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A second specimen of the troodontid Mei, preserved in a bird-like sleeping position. From Gao et al., 2012.

Bone by bone and study by study, paleontologists are learning more than ever before about dinosaurs. But there are still many aspects about prehistoric biology that we know little about. In fact, some of the simplest facets of dinosaur lives remain elusive.

For one thing, we don’t know much at all about how dinosaurs slept. Did Apatosaurus doze standing up or kneel down to rest? Did tyrannosaurs use their tiny, muscular arms to push themselves off the ground after a nap? And, given the discovery of so many enfluffled dinosaurs, did fuzzy dinosaurs ever cuddle up together to stay warm on chilly Mesozoic nights?

Since we can’t observe living non-avian dinosaurs directly, some of these questions have to remain in the realm of speculation. But a handful of fossils have shown us that at least some dinosaurs curled up just like birds. In 2004, Xing Xu and Mark Norell described the tiny, early Cretaceous dinosaur Mei long–a feathery troodontid dinosaur with big eyes and a little switchblade claw on each foot. What made Mei special, though, was the way the dinosaur was preserved.

Many articulated dinosaur skeletons are found in the classic dinosaur death pose, with their tails tilted up and their necks thrown over their backs. The nearly-complete skeleton of Mei was different. The foot-long dinosaur rested its head over its folded arms, and its tail wrapped around the dinosaur’s torso. Mei died sleeping in a roosting position similar to that of modern birds. The dinosaur’s name, which means “sleeping dragon,” is a tribute to the behavior.

Now another Mei specimen has confirmed that the first find was not a fluke. Last week, paleontologist Chunling Gao, of the Dalian Natural History Museum in China, and colleagues described a second, slightly smaller Mei that was preserved in a nearly identical sleeping position. Much like the first, this Mei probably died in a prehistoric ashfall that both killed and preserved the dinosaur in delicate detail without jarring the snoozing troodontid out of position. Some feathery, non-avian dinosaurs not only looked like birds, but they slept like them, too.

The two Mei specimens aren’t the only dinosaurs found in such positions. Gao and colleagues also point out that a specimen of another troodontid found in the Cretaceous rock of Mongolia, Sinornithoides youngi, was found in the same sort of sleeping position. And while not mentioned by the authors of the new study, the sleeping positions of Mei and Sinornithoides remind me of the early Jurassic dinosaur Segisaurus. Described in 1936, the partial skeleton of Segisaurus was found with its legs tucked beneath its body and arms apparently in a resting position. Perhaps this dinosaur, too, died while dozing, and records an even older record of how dinosaurs rested. Such glimpses are rare, but they help fill in some of the most elusive moments in Mesozoic history.

[Check out artist Julius Csotonyi's blog for a lovely new illustration of the second Mei specimen.]

Reference:

Gao C, Morschhauser EM, Varricchio DJ, Liu J, Zhao B (2012). A Second Soundly Sleeping Dragon: New Anatomical Details of the Chinese Troodontid Mei long with Implications for Phylogeny and Taphonomy. PLOS One DOI: 10.1371/journal.pone.0045203

“Feathers” by Randall Munroe, from http://xkcd.com/

Anyone who regularly reads this blog knows that there’s a very easy way to make me annoyed–all you have to do is start whining about how dinosaurs are less cool since paleontologists discovered that many non-avian species sported tufts and coats of fluff, fuzz, bristles and feathers. My reaction is usually along the lines of “Brian SMASH!” Even though I understand that some people find scaly, monstrous dinosaurs aesthetically appealing, I have no patience for the callow assertion that science has somehow ruined dinosaurs through the addition of plumage.

Cartoonist Randall Munroe summed up my feelings–albeit in a more concise and positive way–this week at XKCD. Restoring dinosaurs with protofuzz and feathers isn’t just about giving Tyrannosaurus, Velociraptor and company a new look. Dinosaur feathers, and feather-like structures, are allowing paleontologists to think of dinosaurs in new ways. In particular, Munroe cites a PLoS One study about how feathers may have played into the predatory behavior of sickle-clawed dromaeosaurs  such as Deinonychus. According to paleontologist Denver Fowler and co-authors, Deinonychus may have used its famous “killing claw” to pin down small prey just like modern hawks and eagles do. More than that, the avian raptors flap to help stabilize themselves while immobilizing their prey, and Deinonychus–almost certainly a feathered dinosaur–may have done the same.

