Hesperornis was a large bird reaching up to two metres or 6.5 feet in length. It had a long, thin, conical beak filled with tiny sharp teeth, tiny wings which were virtually useless, and powerful hind legs primarily for swimming. Though Hesperornis were cumbersome and slow-moving on land, in the water, they were very fast and powerful swimmers in the water.
Hesperornis nested in onshore rookeries, like modern penguins, and seemed to hunt in the nearby coastal waters for food. They were aggressive towards other animals when panicked, or when these animals entered their rookeries; but Hesperornis were also easily distracted from these other animals by splashes, and were (according to Helen Cutter) highly unintelligent and thus easy to distract.
Incursions and encountersAnomaly into the Dexter house's flooded basement in the present. There, when a plumber investigated the cellar, the Hesperornis attacked him. It is unknown what happened to the Hesperornis after, but it had apparently disappeared from the cellar by the time the Home Office and Special Forces had arrived at the Dexter residence.
When Nick Cutter swam through the Anomaly into a bay in the Cretaceous, a flock of Hesperornis were swimming around him. On the shoreline, Cutter encountered a rookery of numerous Hesperornis. Two investigated him and Helen Cutter, but Helen distracted them by throwing a stone into the nearby water.
A skeletal diagram of a Hesperornis was present on a backlit information board in one of the labs at the Anomaly Research Centre.
Hesperornis (name meaning "western bird") lived from approximately 84 to 78 million years ago. It was a large bird at approximately 6 feet in length, with a toothed beak for holding prey, almost no wings, and powerful hind legs for swimming. The toes were probably lobed rather than webbed as in today's grebes like in these the toes could rotate well, which is necessary to decrease drag in lobed feet but not in webbed ones like in loons where the toes are simply folded together. Like many other Mesozoic birds such as Ichthyornis, Hesperornis had teeth in its beak which were used to hold prey, most likely fish. In the hesperornithiform lineage they were of a different arrangement than in any other known bird or in non-avian theropod dinosaurs, with the teeth sitting in a longitudinal groove rather than in individual sockets in a notable case of convergent evolution with Mosasaurs.
- The coloration of the Hesperonis in Primeval was similar to that of the Hesperornis in Sea Monsters - A Walking with Dinosaurs Special.
- In Series 1, Hesperornis appeared in the episode recaps of every episode but the first.
- The Primeval merchandising mispelled Hesperornis as Hesperonis, missing out the second "r".
- In Primeval, Hesperornis was shown to be capable of standing upright; in reality its legs were too articulated for swimming to allow an erect stance on land.
- The Hesperornis in Primeval had scales while the real life Hesperornis would have had feathers.
- It could be seen that in the Primeval Hesperornis that its upper beak was full of teeth; but in reality, Hesperornis did not have that many teeth in its upper jaw, lacking teeth in the front of its beak. Plus, the teeth were much smaller in the real life animal than its depiction in the series.
- In Primeval, Hesperornis was depicted as visually interpreting its environment and focusing on something by looking ahead in the manner of a creature with binocular vision. In real life, an avian animal like Hesperornis would have more likely had monocular vision through which its vision was focused on what was on either side of its head.