If geochronology is “the science of determining the ages of earth materials” (according to the center’s website), then Renne must know his gnat’s eyebrow. For those of us lay-folk, it’s about 5,000 years.

Renne and his colleagues have a new paper in Science pinpointing the dates of both the impact and the dinosaur extinction, placing them within the same time of each other—providing evidence, once again, for an asteroid or comet impact being the cause of extinction.

The 110 mile-wide Chicxulub (cheek’-she-loob) crater, off the Yucatan coast of Mexico, is likely the result of a six-mile in diameter asteroid or comet. Using and refining a technique called argon-argon dating, the scientists determined that the impact occurred 66,038,000 years ago, plus or minus 11,000 years.

The same argon-argon dating put the dinosaur extinction at 66,043,000 years ago, with the same margin of error.

The first link between the impact event and dinosaur extinction was published in 1980 by UC Berkeley’s Luis and Walter Alvarez. Since then, many other scientists have supported or refuted the theory, sometimes putting the extinction several hundred thousand years before the impact.

“When I got started in the field, the error bars on these events were plus or minus a million years,” says UC Berkeley paleontologist William Clemens. “It’s an exciting time right now, a lot of which we can attribute to the work that Paul and his colleagues are doing in refining the precision of the time scale with which we work.”

Despite the synchronous impact and extinction, Renne cautions that the impact was not the sole cause of extinction. Dramatic climate variation over the previous million years, including long cold snaps amidst a general Cretaceous hothouse environment, probably brought many creatures to the brink of extinction, and the impact kicked them over the edge.

“These precursory phenomena made the global ecosystem much more sensitive to even relatively small triggers, so that what otherwise might have been a fairly minor effect shifted the ecosystem into a new state,” Renne says. “The impact was the coup de grace.”

This isn’t just a question heard on the playground; it’s asked by scientists as well. Though many agree that the mountain-sized asteroid (that left the now-buried Chicxulub impact crater on the coast of Mexico’s Yucatan Peninsula) ultimately caused the mass extinction, some scientists continue to argue about the health of dinosaurs before the event.

A new study this week, published in the Proceedings of the National Academy of Sciences looks at the health of ecosystems from 13 million to 2 million years prior to the impact. The Academy’s Peter Roopnarine and his colleagues constructed food webs to examine the communities that lived in North America at the time.

In fact, Peter has spent the past eight years constructing paleo food webs, looking comprehensively at who lived in a particular place at a particular time, what they ate and who ate them. It’s a great way to see into the past.

Constructing these food webs involves looking at the fossil record—bone damage, stomach contents, etc.—and creating computer models. Peter says this involves a lot of data. He finds ecologically similar species that occupied a similar space and time and likely shared the same predators and prey. He starts by creating links between them and then builds from there.

Peter also looks at the present to reconstruct the past. He looks at how predators and prey are distributed in a modern ecosystem—there will only be so many top predators, for example, yet there will be many organisms at the bottom of the food web. Peter says that modern food webs are dominated by specialists—those that just eat a few organisms. But as you follow along, you’ll find a few generalists that consume an assortment of items.

Like a baseball statistician looking to build a new team, Peter uses these numbers and statistics from current players to input into his model to map past scenarios.

But, of course, the food web he models represents just one possibility of how that ecosystem looked at that time. So Peter runs the model thousands of times to get the average picture of the dynamics of the food web.

Now back to the dinosaurs… Running the food webs for 13 million years before the impact, Peter and his colleagues found that the ecosystems had been changing in North America. There was less diversity of species with lots of smaller vertebrates and very, very large herbivores. These changes could have taken place when the interior western seaway dried up, which would have affected climate and vegetation.

This doesn’t mean that the ecosystems were fragile by any means. Peter says. They were fine, robust even. Perhaps just a little less robust closer to the impact.

But when the impact hit, these small differences played a significant role, Peter explains. The ecosystems were a bit more vulnerable. “It was a bad time to be alive.”

“Our study suggests that the severity of the mass extinction in North America was greater because of the ecological structure of communities at the time,” notes Peter’s colleague and lead author of the paper Jonathan Mitchell of the University of Chicago.

And these small changes in the past can help us learn what might happen in future major events, says Peter. “What do extreme occurrences look like, where is our vulnerability, what does a recovery look like? Besides shedding light on this ancient extinction, our findings imply that seemingly innocuous changes to ecosystems caused by humans might reduce the ecosystems’ abilities to withstand unexpected disturbances.”

Pegomastax was a heterodontosaur, a genus of herbivores that lived when the supercontinent Pangaea had just begun to split into northern and southern landmasses. The single specimen of the new species was originally chipped out of red rock in southern Africa in the 1960s and discovered in a collection of fossils at Harvard University by Sereno decades ago, but only described now. (In an article in the New York Times, he describes how apologetic he is for not getting to it sooner.)

