Old Bones - a blog about Vertebrate Paleontology

Another guest post today from Allison L. Beck!

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

1. Xu X, et al. 2012. A gigantic feathered dinosaur rom the Lower Cretaceous of China. Nature 484: 92-95. 

2. Bakker RT. 1972. Anatomical and ecological evidence of endothermy in dinosaurs. Nature 231: 81-85. doi:10.1038/238081a0

3. de Ricqles AJ. 1974. Evolution of endothermy: Histological evidence. Evolutionary Theory 1: 51-80.

4. Erickson GM, et al. 2004. Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs. Nature 430: 772-775.

5. Chiappe LM, et al. 1998. Sauropod dinosaur embryos from the Late Cretaceous of Patagonia. Nature 396: 258-261.

6. 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.

7. 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.

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I thought I would try and start having some shorter, more conversational posts, instead of the instructive tomes I've been writing.  This is for a couple reasons: 1) I have the feeling that the longer entries are a bit TOO long, and maybe even a bit dry; 2) it's hard to compose those longer entries - there's research to do, figures to make, etc.  Shorter entries, means more blog posts... hopefully...

So what I thought I'd start doing is talking a bit about "the life of a paleontologist", at least from my point of view, which I think is fairly typical.  I'll talk about myself and other sorts of paleontologists so that hopefully a student, or even a youngster, hoping to get into the field, will know what it takes, and what it will entail.

This week I was actually doing some paleontology, which can sometimes be rare.  Since most of us are academics of a sort (instructors at institutions of higher education), we don't actually put "Paleontologist" on our business cards; often it's "Professor" or "Instructor".  We spend a significant portion of our time teaching, grading, being on committees, advising students, etc. that any academic would do.  Right now I'm teaching a summer school class in Human Dissection Anatomy.  Teaching anatomy is a typical position for a certain class of paleontologists, but the reasons behind that are a story for another time.

As I was saying, this week I was doing some paleo, specifically heading to a museum - the San Diego Natural History Museum (SDNHM).  I'm in Los Angeles, so this was a long day of driving - 2+ hours in the morning, the same in the evening at the end of the day.  Museums are meccas for paleontologists in this way.  The displays the public sees at museums are usually a very small portion of the collections housed there.  The back rooms are filled with banks of drawers filled with more fossils, skins, specimens preserved in alcohol, archeological bits, etc.  For every piece you see on display, there are probably 10's, if not 100's more, in the back.  Most of these aren't "museum quality", meaning that they wouldn't look good on display, and the public wouldn't get much out of them.  But they are precious to the researchers in various fields that flock to see them.  This highlights the dual duties of museums - to inform the public and act as centers for academic research.  Both are equally important.  They help push science forward, while presenting the findings to the public.

This has been one of my favorite parts about being a paleontologist.  Going to museums, opening drawers, not knowing what you'll find.  Often the work is a little dull, comparing specimens, taking measurements, etc.  Every once in a while though you get a gem.  You realize you're looking at a new species, or you notice an interesting feature not documented before.  Really though, for me, it's just opening that drawer and seeing all the knowledge in there, waiting to be discovered.  Every fossil is a data point to help us build the tree of life and reconstruct the grand past of this planet.

A drawer of Eocene (about 40 million years ago) fossils at the San Deigo Natural History Museum

I'll continue next time with what I did while I was at the museum, and about the people that work at the museum.

What is a bird? It sounds like a simple question, because we all have some general idea of what a bird is. You probably make that impression based on characters you associate with birds - birds have feathers, most of them fly, they lay eggs, etc. This kind of definition goes back to what I discussed in my post about turtles - defining groups of animals (taxa) based on collections of characters that they all share. This is all well and good for modern birds, but what happens as we get closer to the ancestral bird, the common ancestor of all birds. There things get tricky, as there were probably a number of taxa around that were 'almost birds'. These taxa may have had some, or even most, of the characters we use to define birds, and you may have even called them a bird on first impression (if you had been around the 150 million years ago when birds first arose), but they weren't birds.

