The Cretaceous Peak

The Cretaceous ran from about 145 to 66 million years ago, the longest stretch of the dinosaur age and the last. Two slow changes shaped it. First, flowering plants spread across the land, adding flowers, fruit, and broad leaves to a world that had been ferns, cycads, and conifers. Second, the old supercontinents kept pulling apart, opening seaways between the northern and southern landmasses.

That second change is the key to this chapter. When a sea cuts a continent in two, the animals on either side stop breeding across the gap and start evolving separately. Give that process tens of millions of years and you get whole faunas that share an ancestry but look like strangers. The Cretaceous is dinosaurs at their most divided, and so at their most varied.

The northern continents, especially western North America and Asia, hold the dinosaurs most people picture first. Here the tyrannosaurA large-headed, small-armed predatory theropod lineage (T. rex and kin).s rose to the top. Tyrannosaurus itself was one of the last of them, a bipedalWalking on two legs. carnivore with a massive skull, a long counterbalancing tail, and famously short two-fingered arms. The bones show it preyed on hadrosaurA 'duck-billed' dinosaur with batteries of grinding teeth; often crested.s and young armored herbivores and scavenged when it could. Whether it leaned more toward hunting or scavenging was argued for decades; most researchers now accept it did both.

Sharing that landscape were the ceratopsianA horned, frilled plant-eater like Triceratops.s, the horned and frilled plant-eaters. Triceratops carried three facial horns and a great bony shield, and lived right up to the end of the period. Bones bearing healed wounds and bite marks show it really was attacked, and on at least one skeleton the front limbs carry breakage and puncture marks from Tyrannosaurus.

The north also held the dromaeosauridA 'raptor' — a feathered theropod with a big sickle claw on each foot. raptors, small sharp predators with a sickle-shaped claw on each second toe. Velociraptor, from Mongolia, was about the size of a turkey, not the man-sized hunter of the films; those were modeled on its larger North American relative Deinonychus. The study of Deinonychus in the late 1960s reshaped how scientists pictured dinosaurs, swapping the old image of plodding reptiles for active, agile animals and helping launch the dinosaur renaissance.

No skin has ever been found with Deinonychus, so its covering is inferred rather than recorded. Velociraptor is better off: quill knobs, the attachment points for wing feathers, were reported on one forearm, and feathered relatives like Microraptor make a feathered Velociraptor very likely.

Feeding alongside them were the duck-billed hadrosaurA 'duck-billed' dinosaur with batteries of grinding teeth; often crested.s, the great herbivores of the north. Parasaurolophus carried a long curved tube crest swept back off its skull. Edmontosaurus lacked that hollow crest, was one of the largest hadrosaurs of all, and was still grazing the floodplains alongside Triceratops and Tyrannosaurus in the final scene of the Cretaceous.

South of the widening sea, on the southern continents, evolution ran toward sheer size. In North Africa lived Spinosaurus, a sail-backed theropodA mostly meat-eating, bipedal dinosaur. Birds are living theropods. with a long, low, crocodile-like skull and straight conical teeth. It is among the largest meat-eaters known, and the bones suggest a generalist that took both aquatic and land prey, biased toward fishing, perhaps a Cretaceous version of a grizzly bear at a river.

In Patagonia hunted Giganotosaurus, a giant of a different family whose remains have made it a rival to Tyrannosaurus for the title of largest land predator. The honest answer is that we cannot be sure which was bigger: the Giganotosaurus skeleton is only about 70 percent complete, so its size depends heavily on how the missing parts are reconstructed, and the largest published estimates may be exaggerated.

Largest of all were the herbivores they shared the land with. Argentinosaurus, a titanosaurA late, often armoured sauropod group; includes the largest dinosaurs. from Argentina, is among the biggest land animals that ever lived, possibly the very biggest. The catch is that it is known from only a handful of bones, some back vertebrae, part of the hips, a rib, and limb bones, so even its length is debated rather than measured.

Variety in the Cretaceous was not only about size and teeth. Some lineages went somewhere genuinely strange. Therizinosaurus, from Mongolia, was a theropodA mostly meat-eating, bipedal dinosaur. Birds are living theropods., a relative of the meat-eaters, but it grew the longest hand claws of any land animal, and used them on a slow, long-necked, pot-bellied plant-eater with a beak. When the first claws were found in 1948 they were so unlike a predator's that they were taken for the remains of a giant turtle.

The horned dinosaurs branched into their own variety too. Styracosaurus wore four to six long spikes off the rim of its neck frillThe bony shield extending back over the neck in ceratopsians., plus a cheek horn on each side and a single horn on its nose, a crown whose purpose, like most ceratopsian ornament, has been argued over for a century.