As it turned out, I found that in T. rex the V1 exit was bored precisely to avian specifications. We had expected to see it in coelurosaurs from the Cretaceous; finding it in carnosaurs, however, was a surprise. Further investigations showed that no Jurassic dinosaur had the avian exit arrangement, so this character seemed to mark one cluster of advanced Cretaceous families, some big, some small.
Over the next two years I continued my low-tech study of skulls, cranially abusing any number of large dinosaurs, while Phil conducted similar examinations of members of the Troödon family, small, slender- snouted, ostrichlike predators that include Osborn’s Saurornithoides. To examine the skulls of these creatures, Phil used a technique similar to mine but relied on fine needles instead of crude coat hangers. Meanwhile Andy took a far more sophisticated route, employing a tool never before used in the study of dino bones: the CT scan.
After borrowing an albertosaur skull from the Royal Ontario Museum, Andy obtained access to a local hospital CT scanner and passed the fossil through it. As we expected, the coat hanger and the CT scan yielded the same results: the V1 wiring pattern was indeed avian. Soon we were running other dinosaur skulls through the scanner, including the troödonts and the unusual specimen from Cleveland that we had named Nanotyrannus.
What we noticed right away was that, in addition to having an avian V1 arrangement, all the creatures were airheads. Modern birds have air chambers carved into their skull bones, and wide air ducts connect their cranial chambers to their windpipe and lungs (birds have air chambers nestled next to the intestines and liver too). Again, while such avian traits were not unexpected in advanced coelurosaurs, conventional wisdom said they shouldn’t be present in carnosaurs. However, the CT scans revealed that both the albertosaur and the nanotyrannosaur could match the coelurosaur in airheadedness. Further study of skull fragments from the T. rex revealed that even this mighty five-ton killer, which we’ve come to think of as the archetypal carnosaur monster, had pneumatic plumbing inside the skull.
Not only did this natural ductwork lead us to believe that carnosaurs, advanced coelurosaurs, and birds were more closely related than anyone had imagined, it also suggested that both big and little dinosaurs shared an unexpected feature: hot-bloodedness. One purpose of cranial air passages could be to help keep internal tissues cool. However, unless an animal’s metabolism runs hot in the first place, there’s no need for such organic air-conditioning.
As we compared cranial plumbing in skull after skull, we became convinced that the fundamental split between coelurosaur and carnosaur, believed in by Osborn a century ago and by most scientists since, had probably never happened. A little 20-pound Saurornithoides and a giant T. rex seemed to be small and large versions of one and the same advanced dinosaurian stock.
So we started coming up with a heterodox version of carnivorous dinosaur history. According to our new way of thinking, meat-eating dinosaurs evolved like this: An evolutionary improvement would suddenly appear in a species, and right away the new design would be incorporated into a wide array of descendant species, some very large, some very small. Repeated many times over 160 million years, this process was like a multiple skyrocket used on the Fourth of July--the main rocket keeps on going vertically, giving off sideways bursts of light every 100 feet.
Approximately 140 to 160 million years ago birdlike innovations appeared in the burst and expressed themselves in large and small dinosaurs alike. Suddenly a whole range of animals exhibited airheadedness, solitary V1 nerves, and other avian features. Some of those creatures, perhaps including Archaeopteryx, actually were birds, while others stayed on the ground, keeping their birdness in terrestrial mode. The descendants of these bird-dinos--Nanotyrannus, Troödon, and others--would die out at the end of the Cretaceous, while the true birds would fly on into the evolutionary future. The theory was not unlike Osborn’s, but it included one important difference: the avian innovations that he thought to be the exclusive province of the smaller, fleeter animals were, in fact, distributed in a much more egalitarian way. The idea that the little coelurosaurs had a monopoly on all these desirable features was quite likely a myth.
Of all the skulls we studied, from both the large and small flightless birds, by far the most startling was that of Nanotyrannus. The CT scans not only confirmed that this was indeed a new species of tyrannosaur, they also suggested that it was an extraordinarily high-tech one, possessing some traits seen only in coelurosaurs and some seen in no other dinosaurs at all. For example, earlier skull studies had revealed that all tyrannosaurs had a characteristic arrangement in which the vertical bone that makes up the midline bulkhead between the eyes is tilted up. But our scans showed that in the Nano-T this midline bone, known as the parasphenoid, goes straight forward like a long, sharp spearpoint; this is exactly the arrangement that we see in Saurornithoides and Troödon.
