He also found early horses , the Cretaceous and Jurassic dinosaurs Apatosaurus and Allosaurus , and described the toothed birds of the Cretaceous ; Ichthyornis and Hesperornis . He also found early horses , flying reptiles , the Cretaceous and Jurassic dinosaurs ; Apatosaurus and Allosaurus , and described the toothed birds of the Cretaceous ; Ichthyornis and Hesperornis .