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A restoration of the skull of Sinornithosaurus highlighting the traits suppossedly indicative of a venomous bite. From the PNAS paper.

Though its dinosaurs looked pretty good, Jurassic Park was not particularly accurate as far as science was concerned. One of the real howlers that sent paleontologists reeling was the decision to make Dilophosaurus, one of the largest of the early predatory dinosaurs, the dinosaur equivalent of a spitting cobra. There was no evidence for it, but a new study published in PNAS suggests that an entirely different kind of dinosaur might have had a venomous bite.

Sinornithosaurus was one of the first feathered dinosaurs to be discovered. Covered in at least two types of feathers, it was a small dromaeosaur, or a relative of larger predators such as Velociraptor and Deinonychus. According to the new study by Enpu Gong, Larry Martin, David Burnhamb and Amanda Falk, however, Sinornithosaurus differed from its more famous cousins in that it had space for a venom gland and grooved teeth capable of delivering venom into the body of its prey.

When looking at the skull of Sinornithosaurus, the researchers believed they found traits commonly associated with venomous reptiles (namely lizards and snakes) such as a space in the skull for a venom gland, a hole in the jaw that would allow the venom to ooze out, and grooved teeth would channel the venom directly into the wounds of the dinosaur’s prey. If all this is accurate, the authors speculate, then Sinornithosaurus was probably armed with a kind of venom that would have immobilized its prey and allowed it to eat it at its leisure.

These are some pretty fantastic claims, but do they hold up to scrutiny? Dinosaurs belong to a wider group of reptiles called archosaurs which also includes crocodiles and birds. To date no evidence has ever been found of a venomous archosaur. But in the introductory portion of the paper, Sinornithosaurus is described as an “avian dromaeosaur” that was part of the “early avian radiation.” That means the authors are suggesting that Sinornithosaurus was not a dinosaur at all but a true bird that became secondarily flightless. Despite the overwhelming evidence that birds are dinosaurs, some scientists disagree, and the only way some of these critics (such as Larry Martin, one of the authors of the new paper) can make sense of feathered dinosaurs it to claim that they really were birds which were unrelated to dinosaurs.

The significance of this alternate view is that if Sinornithosaurus was a venomous bird derived from a more ancient stock of lizard-like reptiles (and hence unrelated to dinosaurs) it would have been evolutionarily closer to lizards and snakes, two groups that we know have venomous members. This association would not be proof positive that Sinornithosaurus had a venomous bite, but it would make it seem more likely that Sinornithosaurus was also venomous. Unfortunately for the authors, though, Sinornithosaurus was a feathered dinosaur that was only a cousin of some of the earliest avians (which were flying around during the time it lived). The existence of a venomous archosaur is still an extraordinary claim that requires extraordinary evidence.

As things presently stand that evidence has not been provided. The authors did not compare the skull of Sinornithosaurus with those of other predatory dinosaurs to see if the same traits occurred among other dinosaurs in differing combinations (perhaps having grooved teeth, for example, but not a “venom gland”).

There could still be alternate explanations for some of the traits they found. Other theropod dinosaurs have grooves in their teeth which appear to have reduced mechanical stress as they bit into prey. The tips of the teeth would penetrate into the prey but the grooves would provide a space for air so that the amount of suction on the tooth would be reduced as it was being removed. Likewise, the “venom gland” could just be an extension of an opening in the skull seen in many theropod dinosaurs and does not necessarily require a new explanation. The authors of the new paper did not discuss alternative hypotheses, and most of the structures they interpret as being indicative of a venomous bite can be otherwise explained.

The situation is made all the worse by a confused press release about the research generated by the University of Kansas. Even though the team did not actually discover Sinornithosaurus (it was named in 1999), the press release proclaims “Venomous prehistoric ‘raptor’ discovered by research team from KU and China.” And, as expected, Sinornithosaurus is not presented as a dinosaur but was instead described as “a venomous bird for all intents and purposes” by Larry Martin.