Deinonychus might have flapped its arms to help restrain prey. Art by Emily Willoughby, image from Wikipedia.

We can’t know for sure whether Deinonychus killed prey like a big, grounded version of a hawk. But it’s possible. Either way, though, studies like these show that prehistoric dinosaur feathers are allowing paleontologists to look to modern birds to generate new hypotheses and tease out previously-unknown aspects of dinosaur lives. As I’ve mentioned before, feathers are the key to figuring out dinosaur colors. How wonderful is that? Again, Munroe says it better than I can: “The past keeps getting cooler!”

Post script: Munroe isn’t the only cartoonist to take on dinosaurs this week. FoxTrot’s Bill Amend had a few suggestions for the Smithsonian National Museum of Natural History’s dinosaur hall renovation. Paleontology curator Matt Carrano responded to the idea of installing a “Tourist Chompsognathus” at our Around the Mall blog.

I adore feathered dinosaurs. It feels a little strange to say that, but it’s true. Few things make me happier than seeing delicately-rendered restorations of theropods covered in fuzz and ceratopsians with some accessory bristles. The various bits of plumage–from quill-like structures to true feathers–make dinosaurs look even more wonderful and fantastic than the drab, scaly monsters I grew up with. And who wouldn’t love a fluffy like dinosaur like Sciurumimus, perhaps the cutest dinosaur of all time?

Of course, not everyone feels the same way. There are some people who want their dinosaurs to be scaly, scaly, scaly, science be damned. They weep, wail and gnash their teeth whenever a new study suggests that another branch of the dinosaur family tree might have been adorned with plumage. It’s as if they expect the Dinosauria to be consistent with an unchanging canon–sci-fi and comic fans suffer a similar apoplexy when one of their favorite characters deviates from their most cherished storyline.

io9′s “We Come From the Future” show recently debated whether science had “ruined” dinosaurs by decorating so many non-avian species with feathers. (Remember–birds are dinosaurs, too, and there have been some very scary birds in the history of life on earth). Granted, some restorations of feathery dinosaurs really do look stupid, and the minor plumes on the heads of Jurassic Park III‘s Velociraptor didn’t really help.

The show’s point-counterpoint debate on the matter isn’t totally serious, and it’s a way to get a tidbit of science out to a wider audience. That’s a good thing. All the same, I’m pretty sick of people who complain that feathers somehow detract from dinosaurian magnificence. How immature can you get? We all love the dinosaurs we first meet as kids, and, for many of us, those leviathans were drab and scaly. But those earlier versions have been slit from stem to stern by more active, colorful and complex dinosaurs, many of which had some kind of feather-like body covering. Which would you prefer? The scaly, sluggish pot-bellied Tyrannosaurus of the mid-20th century, or a svelte, agile predator that has a few patches of fuzz?

Don’t misunderstand me here. I’m not saying that all dinosaurs looked like big chickens. Dinosaurs exhibited an array of body structures–from simple, fuzzy tubes to bristles and full-on flight feathers. Some species, like modern birds, even exhibited several different types of feathers. The weird Beipiaosaurus, for one, had fuzzy protofeathers on much of its body but also had a sort of tail fan created by a different feather type. And “feathered dinosaur” doesn’t mean that the animal was entirely cloaked in plumage. Take Psittacosaurus, for example–this little ceratopsian was a very, very distant relative of birds and had a row of bristles along its tail. The structures were probably visual signals, and I have no doubt that same was true among other dinosaurs. Feathers aren’t just about flight or insulation, but they’re also important in display and communication.

And feathers are the key to dinosaur color. I’m still awestruck that we can recreate the colors of creatures that have been extinct for tens of millions of years. By comparing the microscopic details of prehistoric dinosaur feathers to the feathers of modern birds, we can finally answer that most persistent of paleo questions. That fact, alone, makes feathered dinosaurs especially magnificent.

I’m weary of this Portlandia-esque attitude that dinosaurs are over if they’re feathered. Please. New scientific discoveries are allowing us to gain unprecedented insights into the biology of dinosaurs, including the lives of the fluffy species. Feathers are just part of that bigger picture, and I’m ecstatic that paleontologists are reconstructing dinosaurs in ever-greater detail. The point is this. Feathered dinosaurs are awesome. Deal with it.

