Hesperornis (Western Bird) is a genus of cormorant-like ornithuran seabird that lived in North America, Russia,[6][7][8] and southern Sweden,[9] 83.6 - 72 million years ago, during the Campanian stage of the Late Cretaceous Period. Remains found in Canada indicate that Hesperornis may have lived up to the early Maastrichtian stage of the Late Cretaceous Period.[10] The animal depicted on Prehistoric Planet may be any of the eight species that lived near the Western Interior Seaway of North America, but it is likely Hesperornis regalis, the type species.
Hesperornis is a large seabird that grew approximately 1.5 - 1.8 meters (5 - 6 feet) in length,[1][PhP 1] and weighed around 10.6 kilograms (23 pounds).[2] It is virtually wingless, but it makes up for it with its streamlined body and powerful hind legs that helped it swim quickly. Hesperornithiform toes are believed to be lobed similar to those of modern-day grebes, hence, Prehistoric Planet depicts Hesperornis with lobed feet.[11] However, a study in 2019 suggests that webbed toes for hesperornithiforms were just as likely as lobed toes. Furthermore, the same study brings up how hesperornithiforms have long been considered similar to grebes and loons, which were ungainly on land, but analysis of hesperornithean hindlimbs suggest that they had more in common with the upright-walking cormorants and diving ducks, hence, while hesperornitheans were meant more for a life at sea, they were more competent in terrestrial locomotion than once thought.[12]
Growth[]
Hesperornis reached skeletal maturity a year after hatching, as indicated by analysis of its bone microstructure, which showed that their bones underwent rapid, sustained growth without pauses or interruptions, in sharp contrast with the cyclical growth of enantiornithine birds. Consequently, this means that their bones do not record whether they migrated during winter, or simply stayed where they were and endured the cold months, though it is possible that Hesperornis hatchlings simply reached their full size before their first winter, or that their bone microstructure lacked the necessary microstructure to record changes in growth related to migration or overwintering. Overall, these studies challenge the reliability of using bone microstructures to infer the effects of behavior and climate on avian growth, and also suggest how growth dynamics in birds differed from those of non-avian dinosaurs.[3][4]
Jaws and Dentition[]
“ There's no escape for any unfortunate fish once it's caught by this beak, full of needle-sharp teeth. ”
Like many other Mesozoic birds, Hesperornis had teeth in its hooked, keratinous beak, fit for gripping fish. As with all hesperornithiform birds, Hesperornis had its teeth placed in longitudinal grooves, making it similar to some ctenochasmid pterosaurs like Pterodaustro, which possesses a thousand bristle-like teeth in its lower jaw. This trait is known as aulacodonty, where an animal's teeth are either placed in grooves rather than individual sockets, or possess sockets not made of bone. The palate (mouth roof) of Hesperornis has small pits for the teeth of the lower jaw to fit in when the mouth is closed.[5]
Prehistoric Planet's Portrayal[]
Hesperornis concept art by Gabriel N. Ugueto
Prehistoric Planet depicts Hesperornis with feathers in white, gray, and black tones, providing it with a countershaded coat. Most animals, predator and prey alike, are known to rely on countershading to effectively camouflage themselves, blending in with darkness when seen from above, and blending in with light when seen from below. The integument of Hesperornis resembles those of puffins, penguins, and even sharks, which come in dark grays and blacks opposite their white or light gray undersides. Hesperornis does have some accents on its coat in the form of whitish speckles on its back along with white eyebrows and cheeks. Its yellow-orange eye rings, beak, and throat also further stand out from its black, white, and gray appearance.
“ By day, the prehistoric ocean provides opportunities for a very different type of hunter. In the warm, shallow seas of North America, fish numbers can almost match the nocturnal lantern fish shoal. And they are a magnet for 6 foot-long Hesperornis. ”
Eight recognized Hesperornis species are known from the Western Interior Seaway of North America. The ninth and largest species (and largest hesperornithine bird overall), Hesperornis rossicus (along with a possible tenth Hesperornis species), lived in various parts of Russia[6][7][8] as well as the Kristianstad Basin of northeastern Skåne, the southernmost province of Sweden,[9] extending the possible range of this seabird to other parts of the Turgai Strait, the Eurasian equivalent of the Western Interior Seaway, covering areas north of the Caspian Sea all the way to the "paleo-Arctic" region, separating Europe from Asia.