Are venomous dinosaurs a possibility? Absolutely, but in order to confirm their existence, particularly strong evidence is required. The new study, while interesting, does not include compelling evidence that Sinornithosaurus or any other dinosaur was venomous. For another take on this story, see Ed Yong’s piece at Not Exactly Rocket Science.

Earlier this month, the Discovery Channel premiered the four-part documentary miniseries Clash of the Dinosaurs. I was not all that impressed. It was good to see some scientists get some air time, but overall the hyperbole and repetitiveness of the show made watching it a bit of a chore.

Some of the scientists involved in the show were not too pleased with the final product, either. Sauropod expert Matt Wedel was one of the talking heads on the program and he recently presented an entire litany of problems with the show.  The most egregious error? The distortion of one of Wedel’s quotes through some careful editing by the show’s creators.

The basic story is this. For years there has been a myth that dinosaurs had a “second brain” in their rumps since the ones in their heads were so tiny. The production team behind the show had heard about it and wanted to include it in the show, but Wedel tried to set them straight. In an e-mail sent during the show’s production he explained that vertebrae in the sacral (hip) region of sauropods do show an expansion, but so do the sacral vertebrae of many other vertebrates (including yours and mine). The significance of this is that many basic body movements are controlled by the spinal cord rather than the brain itself, so the expansion of the spinal cord in some areas is for the purpose of accommodating more cells that can regulate these movements on their own. The brain only gets involved when you need to think about where you are stepping. On top of that, there was a second expansion in sauropods that might be similar to a similar one in living birds that contains something called a glycogen body, though what this feature would have done in dinosaurs is unknown. Generally speaking, though, there is no evidence that dinosaurs had any sort of second brain.

But the production team really wanted to talk about the “second brain,” and during Wedel’s on-camera interview they asked about it. He explained that it was an old story but there was not anything to it. In the editing room, however, someone chopped up his quote to make it sound as if he endorsed the idea, and that clip wound up in the final show.

Wedel brought this (and other errors) to the attention of the production company. Their response? They went ahead with the “second brain” myth because their audiences would not be able to understand what scientists really knew about the expansion of the spinal cord. They quote-mined Wedel, they said,  to meet the “demands of [their] audience.” In pushing ahead with the story they wanted to tell, Wedel argues, they not only misled the public but made him tell a lie.

Wedel brought his case directly to the Discovery Channel, and they agreed to change the show so that he would not be saying something that was not true. Wedel will still be in the show, but if you see the show on television in the future or on DVD, you will not see him talking about second brains in dinosaurs. It is wonderful that the turnaround on this problem was so fast, and I am glad that that Discovery Channel listened carefully to the complaints of a scientist.

Even so, the fact that this happened at all is troubling. Documentary filmmakers—who want to bring scientific discoveries to the public—should be just as concerned with accuracy as scientists themselves. Not all production companies are the same, and I am sure some are better than others (see some of the comments on Wedel’s first post), but in this case the desire to perpetuate a myth caused the filmmakers to twist a scientist’s words. This is a shame.

Cruisin' the fossil freeway, by Ray Troll. On view in Cruisin' the Fossil Freeway, December 19, 2009 – May 3, 2010, Burke Museum, Seattle.

Who’s up for a fossil road trip? In 2007  paleoartist Ray Troll and paleobotanist Kirk Johnson published a travelogue of their journey to check out the fossils of the American west called Cruisin’ the Fossil Freeway. It is not your average paleo book. Filled with Troll’s whimsical pop-art illustrations (how many dinosaur books feature sauropods crossing the road?) and Johnson’s entertaining descriptions of their journey, the book is simultaneously entertaining and informative. As I wrote when the book was first released:

From a backstage meeting with Ziggy Marley about a potentially new species of ancient marijuana to tracking down the ever-elusive fossilized tooth whorls of the shark Helicoprion, the book places the reader in the cab with the authors as they search the American west for fossil bones, ancient footprints, and a good country-fried steak. As any good paleontologist knows there’s much more to the science than bones collecting dust in museum drawers. Paleontology has an incredibly rich and adventurous history, and there’s no better way to pay homage to the great bone sharps of the past but to hop in the truck and kick up dust on the way to find the perfect ammonite or Triceratops skull. During the course of their trek the authors stop in to see local rock hounds, professional paleontologists, amateur fossil freaks, and others, illuminating the ever-changing landscape between searches for a decent hotel and a hot meal. What makes this book truly enjoyable, though, is that the authors realize that every fossil has at least two stories to tell; one being the story of the actual organism exhumed from the rock, but also the tale of its discovery (and in the case of some like the Tyrannosaurus “Sue,” even controversy).

