Neornithes

Neornithes is a subclass of birds that includes all modern and fossil birds except for the Archaeornithes. This group represents the lineage that gave rise to modern birds at the end of the Cretaceous period.12

Birds evolved from reptilian ancestors, specifically from a group of dinosaurs called theropods. This evolutionary path is supported by shared characteristics between theropods and birds, including the presence of feathers, which likely first evolved for insulation or display before becoming crucial for flight.34

The development of active flight in birds involved an increase in flight muscle size and the evolution of a keeled sternum for muscle attachment. Over time, feathers became specialized, and skeletal structures adapted for flight, leading to the diverse array of bird species seen today. Modern birds, belonging to the infraclass Neornithes, diversified significantly during the Cretaceous and subsequent periods.52