by Becky Crew
“Your Majesty?”
“ … My father …”
“Yes … ?”
“Nope, it’s gone. But that makes sense, because I’m the only person in this castle with a head too small to fit a reasonably sized brain inside, so why the hell aren’t you people doing my thinking for me? As King of Minorca by virtue of being the most giant animal here, I command you to go out there and sit on our enemies. How do you think I became king in the first place? Wits? And tell the kitchen I’m hungry again.”
WHAT A DIFFERENCE A few million years can make. We may be used to the relatively manageable bundles of fur and breeding that are modern rabbits, but no one was prepared for their ancestors to be hulking great stubby-eared monsters that can’t jump. In 2011, a team of paleontologists led by Josep Quintana from the Institut Català de Paleontologia in Barcelona announced the discovery of a new species of extinct rabbit called Nuralagus rex—literally, “King of the Rabbits.”
During the Late Neogene period, which ended about 2.5 million years ago, N. rex lived on Minorca, a small Spanish island belonging to the Balearic archipelago in the western Mediterranean Sea. So strange was N. rex that it took years after a complete skeleton of one was extracted from slabs of red limestone for researchers to identify it as a massive, lumbering rabbit. “When I found the first bone, I was nineteen years old. I was not aware what this bone represented. I thought it was a bone of the giant Minorcan turtle!” says Quintana.
Publishing in the Journal of Vertebrate Paleontology, the researchers described N. rex as having weighed around 27 pounds—six times the size of a regular European rabbit (Oryctolagus cuniculus). And unlike the long, flexible spine that modern rabbits have, N. rex had a compact, rigid backbone and short legs, which meant it couldn’t hop, or even run. More likely it was slow and hunchbacked, and spent its days burrowing for roots and lazing about on the Minorcan beach. It also had a compact head with beady eyes, short ears, and a small brain so, unlike modern rabbits with their keen eyesight, acute hearing, and incredible agility, the King of the Rabbits showed no sign of needing to spot and escape from predators.
According to the researchers, N. rex represented one of the earliest known cases in mammals of what’s known in evolutionary biology as Foster’s, or island, rule. This rule predicts that the smaller the island, the animals that are usually the large ones will grow progressively smaller, due to a lack of resources. And the animals that are usually the smaller ones, such as rabbits, will become bigger than their diminutive peers on the mainland due to a lack of predators. Because it shared Minorca with a few bats and mice, N. rex had little need to expend its energy on keeping watch for predators, so could instead concentrate all its efforts on growing huge.
Microraptor Shows Its True Colors
MICRORAPTOR
“Strip-o-gram!”
“He’s here! He’s here! He’s—Oh.”
“You’re disappointed?”
“Well I was kind of hoping for someone a little more, I don’t know, colorful. You’re sleek and sophisticated, I’ll give you that, but this is a hen’s night, not a fucking board meet—Oh my.”
“What?”
“Your feathers just caught the light and look, I’ve got goosebumps … Why aren’t you dancing!”
IN RECENT YEARS, SCIENTISTS have been developing new techniques that allow them to observe the composition of a fossilized feather down to its very molecules, which means we can finally discover the colors of the earliest birdlike creatures on Earth. In early 2012, researchers led by evolutionary biologist Ryan Carney from Brown University discovered that the feathers of the iconic birdlike dinosaur Archaeopteryx were structurally identical to those of modern birds. Which is pretty incredible, considering Archaeopteryx is 150 million years old.
The study of fossilized melanosomes—pigment granules that contain melanin, the most common light-absorbing pigment that gives skin, feathers, hair, eyes, scales, and certain internal membranes their color—is very young. Up until 2006, researchers had been misidentifying fossilized melanosomes as bacteria due to their similar, sausage-like shape. But then molecular paleobiologist Jakob Vinther from Brown University identified the presence of ancient melanosomes in the ink sac of a fossilized squid, which opened up a whole new chapter in the study of extinct birds and feathered dinosaurs.