Archosaur skull changes (juveniles on the left, adults on the right). While there was a significant amount of change between the juvenile and adult skulls of alligators (top) and the non-avian dinosaur Coelophysis (middle), there was little change between the juvenile and adult skulls of early birds such as Archaeopteryx (bottom) and their closest dinosaur relatives. From Bhullar et al., 2012.

Birds are dinosaurs. That much is certain. That deep connection, bolstered by fossil finds and theoretical frameworks, has made dinosaurs seem more bird-like than ever expected. From feathers to nesting behavior, many aspects of avian natural history are now known to have originated among non-avian dinosaurs.

But non-avian dinosaurs were not just like birds in every respect. The way many dinosaurs grew is vastly different from the way avian hatchlings mature. Take Triceratops, for example. Even if we ignore the controversial “Toroceratops” hypothesis—which suggests drastic skull transformation late in the life of the horned titan—the reconstructed growth trajectory for this dinosaur involves major skull changes. The horns of young Triceratops curved backward but reoriented as the animal grew to point forward. And the little ornaments around the fringe of the dinosaur’s frill, called the epiossifications, transformed from high, pointy spikes to flattened structures.

Granted, Triceratops was about as far from being an ancestor of birds as is possible while still being a dinosaur. But major transformations seem to have been the norm among dinosaurs, from Tyrannosaurus to Diplodocus to Edmontosaurus. Juvenile dinosaurs had significantly different skull shapes from adults of the same species, and in some cases, it seems that juvenile dinosaurs were occupying different habitats or consuming different food than more mature individuals. (This would be a prehistoric example of what ecologists call niche partitioning.)

Despite the fact that birds represent the only surviving dinosaurian lineage, though, their growth pattern is different. Instead of going through a period of protracted change, as with non-avian dinosaurs, the skulls of young birds are anatomically almost identical to those of adults. And birds take a much faster, more direct route to maturation—many bird species grow to adult size in a year or less. As a new Nature study by Bhart-Anjan Bhullar and collaborators suggests, this feature of bird life can be traced back to ancient transformations that effectively locked bird skulls into a permanent juvenile anatomy.

Bhullar and co-authors used a technique called geometric morphometrics to survey the degree of skull change among birds, various non-avian theropods, the archaic archosaur Euparkeria and the modern American alligator. By tracking landmarks on the skulls in virtual models, the researchers were able to quantify how much the skulls of particular creatures changed. As expected, most non-avian dinosaurs retained the ancestral growth pattern—juvenile skulls were significantly different from adult skulls, regardless of how big those dinosaurs were.

The dinosaurs most closely related to birds showed a different pattern. The eumaniraptoran dinosaurs—the group that contains the sickle-clawed, feathery deinonychosaurs as well as birds—had skulls that looked more juvenile in form, and there was less change in shape between youngsters and adults. A juvenile skull form was undergoing little modification through maturity. Biologists know this as paedomorphosis, when descendent species resemble the juvenile stages of their ancestors.

What could trigger this kind of change? That’s difficult to say. Paedomorphosis is a result of modifications to growth, a developmental phenomenon regulated by particular genes. Bhullar and collaborators suggest that something happened to truncate the development of eumaniraptoran dinosaurs, which included the ancestors of birds. Quirks of development caused these dinosaurs to mature in a juvenile form. And while birds continued this trend in their evolution, its first glimmerings can be traced back to their non-avian ancestors. Non-avian dinosaurs are the key to understanding how birds came to be.

Reference:

Bhullar, B., Marugán-Lobón, J., Racimo, F., Bever, G., Rowe, T., Norell, M., & Abzhanov, A. (2012). Birds have paedomorphic dinosaur skulls Nature DOI: 10.1038/nature11146

A large stone slab containing mudcracks and many footprints left by small theropod dinosaurs, as illustrated in Hitchcock's "Ichnology of New England."

Edward Hitchcock was one of America’s first dedicated dinosaur paleontologists. He just didn’t know it. In fact, during the latter part of his career, he explicitly denied the fact. To Hitchcock, the tracks skittering over red sandstone in the Connecticut Valley were the marks of prehistoric birds from when the Creation was new. Hitchcock could not be dissuaded. As new visions of dinosaurs and the notion of evolution threatened to topple his life’s work, the Amherst natural theologian remained as immutable as the fossil footprints he studied.

