>>Early evolution of birds

Early evolution of birds

Aves Archaeopteryx Pygostylia Confuciusornithidae Ornithothoraces Enantiornithes Ornithurae Hesperornithiformes Neornithes Basal bird phylogeny simplified after Chiappe, 2007

During the Cretaceous Period, birds diversified into a wide variety of forms. Many of these groups retained primitive characteristics, such as clawed wings and teeth, though the latter was lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest birds retained the long bony tails of their ancestors (birds such as Archaeopteryx and Jeholornis), more advanced birds shortened the tail with the advent of the pygostyle bone in the clade Pygostylia.
The first large, diverse lineage of short-tailed birds to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was the reverse of the condition seen in modern birds. Enantirornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. More advanced lineages also specialized in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds"). One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. Despite their extreme specializations, the Hesperornithiformes represent some of the closest relatives of modern birds.