In January 2012, Vinther and Carney published a paper in Nature Communications describing the results of their examination of the molecular structure of a fossilized Archaeopteryx feather. The well-preserved middle-wing feather also happened to be the first Archaeopteryx specimen ever described, unearthed in 1861 by paleontologist Hermann von Meyer near Solnhofen in Germany. Using a method based on the one Vinther had used years earlier, and a highly powerful type of scanning electron microscope at the Carl Zeiss Laboratory in Oberkochen, Germany, the researchers figured out how to image individual one-micron-long melanosomes. (There are 25.5 microns in one thousandth of an inch.) By comparing these melanosomes to those in the feathers of eighty-seven modern bird species, the biologists concluded that their Archaeopteryx feather was likely to have been black, with a 95 percent certainty.
Because black pigmentation is packed with melanin, which is a relatively robust polymer known to make the keratin in feathers thicker and around 40 percent harder, the researchers suggested that this meant Archaeopteryx was a well-equipped flyer. Filled with tough, black pigment, the flight feathers were likely protected against the motion of repetitive flight and the air particles that would continuously whoosh past. So Archaeopteryx had strong, durable feathers, but what Carney and Vinther’s research couldn’t answer was the question of whether it flew using powered flapping or simply gliding.
Just a few months after this paper was published, a separate research team published the results of an investigation into the color of Microraptor’s plumage. Microraptor was an exquisitely feathered, magpie-sized dinosaur with four wings and a long plume of tail feathers, living 130 million years ago in the region that is now northeastern China. The team, led by biologist Matthew Shawkey from the University of Akron in Ohio, analyzed the melanosome properties and arrangements in fossilized feathers from a previously undescribed but extremely well-preserved Microraptor specimen housed in the Beijing Museum of Natural History. “This type of preservation, while rare, is relatively common in fossils from this area of China,” says Shawkey.
Not only did Shawkey’s team find that Microraptor had robust, black feathers, similar to Archaeopteryx’s 20 million years prior, but unlike Archaeopteryx, these feathers were glossy like a raven’s.
The way keratin, melanin, and sometimes air are layered in the microstructure of a feather determines its glossiness, or iridescence. When the researchers compared the arrangement of the melanosomes in Microraptor’s preserved feathers to those of more than 170 modern birds of various colors, they discovered that they were most similar to the melanosome arrangements in modern iridescent feathers. As iridescence is known to play a key role in sexual signaling and display in modern birds, Shawkey’s team, who published their findings in a March 2012 issue of Science, suggested that the same was true for Microraptor. “Iridescence probably has multiple functions, but the one that is most well studied is sexual signaling,” says Shawkey. “Some studies suggest that females prefer to mate with more brightly colored iridescent males, and there is considerable dimorphism (the difference in color between males and females, females being duller) in many iridescent species, suggesting sexual selection.”
So Microraptor, draped in a glossy cloak of black, iridescent feathers that extended well beyond its tailbone, appears to have been built for sexual display, and the implications of research such as Shawkey’s was not lost on Carney. “The importance of this goes beyond simple aesthetics, as the presence of iridescence in Microraptor provides clues as to how the dinosaur might have lived and used its plumage,” he said.
What remains to be discovered is that considering Archaeopteryx, Microraptor, and a similarl
y small, Chinese feathered dinosaur called Anchiornis all had at least some dark plumage, this trait could have been present in the common ancestor of birds and their closest dinosaur relatives. “I think that’s pretty reasonable,” says Shawkey, “particularly given the ubiquity of melanin-based color in modern birds—literally every single clade (a group containing a species and descendants, both living and extinct) of modern birds has at least some melanin-based color.”
Night Vision for Hunters
VELOCIRAPTOR
“Yeah, I mean, I get why we hunt at night, I do, I get it. But I can’t help feeling like a bit of a serial killer, you know?”