It’s name means “thick jaw from Africa,” which describes the cat-sized species well. Pegomastax had a short parrot-shaped beak up front, a pair of stabbing canines, and tall teeth tucked behind for slicing plants. The tall teeth in upper and lower jaws operated like self-sharpening scissors, with shearing wear facets that slid past one another when the jaws closed. The parrot-shaped skull, less than three inches long, may have been adapted to plucking fruit.

The canine teeth in heterodontosaurs often lead scientists to believe that these small dinosaurs ate meat or at least insects, but Sereno says they were more likely used in self-defense and competitive sparring for mates.

Another bizarre feature of Pegomastax are the porcupine-like bristles that likely covered its entire body. Sereno imagines these heterodontosaurs scampering around in search of suitable plants, looking something like a “nimble two-legged porcupine.”

A nimble two-legged porcupine that nips? As a pet? No, thanks.

Image: Tyler Keillor

A team of scientists, led by Ryan M. Carney at Brown University, have now taken that first Archaeopteryx fossil and analyzed the famous dinosaur-bird’s color and flight abilities.

A few years ago, some of the same scientists found a way to determine the color of feathered dinosaurs by looking at melansomes in fossils. As Carl Zimmer describes in his Discover blog:

Depending on the size, shape, and spacing of melanosomes, they can produce a range of hues. It turns out that melanosomes are incredibly rugged, sometimes enduring for millions of years.

(Science in Action covered this research here.)

Placing the fossil in a powerful type of scanning electron microscope, Carney and his colleagues measured the length and width of the Archaeopteryx’s sausage-shaped melanosomes, roughly 1 micron long and 250 nanometers wide. To determine the melanosomes’ color, the team compared the Archaeopteryx melanosomes with those found in 87 species of living birds, representing four classes: black, gray, brown, and a type found in penguins. “What we found was that the feather was predicted to be black with 95 percent certainty,” Carney says.

Next, the team sought to define the melanosomes’ structure with greater accuracy. For that, they examined the fossilized barbules—tiny, rib-like appendages that overlap and interlock like zippers to give a feather rigidity and strength. The barbules and the alignment of melanosomes within them, Carney said, are identical to those found in modern birds.

“If Archaeopteryx was flapping or gliding, the presence of melanosomes would have given the feathers additional structural support,” explains Carney. “This would have been advantageous during this early evolutionary stage of dinosaur flight… We can’t say it’s proof that Archaeopteryx was a flier. But what we can say is that in modern bird feathers, these melanosomes provide additional strength and resistance to abrasion from flight, which is why wing feathers and their tips are the most likely areas to be pigmented.”

The research was published today in Nature Communication.

Illustration: NobuTamura

ScienceNOW reports that:

Tails are often an enigma; many creatures have them, but scientists know little about their function, particularly for extinct species. Dinosaur tails are no exception. Researchers have speculated that some species’ tails were used in fighting, whereas others for stability.

Our friend Robert Full and his colleagues at UC Berkeley found how when leaping, red-headed African Agama lizards swing their tails upward to prevent them from pitching head-over-heels into a rock. You can see a video of this feat here.

“We showed for the first time that lizards swing their tail up or down to counteract the rotation of their body, keeping them stable,” says Full. “Inspiration from lizard tails will likely lead to far more agile search-and-rescue robots, as well as ones having greater capability to more rapidly detect chemical, biological or nuclear hazards.”

While Full is a biology professor, he is no stranger to robots, Scientific American reports.

These are just the latest developments in Full’s full-on flirtations with robots. He has worked with engineers since the mid-1990s when he helped to develop the crab-inspired Ariel, a minesweeping robot… that can look for buried explosives in surf zones. In 2008 Full co-founded the Center for Integrative Biomechanics in Education & Research (CiBER) at University of California, Berkeley, to further integrate the work of biologists and engineers when designing technology.

“Engineers quickly understood the value of a tail,” UC Berkeley engineering graduate student Thomas Libby explains. “Robots are not nearly as agile as animals, so anything that can make a robot more stable is an advancement, which is why this work is so exciting.”

Full and his team received a surprise benefit from the lizard tail research: understanding how dinosaur tails function.  The new research tested a 40-year-old hypothesis that the two-legged theropod dinosaurs—the ancestors of birds—used their tails as stabilizers while running or dodging obstacles or predators.

Indeed, just like the velociraptor depicted in the movie Jurassic Park, these agile dinosaurs may also have used their tails as stabilizers to prevent forward pitch, Full says. “Muscles willing, the dinosaur could be even more effective with a swing of its tail in controlling body attitude than the lizards.”

