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Henry De la Beche's cartoon "Awful Changes", lampooning Charles Lyell's concept that dinosaurs and other prehistoric animals may one day reappear. From Wikipedia.

When the American Museum of Natural History’s paleontologist William Diller Matthew published his book Dinosaurs in 1915, no one understood how the famous Mesozoic creatures originated or went extinct. Both the beginning and end of the “Age of Dinosaurs” were mysterious. Yet, tucked away in a footnote, Matthew made a suggestion that dinosaur-like animals might return someday.

Matthew based his speculation on the idea that the ancestors of dinosaurs resembled modern day lizards. His summary was somewhat contradictory—the long-legged, “more or less bipedal” dinosaur ancestors he described would have looked significantly different than living reptiles—but Matthew affirmed that the immediate precursors of dinosaurs “were probably much like the modern lizards in size, appearance, and habitat.” Following from that connection, he wrote:

If some vast catastrophe should today blot out all the mammalian races including man, and the birds, but leave the lizards and other reptiles still surviving, with the lower animals and plants, we might well expect the lizards in the course of geologic periods to evolve into a great and varied land fauna like the Dinosaurs of the Mesozoic Era.

Matthew was not the first to suggest that dinosaurian creatures might reappear. In the first volume, published in 1830, of his influential book Principles of Geology, the Scottish geologist Charles Lyell linked the character of the Earth’s fauna to climate over long geological cycles. The dinosaurs and other prehistoric reptiles had thrived during a long-lasting summer, and should the Earth’s geology again approach the state it had been in during that time, lush forests inhabited by dinosaurs and pterosaurs might return. Life was closely tied to climate, Lyell thought, and climate was regulated by gradual geological change.

(This was not an evolutionary connection, but an argument that particular kinds of organisms were closely tied to certain climates. The mechanism by which dinosaurs might reappear was not specified by Lyell.)

Lyell’s idea of Iguanodon and Megalosaurus once again roaming the earth was lampooned by his colleague Henry De la Beche in a cartoon called “Awful Changes.” Set in some unknown future time, a highly-intelligent Ichthyosaurus takes up the mantle of paleontologists and, presenting a human skull to an assembly of marine reptiles, says: “You will at once perceive that the skull before us belonged to some of the lower order of animals; the teeth are very insignificant, the power of the jaws trifling, and altogether it seems wonderful how the creature could have procured food.” The thought that ichthyosaurs, dinosaurs, or pterosaurs would suddenly reappear someday was absurd.

Neither did Matthew escape criticism. The naturalist John Burroughs was incredulous at the suggestion that something like “Brontosaurus” might wallow in stagnant, warm swamps in a hypothetical future. In a letter printed in Natural History, the magazine published by the AMNH, Burroughs rhetorically asked: “Does not the evolutionary impulse run its course? Can or will it repeat itself?” Burroughs answered with an emphatic “no.” Dinosaurs were specialized to the unique conditions of their time—part of the gradual evolutionary development of the planet—and the Earth could no more revert to a previous state, the naturalist concluded, than a fruit reverse the process of ripening.

Matthew’s lengthy response was printed right below the critical letter. The offending passage was simply a bit of speculation, Matthew replied, and it had nearly been cut out of the manuscript. Yet, since Burroughs was inspired to write a letter about it, clearly the suggestion that dinosaur-like organisms might reappear was a subject of interest worth considering.

In fashioning his reply, Matthew followed in Lyell’s tradition. Life had not been moving in a straight-line direction from beginning to an unforseen end. Our planet has instead gone through a series of cyclical changes which have influenced the evolution of life on Earth. Evolution has proceeded along some “upward steps,” Matthew conceded, but these have been within the context of environments that have appeared over and over again through time. Dinosaurs, for example, were thought to inhabit a world generally similar to that of the present day despite all the intervening changes between their time and ours, and so the objection that the world was too different could be set aside.

But Matthew was not proposing the return of true dinosaurs. He was considering the evolution of dinosaur-like creatures from modern reptiles. If mammals and birds were suddenly wiped out, then the field would be open for reptiles to proliferate and evolve:

Certainly such an expansive evolution of the lizards with their higher competitors removed would not cause the huge Brontosaurus to reappear on earth. But it might—if we accept the modern theory of geologic history—bring about the appearance of gigantic wading or amphibious reptiles equally huge and equally innocuous, although probably not at all like a Brontosaur in appearance.

Life will continue to change over time. That is inevitable. How life will evolve is another matter. There is no pre-determined evolutionary pathway or trajectory. The history of life on Earth is heavily influenced by contingency—what came before provides context for what comes after—and there is no inherent direction that guarantees the reappearance of dinosaurs or dinosaur-like animals.