It was one of the inspirations for me to finally head out west this past summer to see some of the fossil sites firsthand, and now Cruisin’ the Fossil Freeway has also inspired an exhibit of the same name at the Burke Museum of Natural History in Culture in Seattle. In addition to some of Troll’s art, the exhibit will include some of the fossil stars of the book, from gigantic carnivorous “pigs from hell” to dinosaurs. It will open tomorrow, December 19, and run through May 2010, after which it will go on a road trip of its own to other museums in the United States. If you live near Seattle and get to check it out, let us know in the comments what you think!

A skeletal restoration of Tawa hallae. Almost the entire skeleton was found. From the Science paper.

Part of what so fascinates us about dinosaurs is that they came in such a wide array of forms. Stegosaurus, Velociraptor, Brachiosaurus, Triceratops, Spinosaurus and more; they were all very different creatures. Yet we also know that dinosaurs share a common ancestry. If we had the bones of every dinosaur that ever lived we could start at any point and trace the evolution of dinosaurs to the last common ancestor of the entire group.

Unfortunately the fossil record does not contain a 100 percent complete record of ancient life. Only a very few creatures ever became fossilized, and of those even fewer have been found by scientists. Early dinosaurs, especially, are very rare, but in last week’s issue of Science a team of paleontologists announced the discovery of a dinosaur that helps explain that origins of one of the great branches of the dinosaur evolutionary tree. Named Tawa hallae, it provides some crucial clues to how predatory dinosaurs evolved.

Since the end of the 19th century it has been known that there are two major groups of dinosaurs. There were the ornithischians (the hadrosaurs, horned dinosaurs, ankylosaurs, and a few others) and the saurischians (the theropods and sauropodomorphs). Tawa was close to the origin of theropod dinosaurs, and by comparing it to other early dinosaurs paleontologists were able to refine their ideas about early saurischian evolution.

The bones of Tawa were found in approximately 215-million-year-old rocks of the Chinle Formation in New Mexico. This was a time when early dinosaurs were diversifying, but were not yet like the giants of the Jurassic and Cretaceous. Instead Tawa was a relatively small dinosaur that shared many traits in common with other early theropods such as Coelophysis, thus placing it as part of the early radiation of predatory dinosaurs.

What is even more interesting, however, is that the nearly complete remains of Tawa allowed the scientists to confirm other previously ambiguous dinosaurs as theropods. Since the time of their discovery, the evolutionary positions of Eoraptor and Herrerasaurus have been controversial, but their shared resemblances with Tawa confirm that they were among the earliest theropod dinosaurs. This not only allows scientists to better understand the origins of predatory dinosaurs, but to refine hypotheses of what to look for in the common ancestor of theropods and the sauropodomorphs. In fact, Tawa appears to be part of a radiation of early dinosaurs that migrated from what is now South America into what we presently call North America, confirming that the deposits of South America is probably still some of the best places to look for the earliest dinosaurs.

There is little doubt that the origin of theropods, and dinosaurs in general, will be debated for some time to come, but Tawa has helped to put some early forms in their place. For more on this new discovery, see Bill Parker’s post at Chinleana.

A restoration of the feathered dinosaur Microraptor flying through the trees. From Flickr user Cryptonaut.

One of the most important insights Charles Darwin had was that evolution does not follow a pre-ordained path. There is no evolutionary endpoint that organisms are striving toward. The “endless forms most beautiful” we observe in nature are both shaped by adaption to local conditions and constrained by the contingencies of their history, and it is impossible to predict what life might look like 1 million, 10 million, or 100 million years from now.