WITH ITS SLEEK PHYSIQUE, serrated teeth, and a large, sickle-shaped claw on each hind foot, Velociraptor was built to kill. And new evidence suggests that this feathered carnivore would wait till nightfall before beginning the hunt.
Unlike mammals and crocodiles, all dinosaurs, birds, and some reptiles have a bony ringlike structure around the eye called a scleral ring. This helps to reinforce the structure of the eye, but can also reveal a lot about the habits of its owner, which is particularly important if they happen to be extinct. Postdoctoral researcher Lars Schmitz and vertebrate paleobiologist Ryosuke Motani from the Department of Evolution and Ecology at the University of California, Davis, figured out how to use the scleral ring to determine when a particular dinosaur was most active.
Nocturnal creatures such as the Philippine tarsier, the aye-aye, and most owls have very large eyes relative to their body size. This provides them with an increased retinal surface that can pick up as much of the very limited light available to them at night as possible. With this in mind, scientists can make the assumption that if an animal’s scleral rings are thin with a wide hole in the center, it is nocturnal, as the large hole allows for a larger retina, whereas thicker scleral rings with a narrower hole in the middle would belong to an animal that is most active during the day. And those animals that tend be active during both day and night will have scleral rings somewhere in the middle. “The larger the opening, the greater the total ‘amount’ of light that can pass through it,” says Motani. “Night-active animals need to collect more light than day-active animals so that the image formed on their retina is bright enough to stimulate the light-sensitive cells there.”
Motani and Schmitz measured the size of the eye socket and the inner and outer dimensions of the scleral ring in 33 Mesozoic fossilized dinosaurs, pterosaurs, and ancestral birds from 250–65 million years ago, plus those of 164 living species to check the accuracy of the method. They developed a new computer model that takes these measurements into account, plus the evolutionary relationships of the different species, to make predictions about what stage in the day each species was likely to have been most active. The animals could be categorized as follows: diurnal (day-active), nocturnal (night-active), cathemeral (day- and night-active), and crepuscular (twilight-active). “It was known that night-active animals tended to have larger eyes than their day-active counterparts, and also that these eyes had larger pupil openings for a given eye size,” says Motani. “What we did not know was if the difference between night- and day-active animal eyes was sufficiently large to allow estimation of diel (the period in a 24-hour day) activity patterns in dinosaurs.”
According to the researchers, who published the results in Science in early 2011, measurements of the scleral rings ranged from as small as 0.4 inches in Pterodactylus antiquus, a small pterosaur with a 39-inch-long wingspan and a soft tissue crest on the back of its head and webbed feet, to a whopping 3.7 inches in Saurolophus osborni, a rare herbivorous species of duckbilled dinosaur from Canada that grew to almost 33 feet long. They reported that the latter is twice the size of the emu’s scleral ring, which measures 1.4 nches, but is less than half the size of scleral rings found in giant marine reptiles called Ichthyosaurs.
They found that the majority of flying creatures in their sample, which included pterosaurs and ancestral birds such as Archaeopteryx, were diurnal. All herbivores, including Diplodocus and Protoceratops, were found to be cathemeral, except for the smallest analyzed herbivore, Agilisaurus louderbacki, an agile, beaked dinosaur that would likely dash on two legs and feed on all fours. Schmitz and Motani suggest that this trend was driven by the animals’ size and diet. Herbivorous mammals exceeding a body mass of 933 pounds need about twelve hours per day to forage for food, but they must also avoid the hottest parts of the day. This means to get enough food without overheating, they often need to straddle daytime and nighttime, just like today’s megaherbivores such as elephants.