Strangely, though, we now know that the origin of dinosaurs at least partially owes to a devastating mass extinction that took place approximately 251 million years ago. Our own ancestors and close relatives among the weird and wonderful synapsids were the dominant terrestrial vertebrates just prior to this time, but they were almost entirely wiped out. This set the stage for a proliferation of other creatures, including the ancestors of dinosaurs. Then, 65 million years ago, the non-avian dinosaurs were wiped out in another mass extinction, allowing for the evolutionary radiation of mammals. These rare, global-scale catastrophes wipe out some species and fortuitously provide new opportunities for surviving lineages. If we were to suffer another event right now, of the kind Matthew wondered about, who knows what life would look like 10, 50, or 100 million years from now?

The 165-million-year-old spider Nephila jurassica. (Scale bar is 5 mm). From Selden et al., 2011.

Just in time for Halloween 2008, several gruesome spider photographs popped up in the news. The shots recorded two incidents—both of which took place in Queensland, Australia—of huge golden orb weaver spiders eating birds that had flown into the webs of the arachnids. Birds aren’t exactly a staple of the golden orb weaver diet, but the spiders make the most of the occasional bad luck of small birds.

I was reminded of the photos by a discovery announced just this week in Biology Letters. Not only have paleontologists discovered the fossil of a large orb weaver spider, but it comes from Jurassic fossil beds well-known for small, feathered dinosaurs.

Named Nephila jurassica by Paul Selden, ChungKun Shih and Dong Ren, the newfound orb weaver dates to about 165 million years ago and is the largest fossil spider yet discovered. Its leg span stretched about 6 inches, which makes it comparable in size to females of some living golden orb weaver species. The fossil specimen was also a female—indicated by the presence of an organ called the epigyne—but, unlike modern-day golden orb weavers, males may have been just as big.

In living species, male golden orb weavers are much, much smaller than females. This difference in anatomy between the two sexes is known as sexual dimorphism. Based on a 130-million-year-old fossil of a male golden orb weaver—called Cretaraneus vilaltae—found in Spain, however, it seems that this was not always a characteristic of the group. The fossil male spider from Spain was large and probably did not differ in size from the female. If this is true, then the larger Jurassic species described by Selden and colleagues probably had males and females of equal size, too.

These spiders wove their silken nets in warm Jurassic forests filled with insects, but I wonder if they ever caught dinosaurs by accident. Nephila jurassica was found in the Daohugou Beds of northeastern China. These rocks have also yielded the finely-preserved remains of several feathered dinosaurs—Pedopenna, Epidexipteryx and Scansoriopteryx. Of these, both Epidexipteryx and Scansoriopteryx were small dinosaurs—between the size of a sparrow and a pigeon—that may have spent at least some of their time in the trees. They could not fly and so would not accidentally flutter into a golden orb weaver web, but I wonder if unlucky Scansoriopteryx or Epidexipteryx sometimes blundered into the spider webs and became prey for the arachnids. Barring the discovery of a truly spectacular fossil, we will probably never know, but, if it did occur, the frightening photos of Australia are glimpses of interactions between feathered dinosaurs and spiders that stretch back a very long way.

References:

Selden, P., Shih, C., & Ren, D. (2011). A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China Biology Letters DOI: 10.1098/rsbl.2011.0228

A plastic dinosaur battle spotted in Washington D.C. Photo courtesy Amanda Fiegl.

Most of the dinosaur sightings readers have sent in are of big creatures, but this week we have dinosaurs on a smaller scale.

Following up her last submission of a giant pliosaur—which wasn’t a dinosaur, but a fearsome Mesozoic marine reptile—former Food & Think blogger Amanda Bensen (now Fiegl) has sent us a photo of a dinosaur action scene from Washington, D.C. I like it. It’s a nice mix of some of the old, grotesque dinosaur toys I had as a kid and some of the newer models. I wonder which side will win? The grumpy, outdated dinosaurs, or their supercharged replacements?

Have you seen a dinosaur (or other ancient beast) in an unusual place? Please send us your sighting via dinosaursightings@gmail.com!

The skull of Deinonychus. Notice the reconstructed sclerotic ring inside the eye opening. From Wikimedia Commons.

Randall Munroe, the creator of the webcomic XKCD, isn’t going to like this one bit. Fear of attack by Velociraptor is a running theme in the science-themed series—lazy computer programmers should be especially wary—and two separate discoveries announced last week gave those with a phobia of raptors good reason to barricade the doors and windows. Not only did Velociraptor have an excellent sense of smell, but they also hunted at night.

We don’t know for sure what dinosaur eyes looked like. The soft-tissue structures rotted away between the time of death and preservation. But there was one feature of the skull that allowed paleontologists Ryosuke Motani and Lars Schmitz to approach the question of whether some dinosaurs were active in the dark—a circle of bones called the sclerotic ring.

Though relatively rare in the dinosaur fossil record, sclerotic rings can give paleontologists a general picture of eye size and shape. This is because the bone surrounded the pupil and the iris of the eye. Birds, lizards, and other vertebrates have this feature, too, and the details of the sclerotic ring are closely associated with the light conditions when an animal is active.