More important than its Troödon parasphenoid, however, was the Nano-T’s Troödon brain. There’s a widespread misconception (helped along by Gary Larson’s delightful Far Side cartoons) that all dinosaurs were pea- brained. It’s true that T. rex had a small brain compared with that of an elephant or a rhino of the same weight. But since the 1970s a wide array of smaller dinosaurs have been shown to have big brains, as large as those of birds of the same body bulk.
How do we know how big their brains were? Although brain tissue is the first to rot out of a carcass after death, the brain can leave indelible marks on fossil bone, revealing much about its size and shape. Big-brained dinosaurs had brain hemispheres that fit tightly into the braincase bones, so a clean, clear imprint is left on the inner bone surfaces. Small-brained species, T. rex included, had small, loose-fitting brains, covered with a thick layer of connective tissue. The interior of their cranium thus doesn’t show the brain shape clearly. Before we peered inside the head of Nano-T, no big dinosaur (over 1,000 pounds) had been found with a large, tight-fitting brain. As the Nano-T skull had its inner secrets exposed by the CT scanner, however, it was obvious that this species was the exception. The inner braincase bones showed a snug cerebral fit. This tyrannosaur had been outfitted with formidable mental equipment.
The list of Nano-T’s unexpectedly advanced features didn’t end with braininess, airheadedness, and bird nerves. Even more surprising was its snout architecture. The scan showed paper-thin sheets of bone running along the inner snout wall, curled around like bony cannoli (the Italian pastry). No one had seen such clear evidence of these structures, known technically as turbinals, in dinosaurs, but they are standard in some modern mammals. Dogs and hyenas, for example, owe their acute sense of smell to turbinals covered by thin layers of sensory tissue; the curl of the bony sheets increases the surface area of tissue that can be packed into the snout. To accommodate such a razor-sharp smell apparatus, the modern mammals have huge olfactory bulbs in their brains. Our CT scans revealed that the Nanotyrannus was similarly well-equipped: the braincase area was spacious enough to contain an olfactory bulb ten times bigger than that of the average modern bird, relative to head size.
The Nano-T was equipped with other superlative sensory hardware as well. Bones in the inner ear appear to have been surrounded by air chambers that would have increased sensitivity to low-frequency sounds made by rustling prey. Grooves on the cheekbones showed that the outer ear canals wrapped around the head so that both ears pointed forward. Like an old-fashioned hearing trumpet, these canals carried in sound from the front, providing the animal with stereo hearing that would have made it much better able to pinpoint the distance and direction of prey.
The Nano-T’s vision may have been similarly sharp, as indicated by its huge owl-like eye sockets and the prominent optic lobes of its brain. Since the brain wiring was very birdlike, it’s also reasonable to assume that Nanotyrannus had the full range of color vision possessed by hawks, eagles, and other modern birds.
And once the animal’s acute sense of smell, hearing, and sight helped it apprehend prey, its unusual jaw architecture would have let it dispatch what it caught very efficiently. Because of the extreme narrowness of the Nano-T’s muzzle, anything the animal bit down on would likely droop out over either side of its mouth. While this meant that less food would be contained in any one mouthful, it also meant that both of the animal’s parallel rows of teeth would get hold of the prey, greatly increasing gripping power. What’s more, the CT scans revealed that the Nano-T’s tooth crowns were nearly all the same height and were packed together tightly with few spaces in between. This would have provided an efficient, pinking shears- like bite, far more effective than what the T. rex could achieve with its irregular, gap-filled smile.
We may never know precisely which meat-eating dinosaur was the closest to modern birds, though more CT scans from T. rex and other large predators will help. For now, however, Nanotyrannus--with one foot in the carnosaur camp, one foot in the coelurosaur camp, and an eye cocked toward the avian future--is almost all the proof we need to claim that the old distinctions between big dinos, small dinos, and their feathered descendants must be discarded as obsolete.
A living Nano-T must have been an extremely alert and deadly predator. Scanning the Cretaceous meadows and forests for the faintest movement, sound, or scent of prey, the Nano-T represented the highest technology Darwinian processes had built into a carnivore. Of course, to the unfortunate creatures that populated the Cretaceous world alongside Nanotyrannus, arcane questions of speciation, relatedness, and genetic adaptations would have been utterly beside the point. As far as they were concerned, the swift, fierce, sharp-eyed predator stalking the ancient plains was simply the roadrunner from hell.