For a long time the archetypal ancestral bird was Archaeopteryx. Even many lay-people have heard of this amazing fossil, mostly because of its exquisite preservation. Found in the mid-1800's in Germany, the fossils of Archaeopteryx come from the Solnhofen limestone. This fine-grained muddy rock is a gold-mine of fossils. Probably deposited in a calm lagoon, which allowed for fairly rapid deposition with relatively little disturbance, the fossils found within its layers are amazingly well preserved. Not only has Archaeopteryx been found there, but so are fossils of small dinosaurs, pterosaurs (winged reptiles that shared the world of the dinosaurs), invertebrates like horseshoe crabs and starfish, and countless plant fossils. There are also tracks of some of the animals, preserved as they walked across the muddy bottom of the lagoon. There's a great German word for a fantastic collection of fossils like this - a Lagerstatten (German for "storage place").

One of the specimens of Archaeopteryx from Solnhofen (image from Wikipedia). Note the impressions of feathers.

The Archaeopteryx fossils from Solnhofen are amazing for their impressions of feathers. The skeletons are pressed between layers of the muddy limestone, and although all the soft tissues, such as the feathers, are gone, they left impressions in the mud before rotting away. The same is true of many of the fossils from these deposits; there are wing-skin impressions from the pterosaurs and body impressions of soft-bodied molluscs.

It was these feather impressions that earned Archaeopteryx the title of the 'earliest bird'. While still retaining some primitive characters from its dinosaurian ancestors, such as teeth and a bony tail, there were enough bird characters, feathers being the biggest one, to call it a bird. Now a new fossilfrom China has knocked Archaeopteryx from its avian pedestal.

Reconstruction of Archaeopteryx by Heinrich Harder - 1916 (from Wikipedia)

Birds have long been known to be related to dinosaurs. Even Charles Darwin remarked on this connection in the 6th edition of his "Origin of Species", and Archaeopteryx was proof of this connection. It was actually thought to be a dinosaur, until someone noticed the feather impressions surrounding the bony fossilized remains. Specifically, birds are thought to be related to theropod (two-legged, meat-eating) dinosaurs, like Velociraptor and Compsognathus made famous in the "Jurassic Park" movies (Note: The famed dinosaur in the first of those movies was actually based on Deinonychus, a species closely related to Velociraptor; Velociraptor is much smaller than the critters shown in the movies, but it has a cooler name than Deinonychus). Technically birds ARE dinosaurs, which is why some paleontologists have taken to calling what the public would normally just call 'dinosaurs', 'non-avian dinosaurs'.

A passage from "Origin of Species" where Darwin talks about Archaeopteryx (from Darwin Online). Even he knew birds were dinosaurs!

This new fossil from China, dubbed Xiaotingia by Xing Xu and the other authors of the study, is closely related to Archaeopteryx. The analysis the authors did found this relationship, but more dramatically, showed that these taxa are closer to the theropod relatives of birds, and not to birds themselves. We know that many of these theropods had feathers, but were not birds (thereby removing one of the seemingly unique features of birds), so Archaeopteryx and Xiaotingia are just another group of feathered dinosaurs. The authors admit that their analysis is somewhat tentative in the robustness of its findings. The findings make some profound implications about the evolution of flight. It's thought that Archaeopteryx was capable of powered (as opposed to gliding) flight, so the new arrangement implies that either flight evolved twice, or was primitive for that theropod group.

Phylogeny showing the relationships of Archaeopteryx and Xiaotingia to birds and "non-avian" dinosaurs.  The dotted lines represent two possibilities for their relationships - one closer to birds, the other closer to small theropod dinosaurs.  The paper by Wu and colleagues suggests the latter.

So the question "What is a bird?" has gotten even more muddied (like the bottom of the Solnhofen lagoon), and only more fossils will tease out the answer.