The research is published in the recent edition of Nature.

Image: Robert Full lab, UC Berkeley

Ok. No more movie voice. But where did the large sauropod Camarasaurus roam? A new paper in Nature may be closer to discovering the truth.

Scientists believe that some dinosaurs migrated seasonally for food, and now Henry Fricke and his colleagues at Colorado College may have the evidence—thirty-two Camarasaurus teeth.

Camarasaurus were enormous. With an average length of 50 feet, they ate about twice as much as today’s modern elephants—almost 1,000 pounds of food each day! As Fricke told Nature News,

They are huge—they would probably have eaten themselves out of house and home if they stayed in one place.

According to the fossil record, Camarasaurus inhabited the dry plains of western North America. The teeth the researchers sampled were all found in Wyoming and Utah, where the ancient seasonality record is pretty clear. ScienceNOW explains:

During the wet season, the prehistoric lowland basins of Wyoming and eastern Utah were flat, open habitats carpeted with ferns and stands of conifers. The researchers propose that the dinosaurs left this area at some point during the year, probably during the dry season, when the smorgasbord of tasty plants closed.

How did the researchers extract the evidence of movement from the Camarasaurus teeth? By measuring the ratio of isotopes within the enamel, New Scientist explains.

The ratio of isotopes is determined by the water the dinosaurs drank. [Fricke] found the ratio in teeth was different to that in carbonate rock from the floodplain—which carries the signature of the water it formed in. This suggests that Camarasaurus sometimes left the area.

Fricke and his team suspect the dinosaurs headed to higher, wetter ground seasonally, when the plains were dry, possibly traveling up to 180 miles.

The team plans to sample the teeth of other dinosaur species next. They have a hunch that where the herbivores traveled, the carnivores were close behind.

Image credit: Dmitry Bogdanov/Wikipedia

Paleontology graduate student Ryan McKellar discovered a wide range of feathers among the vast amber collections (more than 4,000 pieces!) at the Royal Tyrrell Museum and other Canadian museums. Much of these stem from Canada’s most famous amber deposit, near Grassy Lake in southwestern Alberta.

Out of the 4,000 pieces examined, McKellar turned up 11 that contained feather specimens.

While no dinosaur or avian fossils were found in direct association with the amber feather specimens, the feathers look to belong to both groups—avian and non-avian. The New York Times describes the feather structures that led to this conclusion.

One specimen of so-called proto-feathers [belonging to the non-avian dinosaurs] had a single bristlelike filament and some simple clusters. Others were complex structures with hooklike barbules that act like Velcro; in modern birds, this keeps feathers in place during dives. Still other specimens revealed feather patterns for flight and underwater diving.

The non-avian dinosaur evidence could point to small theropods as the source of the feathers. Some of the feather specimens with modern features resemble those of modern birds like the Grebe, which can swim underwater. The feathers can take on water giving the bird the ballast required to dive more effectively.

McKellar says the Grassy Lake find demonstrates that numerous evolutionary stages of feathers were present in the late Cretaceous period and that plumage served a range of functions in both dinosaurs and birds.

Nature News describes this evolution in detail:

Feathers’ evolutionary origin remains murky, but palaeontologists propose that they started off as simple, flexible filaments similar to those in the coat of ‘dino fuzz’ that covered the small predatory dinosaur Sinosauropteryx. From there, feathers adapted to become complex branching structures, eventually culminating in the asymmetrical flight feathers of the early bird Archaeopteryx and its living relatives.

The amber preserves microscopic detail of the feathers and even their pigment. McKellar describes the colors as typically ranging from brown to black. The ancient amber cannot be broken to discover more about the color, but high-resolution X-ray imaging could provide more information about the feathers’ melanosomes in the future.

The research was published today in the journal Science.

Image: Science/AAAS

In this corner, weighing in at 2,600 pounds, a 17-foot tall mammal, the giraffe.

And in this corner, weighing in at about 60 pounds, a 6½-foot pachycephalosaur, Stegoceras validum.

And judging this fierce competition, researchers from the University of Alberta.

Bell rings. Head-butting ensues… Well, virtually, anyway.

And the winner, published this week in PLoS ONE: the Stegoceras.

The small dinosaur with the domed head may have only been the size of a German Shepherd, but it was basically built for head-butting, the scientists discovered.

Actually, giraffes are terrible at head-butting. “They swing their necks at each other and try to hit each other in the neck or the side,” says study co-author Eric Snively, PhD. If giraffes do manage to butt heads, they can knock each other out because “their anatomy isn’t built to absorb the collision.”

Using CT scanning and a new statistical method for diagnosing behavior in fossil animals, the researchers compared the bony-headed dinosaur with many other modern ungulates (hoofed animals) that engage in different kinds of combat.

