Birds Love

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 in reverse to that of modern birds. Enantiornithes 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 specialised 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, 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.