Hesperornithiformes

Hesperornithiformes are an extinct order of aquatic, flightless birds that lived during the Late Cretaceous, roughly 95 to 66 million years ago. Fossils are most common in the marine settings of the Western Interior Seaway of North America but have also been found in Europe. They represent some of the earliest diving birds and are notable for their toothed beaks.

Morphology and ecology: Hesperornithiformes had long bodies, strong hind limbs, and feet adapted for propulsion in water, enabling efficient pursuit of prey underwater. Their wings were greatly reduced or possibly absent, and the sternum was not keeled, all features consistent with a flightless lifestyle. They retained teeth along the jaws, a primitive trait among birds, which aided in catching slippery fish.

Taxonomy and relationships: The order is divided into several families, including Hesperornithidae (for example, Hesperornis and Baptornis) and Parahesperornithidae (including Parahesperornis). They are part of Aves and are viewed as more primitive than modern Neornithes, occupying a distinct evolutionary branch that converges on aquatic avian life.

Paleoenvironment and extinction: Hesperornithiformes inhabited coastal and offshore habitats, often in brackish or marine waters. They went extinct at the end of the Cretaceous period, at the Cretaceous–Paleogene boundary, along with many other marine and terrestrial groups. Their fossils contribute important evidence about the diversity and adaptation of prehistoric seabirds and the early evolution of diving strategies in birds.