“Our analyses are the closest we can get to observing their behavior. In a way, we can get ‘inside their heads’ by colliding them together virtually. We combined anatomical and engineering analyses of all these animals for a pretty thorough approach,” says Snively. Sounds like fun.

Most current head-butting animals, like big horn sheep and musk ox, have domes like a good motorcycle helmet. “They have a stiff rind on the outside with a sort of a spongy energy absorbing material just beneath it and then a stiff, really dense coat over the brain,” says Snively.

The Stegoceras had an extra layer of dense bone in the middle. According to co-author Jessica Theodor, PhD, “It’s pretty clear that although the bones are arranged differently in the Stegoceras, it could easily withstand the kinds of forces that have been measured for the living animals that engage in head butting.” Making it a better head-butter than the best living head-butters!

And why all the head-butting? It’s is form of male-to-male competition for access to females, says Theodor. Or, as Wired UK put it:

Ladies love a good head-butter.

Nice work, Stegoceras.

Image: Eric Snively, Jessica M. Theodor,  PLoS ONE 2011

Looking at a 70 million year-old fossil skull of their closest cousin, Tarbosaurus bataar, researchers determined that young tyrannosaurs had a very different lifestyle than adults.

The skull, found in Mongolia, was part of an almost complete skeleton, missing only the neck and a portion of the tail. Based on careful analysis of the microstructure of the leg bones, co-author Andrew Lee of Midwestern University estimated that the juvenile was only 2 to 3 years old when it died. It was about 9 feet in total length, about 3 feet high at the hip and weighed about 70 pounds. In comparison, Tarbosaurus adults were 35 to 40 feet long, 15 feet high, weighed about 6 tons and probably had a life expectancy of about 25 years.

“This little guy may have been only 2 or 3, but it was no toddler…although it does give new meaning to the phrase ‘terrible twos,’” says Ohio University paleontologist and co-author Lawrence Witmer. “We don’t know to what extent its parents were bringing it food, and so it was probably already a pretty capable hunter. Its skull wasn’t as strong as the adult’s, and would have had to have been a more careful hunter, using quickness and agility rather than raw power.”

According to lead author Takanobu Tsuihiji of the National Museum of Nature and Science in Tokyo, “The younger animals would have taken smaller prey that they could subdue without risking damage to their skulls, whereas the older animals and adults had progressively stronger skulls that would have allowed taking larger, more dangerous prey.”

Co-author Mahito Watabe also speculates that “the young juvenile Tarbosaurus would have hunted smaller prey, perhaps something like the bony-headed dinosaur Prenocephale.”

“It’s one of the secrets of success for tyrannosaurs—the different age groups weren’t competing with each other for food because their diets shifted as they grew,” says Witmer.

The study is exciting, reports ScienceNow, because:

Such large dietary differences between a juvenile well past infancy and an adult are rare in the animal kingdom and unprecedented in the world of dinosaurs, the researchers say. If paleontologists didn’t know as much as they do about Tarbosaurus, say the authors, they would think this youngster belonged to a separate species.

This conclusion overturns the conventional wisdom that dinosaurs stayed active by day while early mammals scurried around at night. “It was a surprise, but it makes sense,” co-author Ryosuke Motani said.

Motani’s research focused on the scleral ring. Dinosaurs, lizards, and birds all have a bony ring in their eye, called the scleral ring (a structure not found in mammals or crocodiles). The researchers measured the inner and outer dimensions of this ring, plus the size of the eye socket, in 33 fossils of dinosaurs, ancestral birds, and pterosaurs. They took the same measurements in 164 living species.

In living species, diurnal animals have a small opening in the middle of the ring. In nocturnal animals, the opening is much larger. Cathemeral animals—active both day and night—tend to possess a scleral ring opening somewhat in between.

By looking at the living species, the UC Davis team confirmed that eye measurements accurately predict whether animals are active by day, by night, or around the clock. They then applied the technique to fossils from herbivorous and carnivorous dinosaurs, flying reptiles called pterosaurs, and ancestral birds.

The measurements revealed that the big plant-eating dinosaurs were active day and night, probably because they had to eat most of the time, except for the hottest hours of the day when they needed to avoid overheating. Modern megaherbivores such as elephants show the same activity pattern, Motani said.

Velociraptors and other small carnivores hunted at night. Flying creatures, including early birds and most pterosaurs, probably flew around in the daytime.

Nature News and Discover mention that this discovery could put the famous fossil of a Velociraptor battling Protoceratops in a new light. From Nature News:

…it provides even more information, suggesting that Velociraptor, a nocturnal predator, came upon the Protoceratops while it was resting at night.

Talk about a midnight snack!

Image courtesy of Kabacchi/Wikimedia