Hitchcock was not the first to wonder about the prehistoric imprints. Members of the Lenape, a Native American group in Canada and the northeastern United States, had seen the bizarre, three-toed tracks and ascribed them to monsters and other beings. These were the footsteps of creatures that ruled the world before humans came to dominance. European settlers and their descendants had to stretch their mythology a little more to accommodate the tracks. Some thought such tracks might have been left by Noah’s raven after the biblical deluge, although many simply called them “turkey tracks” and apparently were little concerned with where they had come from.

It wasn’t until 1835 that James Deane, a doctor with a curiosity for natural history, found out about a sample of the peculiar tracks near Greenfield, Massachusetts. He knew that they represented prehistoric organisms, but he wasn’t sure which ones. He wrote to Hitchcock, then a geology professor at Amherst, to inquire about what could have left such markings in stone. At first Hitchcock didn’t believe Deane. There might be some quirk of geological formation that could have created track-like marks. But Deane was persistent. Not only did he change Hitchcock’s mind, but the geologist became so enthusiastic that he quickly became the most prominent expert on the tracks—a fact that frustrated Deane and led to tussles in academic journals over who really was the rightful discoverer of the Connecticut Valley’s lost world.

Hitchcock began publishing about the peculiar trace fossils in 1836. He was confident from the very start that they must have been created by prehistoric birds. (He was so enthused by the idea he even wrote poetry about the “sandstone birds.”) No variety of creature matched them better. The word “dinosaur” had not even been invented yet; the British anatomist Richard Owen would establish the term in 1842. The few dinosaurs that had been found, such as Iguanodon, Megalosaurus and Hylaeosaurus, were known only from paltry remains and all were believed to have been enormous variations of lizards and crocodiles. Dinosaurs were a poor fit for the tracks, and became even worse candidates when Owen gave them an anatomical overhaul. Owen not only named dinosaurs, he re-branded them as reptiles with mammal-like postures and proportions. The huge sculptures of the Crystal Palace exhibition, created with the help of artist Benjamin Waterhouse Hawkins, are a testament to Owen’s view of dinosaurs as reptiles that had taken on the anatomical attitudes of rhinoceros and elephants.

But Owen and other paleontologists did not agree with Hitchcock’s interpretation. They argued that the tracks could have been made by some unknown variety of amphibian or reptile. This was not so much because of the anatomy of the tracks—anyone could see that they were made by creatures with bird-like feet—but because no one thought that birds could have lived at so ancient a time or grown large enough to make the biggest, 18-inch tracks Hitchcock described. Even though early 19th century paleontologists recognized that life changed through the ages, they believed there was a comprehensible progression in which so-called “higher” types of creatures appeared later than others. (Mammals, for example, were thought to have only evolved after the “Secondary Era” when reptiles ruled since mammals were thought to be superior to mosasaurs, ichthyosaurs, and other creatures of that middle time.)

Hitchcock remained steadfast, and his persistence was eventually rewarded with the discovery of the moa. These huge, flightless birds recently lived on New Zealand—they were wiped out more than 500 years ago by humans—and in 1839 Richard Owen rediscovered the birds through a moa thigh bone. He hypothesized that the bone must have belonged to a large, ostrich-like bird, and this idea was soon confirmed by additional skeletal bits and pieces. Some of these ratites stood over nine feet tall. When the news reached Hitchcock in 1843, he was thrilled. If recent birds could grow to such sizes, then prehistoric ones could have been just as large. (And, though Hitchcock died before their discovery, preserved moa tracks have a general resemblance to some of the largest footprints from the Connecticut Valley.) Opinion about the New England tracks quickly changed. There was no longer any reason to doubt Hitchcock’s hypothesis, and paleontologists hoped that moa-like bones might eventually be found to conclusively identify the trackmakers.

Lacking any better hypotheses, Hitchcock prominently featured his avian interpretation of the three-toed tracks in his 1858 book The Ichnology of New England. It was a gorgeous fossil catalog, but it also came at almost precisely the wrong time. Gideon Mantell, the British doctor and paleontologist who discovered Iguanodon, was beginning to wonder if some dinosaurs primarily walked on their hind limbs in a bird-like fashion, and the Philadelphia polymath Joseph Leidy described Hadrosaurus, a dinosaur certainly capable of bipedal locomotion on account of having shorter forelimbs than hindlimbs, the same year that Hitchcock’s monograph came out. Dinosaurs were undergoing another major overhaul, and the few that were known at the time were being recast as relatively bird-like creatures. Even worse for Hitchcock, the following year another student of the Connecticut Valley tracks, Roswell Field, reinterpreted many of the footprints and associated traces as being made by prehistoric reptiles. Especially damning was the fact that deep tracks, left when the creatures sunk into the mud, were sometimes associated with drag marks created by a tail. Hitchcock’s tableau of ancient Massachusetts moas was becoming increasingly unrealistic.

