When Dinosaurs Roamed America

When Dinosaurs Roamed America was first revealed on the Discovery website in June 2001, originally with a July 13 airdate set.[5] It was also announced that clips from the program would be added to the website for the upcoming show.[5] Nine different clips were available, which showed bits of the five segments of When Dinosaurs Roamed America.[6] The site for When Dinosaurs Roamed America lasted until September 2008, when it became a redirect to the main Dinosaurs page on the Discovery website.

• Coelophysis
• Rutiodon
• Desmatosuchus
• Icarosaurus
• Traversodont
• Mammal (live-acted by an eastern quoll)
• Locust

The program starts in the Late Triassic, near modern-day New York City in the Newark Supergroup. The narrator explains how the Permian mass extinction led to new forms of life, including, eventually, the most extraordinary creatures ever to walk the planet, the dinosaurs. The camera tracks a Coelophysis through the woods. The program depicts Coelophysis as preying mainly on small animals, such as insects and Icarosaurus. It encounters other, larger non-dinosaurian archosaurs such as Rutiodon and Desmatosuchus. The quick Coelophysis is portrayed as a very successful inhabitant of this world.

• Dilophosaurus
• Coelophysis kayentakatae (identified by its synonym Syntarsus)
• Anchisaurus

The program moves on to the Early Jurassic of Pennsylvania, showing a pack of Syntarsus.[note 1] These dinosaurs, closely related to Coelophysis, are hunting the primitive herbivorous dinosaur Anchisaurus; one of the Syntarsus is killed when the initial attack fails, and they are chased away by a Dilophosaurus, which kills the Anchisaurus to feed her young. A second Dilophosaurus arrives as well, and after a short but vicious fight it is driven away, allowing the first Dilophosaurus and her young to feed in peace. The narrator then explains Syntarsus and Dilophosaurus will become the gigantic carnivores such as Allosaurus and Tyrannosaurus, while Anchisaurus will become the sauropods.

• Apatosaurus
• Dryosaurus
• Ceratosaurus
• Stegosaurus
• Camarasaurus
• Allosaurus
• Pterosaur (possibly represented by either Comodactylus or Rhamphorhynchus)

The show skips to prehistoric Utah during the Late Jurassic period. The region has been engulfed by a severe drought as the seasonal rain has failed to arrive. A predatory Ceratosaurus stalks a family of Dryosaurus, including a mother and two youngsters. The predator eventually breaks its cover and charges after the small dinosaurs as they scatter. The Dryosaurus can only flee but the Ceratosaurus is quicker than they are and catches up, grabbing and killing one of the juveniles. The Ceratosaurus feasts while the others escape into a grove of pine trees and run into a herd of sauropods called Camarasaurus. They will be safe around the gentle giants. A male Stegosaurus fights off the attacking Ceratosaurus, the same individual from the earlier sequence, later on using his spiked tail and follows a female Stegosaurus by constantly displaying his plates. Eventually, the female decides that he is a healthy individual and the two mate. With the onset of the rainy season, a herd of Apatosaurus arrive, followed by an Allosaurus who launches an assault against the herd while the sauropods graze but is unsuccessful due to their size and strength. The Allosaurus subsequently kills the Ceratosaurus that attacked the Dryosaurus and finally is able to feed. Later while on the move, one Apatosaurus stumbles over a fallen tree and tumbles off of a 20 foot high cliff. The sauropod is injured with a broken leg, and its agonized bellowing is picked up by a pack of hungry Allosaurus. When the Allosaurus arrive, they begin to eat the sauropod alive.

• Dromaeosaur [note 2]
• Nothronychus
• Zuniceratops
• Suskityrannus (identified as coelurosaur)

The program then shows a forest located in New Mexico during the Middle Cretaceous period. Small predatory coelurosaurs scamper through the foliage and steal pieces of meat from a dead Zuniceratops. This smaller cousin of Triceratops had been killed by raptors and the hungry dinosaurs are feasting. The raptors chase off a lone dromaeosaur as it tries to steal some meat. The lone dromaeosaur then tries to attack a grazing Nothronychus, only to be slashed by its long claws and knocked over. Uninjured, the raptor retreats. An old male Zuniceratops is battling for dominance with a younger but healthier male whilst the herd members look on. The younger dinosaur gores the older herd leader with its right horn, wounding its frill. The sounds of battle are picked up by raptors and the hungry creatures follow the sounds to a clearing and watch from the foliage as the battle plays out before attacking.

