 
 <prompt> Wetin be di group wey cause birds to dey exist? Based on di question wey dey before dis one, write small tori wey contain di answer. E fit be 1 - 20 sentences. Context: <response>: The fossil record dey show say birds na di last dinosaurs wey survive, dem evolve from ancestors wey get feather inside di theropod group of saurischian dinosaurs. True birds first show face during di Cretaceous period, about 100 million years ago. DNA-based evidence dey find say birds diversify well well around di time wey di Cretaceous–Paleogene extinction event happen wey kill all other dinosaurs. Birds for South America survive dis event and come migrate go other parts of di world through multiple land bridges while dem dey diversify during times wey global cooling dey happen. Primitive bird-like dinosaurs wey no dey inside class Aves proper, for di bigger group Avialae, don dey found since di mid-Jurassic period. Many of dis early "stem-birds", like Archaeopteryx, no fit fly well well yet, and many still get primitive characteristics like teeth for mouth instead of beaks, and long bony tails. 