If Hitchcock ever doubted his interpretation, he never let on. He reaffirmed his conclusions and modified his arguments in an attempt to quell dissent. In his last book, A Supplement of the Ichnology of New England, published in 1865, a year after his death, Hitchcock used the recently discovered Jurassic bird Archaeopteryx as a way to save his interpretation. Tail drags were no obstacle to the bird hypothesis, Hitchcock argued, because Archaeopteryx was generally regarded as being the primordial bird despite having a long, reptile-like tail. Perhaps such a bird could have been responsible for the trace fossils Hitchcock called Anomoepus, but the tail drags left by the animals that dwelled in Jurassic New England were also associated with tracks indicating that their maker walked on all fours. In response, Hitchcock cast Archaeopteryx as a quadrupedal bird—a representative of a new category different from the classic, bipedal bird tracks he had promoted for so long.

Other paleontologists took a different view. If Archaeopteryx looked so primitive and lived after the time when the red Connecticut sandstone was formed, then it was unreasonable to think that more specialized, moa-like birds created Hitchcock’s tracks. Furthermore, a few bones found in a Massachusetts quarry of roughly the same age in 1855 turned out to belong to a dinosaur—a sauropodomorph that Othniel Charles Marsh would later name Anchisaurus. The bird bones never turned up, and all the while dinosaur fossils were becoming more and more avian in nature. By the 1870s the general paleontological opinion had changed. New England’s early Jurassic was not filled with archaic birds, but was instead home to dinosaurs which were the forerunners of the bird archetype.

Our recent realization that birds are the direct descendants of one group of coelurosaurian dinosaurs has led some of Hitchcock’s modern day fans to suggest that he was really right all along. In an essay for the Feathered Dragons volume, paleontologist Robert Bakker extolled Hitchcock’s scientific virtues and cast the geologist’s avian vision for the tracks as essentially correct. Writer Nancy Pick, in her 2006 biography of the paleontologist, wondered, “What if Hitchcock clung to his bird theory because he was right?” But I think such connections are tenuous—it is a mistake to judge Hitchcock’s work by what we have come to understand a century and a half later.

While Bakker is right that Hitchcock stuck to his bird hypothesis early on because dinosaurs were not known in the 1830s to 1850s to be suitably avian, this does not explain why Hitchcock refused to entertain a dinosaurian origin for some of the tracks when evidence for such a connection began to accumulate. By sticking to the same point, Hitchcock went from being right to being so wrong that he tried to fit creatures like Archaeopteryx into the footprints to preserve his point. More importantly, though, Hitchcock promoted a variety of creationism that we would probably label as intelligent design today—he detested the idea of evolution by means of natural selection that Charles Darwin articulated in 1859. Hitchcock would not have accepted the idea that birds are the evolutionary descendants of dinosaurs. He likely would have rejected the idea of avian dinosaurs that some writers wish to attribute to him.

Hitchcock himself acknowledged that he was a stubborn man. Perhaps his obstinacy prevented him from accepting new ideas during a critical period of change within geology, paleontology and natural history. We may never know. Unless a letter or journal entry articulating his thoughts on the subject appear, his anti-dinosaur interpretation will remain a mystery. All we know for sure is that, regardless of whether he agreed with the label or not, Hitchcock was one of the first interpreters and promoters of North American dinosaurs.

References:

Bakker, R. 2004. “Dinosaurs Acting Like Birds, and Vice Versa – An Homage to the Reverend Edward Hitchcock, First Director of the Massachusetts Geological Survey” in Feathered Dragons. Currie, P.; Koppelhus, E.; Shugar, M.; Wright J. eds. Bloomington: Indiana University Press. pp. 1-11

Pick, N. and Ward, F. 2006. Curious Footprints: Professor Hitchcock’s Dinosaur Tracks & Other Natural History Treasures at Amherst College. Amherst: Amherst College Press.

Switek, B. 2010. Written in Stone. New York: Bellevue Literary Press. pp. 91-104