It is unknown whether Hesperornis migrated in response to wintertime or stayed in the same place all year-round, as their bones, which suggest that they reached maturity within a year of hatching, bore no signs of cyclic growth marks, zonation, or differences in vasculature, a similarity they share with living brush-tailed penguins.[4]
Paleofauna[]
“ Xiphactinus, known as 'X-fish'. A feeding opportunity soon attracts them in large numbers. At over 17 feet long, they are one of the largest and fastest fish in the ocean. ”
Similar to penguins, Hesperornis is capable of remaining submerged in the sea for extended periods of time to hunt for fish. This ability, along with its small size and "warm-blooded" metabolism, allows it to be an efficient and agile hunter. Even so, as a small animal, Hesperornis was likely quite low in the overall food web of the ecosystems it inhabited, considered a light snack by predators like Squalicorax (also known as the "crow shark"), the ichthyodectid bony fish Xiphactinus, Cretoxyrhina (also known as the "Ginsu shark"), various mosasaurs, and other large hunters.
Hesperornis concept art by Gabriel N. Ugueto, featuring Dolichorhynchops in the background
“ The Hesperornis have only one option; to swim for their lives. Xiphactinus is faster; Hesperornis is more agile. ”
Fortunately, despite not being as big or as fast as these threats, the same traits that make Hesperornis an effective hunter also allow it to evade its pursuers with quick, acrobatic maneuvers. In 2016, a Hesperornis leg bone discovered in the Pierre Shale in the 1960s was examined. It bore bite marks left by a polycotylid plesiosaur (possibly a young Dolichorhynchops). Despite suffering grievous injuries, the bird, a juvenile by the time of the attack, managed to escape from its hunter's grip, endure the resulting infection to its condyle, and continue growing until it reached skeletal maturity. This proved to be an exceptional discovery, for not only does it preserve a predator-prey relationship, but it also showcases the animal's unexpected survival into adulthood, a rare, unlikely outcome for supposedly-frail organisms like birds.[13] Given that Dolichorhynchops appears to have originally been planned to appear alongside Hesperornis in the second segment of Oceans,[PhP 2] it is possible that Prehistoric Planet had, at one point, intended to reference the evidence that polycotylids preyed on Hesperornis.
Hesperornis may have lived up to the earliest point of the Maastrichtian stage as implied by fossils in Canada,[10] but it ultimately died out long before the Cretaceous-Paleogene Extinction Event that wiped out three-quarters of all life on Earth by the end of the Cretaceous Period, 66 million years ago. The circumstances surrounding the extinction of Hesperornis remain unknown. Some members of the same ecosystem, like Cretoxyrhina, perished before the end of the Campanian stage due to the shrinking of the Western Interior Seaway by that time, coupled with competition against larger and more powerful mosasaurs worldwide. It is therefore reasonable to hypothesize that Hesperornis succumbed due to similar reasons, like changes in their chosen environmental biomes, competition against more efficient piscivores (fish eaters) aiming to occupy the same niche, or possibly even the disappearance of preferred prey that were also driven to extinction by these various factors.
Appearance[]
The second segment of Oceans focuses on what happens during the day in the warm, shallow seas of North America. A massive shoal of fish attracts many Hesperornis, the flightless seabirds catching several fish with their beaks full of needle-sharp teeth. This great gathering of fish also attracts several Xiphactinus, which swallow mouthfuls of fish whole.
“ At first, there is plenty for everyone. But as fish numbers dwindle, the X-fish turn their attention elsewhere. Predator is about to become prey. ”
Suddenly, several Hesperornis are attacked by Xiphactinus, with some even being swallowed whole. The surviving Hesperornis frantically attempt to escape the predators with evasive maneuvers. While Xiphactinus is faster, Hesperornis is more agile, allowing some of the birds to repeatedly dodge their pursuers just a second before getting grabbed and devoured.
“ But in the eyes of an X-fish, anything is on the menu... even their own kind. ”
Following this, one Xiphactinus is shown chewing on the body of a smaller X-fish, for their voracious appetites meant that they are willing to hunt anything they can reasonably tackle. With the shoal of fish reduced to nothing, the banquet is over, and the predators move on.