Modern-day nocturnal animals tend to have wide sclerotic rings with a very large aperture in the middle relative to eye size. Animals that are more active during the day (diurnal), on the other hand, have smaller apertures relative to their eye size. By tracking this association, Motani and Schmitz were able to detect that dinosaurs were active during all times of the day.

(The study also included analysis of pterosaurs and other archosaurs, but I am going to restrict my comments to the findings about dinosaurs here.)

As a group, the dinosaurs did not all neatly fall into nocturnal and diurnal groups. Herbivorous dinosaurs, in particular, appear to have been cathemeral—they would have been active over short periods of time during the day and night. Rather than foraging continuously from dawn until dusk, herbivorous dinosaurs such as the hadrosaurs  Corythosaurus and Saurolophus, the small ceratpsian Protoceratops, the sauropodomorph Plateosaurus and the sauropod Diplodocus were probably most active during the early, cool parts of the day and then again around twilight.

Small, predatory dinosaurs were different. Almost all the carnivorous dinosaurs that were examined had sclerotic rings consistent with a nocturnal lifestyle, including Juravenator, Microraptor and—you guessed it—Velociraptor. Based upon the inferred night-hunting habits of Velociraptor and the cathemeral pattern of Protoceratops, Motani and Scmitz suggest that the deadly encounter between the two species immortalized in the “fighting dinosaurs” specimen probably happened at twilight or in low-light conditions.

Not all theropod dinosaurs stalked prey by night, though. The small predator Sinornithosaurus appears to have had the more varied schedule seen among the herbivores, and this was also found for the omnivorous “ostrich mimic” dinosaurs Garudimimus and Ornithomimus. Early birds—the descendants of small, feathered theropods—were different. Every species in the study—Archaeopteryx, Confuciusornis, Sapeornis and Yixianornis—had eyes specialized for daytime activity. Perhaps, during early bird evolution, there was a transition from nocturnal ancestors to flying descendants active during the day.

These findings change our perspective of what Mesozoic life was like. Dinosaurs were thought to be mostly active during the day, with small mammals—including our ancestors and cousins—coming out at night. Now it seems that the Cretaceous nights were not as safe as had been presumed. With so many agile predatory dinosaurs around, mammals would have much to fear during the nighttime hours.

Then again, the idea that Mesozoic mammals scurried through the night is an assumption based upon the idea that dinosaurs were stomping around during the day. Studies of the mammals themselves will be needed to see how their activity overlapped with that of the dinosaurs. Since mammals lack sclerotic rings, though, some other technique will have to be used. Further studies of dinosaurs will be required, too. Conspicuously missing from the study were large-bodied predators akin to Allosaurus and Albertosaurus. When these giants hunted, and when the mammals under their feet were active, awaits future study.

For more, see Schmitz’s own post on the research at his blog and Ed Yong’s report at Not Exactly Rocket Science.

References:

Motani, R., & Schmitz, L. (2011). PHYLOGENETIC VERSUS FUNCTIONAL SIGNALS IN THE EVOLUTION OF FORM-FUNCTION RELATIONSHIPS IN TERRESTRIAL VISION Evolution DOI: 10.1111/j.1558-5646.2011.01271.x

Schmitz, L., & Motani, R. (2011). Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology Science DOI: 10.1126/science.1200043

The way I look at dinosaurs now isn’t the same way I looked at them when I was five or 10. Like the above video from a Sydney school shows, kids still feel that mix of joy and fright when they get up-close-and-personal with dinosaurs. That kind of interaction can be used to educate—as museums such as the Natural History Museum of Los Angeles and the Utah Museum of Natural History do with their dinosaur shows—and it can also be tapped for theme park scares.

Though the video shows an actor in a dinosaur puppet costume, it reminds me of the popular robotic dinosaur displays I saw when I was about the same age. I was simultaneously enthralled and terrified by them. Years before computer-animated dinosaurs were a regular staple of TV and movies, they were the closest thing to living dinosaurs I had ever seen. I still remember peeking out from behind a wall at the robotic Tyrannosaurus and Triceratops in a temporary exhibit at the Morris Museum, fearing that they might snatch me up and eat me if I got too close.

I have mixed feelings about those animatronic dinosaurs. As Stephen Jay Gould pointed out in his essay “Dinomania” in Dinosaur in a Haystack, the ranks of jerking, growling robots are welcomed into zoos and museums in the hope that visitors will then wander among more educational exhibits and learn something before leaving—but this is more of a hope than a reality. Presented in the right way, galleries of animatronic dinosaurs could be very educational, but often they are more akin to theme park attractions than anything else.

That’s the trouble with dinosaurs. Not only were they living animals that are objects of scientific study, but they are also malleable cultural icons that can terrify as much as enlighten. Mixing the two—using their monstrous appearance to educate—is a tricky act.

[Hat-tip to Love in the Time of Chasmosaurs for the video]