It is thought that Ichthyornis was the Cretaceous ecological equivalent of modern seabirds such as gulls, petrels, and skimmers.
An average specimen was the size of a pigeon, 24 centimetres (9.4 in) long, with a skeletal wingspan (not taking feathers into account) of around 43 centimetres (17 in), though there is considerable size variation among known specimens, with some smaller and some much larger than the type specimen of I. dispar.

Ichthyornis is notable primarily for its combination of vertebrae which are concave both in front and back (similar to some fish, which is where it gets its name) and several more subtle features of its skeleton which set it apart from its close relatives.
Ichthyornis is perhaps most well known for its teeth.
The teeth were present only in the middle portion of the upper and lower jaws.
The jaw tips had no teeth and were covered in a beak.
The beak of Ichthyornis, like the hesperornithids, was compound and made up of several distinct plates, similar to the beak of an albatross, rather than a single sheet of keratin as in most modern birds.
The teeth were more flattened than the rounded teeth found in crocodilians, though they became wider towards the base of the crown.
The tips of the teeth were curved backward and lacked any serrations.

The wings and breastbone were very modern in appearance, suggesting strong flight ability and placing it with modern birds in the advanced group Carinatae.
Unlike earlier avialans such as the enantiornithines, the species appears to have matured to adulthood in a rather short, continuous process.
It is thought that Ichthyornis was the Cretaceous ecological equivalent of modern seabirds such as gulls, petrels, and skimmers. An average specimen was the size of a pigeon, 24 centimetres (9.4 in) long, with a skeletal wingspan (not taking feathers into account) of around 43 centimetres (17 in), though there is considerable size variation among known specimens, with some smaller and some much larger than the type specimen of I. dispar.
Ichthyornis is notable primarily for its combination of vertebrae which are concave both in front and back (similar to some fish, which is where it gets its name) and several more subtle features of its skeleton which set it apart from its close relatives. Ichthyornis is perhaps most well known for its teeth. The teeth were present only in the middle portion of the upper and lower jaws. The jaw tips had no teeth and were covered in a beak. The beak of Ichthyornis, like the hesperornithids, was compound and made up of several distinct plates, similar to the beak of an albatross, rather than a single sheet of keratin as in most modern birds. The teeth were more flattened than the rounded teeth found in crocodilians, though they became wider towards the base of the crown. The tips of the teeth were curved backward and lacked any serrations.
The wings and breastbone were very modern in appearance, suggesting strong flight ability and placing it with modern birds in the advanced group Carinatae. Unlike earlier avialans such as the enantiornithines, the species appears to have matured to adulthood in a rather short, continuous process.
Ichthyornis