Tag: dinosaurs

Preserved protein from an 80-million-year old dinosaur support the dinosaurian origin of birds

May 6, 2009 • 6:16 am

In this week’s Science we find a paper by Schweitzer et al. (total of 16 authors!) that has a quite remarkable result. (See the one page summary by Robert Service here.)  The upshot is that protein-sequence data from an 80-million-year old duckbilled dinosaur supports the dinosaurian origin of birds.

This story has a bit of a tortuous background.  First of all, we’ve known for a long time that birds evolved from gracile theropod dinosaurs; the fossil and anatomical evidence is given in WEIT.  But for some folks, DNA-based data is more convincing than is the fossil record.  And molecular evidence is what Schweitzer et al. provided.

In 2007, her group published a paper purporting to show that fragments of the protein collagen (a structural protein found in blood vessels and connective tissue), taken from a 68-million-year old fossil of T. rex, showed that the protein sequence (which reflects the DNA sequence) was more similar to that of birds than to that of modern reptiles. This strongly suggests that birds evolved from a lineage of dinosaurs that had already branched off of the lineage that gave rise to modern reptiles.  This meant that birds and dinosaurs are each other’s closest relatives compared to say, turtles or iguanas. In fact, many systematists say that birds are dinosaurs, which is the conclusion you’re forced to if you’re a hidebound cladist.  I mentioned this paper in WEIT in footnote 11 on p. 237 (this was probably the last thing I put in the book). Of course this result was no surprise to paleontologists.

As recounted by Service, this result was sharply questioned by other researchers, who claimed that the protein sequence was due to contamination; many others thought it was hard to believe that any protein could survive for so many millions of years.  I was a bit depressed about this, because I had already put the result in my book and there was no opportunity to change it before publication.  Also, it was just such a cute result — the kind of new finding that gets our juices flowing as scientists.

Well,  Schweitzer and her team persisted, and her new result supports the old one.  This time the group extracted protein fragments from the femur of a duck-billed dinosaur, Brachylophosaurus canadensis, collected in Montana. Great care was taken to prevent contamination.  They found that some of the elements in the demineralized bone bound to  antibodies made against bird collagen, indicating that collagen fragments similar in sequence to those of birds were present in the bone.  The same was found with antibodies for hemoglobin and two other structural proteins.

Some biochemical wizardry on bone extracts identified eight fragments of collagen, and their sequences (also determined in a separate lab to preclude contamination) were determined.  Sure enough, they were similar to the T. rex fragments that were questioned earlier, and were more similar to collagen sequences of modern birds than to those of modern reptiles. (See the phylogeny below.)  Note that there’s a small disparity between the fossil and biochemical evidence:  T. rex, as a theropod (the group that gave rise to modern birds), should be more closely related to modern birds than is B. canadensis.  This discrepancy might be an artifact of not having a complete protein sequence.

Let’s be clear: the phylogeny that we get doesn’t really tell us anything we didn’t know before.  Birds are highly-evolved dinosaurs, and that was already confirmed by the fossil record.  Still, it’s nice to have this molecular confirmation, and perhaps the most surprising result is the ability to determine the sequences of protein fragments that have survived for millions of years.  If this can be done with other fossils, we’ve suddenly gained the ability to solve many long-standing puzzles about ancestry and evolutionary relatedness.

phylogeny

Phylogeny showing closer relationship between dinosaurs and birds (Gallus = chicken, Struthio = ostrich) than between either of these groups and modern reptiles (alligators and Anolis lizards).  Ergo, birds are dinosaurs.


hadriosaur

Brachylophosaurus canadensis, the duck-billed dinosaur whose proteins were sequenced.  Illustration by Julius T. Csotony from Science article.

Schweitzer, M. H. et al.  2009. Biomolecular Characterization and protein sequences of the campanian hadrosaur B. canadensis. Science 324:626-631.

Feathered dinosaurs

March 2, 2009 • 10:01 pm

by Greg Mayer

One of the most exciting developments in paleontology in the past ten years or so has been the discovery that many species of theropod dinosaurs had feathers.  The earliest discoveries were quite controversial.  At the 1997 meetings of the Society of Vertebrate Paleontology in Chicago, a paper was read criticizing the interpretation of the skin structures on these fossils as feathers. In response, Phil Currie, one of the team working on the fossils,  presented an impromptu rebuttal paper later the same day, a rather unusual development for a normally tightly scheduled scientific meeting.  I was not convinced they were feathers myself until a while later, when a number of fossils of the new forms were brought to the Field Museum, and I was able to see them for myself– they had feathers!  One of the strangest of these feathered dinosaurs was Microraptor gui, which had both its forelimbs and hindlimbs modified into feathered wings.  It seems to exemplify the remark of J.B.S Haldane, the British geneticist who was one of the founders of the modern synthesis, “that the universe is not only queerer than we suppose, but queerer than we can suppose.”  Jerry highlights Microraptor in chap. 2 of WEIT, and notes a NOVA program on PBS, “The Four-winged Dinosaur”, that has a great website with interactives and videos, including the entire program. Originally airing last year, it was just recently shown again on my local PBS station, so check to see if it may be showing again in your area too.

Most of the specimens of feathered dinosaurs, as well as many true birds, have come from the fossil beds of Liaoning in northeastern China. The American Museum of Natural History has a nice website on the Liaoning fossil biota.  The Liaoning deposits have become one of the most important and interesting of what are called Lagerstatten (singular: Lagerstatte), a German word for a fossil deposit with extraordinary conditions of preservation. Such deposits, because they reveal structures (such as soft parts like feathers) and organisms (those lacking hard parts) otherwise missing from the fossil record, are often of crucial importance in studying the history of life on Earth.  Other famous Lagerstatten include the Pre-Cambrian Ediacara Hills of Australia, the Cambrian Burgess Shale in British Columbia and Chengjiang in China, and the Jurassic Solnhofen Limestone of Bavaria. These Lagerstatten have revealed, respectively, an early multicellular fauna, the Cambrian Explosion, including the earliest vertebrates, and Archaepoteryx, the first bird.

More on the evolution of flight

January 30, 2009 • 4:51 pm

As I discuss in WEIT, the evidence shows that birds evolved from theropod dinosaurs–gracile, carnivorous beasts that walked on two legs. Some of the important evidence comes from Chinese fossils showing theropods with various types of feathers. The incipient stages of feather evolution appears to be filamentous feathers (T. rex might well have been covered with fluff!), implying that flight feathers originated as devices to help insulate the theropods. In a recent paper in Nature, however, F. Zhang et al. found a theropod fossil (Epidexipteryx hui), about 160 million years old, which had not only the downy feathers (feathers completely unsuitable for flight) but also four very long tail feathers that could not have been used for either flight or insulation. E. hui also showed a number of morphological features that seem to make it closely related to modern birds. The most likely explanation for these tail feathers is that they were ornaments–ornaments that evolved for either species recognition or via sexual selection. The evolution of flight, then, may have begun with feathers that were used for display. A somewhat fanciful reconstruction of the beast is shown below.

Epidexipteryx hui (reconstruction)
Epidexipteryx hui (reconstruction)