The injured Zuniceratops is attacked by the pack of dromaeosaurs and is fatally wounded. Another Zuniceratops headbutts the attacking dromaeosaur and tosses it off the struggling male. The dromaeosaurs retreat but the old Zuniceratops will not last long. Weeks later, a thunderstorm blows in and lightning illuminates the darkened skies. Panicked dinosaurs scatter but the old Zuniceratops cannot get up. As it sounds its distress call, the dromaeosaurs return joined by a fourth member and surround the wounded dinosaur. The raptors attack and soon kill it. Meanwhile, lightning ignites the dry vegetation. Fire springs up, and most of the dinosaurs scatter in all directions. Zuniceratops panic for safety and the Nothronychus follows. However, the feasting dromaeosaurs are too distracted by eating and fire surrounds the region. The four raptors burn to death along with their prey. But some other raptors do flee and make it to safety along with some of the other creatures.

• Tyrannosaurus
• Triceratops
• Edmontosaurus (identified as its former synonym Anatotitan)
• Quetzalcoatlus
• Ornithomimus (unidentified; revealed on website)
• Purgatorius (live-acted by opossums)
• Turtle (live-acted)
• Bird (live-acted)
• Spider (live-acted)

The program explains that dinosaurs similar to Zuniceratops evolved into the famous Triceratops. In the Late Cretaceous, Anatotitan and Triceratops browse on a rolling grassland bordered by tropical jungle, while Ornithomimus peck at roots and other plants in the area. Flying pterosaurs such as Quetzalcoatlus soar overhead, looking for carcasses. A young Tyrannosaurus rex arrives on the scene, and the Triceratops form a defensive circle around the juveniles and display their powerful horns whilst the Tyrannosaurus attempts to get through and roars repeatedly but the Triceratops stay to fight instead of fleeing. Unable to get past the horns of the defensive Triceratops, the Tyrannosaurus attacks a Quetzalcoatlus, but the pterosaur launches off and flies away from the hungry theropod. The Tyrannosaurus goes back into the trees and the Triceratops become less agitated.

At night, the young Tyrannosaurus returns to its family, and the mother Tyrannosaurus chastises two of her young after they hit her scarred leg while sparring. The next day, the young Tyrannosaurus and its siblings are taught by their mother to hunt. They target a herd of Anatotitan grazing in a forest clearing and after bursting from the trees, the herd scatters and flees. The three Tyrannosaurus chase after one individual and it runs straight into the forest where the mother emerges from the bushes, and, grabbing it, killing the unfortunate hadrosaur by breaking its neck. She then roars before the feast begins.

As they begin to feast, a huge asteroid, 6 miles across, hurtles towards the planet at 45000 miles an hour and as it enters the atmosphere friction turns it into a blazing missile. The asteroid crosses the ocean in just 4 minutes, crashing into the Gulf of Mexico. The impact gouges out a crater 120 miles wide and sends an incandescent plume of dust, glass and ash into the atmosphere which falls back to earth as fiery debris. The blast wave radiates outward from the impact in a circle and in minutes, everything for hundreds of miles is incinerated by the intense heat or blown apart by the blast wave. Dinosaurs in the region are vaporized in a matter of minutes.

In North America, plants and animals suffer a different fate. Some are incinerated by the mounting heat whilst others succumb to shock waves generated by the collision. Fleeing Triceratops and Anatotitan are caught by the speeding ejecta cloud and destroyed. The feasting Tyrannosaurs watch in horror as a burning blast wave hurtles towards them and flee as pieces of fiery rock rain down. Eventually, all the region's dinosaurs die including the Tyrannosaurus family. A few hours after impact, a heavy cloud of dust and ash settles over America, and temperatures drop as sunlight and heat can no longer reach the surface of the planet. Gasses such as nitrogen and carbon dioxide are burned by the heat and are washed out of the atmosphere as acid rain.

Two months after impact, the sun finally reaches the surface as the heavy cloud of ash clears away. The disaster is over, but 90% of all leaf bearing trees, ferns, vines and plants have been obliterated, and 70% of the animals have died out. Most dinosaurs are extinct.

Despite the depressing and traumatic event, life is described as being resilient, and a turtle is shown emerging from the water and a bird flies overhead, explained by the narrator as the only dinosaurs left. Out of the ashes and charred debris, several small possum-like mammals (Purgatorius) emerge, and the narrator explains that small mammals such as these will eventually evolve into humans, and think back in awe to a time "when dinosaurs roamed America".