Hesperornis

MEANING: Western bird

PERIOD: Late Cretaceous

CONTINENT: North America

The cormorant-like Hesperornis reached up to 1.8 m in length, had virtually no wings, and swam with its powerful hind legs. It had teeth present along nearly the entire lower jaw and the back of the upper jaw. Studies of the bone surface show that at least the tips of the jaws supported a hard, keratinous beak similar to that found in modern birds.

Hesperornis is from the Late Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Late Cretaceous making up roughly the second half of it, lasting from about 100 to 66 million years ago. It was a time of significant evolutionary change, with dinosaurs reaching their greatest diversity before the mass extinction at the end of the Cretaceous.

The Cretaceous was a period with a relatively warm climate, though the Late Cretaceous experienced a global cooling trend, caused by falling levels of carbon dioxide in the atmosphere. The continents were nearing their present positions, but high sea levels flooded low-lying regions, turning Europe into an archipelago, and forming the Western Interior Seaway in North America. These seas were home to a variety of marine reptiles, including mosasaurs and plesiosaurs, while pterosaurs and birds shared the skies.

On land, dinosaurs continued to thrive and diversify during the Late Cretaceous, producing many of the most well-known goups, including tyrannosaurs, hadrosaurs, and pachycephalosaurs. Established Cretaceous dinosaur clades like the ceratopsians, ankylosaurs, and dromaeosaurs continued to flourish. Sauropod species consisted almost exclusively of titanosaurs, which seemed to be confined to the Southern Hemisphere for much of the Late Cretaceous. Flowering plants and grasses diversified and spread, becoming the dominant flora similar to what we see today.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous-Paleogene (K-Pg) extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. This event, likely triggered by an asteroid impact, is marked by the abrupt K-Pg boundary, a distinct geologic layer separating the Mesozoic and Cenozoic Eras. In its aftermath, mammals and avian dinosaurs rapidly diversified, becoming the dominant land animals of the Cenozoic Era.

Hesperornis is a paravian. The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of theropod dinosaurs named Paraves. The Archaeopteryx has famously been known as the first example of a bird for over a century, and this concept has been fine-tuned as better understanding of evolution has developed in recent decades.

Like other theropods, all paravians are bipedal, walking on their two hind legs. Most of the earliest groups were carnivorous, though some smaller species are known to have been omnivores. Paravians generally have long, winged forelimbs, though these have become smaller in many flightless species. The wings usually bore three large, flexible, clawed fingers in early forms. Over time, the fingers became fused and stiffened in some lineages, and the claws reduced or lost. An increasingly asymmetric wrist joint allowed the forelimbs to elongate and an elaboration of their plumage eventually allowed the evolution of flapping flight possible.

Birds, as we know them today, are the only surviving lineage of dinosaurs. As these early paravian dinosaurs continued to evolve through the Late Jurassic and into the Cretaceous, they developed lighter skeletons, improved respiratory systems, and more specialized feathers, all of which contributed to better aerial capabilities. The classic features we associate with modern birds - beaks, powerful flight muscles, and highly refined feathers - gradually emerged over millions of years. By the end of the Mesozoic, true birds had diversified into a wide range of ecological roles, from fish hunters to seed eaters. While most dinosaur lineages vanished in the Cretaceous-Paleogene extinction event, this small, feathered group managed to survive and radiate into one of the most successful vertebrate clades alive today.