cup of water cools relative to a pot). Additionally, some paleontologists started looking at the histology, or microscopic structure, of dinosaur bone, and noticed that it has a lot of similarities to that of living endothermic mammals (3). Although some of their ideas have been modified, there is no doubt that these works opened up a new era of dinosaur paleobiology.
The old view of Tyrannosaurus rex. In this original Charles R. Knight mural, T. rex was a slow, lumbering, scaly animal.
Studies on dinosaur bone histology continue today, and well-studied fossils, such as T. rex, have offered us new insight into the workings of a dinosaur body. We now know that the famous Field Museum T. rex named SUE, was but a teenager, and that Tyrannosaurus grew fast early in its life to reach a large size, then growth slowed (4). The enormous long-necked sauropod dinosaurs, like Apatosaurus grew from tiny eggs, no bigger than a foot or so in diameter (5).
Studies in biomechanics, the application of engineering principles to biological systems, have brought us the knowledge that T. rex and other theropods did not actually drag their tails, but stood with their tails sticking straight out, balancing over their feet. Indeed, even the gargantuan, long-necked sauropods, the largest creatures ever to walk the earth, most likely had complex arrangements of muscle and connective tissue allowing them to hold those long tails straight out from their bodies. Fossilized trackways of sauropods rarely show evidence of tail drag marks, and recent studies have demonstrated that it was physically possible for these animals to not only support those tails and necks, but possibly even to rear up on their hind legs (6).
Finally, we know that many theropds were not likely covered entirely in dry, scaly skin, but rather had a feathery covering, much like the down that covers baby chicks to help keep them warm. Find after find of feathered dinosaurs have been announced in the last two decades. Having a feathery insulation makes sense if you are small, like many theropod ancestors of today's birds. Feathers are also great for showing off to a mate, and some may have functioned that way in dinosaurs as well. In fact, new work on the mineral composition of some preserved feathers has allowed paleontologists to identify the actual color of some of these feathers (7)!
Artist's conception of Yutyrannus, which demonstrates the dynamic, nimble theropods of current understanding. Image by Brian Choo.
Dinosaurs have been studied by paleontologists for over 150 years, and for most of that time we held the old view of dinosaurs. So why, in such a short period of time, have we remodeled our dinosaurs? One of the main reasons is that more people have been collecting fossils. Paleontologists have continued to scour North America and Europe, and in more recent years, expeditions to South America, Africa, Asia and even Antarctica have been funded. More fossils always lead to more information. But perhaps more surprisingly, politics plays a role. Partnerships between scientists from African countries (particularly in the Sahara Desert and Madagascar) and western paleontologists with financial resources have produced fruitful expeditions and increased known dinosaur diversity immensely. Additionally it wasn't until the 1990s that large swaths of China's Gobi Desert were opened up for collecting by western paleontologists, in collaboration with their Chinese counterparts. Finally, new technology, such as CT scanning, has allowed us to look inside fossils and examine them more closely than ever before.
Dinosaurs truly are far removed from the lumbering evolutionary dead-ends we once thought them to be. They are every bit as diverse and dynamic as the animals we are familiar with today.
Further Reading and References
- Xu X, et al. 2012. A gigantic feathered dinosaur rom the Lower Cretaceous of China. Nature 484: 92-95.
- Bakker RT. 1972. Anatomical and ecological evidence of endothermy in dinosaurs. Nature 231: 81-85. doi:10.1038/238081a0
- de Ricqles AJ. 1974. Evolution of endothermy: Histological evidence. Evolutionary Theory 1: 51-80.
- Erickson GM, et al. 2004. Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs. Nature 430: 772-775.
- Chiappe LM, et al. 1998. Sauropod dinosaur embryos from the Late Cretaceous of Patagonia. Nature 396: 258-261.
- There is a lot of work on sauropod bodies, but some information on tail drags can be found here. For a scientific review of locomotion in these long-necked giants, including rearing, see: Carrano MT. 2005. The evolution of sauropod locomotion. In: Curry Rogers KA and Wilson JA. 2005. The Sauropods: Evolution and Paleobiology. University of California Press. pp. 229-251.
- For information on the new techniques and new capacity to identify color in dinosaurs, see a recent article in the New York Times. The first identification of feather color was described in: Zhang F, et al. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463: 1075-1078. and the first evidence of display coloration in feathers was just reported in: Li Q, et al. 2012. Reconstruction of Microraptor and the evolution of iridescent plumage. Science 335: 1215-1219.