But Velociraptors and all other terrestrial predators tested turned out to be mostly night hunters, which is the pattern seen in extant mammalian carnivores. This supports a previous study carried out in 2008 by evolutionary development biologist Martin Kundrát from Uppsala University in Sweden and Jiří Janáček, a biomathematics expert from the Institute of Physiology at the Academy of Sciences of the Czech Republic in Prague. Publishing in Die Naturwissenschaften, the researchers speculated that, based on the brain structure of Conchoraptor gracilis, this small, beaked hunter likely had excellent hearing—“an adaptation required for accurate detection of prey and/or predators under conditions of low illumination.” Unfortunately, bigger carnivores such as Tyrannosaurus rex have not provided us with sufficiently well-preserved scleral rings, so we remain in the dark about what time of day they liked to hunt.
Horses the Size of House Cats
SIFRHIPPUS
“What is this, a litter box? Look, I know I’m cat sized, but I’m still a horse. I have my pride. Now would you kindly move away from the cat door? I have appointments.”
FIFTY MILLION YEARS AGO, horses were the size of house cats. Sifrhippus, the earliest known horse, first appeared in the fossil record in North America about 56 million years ago. This coincided with what’s known as the Paleocene–Eocene Thermal Maximum (PETM), an extremely brief but critical time during which many of the most primitive ancestors of today’s mammals appeared, including the first horses, cows, and various primates. Over the 175,000-year period of the PETM, the average global temperatures shot up by about 4–6 degrees Celsius (7–11˚F), caused by a massive release of carbon dioxide into the Earth’s atmosphere and oceans. About a third of all mammal species at the time shrank significantly in response to this massive climate change event—one of the most rapid warming episodes in Earth’s history. Sifrhippus reduced its body size from just over 11 pounds in weight to no more than 8.5 pounds.
While scientists have known for a long time that animals have a tendency to be smaller in hot climates and larger in cold climates—a trend known as Bergmann’s rule, named after nineteenth-century German biologist Christian Bergmann, who in 1847 was one of the first people to propose it—what was unclear was whether the difference in size had to do directly with the temperature, or the availability of resources.
So a team of researchers led by Ross Secord, assistant professor in vertebrate paleontology and paleoclimatology from the University of Nebraska–Lincoln and Jonathan Bloch, associate curator of vertebrate paleontology for the Florida Museum of Natural History at the University of Florida, decided to examine the teeth of Sifrhippus to figure it out. “The first horse that came to North America was around the size of a small dog, like a mini schnauzer, then they immediately started to shrink to the size of a small cat. It took about 100,000 years—that’s fast,” says Bloch. “Various theories [exist] for why this change has happened. We’ve known that this interval is special, it’s the first appearance of these very important mammals, so that’s why I’ve focused on it for the past nine years. I’ve been trying to understand what’s happening to the animals during this time.”
Bloch, Secord, and colleagues had been working in the Bighorn Basin in north central Wyoming for almost ten years, examining the fossilized horse teeth from the PETM buried at the site. By analyzing the size of the teeth and the oxygen and carbon isotopes within (an isotope is a variant of a ch
emical element that scientists regularly use to figure out what the environment was like during a certain period of time), they teased out the progression of the ancient horse’s size and the corresponding temperature of its environment. They published the results in a 2012 issue of Science. “What makes this really special [is] from the composition of the teeth of the mammals, we could back out the temperature that the animals lived in,” says Bloch, “and were able to show how the animal changed during the time, and how the shifts in size correlated with the shift in temperature in something like a horse.”
What the researchers found when they plotted the size of Sifrhippus through time was quite stunning—a very clear, close correlation between the temperature of the environment and the horse’s sudden dip in size during the PETM, before increasing to almost 15 pounds during the following 45,000 years when the temperature returned to normal. What followed was an incredible diversification of the horse, which branched out from Sifrhippus into an array of species with very different histories. During the Miocene epoch, which spanned from 23 million years ago to just over 5 million years ago, the evolution of the Earth’s grasslands occurred, which changed things for horses dramatically because for the first time ever, they emerged from their homes in the forest. “It’s a whole different story,” says Bloch. Gradually, over tens of millions of years, the tiny first horse reached the solid 1100 pounds it is today.
