Hesperornis

Credit: Jack Wood.

*Hesperornis* is a large flightless bird that lived in the Late Cretaceous in the Western Interior Seaway.

## Description

*Hesperornis* was an exceptionally large sea bird and lacked wings. *Hesperornis* propelled through water with its long hindlimbs. The birds feet were once thought to have been lobed, like modern grebes, rather than webbed. However, studies on *Hesperornis* morphometrics show that webbed toes and lobed toes are equally as likely.

Similar many other Mesozoic birds, *Hesperornis* had a beak which was lined with teeth, sitting in a longitudinal groove rather than individual sockets. This feature was evolved independently evolved in mosasaurs. Teeth are present along the entire dentary, up to the upper maxilla and premaxilla. The predentary lacks teeth. Examinations on the jaws of *Hesperornis* show that the tip of the animals beak was likely covered in a keratinous sheath.

The birds palate bears small pits where teeth would lock into in life. Unlike most birds, *Hesperornis* bore a dinosaur-like joint in its jaws. This feature likely allowed the animal to articulate the mandible on its own, letting the lower teeth disengage.

## Paleoecology

*Hesperornis* primarily dove under the waves into schools of fish to snatch up fish and squid. Hesperornis was likely clumsy on terrestrial surfaces.

## Paleobiology

### Habitat

*Hesperornis* was primarily aquatic, living along shallow marine shelf seas like the Western Interior Seaway, Turgai Straight and the North Sea. Then, these waters were much warmer; subtropical. However, some *Hesperornis* have been found in inland bodies of freshwater, such as the Foremost Formation, suggesting populations moved partially or fully away from saltwater deposits at some point. H. altus is exclusive to Judith River Formation freshwaters.

### Propulsion and Gait

In terms of limb length, socket positioning and the hips form, *Hesperornis* likely swam like the modern loon. This entails swift underwater movements and awkward waddling on land. Also like the common loon, the legs were likely encased inside the bodies wall to the ankle, causing its lower legs to jut out from under its body located near its tail. This would have prevented the necessary standard bipedal gait needed for walking. Rather than walking, *Hesperornis* likely dragged itself forward via its legs, sliding its chest against the ground. However, recent studies suggest that *Hesperornis* functioned more similar to the (admittingly bipedal) cormorant.

### Juveniles

Juvenile *Hesperornis* grew very quickly at a constant rate until maturity. This is the case for modern birds, excluding enatiornithes.

### Pathology

One *Hesperornis* leg bone discovered in the 1960s, examined by David Burnham and Bruce Rothschild et al., shows prominent bite marks along the bone caused by an immature polycotylid plesiosaur, possibly *Dolichorhynchops* or similar genera. The condyle shows specific signs of infection and healing, meaning the bird escaped its predator and survived till fossilization. This finding was published in 2016, from the journal Cretaceous Research.

## Species

* *H. regalis*
* *H. crassipes*
* *H. gracillis*
* *H. altus*
* *H. montana*
* *H. rossicus*
* *H. bairdi*
* *H. chowi*
* *H. macdonaldi*
* *H. mengeli*
* *H. lumgairi*

## References

* https://www.britannica.com/animal/Hesperornis
* https://www.researchgate.net/publication/235963010\_Cretaceous\_foot-propelled\_bird\_Hesperornis\_its\_own\_way
* https://www.nationalgeographic.com/animals/facts/hesperornis
* https://www.mindat.org/taxon-P96488.html
* https://www.nature.com/articles/043176a0
* Dorling Kindersley Encyclopedia of Dinosaurs and Prehistoric Life, p. 142