But paleontologist Simon Conway Morris begs to differ. He argues that some evolutionary outcomes are inevitable. If this were true then it could be argued that our species had been planned all along, thus allowing for a closer correspondence between cherished religious beliefs and what we know about nature.

Conway Morris makes his case primarily through identifying evolutionary convergences, a natural phenomenon in which two unrelated organisms independently evolve the same trait. If a particular trait has evolved multiple times, Conway Morris argues, then it can be treated as an inevitable outcome of evolution which therefore suggests (in his view) that life is being pulled in a particular direction by some supernatural force. In his latest exposition of this idea, published in the journal Naturwissenschaften, Conway Morris appeals to the evolution of birds to help support his thesis.

According to Conway Morris, “birds” evolved at least three times. In addition to the earliest recognized bird Archaeopteryx, the four-winged dromaeosaur Microraptor and its relative Rahonavis might be considered dinosaurs that independently acquired some degree of flight. For Conway Morris the convergent evolution of flying dinosaurs several times means that evolution is following a predictable pathway; if birds did not evolve from one lineage of feathered dinosaurs then they certainly would have evolved from another.

But there are some severe problems with this interpretation. First, it is still not entirely clear how Archaeopteryx, Microraptor, and Rahonavis flew, if they could fly at all. They were all small, bird-like dinosaurs that possessed what we call “flight feathers,” but this does not mean that they all flew or flew the same way. They may have been gliders rather than fliers, especially Microraptor, and while each is relevant to understanding the origin of birds we cannot honestly interpret each as definite evolutionary step towards birds of today.

Indeed, feathers and other “bird” characteristics were widely shared among dinosaurs that were not ancestral to birds. Birds evolved only once, and the alternative lineages Conway Morris sees as anchored to Microraptor and Rahonavis never came to be. He does not address why this might be so, but it has everything to do with the complementary roles of contingency and constraint in evolution.

In many cases, the dinosaurs that possessed “avian” traits such as feathers could not be easily modified into flying creatures. Many were too big, had the wrong kind of feathers, or just did not have a lifestyle in which gliding or flying would have been advantageous to survival and reproduction. Little quirks of evolutionary history and ecology constrained how feathered dinosaurs could be modified from one generation to the next, and it was in only one lineage, by chance, that the circumstances caused the first birds to evolve. Likewise, the fact that birds survived the mass extinction at the end of the Cretaceous could not have been predicted beforehand. Had they perished, we probably would not be spending so much time talking about them now as they would be, in our biased view, just another evolutionary dead end.

Despite all this, however, Conway Morris concludes: “Avian theropods are, therefore, pre-ordained.” I cannot help but think of this as a case of confirmation bias. In battling a straw man, “ultra-Darwinian” view, Conway Morris treats every case of convergence as evidence of inevitability in evolution, thereby ignoring the roles of contingency and constraint in shaping the tree of life. If a group of dinosaurs of similar body shape inherits feathers from a common ancestor, for example, then of course it would be expected that some of them might be adapted in similar ways given their shared characteristics. The resulting convergences would not be the result of inevitability, but based upon the constraints of surviving and reproducing given a common starting state.

Evolutionary convergence is a real pattern, and an interesting one at that, but it has to be understood as being couched within evolutionary history. It is not productive to simply cherry-pick occurrences of traits evolving more than once and then state that it was all meant to be.

At a grand scale, though, I think that the wider diversity of dinosaurs undercuts Conway Morris’ argument for evolutionary inevitability. Dinosaurs have been around for over 230 million years, and during their time on earth they have diversified into a wide array of unique forms that generally have not been duplicated by mammals. There have been some instances of convergence, as between the armored ankylosaurs and the hard-shelled mammals called glyptodonts, but you would think that if evolution was proceeding in a fore-ordained direction most dinosaurs would have made the “next step” to whatever Conway Morris believes should have come after them. Yet no such signal is readily visible. Hence the evolution of dinosaurs (and all other life)  is better viewed through Darwin’s perspective, and I think the notion that dinosaurs were not predestined makes them all the more fascinating.